Genotypic variation in sweet potato under drought: physiological, biochemical, and morphological insights

Variação genotípica em batata-doce sob estresse por déficit hídrico: aspectos fisiológicos, bioquímicos e morfológicos

Abstract

Sweet potato is widely cultivated in tropical and subtropical regions due to its nutritional value and agronomic versatility. Although often regarded as drought-tolerant, the crop is vulnerable to water deficit, which negatively affects yield and storage root quality. Assessing and understanding how sweet potato genotypes respond to water deficit is essential for identifying drought-tolerant candidates and supporting breeding programs aimed at improving resilience in drought-prone environments. This study aimed to evaluate the physiological, biochemical, and morphological responses of sweet potato genotypes under water deficit, with the goal of identifying drought-tolerant genotypes and better understanding their responses to water deficit. A completely randomized design was adopted in a 2×20 factorial scheme with four replications. The first factor consisted of two irrigation regimes (20% and 100% of pot capacity), and the second of 20 sweet potato genotypes. Parameters related to chlorophyll fluorescence, biomass accumulation, water potential, gas exchange, biochemical markers, and secondary metabolites were evaluated. Results revealed considerable genotypic variability in response to water deficit. Drought stress affected physiological traits associated with photosynthetic performance and oxidative metabolism, although the intensity of responses varied among genotypes. Principal component analysis highlighted contrasting adaptive strategies involving physiological regulation, antioxidant responses, and root growth adjustments under drought conditions. Genotypes ‘Luiza’, ‘Canadense’, ‘CIP-420717’, ‘Maria Isabel’, and ‘CIP-440186’ exhibited superior performance and represent promising sources for drought-tolerance breeding in sweet potato. Their superior performance was associated with higher water use efficiency, maintenance of photosynthetic traits, and lower oxidative damage under drought conditions.

Keywords:
Ipomoea batatas (L.) Lam.; abiotic stress; drought; plant physiology; photosynthesis; chlorophyll fluorescence

Resumo

A batata-doce é amplamente cultivada em regiões tropicais e subtropicais devido ao seu valor nutricional e à sua versatilidade agronômica. Embora frequentemente considerada tolerante à seca, a cultura é vulnerável ao déficit hídrico, o que afeta negativamente a produtividade e a qualidade das raízes tuberosas. Avaliar e compreender como diferentes genótipos de batata-doce respondem ao déficit hídrico é essencial para a identificação de materiais tolerantes à seca e para o apoio a programas de melhoramento voltados ao aumento da resiliência em ambientes sujeitos à escassez de água. Este estudo teve como objetivo avaliar as respostas fisiológicas, bioquímicas e morfológicas de genótipos de batata-doce sob déficit hídrico, visando identificar genótipos tolerantes e caracterizar respostas associadas à tolerância ao estresse hídrico. O delineamento experimental adotado foi inteiramente casualizado, em esquema fatorial 2×20, com quatro repetições. O primeiro fator consistiu em dois regimes de irrigação (20% e 100% da capacidade de vaso), e o segundo em 20 genótipos de batata-doce. Foram avaliados parâmetros relacionados à fluorescência da clorofila, acúmulo de biomassa, potencial hídrico, trocas gasosas, marcadores bioquímicos e metabólitos secundários. Os resultados revelaram expressiva variabilidade genotípica em resposta ao déficit hídrico. O déficit hídrico afetou características fisiológicas associadas ao desempenho fotossintético e ao metabolismo oxidativo, embora a intensidade das respostas tenha variado entre os genótipos. A análise de componentes principais destacou estratégias adaptativas contrastantes envolvendo regulação fisiológica, respostas antioxidantes e ajustes no crescimento radicular sob condições de déficit hídrico. Os genótipos ‘Luiza’, ‘Canadense’, ‘CIP-420717’, ‘Maria Isabel’ e ‘CIP-420186’ apresentaram desempenho superior e se destacam como fontes promissoras para o melhoramento da tolerância à seca em batata-doce. O desempenho superior desses genótipos está associado à maior eficiência no uso da água, à manutenção de características fotossintéticas e ao menor dano oxidativo sob condições de déficit hídrico.

Palavras-chave:
Ipomoea batatas (L.) Lam.; estresse abiótico; seca; fisiologia vegetal; fotossíntese; fluorescência da clorofila

1. Introduction

Sweet potato is cultivated worldwide and plays a crucial role in food and nutritional security. Its storage roots, rich in soluble fiber, play a crucial role in regulating blood glucose and cholesterol levels, making it an important food for individuals with diabetes (Ramamoorthy et al., 2022). Additionally, its high nutrient density and low cost make it a staple food in many developing regions (Upadhyay et al., 2026). Furthermore, sweet potato has considerable industrial potential due to its high starch content, being widely recognized as a promising raw material for bioethanol production and other value-added industrial applications (Cabral et al., 2025).

Although sweet potato is often perceived as drought-tolerant, it is in fact highly sensitive to water deficits, particularly during the establishment phase and the development of storage roots. Drought stress disrupts vital physiological processes such as photosynthesis and the translocation of photoassimilates, leading to decreased yield and altered root quality (Guimarães et al., 2021; Sapakhova et al., 2023; Toroco et al., 2023). Drought is among the most critical abiotic stress impacting plant growth and development, with direct repercussions for global agricultural yield (Zhou et al., 2023).

Genotypic responses to drought in sweet potato are highly variable, and understanding the physiological mechanisms underlying this tolerance is essential for breeding cultivars better adapted to water-limited environments (De Melo et al., 2025). In regions characterized by sandy soil and hot climates, drought poses a major constraint to sweet potato production. Characterizing underutilized genotypes in this context provides an opportunity to deepen the understanding of drought tolerance and to support the development of more resilient and productive cultivars.

Numerous studies have evaluated sweet potato genotypes under drought conditions to identify tolerance mechanisms and guide cultivar development (Yooyongwech et al., 2017; Ramamoorthy et al., 2022; Liu et al., 2023). However, there remains a notable gap regarding Brazilian cultivars, especially studies integrating physiological, biochemical, and morphological responses. Given the crop’s role in food security and the increasing challenges posed by climate change, developing drought-tolerant cultivars is essential for sustainable production. This study aimed to characterize the physiological, biochemical, and morphological responses of sweet potato genotypes under water deficit to identify drought-tolerant candidates and contribute to breeding efforts.

2. Material and Methods

The experiment was conducted under greenhouse conditions in Presidente Prudente, São Paulo, Brazil. A completely randomized design was used in a 2 × 20 factorial scheme, with four replications. The first factor consisted of two water replacement levels (20% and 100% of pot capacity). The second factor consisted of 20 sweet potato genotypes previously classified as apparently drought-tolerant.

Twenty genotypes were used, including ten Brazilian commercial cultivars (‘Beauregard’, ‘Canadense’, ‘Japonesa’, ‘Luiza’, ‘Mineirinha’, ‘BRS Cuia’, ‘IAC Clara’, ‘Maria Isabel Unesp’, ‘BRS Cotinga’, and ‘IAC 134 AL01’) and ten accessions from the International Potato Center (‘CIP-420717’, ‘CIP-440224’, ‘CIP-400092’, ‘CIP-440186’, ‘CIP-440178’, ‘CIP-440003’, ‘CIP-440181’, ‘CIP-187012/2’, ‘CIP-440209’, and ‘CIP-420024’). These genotypes had been previously tested and classified as drought-tolerant based on greenhouse evaluation using a methodology adapted from Omotobora et al. (2014) (De Melo et al., 2025).

Sweet potato stem cuttings were standardized to 30 cm in length and planted in 12 L plastic pots filled with a 2:1 mixture of topsoil and washed sand. Basal fertilization was carried out according to crop recommendations (Peressin et al., 2022).

After planting, all pots were irrigated at 100% of pot capacity for five weeks to ensure proper plant establishment. Once the plants had developed leaves and adventitious roots, the irrigation management treatments (20% and 100% of pot capacity) were initiated. These contrasting water regimes were maintained for four weeks to coincide with the onset of storage root formation, a critical stage for water demand in sweet potato.

Irrigation was managed through soil moisture monitoring using time-domain reflectometry (TDR) sensors, previously calibrated. To determine pot capacity, the soil in each pot was saturated with water, covered to prevent evaporation, and allowed to drain by percolation. Moisture readings were taken every 24 hours using TDR until they stabilized, indicating the water retention capacity of the mixture.

2.1. Physiological and growth analysis

Leaf gas exchange was measured on the third or fourth fully expanded leaf, counted from the apex, using an infrared gas analyzer (IRGA, LI-6400XTR, LI-COR, USA) at a light intensity of 1000 μmol photons m⁻²s⁻¹, CO₂ concentration of 400 μmol mol⁻¹, and ΔCO₂ + ΔH₂O below 1%, between 10:00 and 11:00 a.m., at 29 days after the onset of irrigation treatments. The following parameters were recorded: net photosynthetic rate (A, μmol CO₂ m⁻² s⁻¹), transpiration rate (E, mmol H₂O m⁻² s⁻¹), stomatal conductance (g_s, mol CO₂ m⁻² s⁻¹), intercellular CO₂ concentration (Ci, μmol mol⁻¹), leaf temperature (Tleaf, °C), and vapor pressure deficit based on leaf temperature (VPD, kPa). Water use efficiency (WUE = A/E) and instantaneous carboxylation efficiency (EiC = A/Ci) were also calculated.

The following parameters of chlorophyll fluorescence were obtained using saturation pulse method: steady-state fluorescence (F_s), maximum fluorescence (F_ms), quantum yield of PSII [Y(II)], and electron transport rate (ETR). Measurements of initial fluorescence (F₀), maximum fluorescence (F_m), potential quantum efficiency of PSII (F_v/F_m), and the F_v/F₀ ratio were also performed on dark-adapted leaves. All evaluations were performed 30 days after irrigation treatment onset using the Plant Stress Kit (ADC BioScientific).

Leaf flavonoid content was measured using a multi-pigment meter (MPM-100, ADC BioScientific) at 30 days after drought imposition. Leaf water potential (ψ_W) was measured at midday using a Scholander-type pressure chamber (model 1000, PMS Instruments, USA) at 30 days after the onset of water deficit. The same plants were used for both gas exchange and water potential measurements.

At 9 weeks (63 days) after planting, shoots, fibrous roots, and storage roots were harvested, and shoot fresh mass (SFM) was determined. The plant material was dried in a forced-air oven at 65°C until constant weight to determine shoot and root dry mass. Root morphology was analyzed using a root scanner (Epson LA2400) and WinRhizo software (Regent Instruments), recording total length (RL, in cm), average diameter (RD, in mm), and length per volume (RL.V, in cm cm^-3).

2.2. Biochemical stress indicators and secondary metabolites analysis

At the end of the experiment (9 weeks, 63 days), fresh leaf samples were collected for biochemical analysis. Immediately after collection, the leaves were immersed in liquid nitrogen to halt cellular metabolism and preserve biochemical compounds. The material was then stored in an ultrafreezer at −80 °C until the biochemical analyses were performed. Lipid peroxidation was measured as malondialdehyde (MDA) content, using the thiobarbituric acid (TBA) assay following Heath and Packer (1968). Soluble sugar content in the leaves was quantified using the method of Dubois et al. (1956), with glucose as a standard.

The contents of photosynthetic pigments chlorophyll a, chlorophyll b, total chlorophyll, carotenoids, and anthocyanins were determined spectrophotometrically according to Wellburn (1994), and results were expressed in μg mL⁻¹. Free proline in leaf tissues was extracted and analyzed according to the method of Bates et al. (1973). Hydrogen peroxide content was determined following the protocol described by Velikova et al. (2000).

Total phenolic content (μg mL⁻¹) was measured using the Folin-Ciocalteu method and 25% sodium carbonate solution, according to Stagos et al. (2012).

2.3. Statistical analysis

Prior to analysis of variance (ANOVA), assumptions of normality of residuals, homogeneity of variances, and independence of errors were tested. When ANOVA revealed significant differences by the F test, treatment means were grouped using the Scott-Knott test at a 5% significance level. In addition, correlation analysis was performed using the Pearson method. A heat map was also generated to better visualize the relationships among variables. All analyses were conducted in the R software environment.

3. Results

The analysis of variance revealed a significant genotype × pot capacity interaction for RL.V, RL, PER, FEN, MDA, PRO, CAR, ClT, ClB, ClA, iWUE, EiC, E, Ci, and g_s, indicating differential responses of sweet potato genotypes under the evaluated pot capacities (Table 1, Tables S1-S5 – Supplementary Material). Among the traits without significant interaction, NL and FLM showed significant isolated effects for both genotype and pot capacity, whereas DRM and FLA were influenced only by genotype, and ψW and Fv.F0 only by pot capacity. In contrast, ETR and Y were not significantly affected by genotype, pot capacity, or their interaction.

Table 1
Analysis of variance for physiological, biochemical, and morphological traits in sweet potato genotypes under two pot capacities.

	Genotype (G)	Pot capacity (C)	G x C	CV (%)
NL+	2.68**	22.77**	1.40NS	26.21
DRM++	4.67**	0.668NS	1.37NS	19.41
FLM+	2.92**	9.57**	1.03NS	27.89
RL.V++	3.92**	2.29NS	1.88*	20.69
RL++	4.18**	0.19NS	2.76**	16.27
PER++	5.41**	1.19NS	5.07**	21.63
FEN++	12.14**	10.34**	11.31**	15.71
MDA++	5.67**	13.92**	6.99**	0.26
PRO++	2.22**	22.11**	2.13**	0.04
CAR	219.82**	1054.84**	173.15**	2.20
FLA++	2.49*	0.28NS	1.37NS	16.97
ClT	19.25*	92.98**	18.96**	7.88
ClB	4.71**	10.60**	4.81**	15.75
ClA	37.32**	214.74**	36.25**	5.82
iWUE++	5.15**	18.87**	2.18*	12.37
EiC++	1.87NS	2.57NS	2.02*	1.22
E++	2.2*	18.59**	2.04*	20.86
Ci++	1.19NS	11.78**	1.94*	11.11
gs++	1.68NS	5.01*	2.07*	4.91
ψW ++	1.14NS	13.14**	1.27NS	6.52
Fv.F0	1.62NS	14.50**	2.35*	12.67
ETR++	1.44NS	0.12NS	1.52NS	19.81
Y	1.09NS	0.00NS	0.85NS	15.22

• ^*
  Significant at the 5% probability level (p < 0.05).
• ^**
  Significant at the 1% probability level (p < 0.01).
• NS = not significant.
• ⁺
  Data transformed using Log(x + 1).
• ⁺
  +Data transformed using √(x + 1).
• PER – Hydrogen peroxide; MDA – Malondialdehyde; PRO – Proline; PHEN – Phenolics; Y – Quantum efficiency of photosystem II; ClTot – Total chlorophyll; ClA – Chlorophyll a; ClB – Chlorophyll b; CAR – Carotenoids; RL – Root length; RL.V – Root length-to-volume ratio; ψW – Leaf water potential; Fms – Maximum fluorescence; F0 – Minimum fluorescence; Fv/F0 – Stress indicator in Fv/Fm; Flv – Flavonoids; NL – Number of leaves; FLM – Fresh leaf mass; DRM – Dry root mass; gs – Stomatal conductance; Ci – Intercellular CO₂ concentration; EiC – Instantaneous carboxylation efficiency; iWUE – Instantaneous water use efficiency; E – Transpiration.

The summary presented in Figure 1 illustrates the variation in genotype performance for the evaluated traits under contrasting water conditions. The stress indicator Fv/F0, leaf water potential, and electron transport rate were the most affected variables under drought, with nine, seven, and six genotypes, respectively, showing inferior performance under water-deficit conditions.

Figure 1
Summary of the response of genotypes subjected to water deficit (20% pot capacity) compared to well-watered conditions (100% pot capacity). Gray cells indicate similar behavior under both water regimes, red cells indicate a decrease in trait performance under water deficit, and blue cells indicate an improvement in trait performance under water deficit, based on the Scott-Knott test at a 5% probability level. PER – Hydrogen peroxide; MDA – Malondialdehyde; PRO – Proline; PHEN – Phenolics; Y – Quantum efficiency of photosystem II; ClTot – Total chlorophyll; ClA – Chlorophyll a; ClB – Chlorophyll b; CAR – Carotenoids; RL – Root length; RL.V – Root length-to-volume ratio; ψW – Leaf water potential; F_ms – Maximum fluorescence; F₀ – Minimum fluorescence; F_v/F₀ – Stress indicator in F_v/F_m; Flv – Flavonoids; NL – Number of leaves; FLM – Fresh leaf mass; DRM – Dry root mass; g_s – Stomatal conductance; Ci – Intercellular CO₂ concentration; EiC – Instantaneous carboxylation efficiency; iWUE – Instantaneous water use efficiency; E – Transpiration.

In addition to these, photosynthetic traits such as quantum efficiency of photosystem II (Y), stomatal conductance (g_s), and transpiration rate (E) were also considerably impacted, although genotype responses varied (Figure 1).

Among the variables associated with oxidative stress, increases in malondialdehyde (MDA) were observed in genotypes ‘CIP-420717’, ‘CIP-440178’, ‘CIP-440003’, ‘CIP-440209’, ‘BRS Cuia’, and ‘BRS Cotinga’, while hydrogen peroxide (H₂O₂ – PER) increased in ‘CIP-187012/2’, ‘CIP-440209’, ‘BRS Cuia’, and ‘Maria Isabel’, indicating a greater accumulation of reactive oxygen species (ROS) (Figure 1). Conversely, proline (PRO) content increased in ‘CIP-440092’, ‘CIP-187012/2’, ‘Canadense’, ‘Beauregard’, and ‘IAC 134 Al01’, and phenolic compounds (FEN) increased in ‘CIP-400092’, ‘CIP-420024’, ‘Canadense’, ‘Beauregard’, and ‘Maria Isabel’ under stress, suggesting activation of antioxidant defense mechanisms (Figure 1). The analysis of malondialdehyde content, a biomarker of lipid peroxidation, revealed different responses among sweet potato genotypes under drought. Genotypes ‘CIP-420717’, ‘CIP-440178’, ‘CIP-440003’, ‘CIP-440209’, ‘BRS Cuia’, and ‘BRS Cotinga’ exhibited significantly higher MDA content under 20% field capacity, indicating increased oxidative damage (Figure 1). In contrast, genotypes ‘CIP-440186’ and ‘IAC 134 AL01’ showed reduced MDA levels under drought, suggesting higher tolerance to oxidative stress (Figure 1). The remaining genotypes did not show significant changes in MDA content in response to water deficit.

Hydrogen peroxide content also varied among genotypes under drought. Four genotypes (‘CIP-187012/2’, ‘CIP-440209’, ‘BRS Cuia’, and ‘Maria Isabel’) showed significant increases, indicating elevated oxidative stress levels and suggesting higher susceptibility to drought-induced oxidative damage. Conversely, genotypes ‘CIP-440224’, ‘CIP-420024’, and ‘IAC 134 AL01’ showed decreased H₂O₂ content, indicating better tolerance to oxidative stress. Foliar proline content increased significantly in only five genotypes: ‘CIP-400092’, ‘CIP-187012/2’, ‘Canadense’, ‘Beauregard’, and ‘IAC 134 AL01’.

Genotypes ‘CIP-440224’, ‘Mineirinha’, and ‘IAC 134 AL01’ showed reduced phenolic content under 20% field capacity, suggesting a potential decrease in antioxidant capacity. In contrast, ‘CIP-400092’, ‘CIP-420024’, ‘Canadense’, ‘Beauregard’, and ‘Maria Isabel’ exhibited significant increases in phenolic compounds under stress, suggesting enhanced oxidative stress response capacity.

Flavonoid production increased in genotypes ‘CIP-420717’, ‘CIP-400092’, ‘CIP-440178’, ‘Luiza’, ‘Beauregard’, ‘Japonesa’, and ‘Maria Isabel’ under stress conditions. On the other hand, genotypes ‘CIP-440186’ and ‘CIP-440181’ showed reduced flavonoid expression under drought.

Relative content of photosynthetic pigments, including total chlorophyll (ClTot), chlorophyll a (ClA), chlorophyll b (ClB), and carotenoids (CAR), differed significantly among genotypes under drought conditions, with increases observed in some genotypes and reductions in others, suggesting pigment composition adjustments in response to water limitation. High variability was observed in chlorophyll a content among genotypes under both irrigated and drought conditions, whereas chlorophyll b varied less. Under drought, six genotypes (‘CIP-440186’, ‘CIP-440181’, ‘CIP-187012/2’, ‘Canadense’, ‘Maria Isabel’, and ‘IAC 134 AL01’) showed increases in both pigments, while five (‘CIP-420024’, ‘Luiza’, ‘Japonesa’, ‘BRS Cuia’, and ‘IAC Clara’) showed increases only in ClA content.

Total chlorophyll content also varied among genotypes, highlighting distinct adaptive strategies. Half of the genotypes (‘CIP-440186’, ‘CIP-440181’, ‘CIP-187012/2’, ‘Luiza’, ‘Canadense’, ‘Japonesa’, ‘BRS Cuia’, ‘IAC Clara’, ‘Maria Isabel’, and ‘IAC 134 AL01’) had higher total chlorophyll under drought (20% field capacity), indicating drought tolerance. Conversely, ‘CIP-440224’ and ‘Beauregard’ showed higher total chlorophyll under well-watered conditions, suggesting a preference for more humid environments.

Carotenoid content also varied, with most genotypes (‘CIP-440186’, ‘CIP-440003’, ‘CIP-440181’, ‘CIP-187012/2’, ‘CIP-420024’, ‘Luiza’, ‘Canadense’, ‘Japonesa’, ‘BRS Cuia’, ‘IAC Clara’, ‘Maria Isabel’, ‘BRS Cotinga’, and ‘IAC 134 AL01’) accumulating more carotenoids under drought (20% field capacity), suggesting a possible adaptive strategy to oxidative stress.

Morphological traits such as root length (RL), number of leaves (NL), and root dry mass (RDM) exhibited variable responses, indicating differential phenotypic plasticity among genotypes (Figure 1). Some genotypes showed increased root growth under drought conditions. An increase in root length was observed in the genotype Luiza, while root dry mass increased in ‘CIP-440224’, ‘CIP-187012/2’, ‘CIP-440208’, ‘Canadense’, ‘Mineirinha’, ‘Japonesa’, ‘BRS Cuia’, ‘IAC Clara’, and ‘IAC 134 AL 01’, indicating potential adaptive responses to water limitation.

Genotypes ‘Mineirinha’ and ‘IAC 134 AL01’ had higher leaf numbers compared to the others. These two, along with ‘Japonesa’, ‘BRS Cuia’, ‘IAC Clara’, and two CIP genotypes (‘187012/2’ and ‘440209’), also showed higher fresh mass. Regarding DRM, seven genotypes (‘CIP-440224’, ‘Canadense’, ‘Mineirinha’, ‘Japonesa’, ‘BRS Cuia’, ‘IAC Clara’, and ‘IAC 134 AL01’) had superior performance.

The root length-to-volume ratio decreased significantly under drought for genotypes ‘CIP-440186’, ‘CIP-440178’, ‘Japonesa’, ‘BRS Cotinga’, and ‘IAC 134 AL01’. This decline in root parameters suggests that these genotypes were more sensitive to water deficit, showing impaired root development.

Instantaneous water use efficiency (iWUE) remained stable or even increased in some genotypes, suggesting the presence of adaptive mechanisms that optimize the balance between carbon assimilation and water loss under stress. Genotypes ‘CIP-440209’, ‘Luiza’, and ‘IAC 134 AL01’ demonstrated higher iWUE under drought (20% field capacity), indicating greater drought adaptability (Figure 1).

When subjected to 20% field capacity, intercellular CO₂ concentration (Ci) varied among genotypes. Genotypes ‘CIP-440186’, ‘CIP-420024’, ‘Luiza’, and ‘BRS Cuia’ exhibited increased Ci under drought (Figure 1). The instantaneous carboxylation efficiency (EiC) showed varying responses under drought. ‘CIP-420024’ presented a significant reduction in EiC, indicating lower efficiency in converting CO₂ into biomass, while ‘CIP-440003’ showed increased EiC, suggesting higher adaptability to drought. The other genotypes did not show significant changes in EiC.

Transpiration rates decreased in genotypes ‘CIP-440209’, ‘Mineirinha’, ‘BRS Cuia’, ‘IAC Clara’, ‘Maria Isabel’, and ‘BRS Cotinga’ under stress (Figure 1). ‘CIP-440003’ was the only genotype with increased transpiration under drought. Overall, 12 genotypes were not negatively affected by drought in terms of transpiration.

Leaf water potential decreased under drought (20% field capacity) for genotypes ‘CIP-420717’, ‘CIP-440003’, ‘CIP-187012/2’, ‘CIP-420024’, ‘Mineirinha’, ‘Beauregard’, and ‘IAC 134 AL01’ (Figure 1). This decline in leaf water potential, resulting from lower soil water potential, is expected to affect stomatal movement and gas exchange.

Regarding the Fv/F0 ratio (stress indicator in chlorophyll fluorescence analyzes), most genotypes showed no significant difference between water regimes, indicating that the quantum efficiency of PSII was not broadly compromised. However, genotypes ‘CIP-420717’, ‘CIP-440209’, ‘CIP-420024’, ‘Beauregard’, ‘Japonesa’, ‘BRS Cuia’, ‘IAC Clara’, ‘BRS Cotinga’, and ‘IAC 134 AL01’ exhibited lower Fv/F0 under drought, indicating damage to the photosynthetic apparatus (Figure 1).

Overall, PSII quantum yield was not significantly affected by irrigation regime in most genotypes (Figure 1). Only ‘BRS Cuia’ and ‘Maria Isabel’ exhibited reduced Y(II) under drought. Six genotypes (‘CIP-440224’, ‘CIP-440181’, ‘CIP-420024’, ‘Luiza’, ‘Canadense’, and ‘BRS Cotinga’) had lower electron transport rates (ETR) under drought, indicating greater sensitivity.

Under well-watered conditions, a positive correlation was observed among total chlorophyll, chlorophyll a, chlorophyll b, and carotenoids (Figure 2a). A strong positive correlation was also found between root traits (RL and RL.V), which were also positively correlated with F_v/F₀. Among foliar gas exchange traits, a strong positive correlation was observed between EiC and Ci, and between E and iWUE, while E and iWUE showed a strong negative correlation with Ci and EiC.

Figure 2
Correlation coefficient (r) among chlorophyll fluorescence parameters, biomass, water potential, gas exchange, biochemical parameters, and secondary metabolites in sweet potato genotypes under well-watered (100%) and drought-stressed (20%) conditions. PER – Hydrogen peroxide; MDA – Malondialdehyde; PRO – Proline; PHEN – Phenolics; Y – Quantum efficiency of photosystem II; ClTot – Total chlorophyll; ClA – Chlorophyll a; ClB – Chlorophyll b; CAR – Carotenoids; SUG – Total sugars; RL – Root length; RL.V – Root length-to-volume ratio; ψW – Leaf water potential; F_ms – Maximum fluorescence; F₀ – Minimum fluorescence; F_v/F₀ – Stress indicator in F_v/F_m; Flv – Flavonoids; NL – Number of leaves; FLM – Fresh leaf mass; DRM – Dry root mass; g_s – Stomatal conductance; Ci – Intercellular CO₂ concentration; EiC – Instantaneous carboxylation efficiency; iWUE – Instantaneous water use efficiency; E – Transpiration.

Similarly to the well-watered condition, under stress conditions, correlations were found among chlorophylls (a, b, and total) and carotenoids, among root traits, and between Ci and EiC (Figure 2b). A strong positive correlation was observed between leaf water potential and shoot fresh mass. F_v/F₀ was positively correlated with root dry mass and F_m. The photosynthetic rate showed a strong positive correlation with the chlorophyll fluorescence parameters F_m and F_v/F₀. A strong positive correlation was also observed between iWUE and stomatal conductance.

Principal component analysis (PCA) explained 35.42% of the total variance in the first two components (PC1 = 21.18% and PC2 = 14.24%) and 47.43% when considering the first three components (PC3 = 12.01%) (Figure 3). The biplot revealed a clear separation between irrigation regimes, with genotypes under well-watered conditions predominantly distributed on the positive side of PC1, whereas those under water deficit were located on the negative side, indicating a strong influence of water availability on trait expression. In addition, the broader dispersion of genotypes under drought conditions, as indicated by the larger ellipse, suggests greater variability in responses within this environment.

Figure 3
Principal component analysis (PCA) biplot of physiological, biochemical, and morphological traits of sweet potato genotypes under two water regimes (100% and 20% pot capacity). The first two principal components explained 35.42% of the total variance (PC1 = 21.18% and PC2 = 14.24%). Points represent genotypes, colored according to water regime, and ellipses indicate the dispersion of genotypes within each condition. Arrows represent variable loadings, with direction and length indicating the contribution of each variable to the principal components. PER – Hydrogen peroxide; MDA – Malondialdehyde; PRO – Proline; PHEN – Phenolics; Y – Quantum efficiency of photosystem II; ClTot – Total chlorophyll; ClA – Chlorophyll a; ClB – Chlorophyll b; CAR – Carotenoids; SUG – Total sugars; RL – Root length; RL.V – Root length-to-volume ratio; ψW – Leaf water potential; Fms – Maximum fluorescence; F0 – Minimum fluorescence; Fv/F0 – Stress indicator in Fv/Fm; Flv – Flavonoids; NL – Number of leaves; FLM – Fresh leaf mass; DRM – Dry root mass; gs – Stomatal conductance; Ci – Intercellular CO₂ concentration; EiC – Instantaneous carboxylation efficiency; iWUE – Instantaneous water use efficiency; E – Transpiration.

PC1 was mainly associated with physiological variables, showing positive loadings for stomatal conductance (gs) and Fv/Fo, and negative loadings for intercellular CO₂ concentration (Ci) and malondialdehyde (MDA), representing a gradient between higher physiological activity and stress-related responses (Table 2). PC2 was primarily influenced by pigment-related traits, with strong negative loadings for total chlorophyll (ClT) and carotenoids (CAR), indicating variation in photosynthetic adjustment among genotypes (Table 2).

Table 2
Loadings of variables for the first six principal components derived from PCA of physiological, biochemical, and morphological traits of sweet potato genotypes. Although six components are presented for completeness, only the first three (PC1–PC3) were interpreted, as they explained a greater proportion of the total variance and showed clearer biological meaning.

Variable	PC1	PC2	PC3	PC4	PC5	PC6
RL	0.118	0.053	-0.530	-0.032	0.081	0.288
DRM	0.144	-0.124	-0.449	-0.050	0.337	0.148
FlvM	-0.337	0.267	0.062	-0.153	-0.069	0.211
Fv/Fo	0.437	-0.019	-0.068	0.149	-0.167	-0.036
gs	0.355	-0.023	-0.105	-0.313	-0.025	0.411
Ci	-0.422	0.024	-0.258	-0.054	0.046	-0.089
EiC	0.287	-0.148	0.397	0.176	0.254	0.446
EUA	-0.212	-0.197	0.324	0.389	0.354	0.193
Clor_Total	-0.101	-0.626	-0.121	0.069	-0.198	-0.028
CAR	-0.153	-0.608	-0.158	0.026	-0.132	0.038
PER	-0.230	0.037	-0.047	0.254	-0.279	0.431
MDA	-0.317	0.195	-0.197	0.160	0.138	0.371
PRO	-0.066	-0.128	0.186	-0.571	0.098	0.186
FEN	-0.091	-0.051	0.212	-0.289	-0.530	0.260
PH	0.174	0.167	-0.098	0.404	-0.459	0.096
SD	1.78	1.46	1.34
Variance (%)	21.18	14.24	12.01
Cumulative (%)	21.18	35.42	47.43

• PER – Hydrogen peroxide; MDA – Malondialdehyde; PRO – Proline; PHEN – Phenolics; Y – Quantum efficiency of photosystem II; ClTot – Total chlorophyll; ClA – Chlorophyll a; ClB – Chlorophyll b; CAR – Carotenoids; SUG – Total sugars; RL – Root length; RL.V – Root length-to-volume ratio; ψW – Leaf water potential; Fms – Maximum fluorescence; F0 – Minimum fluorescence; Fv/F0 – Stress indicator in Fv/Fm; Flv – Flavonoids; NL – Number of leaves; FLM – Fresh leaf mass; DRM – Dry root mass; gs – Stomatal conductance; Ci – Intercellular CO₂ concentration; EiC – Instantaneous carboxylation efficiency; iWUE – Instantaneous water use efficiency; E – Transpiration.

The third principal component (PC3) revealed an additional pattern not captured by PC1 and PC2. This component was negatively associated with root length (RL) and root dry mass (DRM), and positively associated with instantaneous carboxylation efficiency (EiC), proline (PRO), and phenolic compounds (FEN), indicating a trade-off between structural growth and biochemical and physiological adjustments under drought conditions (Table 2).

4. Discussion

Overall, the results revealed a clear genotype-dependent response to drought, integrating physiological, biochemical, and morphological traits associated with stress tolerance. Genotypes such as ‘Luiza’, ‘Canadense’, ‘CIP-440186’, ‘Maria Isabel’, and ‘IAC 134 AL01’ showed a more favorable response, combining traits such as higher water use efficiency, maintenance of photosynthetic performance, increased accumulation of protective compounds (e.g., proline, phenolics, and carotenoids), and reduced oxidative damage, as indicated by lower MDA and H₂O₂ levels. In contrast, other genotypes exhibited greater sensitivity, characterized by reductions in chlorophyll fluorescence parameters, increased oxidative stress markers, and impaired root development. Additionally, morphological adjustments, such as increased root length in ‘Luiza’ and higher root dry mass in several genotypes, reinforce the importance of belowground traits in drought adaptation. Taken together, these results demonstrate that drought tolerance in sweet potato is a multifactorial trait involving coordinated physiological regulation, antioxidant defense, and morphological plasticity, providing a robust basis for the selection of promising genotypes.

Understanding the physiological and biochemical responses of sweet potato genotypes to water deficit is essential for the development of drought-tolerant cultivars. Water deficit is one of the main limiting factors for agricultural production in various regions worldwide. Although sweet potato is often regarded as a hardy crop, it is also susceptible to the effects of drought, which can compromise yield and quality, especially when stress occurs during critical growth stages (Omotobora et al., 2014; Kivuva et al., 2015; Yooyongwech et al., 2017; Ramírez et al., 2023). These contrasting water regimes were aligned with the initiation of storage root development, a key stage characterized by increased water demand in sweet potato. The results showed that genotypes responded differently to the treatments, highlighting the genetic variability for drought tolerance in sweet potato.

Some genotypes, such as ‘Mineirinha’ and ‘IAC-134 AL01’, displayed higher leaf number, shoot fresh mass, and root dry mass. In general, there was considerable variation among genotypes for the biometric parameters evaluated. Under water deficit conditions, leaf number and size tend to decrease, resulting in reduced leaf area. Leaf water potential and leaf area are closely linked in sweet potato, especially under water deficit. Sweet potato plants limit leaf expansion to conserve water during drought (Huang et al., 2025). Another commonly observed effect is the reduction of both fresh and dry leaf mass (Zhao et al., 2006). Indeed, drought stress causes a wide range of plant responses, from changes in gene expression and cellular metabolism to alterations in growth and productivity (Anjum et al., 2011).

Physiological assessments under controlled drought conditions revealed significant differences in pigment content, chlorophyll fluorescence parameters, and leaf water potential among the 20 genotypes. Most genotypes did not show a reduction in PSII quantum efficiency [Y(II)], suggesting the presence of photoprotective mechanisms that allow excess energy dissipation, minimizing drought-induced damage. Electron transport rate varied among genotypes but was not directly affected by water deficit. In fact, some sweet potato cultivars show minimal physiological changes under drought, indicating strong inherent tolerance (Liu et al., 2023). However, ‘BRS Cuia’ and ‘Maria Isabel’ showed decreased Y(II) under drought, indicating greater sensitivity compared to other genotypes.

F_v/F₀ and Y(II) are widely used as markers of PSII efficiency and photosynthetic performance, functioning as stress indicators. Genotypes maintaining high value for these parameters under drought stress likely preserved the integrity of their photosynthetic apparatus, suggesting higher tolerance. Conversely, those showing significant reductions may have suffered more severe damage, compromising photoshynthesis and potentially affecting their growth and development.

The decrease in leaf water potential observed in genotypes such as ‘CIP-420717’, ‘CIP-440003’, ‘CIP-187012/2’, ‘Mineirinha’, ‘Beauregard’, and ‘IAC 134 AL01’ under 20% pot capacity indicates higher sensitivity to drought. This variation can be attributed to anatomical and physiological traits such as stomatal density, cuticle thickness, and water use efficiency. Even at relatively high water potentials (−0.3 to −0.5 MPa), photosynthetic rates can decline rapidly (Basu et al., 1998). Previous studies with the same crop have shown that reductions in leaf water potential led to declines in chlorophyll content, CO₂ assimilation, stomatal conductance, and transpiration rate (Gajanayake and Reddy, 2016). According to Daryanto et al. (2016), sweet potato undergoes permanent wilting when leaf water potential reaches around −1.3 MPa.

Drought stress in sweet potato leads to a clear relationship between stomatal conductance and photosynthesis. Reduced stomatal conductance under drought is a primary cause of decreased photosynthetic rates, especially in the early stages of stress. As drought persists, non-stomatal factors can also limit photosynthesis (Huang et al., 2022; Huang et al., 2025). Variation in stomatal conductance among sweet potato genotypes suggests different adaptive strategies to drought. Typically, conductance decreases under stress to prevent leaf dehydration, which may reduce photosynthetic activity. Genotypes such as ‘CIP-440209’, ‘IAC Clara’, and ‘BRS Cotinga’ followed this pattern, while ‘CIP-400092’, ‘CIP-440186’, and ‘CIP-440003’ showed increased g_s under drought, indicating better drought tolerance adaptability.

Transpiration rates were not negatively affected by drought in most of the genotypes. Mild drought severity leads to a significant decrease in both stomatal conductance and transpiration rate in sweet potato, primarily due to stomatal closure, which helps the plant conserve water but also limits photosynthesis. Sweet potato varieties with better regulation of these traits can maintain higher productivity (Van Heerden and Laurie, 2008).

Four genotypes (‘CIP-440186’, ‘CIP-420024’, ‘Luiza’, and ‘BRS Cuia’) showed increased intercellular CO₂ concentration (Ci) under drought. Ci is a key physiological indicator of a plant’s ability to fix carbon during photosynthesis. The impairment of photosynthetic traits is directly associated with reduced photochemical efficiency and carbon assimilation under drought stress, indicating functional limitations in both biochemical and structural components of the photosynthetic apparatus. While environmental factors such as water availability and light influence Ci, stomatal aperture remains a central regulator.

Instantaneous carboxylation efficiency (EiC), calculated as the ratio between photosynthetic rate and Ci, reflects how effectively plants use CO₂. It is influenced by the diffusion of CO₂ through the substomatal cavity, cell walls, and membranes into the chloroplasts (Flexas et al., 2012). ‘CIP-420024’ was the only genotype to show reduced EiC under drought.

Water use efficiency (WUE) refers to the biomass produced per unit of water transpired. Stomatal behavior is one of its key determinants. By controlling gas exchange and transpiration, stomata modulate WUE (Concenço et al., 2007). Instantaneous WUE, defined as the ratio of photosynthesis to transpiration (Xu and Hsiao, 2004), was higher in ‘CIP-440209’, ‘Luiza’, and ‘IAC 134 AL01’ under 20% pot capacity. This suggests better adaptation through physiological and biochemical mechanisms that optimize water use. Most genotypes showed no significant differences in iWUE between 100% and 20% pot capacity, indicating lower sensitivity to drought. Under drought, plants often reduce photosynthesis while adjusting water use to optimize survival (Yooyongwech et al., 2013).

Biochemical analyses revealed significant changes in photosynthetic pigments (chlorophylls, anthocyanins, carotenoids), lipid peroxidation (MDA), proline, phenolic compounds, and peroxides. Most genotypes exhibited higher chlorophyll content under 20% field capacity, consistent with a previous study where sweet potato genotypes showed a 23% increase in chlorophyll under drought (Ramamoorthy et al., 2022). This may reflect an adaptive strategy to maximize light capture under reduced photosynthetic rates, as some plants adjust pigment synthesis under stress (Doupis et al., 2013). However, ‘CIP-440224’ and ‘Beauregard’ had lower chlorophyll content under well-watered conditions, possibly due to oxidative stress-related pigment degradation under drought (Sepehri and Golparvar, 2011).

Carotenoid content was higher under drought in genotypes including ‘CIP-440186’, ‘CIP-440181’, ‘CIP-187012/2’, ‘CIP-420024’, ‘Luiza’, ‘Canadense’, ‘Japonesa’, ‘BRS Cuia’, ‘IAC Clara’, ‘Maria Isabel’, ‘BRS Cotinga’, and ‘IAC 134 AL01’. Carotenoids function as light-harvesting pigments and stress regulators by mitigating ROS damage. Carotenoids as xanthophylls are crucial for the assembly of photosystems (Divya et al., 2018; Soares et al., 2019). Anthocyanins, a subclass of flavonoid pigments, are commonly linked to oxidative stress protection. Higher anthocyanin levels are associated with increased drought tolerance (Rodziewicz et al., 2014). However, in this study, anthocyanin content decreased under drought, suggesting reduced capacity to neutralize free radicals and higher oxidative susceptibility (Rice-Evans et al., 1996).

Flavonoids and proline are non-enzimatic antioxidants with essential functions for a higher plant tolerance to abiotic stress. These metabolites are able to neutralize, remove and/or transform ROS, in order to achieve ROS homeostasis in plant cells (Soares et al., 2019). Flavonoid production varied among genotypes, with ‘CIP-420717’, ‘CIP-400092’, ‘CIP-440178’, ‘Luiza’, ‘Beauregard’, ‘Japonesa’, ‘Canadense’ and ‘Maria Isabel’ showing higher levels under water deficit. The antioxidant capacity of flavonoids is due their capacity to direct interaction with ROS but also due to their ability to serve as substrate for different peroxidases enzymes (Agati et al., 2012). Proline, an osmoprotectant amino acid, protects cell membranes and proteins from drought-induced oxidative damage (Delauney and Verma, 1993; Hare et al., 1998). ‘CIP-400092’, ‘CIP-187012/2’, ‘Canadense’, ‘Beauregard’, and ‘IAC 134 AL01’ accumulated more proline under drought, reflecting a common strategy in drought-tolerant plants for maintaining cell integrity and survival. These results corroborate those reported in previous studies addressing that sweet potato drought-tolerant cultivars maintain higher antioxidant capacity, accumulate protective metabolites (e.g., proline, flavonoids), and sustain energy supply through enhanced respiratory metabolism (Yin et al., 2024).

MDA, a lipid peroxidation product, is a marker of oxidative stress. Lower MDA levels suggest greater antioxidant capacity and drought tolerance (Marcińska et al., 2013). In a study by Zhang et al. (2020), early drought stress led to persistent oxidative damage in sweet potato leaves. ‘CIP-440186’ and ‘IAC 134 AL01’ had the lowest MDA accumulation, indicating higher drought tolerance. Twelve other genotypes maintained normal MDA levels under drought, also showing good adaptation.

‘CIP-440224’, ‘CIP-420024’, and ‘IAC 134 AL01’ showed reduced peroxide content under 20% pot capacity. Although drought increases peroxide production, plants activate antioxidant defenses and osmoprotectants to counteract damage. Understanding these responses is crucial for improving drought tolerance.

Overall, biochemical markers revealed contrasting responses to drought. Increased MDA and H₂O₂ content (PER) indicated lower tolerance, whereas higher proline and polyphenol levels were associated with greater resilience. ‘IAC 134 AL01’ stood out with low ROS accumulation and high protective compound levels.

Root length variation among genotypes reflects different adaptive strategies for water uptake. ‘Luiza’ exhibited the longest roots under drought, suggesting an ability to explore deeper soil layers. In contrast, ‘BRS Cotinga’ and ‘IAC 134 AL01’ had greater root development under well-watered conditions, indicating water-dependent growth strategies. Previous studies showed that drought reduces total root length, surface area, and volume (Ramamoorthy et al., 2022).

Sweet potato is an economically and nutritionally important crop and improving its drought tolerance is key for regions with limited water availability. The wide range of physiological, biochemical, and developmental responses observed highlights the complexity of drought tolerance. A multifactorial approach is essential for identifying promising genotypes. While this study provided valuable insights, future field evaluations of yield performance under drought are needed to fully assess agronomic potential.

The separation of genotypes according to irrigation regimes observed in the PCA highlights the strong effect of water availability on plant physiological and biochemical responses. Under drought conditions, the greater dispersion of genotypes indicates increased heterogeneity in responses, suggesting that water deficit amplifies differences among genotypes and reveals distinct adaptive strategies.

The association of PC1 with stomatal conductance and Fv/Fo, together with its negative relationship with Ci and MDA, reflects the balance between photosynthetic performance and stress intensity. Genotypes positioned toward higher g_s and Fv/Fo values likely maintain greater physiological activity, whereas those associated with higher Ci and MDA may be experiencing limitations in carbon assimilation and increased oxidative stress.

PC2, driven by chlorophyll and carotenoids, indicates adjustments in the photosynthetic apparatus under water deficit, which may contribute to maintaining light harvesting and photoprotection. These responses are commonly associated with acclimation mechanisms in plants subjected to abiotic stress.

Importantly, PC3 revealed a complementary pattern, indicating a trade-off between root growth and biochemical defense mechanisms. The negative association with RL and DRM, and the positive association with PRO, FEN, and EiC, suggest that genotypes may adopt different strategies under drought, either investing in root development to enhance water acquisition or activating metabolic and antioxidant pathways to mitigate stress damage.

Together, these results demonstrate that drought tolerance in sweet potato is a multifaceted trait, involving coordinated physiological, biochemical, and morphological responses, rather than a single dominant mechanism.

5. Conclusion

The genotypes ‘Luiza’, ‘Canadense’, ‘CIP-420717’, ‘Maria Isabel’, and ‘CIP-440186’ stood out as more drought-tolerant under the evaluated conditions. Overall, drought tolerance in sweet potato was associated with the maintenance of physiological performance, efficient regulation of water use, and activation of protective biochemical and morphological responses. These findings reinforce the multifactorial nature of drought tolerance and support the selection of promising genotypes for cultivation and breeding programs in water-limited environments.

Supplementary Material

Supplementary material accompanies this paper.

Table S1

Mean values of number of leaves (NL), dry root mass (DRM, in g), leaves fresh mass (LFM, in g), root length-to-volume ratio (RL.V, in m m^3-1), and root length (RL) of sweet potato genotypes grown under two water availability conditions (100% and 20% of pot capacity).

Table S2

Mean values of hydrogen peroxide (PER), total phenolic compounds (PHEN), malondialdehyde (MDA), and proline (PRO) in sweet potato genotypes cultivated under two water availability conditions (100% and 20% of pot capacity).

Table S3

Mean values of carotenoids (CAR), flavonoids (FLA), total chlorophyll (ClT), chlorophyll b (ClB), and chlorophyll a (ClA) in sweet potato genotypes cultivated under two water availability conditions (100% and 20% of pot capacity).

Table S4

Mean values of instantaneous water use efficiency (iWUE), instantaneous carboxylation efficiency (EiC), transpiration rate (E), and intercellular CO₂ concentration (Ci) in sweet potato genotypes cultivated under two water availability conditions (100% and 20% of pot capacity).

Table S5

Mean values of leaf water potential (Ψ_W), Fv/F₀ ratio, electron transport rate (ETR), and quantum efficiency of photosystem II (Y) in sweet potato genotypes cultivated under two water availability conditions (100% and 20% of pot capacity).

This material is available as part of the online article from https://doi.org/10.1590/1519-6984.303785

Acknowledgements

This study was funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), grant numbers 2022/03120-6 and 2023/01704-3.

Data Availability Statement

All datasets supporting the findings of this study are published within the article itself.

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*

e-mail: [e-mail]

Editor:

Takako Matsumura Tundisi

Publication Dates

• Publication in this collection
  05 Oct 2026
• Date of issue
  2026

History

• Received
  15 Dec 2025
• Accepted
  10 Sept 2026