Estimating chill and heat requirement for peach flowering and harvesting season in tropical production areas

ABSTRACT

Low air temperature during the dormancy period promotes uniform flowering in peach trees, while the heat after the full bloom is necessary for fruit development. Rootstock cultivars affect the timing of scion phenological stages, either advancing or delaying their occurrence from bud break to harvest. Given the expansion of stone fruit cultivation in areas with suboptimal chill accumulation and the ongoing climate changes, this research aimed to evaluate effects of 17 clonal rootstocks on chill requirement for full blooming and heat accumulation for fruit development of ‘BRS-Kampai’ and ‘BRS-Rubimel’ cultivars, as well as two own-rooted scion trees. Field trials were conducted in two locations in the state of São Paulo, Brazil. Under the edaphoclimatic conditions of the study, rootstocks affected chill portions accumulated until bud break, number of days from the autumn equinox to break dormant bud stage, number of days from autumn equinox to full bloom stage, length of the fruit development period, and harvest date. Growing degree hours accumulated 30 days after full bloom stage was the single feature that showed no statistical differences among the tested rootstocks. Pearson correlations between degree days accumulated 30 days after full bloom and fruit development period were significant and more stable for ‘BRS-Rubimel’ than for ‘BRS-Kampai’, suggesting that this parameter can be used to estimate the harvest time with precision. Therefore, we concluded that rootstocks significantly influence chill accumulation required for breaking bud dormancy and reach the full bloom stage, as well as the heat accumulation for reach the harvest season.

Key words
chill portions; GDH 30; rootstock; Prunus persica

INTRODUCTION

Peach [Prunus persica var. vulgaris (L.) Batsch] and nectarine (P. persica var. nucipersica Dippel.) are fruit trees from temperate zones, which need a period of low air temperature to stimulate bud development and bloom regularly in the spring season (Hawerroth et al. 2010, Delgado et al. 2024). Throughout the dormant period, trees show no vegetative growth, but they maintain metabolic activities, which allow their resistance to cold temperatures (Petri et al. 2021). Accumulation of low air temperatures during autumn and winter seasons induces uniform flowering to the peach trees. Air temperature is the major environmental factor involved in breaking dormancy of floral buds, although photoperiod (Erez 2024) and rainfall also affect this process (Hawerroth et al. 2010). Satisfying the chill requirement is important for the bud breaking and plenty peach flowering, while the heat is central for fruit growth and to predict harvest date.

Previous studies showed that air temperatures between full bloom and 30 days after this phenological stage are suitable to predict length of fruit development period (Lopez and DeJong 2007). Thus, the growing degree hours (GDH) accumulated during the period of 30 days after the full bloom stage (GDH 30) has strong correlation with harvest date in different stone fruits cultivars (Mimoun and DeJong 1999). Rootstocks, in addition to ensure adequate tree growth and resistance to abiotic and biotic stresses (Ghrab et al. 2014), potentially affect the flowering season, delaying or anticipating the bloom (Beckman et al. 1992, Durner and Goffreda 1992, Yahmed et al. 2016).

The first studies addressing chill requirements for breaking flower buds used 7.2°C as a threshold to start chill accumulation (Weinberger 1950, Petri et al. 2021). However, the advance of knowledge about peach trees physiology showed that breaking bud dormancy is related to a complex metabolic process (Hawerroth et al. 2010, Erez 2024), and the use of a single air temperature threshold is unsuitable. In view of different effects of air temperatures over chill accumulation, diverse models for rest completion of buds in temperate fruit trees were developed around the world. They consider positive, negative or null effects of air temperature (Fishman et al. 1987a, Fishman et al. 1987b, Miranda et al. 2021, Miranda 2023). Besides, the occurrence of high air temperatures during flower bud break season may negate effects of any chill previously accumulated by the tree (Richardson et al. 1974).

The dynamic model, proposed by Fishman et al. (1987a and 1987b), was selected to estimate accumulated chill, measured by chill portions (CP), for bud dormancy breaking. This model considers that the rest completion is reach in two-step process: in the first step, precursor made by low air temperature (~1.67–12.78°C) would be reversible when high air temperature occurs; in the second step, when precursor reaches a critical portion, it turns into a stable factor and does not return to the initial phase (Erez et al. 1990). This model was chosen because it performed well in extensive range of climates and its use is recommended in subtropical regions (Ghrab et al. 2014, Carbonieri and Morais 2015, Miranda et al. 2021). In addition, the dynamic model is recommended to be applied in studies addressing climate changes (Miranda 2023), has low coefficient of variation among the years, and shows high correlation with the Utah model, developed by Richardson et al. (1974; R² = 0.96), and medium correlation with 0–7.2°C model developed by Weinberger (1950; R² = 0.67) (Ghrab et al. 2014).

Since the use of rootstocks may change absorption of water and nutrients, we hypothesized that its use could affect the amount of chill required for breaking bud dormancy and flowering, and the heat essential for harvest of ‘BRS Kampai’ and ‘BRS Rubimel’ peach scions in mild winter region of São Paulo state, Brazil.

Considering the expansion of stone fruit cultivation into regions where chill accumulation is below optimal, this study aimed to evaluate the effects of 17 clonal rootstocks and own-rooted trees on the chill and heat requirements for full bud break, flowering, and harvest of two peach scion cultivars.

MATERIAL AND METHODS

Seventeen selections, species or Prunus cultivars (Table 1) were selected from the Embrapa Prunus Rootstock Germplasm Bank (31°40’42.00”S; 52°27’05.09”W, 54 m a.s.l.), and used as clonal rootstocks for peach based on their reaction to biotic and abiotic factors or other interesting characteristics (Neves et al. 2017, Sobierajski et al. 2021), plus two own-rooted scion trees. All rootstocks were propagated by softwood cuttings under intermittent mist system, and after acclimation and plant growth in bags under greenhouse conditions, they were budded with ‘BRS Kampai’ or ‘BRS Rubimel’ peach scion cultivars by “T inverted” budding method (Mayer et al. 2013). At the same time, own-rooted nursery trees of both peach cultivars were produced by soft cuttings. These scion peach cultivars are for fresh market, have medium tree vigor, semi-vertical tree growth behavior, and chill requirements ranging from 200 and 300 chilling hours to flowering (Raseira et al. 2014), measured according to Weinberger (1950).

Two field trials were installed in August, 2014 in the state of São Paulo, Brazil. The location of Jarinu (23°04’59”S; 46°43’27”W; 800 m. a.s.l.) present Cwa climate, according to Köppen-Geiger’s classification (Alvares et al. 2013), and soil is classified as Inceptisol, according to the Brazilian Soil Classification System (Santos et al. 2018). The location of Jundiaí (23°06’52”S; 46°56’03”W; 740 m a.s.l.) also presents Cwa climate, according to Köppen-Geiger’s classification (Alvares et al. 2013), but soil is classified as Ultisol, according to the Brazilian Soil Classification System (Santos et al. 2018). The trial installed in Jarinu was with trees of ‘BRS Kampai’ trained to a Y system spaced by 5.5 × 1.4 m, while trees of ‘BRS Rubimel’ were planted in Jundiaí and trained to an open vase system (5 × 4 m). Both trials were designed in randomized blocks, with six replications and one tree per plot. All cultural practices were conducted according recommendations of Moura et al. (2014).

Table 1
Identification and genetic origin of 17 selections, species, and cultivars tested as clonal rootstocks for ‘BRS Kampai’ and ‘BRS Rubimel’ peach in mild winter region of São Paulo state, Brazil.

Rootstock	Genetic origin
‘Barrier’	P. persica × P. davidiana
‘Cadaman’	P. persica × P. davidiana
‘Capdeboscq’	P. persica
Clone 15	P. mume
‘Flordaguard’	P. persica × P. davidiana
‘Genovesa’	Prunus salicina
GxN9	P. persica × Prunus dulcis
I-67-52-4	P. persica
‘Ishtara’	(P. cerasifera × P. salicina) × (P. cerasifera × P. persica)
México Fila-1	P. persica
‘Nemared’	P. persica
‘Okinawa’	P. persica
‘Rigitano’	P. mume
‘Santa Rosa’	P. salicina
‘Tsukuba-1’	P. persica
‘Tsukuba-2’	P. persica
‘Tsukuba-3’	P. persica

Bud, flower, and fruit development were measured twice a week from tree pruning until physiological fruit ripening. In each tree, we marked four 1-year-old shoots, which had phenological development evaluated during consecutive seasons (‘BRS Rubimel’: 2016 and 2017; ‘BRS Kampai’: 2016 to 2018). Tree phenological stages were identified according to the E. Bellini scale (Bassi and Monet 2008), including stages from the dormant bud to the commercial ripening. Full bloom was considered when shoots had 50% of opened flowers. Fruit harvest date was considered when fruits reach the physiological stage for ripening.

Date interval was registered in Julian’s days (days of the year—DOY). The number of days to break dormant bud stage (number of days E1), to reach full bloom (number of days E3), and to reach harvest date (number of days E10) were measured from the autumn equinox (in South Hemisphere: March 20^th, 2016; March 21^st, 2017 and 2018). The fruit development period (FDP) was measured from the interval between full bloom and harvest date, according to Marra et al. (2002). The hourly air temperature data were downloaded from an Akso datalogger (model AK172-V2) installed in both experimental orchards, with air temperature sensors placed inside a meteorological shelter.

The CP was obtained from chill_portions function, available in ‘fruclimadapt’ R-package (Miranda et al. 2021, Miranda 2023), in the R software environment (R Core Team 2023). This function requires as parameter DOY when chill accumulation is supposed to start. In this study, we standardized the autumn equinox as an indicator of beginning of chill. The GDH accumulated 30 days after full bloom stage was estimated by the Asymcur model (Anderson et al. 1986). This model put on the begins of heat accumulation when the minimum air temperature rises above a base temperature (Tb). The growth increases with air temperature up to an optimum temperature (To) and ceases when the air temperature is above a critical temperature (Tc). The estimates of GDH were obtained from GDH_asymcur function, available in ‘fruclimadapt’ package (Miranda et al. 2021, Miranda 2023). In addition, we used air temperatures Tb = 7.5°C, To = 26°C, and Tc = 38.5°C as parameters for the GDH_asymcur function, as specified by Marra et al. (2002), for peach fruit development.

The analyses were carried out independently for each location to account for the site-specific climatic conditions, as well as the cultivars and training systems employed. The statistical Shapiro-Wilk’s test (normality), analysis of variance (ANOVA), Tukey’s test and Pearson correlation were calculated at the 5% of significance level using the ‘Agricolae’ package (DeMendiburu 2021), and the plots drawn by the ‘ggplot2’ package (Wickham et al. 2019). All above cited packages were run by R software environment (R Core Team 2023).

RESULTS AND DISCUSSION

Chill portions accumulated in Jarinu (‘BRS Kampai’ trial) were 7.04 (from June 9^th to August 22nd), 6.01 (from June 11^th to August 22^nd), and 5.95 CP (from May 21^st to September 5^th), respectively along the seasons from 2016 to 2018 (Fig. 1). The location of Jundiaí (‘BRS Rubimel’ trial) accumulated 6.79 (from June 5^th to August 23^rd) and 4.03 (from June 11^th to July 18^th) in 2016 and 2017, respectively (Fig. 2). These data shown that both locations present warm winter conditions, with low chill accumulated to stimulate dormant bud breaking. The knowledge of chill accumulated is essential to define species and cultivars most adapted to the local microclimate. In regions where chill requirement is not enough to break bud dormancy, growers can apply tree growth regulators to complete chill requirements (Carbonieri and Morais 2015).

Figure 1
Maximum, medium, and minimum air temperature and chill portions accumulated. Jarinu, São Paulo state, Brazil, (a) 2016, (b) 2017, and (c) 2018.

Figure 2
Maximum, medium, and minimum air temperature and chill portions accumulated. Jundiaí, São Paulo state, Brazil, (a) 2016 and (b) 2017.

The ANOVA showed statistical differences among clonal rootstocks tested to break dormant bud stage, considering the CP and number of days E1, along all seasons evaluated for ‘BRS Rubimel’ trial (Table 2). On the other hand, in ‘BRS Kampai’ trial, rootstocks presented statistical differences for CP (2017) and for the number of days from autumn equinox to the break dormancy stage (2016). This result suggests that rootstocks exert influence in breaking of bud dormancy, but with different sensitivities according to the scion cultivar. Beckman et al. (1992) indicate that differences in bloom date between rootstocks is related to differences in the onset of chill-hour accumulation or in chilling requirement of scion/rootstock combinations.

The ‘BRS Kampai’ trial showed a higher requirement of CP in 2017 for trees budded on ‘Okinawa’ (2.47 CP) and ‘Tsukuba-1’ rootstocks (2.31 CP) compared with other rootstocks (Table 3). The ‘BRS Rubimel’ trial demonstrated a high requirement of CP in 2016, for trees budded on a group of 13 rootstocks, led by the ‘Tsukuba-3’ (5.29 CP). In 2017, this scion also showed a higher chilling requirement when budded on a group of 17 rootstocks (ranging from 4.02 to 3.67 CP) compared with ‘Tsukuba-2’ rootstock (3.31 CP). According to Erez (2024), some horticultural practices may be used to compensate for low chill accumulation, such as reducing vegetative vigor by using dwarfing rootstocks, among other approaches. This is consistent with our findings, in which rootstocks with higher chilling requirements also showed greater plant height and higher nutrient uptake for most nutrient content (Sobierajski et al. 2021), resulting in enhanced vegetative growth performance.

Table 2
Analysis of variance (F test and coefficient of variation—CV%) of chill portions accumulated (CP), of number days from autumn equinox to break dormant bud stage (number of days E1), of number days from autumn equinox to the full bloom stage (number of days E3), of growing degree hours accumulated 30 days after full bloom stage (GHD 30), of fruit development period (FDP), and harvest date (number of days E10) among the own-rooted trees and 17 genotypes tested as clonal rootstocks for ‘BRS Kampai’ and ‘BRS Rubimel’ peach, respectively, in Jarinu and Jundiaí, São Paulo state, Brazil, 2016–2018.

Source of variation	BRS Kampai				BRS Rubimel
	2016	2017	2018		2016	2017
	CP
Rootstock	0.83ns	3.99**	0.89ns		1.57*	2.54*
CV%	3.67	3.93	0.46		16.04	8.47
	Number of days E1
Rootstock	2.15*	1.53ns	1.38ns		2.14*	47.27**
CV%	5.22	5.34	1.53		16.16	8.97
	Number of days E3
Rootstock	3.65**	2.28*	1.14ns		2.51*	1.95*
CV%	5.10	6.69	1.85		12.99	9.60
	GDH 30
Rootstock	1.05ns	1.76ns	1.42ns		1.92ns	1.68ns
CV%	2.52	3.36	0.38		6.97	5.08
	FDP
Rootstock	4.09**	7.40**	4.14**		2.70*	1.47*
CV%	10.14	6.91	12.31		13.15	19.50
	Number of days E10
Rootstock	530.85**	1.40ns	3.60*		0.82ns	74.39**
CV%	0.18	2.69	5.93		2.48	4.20

• ^ns
  p ≥ 0.05;
• ^*
  p < 0.05;
• ^**
  p < 0.01.

Table 3
Chill portions accumulated (CP) from autumn equinox to break dormant bud stage among the own-rooted trees and 17 genotypes tested as clonal rootstocks for ‘BRS Kampai’ (2016 to 2018) and ‘BRS Rubimel’ (2016 and 2017) peach, respectively, in Jarinu and Jundiaí, São Paulo state, Brazil<tfn>*</tfn>.

Rootstock or own-rooted trees	‘BRS Kampai’				‘BRS Rubimel’
	2016ns	2017	2018ns		2016	2017
Own-rooted trees	2.64	1.38 ab	1.92		4.17 b	3.71 ab
‘Barrier’	2.64	1.34 ab	1.92		4.72 ab	3.93 a
‘Cadaman’	2.64	1.97 ab	1.92		4.60 ab	3.67 ab
‘Capdeboscq’	2.64	0.96 b	1.92		4.20 b	3.92 a
Clone 15	2.64	0.96 b	1.92		4.36 ab	3.88 ab
‘Flordaguard’	2.64	1.09 b	1.92		4.51 ab	4.02 a
‘Genovesa’	2.64	1.37 ab	1.92		5.00 ab	3.72 ab
GxN9	2.64	1.97 ab	1.92		4.04 b	3.77 ab
I-67-52-4	2.64	1.13 b	1.92		4.04 b	4.02 a
‘Ishtara’	2.64	1.37 ab	1.92		4.56 ab	4.02 a
México Fila-1	2.64	0.96 b	1.92		4.39 ab	4.02 a
‘Nemared’	2.64	1.16 b	1.92		4.55 ab	3.85ab
‘Okinawa’	2.64	2.47 a	1.92		4.45 ab	3.81 ab
‘Rigitano’	2.69	0.96 b	1.92		4.54 ab	4.02 a
‘Santa Rosa’	2.64	1.67 ab	1.92		4.34 ab	3.90 ab
‘Tsukuba-1’	2.70	2.31 a	1.92		4.22 b	3.73 ab
‘Tsukuba-2’	2.64	1.21 b	1.92		4.63 ab	3.31 b
‘Tsukuba-3’	2.64	1.10 b	1.92		5.29 a	3.93 a
Mean	2.65	1.38	1.92		4.49	3.85

• ^*
  Means followed by the same letter, in the column, do not differ statistically by Tukey’s test (p ≥ 0.05);
• ^ns
  p ≥ 0.05.

‘BRS Kampai’ trees needed between 98.88 (‘Clone 15’) and 105.33 (‘Nemared’) days to complete the period from the autumn equinox to bud break (E1) in 2016 (Table 4). In 2017, this period ranged from 97.50 (‘Capdeboscq’ rootstock) to 106.50 (‘Okinawa’ rootstock) days, and in 2018 from 121.33 (‘BRS Kampai’ own-rooted trees) to 123.09 (‘Tsukuba-2’ rootstock) days, but no statistical differences were observed among rootstocks. For ‘BRS Rubimel’ trial, no significant differences were found among the 15 evaluated rootstocks in 2016, with values ranging from 97 (‘I-67-52-4’ rootstock) to 115 (‘Tsukuba-3’ rootstock) days. Only for ‘GxN9’ and ‘Capdeboscq’ rootstocks, in addition to own-rooted ‘BRS Rubimel’ trees, showed a shorter period to overcome bud dormancy. In 2017, ‘BRS Rubimel’ trees budded on ‘Tsukuba-2’ (197.71 days), ‘Tsukuba-3’ (214.91 days), and ‘Tsukuba-1’ (217.43 days) required significantly more days to complete the period from autumn equinox to bud break compared with the other 15 genotypes. The differences in rootstock behavior regarding bud break may be beneficial to peach growers, as they provide options for selecting rootstocks according to local climatic conditions.

Table 4
Number of days from autumn equinox to break dormant bud stage (number of days E1) among the own-rooted trees and 17 genotypes tested as clonal rootstocks for ‘BRS Kampai’ (2016 to 2018) and ‘BRS Rubimel’ (2016 and 2017) peach, respectively, in Jarinu and Jundiaí, São Paulo state, Brazil<tfn href="tfn06">*</tfn>.

Rootstock or own-rooted trees	‘BRS Kampai’				‘BRS Rubimel’
	2016	2017ns	2018ns		2016	2017
Own-rooted trees	101.64 ab	105.07	121.33		92.17 b	136.00 b
‘Barrier’	104.25 a	102.33	122.74		112.00 ab	142.45 b
‘Cadaman’	100.79 ab	103.67	122.79		107.44 ab	131.65 b
‘Capdeboscq’	102.80 ab	97.50	121.80		91.38 b	141.90 b
Clone 15	98.88 b	99.33	122.00		97.54 ab	136.21 b
‘Flordaguard’	101.61 ab	103.15	121.73		105.67 ab	142.79 b
‘Genovesa’	103.59 ab	102.25	122.00		110.42 ab	145.60 b
GxN9	101.85 ab	105.00	122.00		80.50 b	125.25 b
I-67-52-4	102.50 ab	97.83	122.63		97.00 ab	154.60 b
‘Ishtara’	100.04 ab	104.17	122.00		106.38 ab	142.69 b
México Fila-1	100.79 ab	99.50	122.67		99.83 ab	138.00 b
‘Nemared’	105.33 a	100.67	122.00		105.00 ab	144.17 b
‘Okinawa’	100.30 ab	106.50	122.00		104.27 ab	134.79 b
‘Rigitano’	102.57 ab	99.14	122.45		102.00 ab	137.73 b
‘Santa Rosa’	103.50 ab	103.50	122.67		109.10 ab	145.50 b
‘Tsukuba-1’	104.67 a	105.40	122.00		97.81 ab	217.43 a
‘Tsukuba-2’	102.26 ab	103.60	123.09		108.33 ab	197.71 a
‘Tsukuba-3’	103.50 ab	102.22	122.20		115.00 a	214.91 a
Mean	102.22	102.97	122.25		102.50	150.74

• ^*
  Means followed by the same letter, in the column, do not differ statistically by Tukey’s test (p ≥ 0.05);
• ^ns
  p ≥ 0.05.

Regarding the full bloom stage, the ANOVA showed statistical differences among the rootstock effects considering the number of days from autumn equinox to the full bloom days (number of days E3), except in 2018 for ‘BRS Kampai’ trial (Table 2). The GDH 30, nonetheless, showed no statistical differences among the rootstock effects along with most seasons evaluated for both scion cultivars, except for ‘BRS Rubimel’ trial, in 2017.

The number of days E3 for ‘BRS Kampai’ trial, in 2016, ranged from 108.79 to 117.26, respectively, for Clone 15 and ‘Barrier’ rootstocks (Table 5). The largest group was formed by 13 rootstocks that needed higher number of days, ranging from 111.91 to 117.26 days. Similarly in 2017, a large group with 17 cultivars, led by own-rooted trees of ‘BRS Kampai’, needs more days to reach full bloom stage (between 99.10 and 108.25 days) than trees budded on ‘Nemared’ (98.81 days). Considering ‘BRS Rubimel’ trial, the largest group of rootstocks needed between 104.14 (I-67-52-4 rootstock) and 122.73 (‘Tsukuba-3’ rootstock) days in 2016; and between 135.25 (GxN9 rootstock) and 163.25 (I-67-52-4 rootstock) days, in 2017. Comiotto et al. (2013) evaluated the influence of six rootstocks on phenology of ‘Maciel’ and ‘Chimarrita’ peach scion, in Bento Gonçalves, RS, Brazil (29°09’44”S; 51°31’50”W; 640 m a.s.l.). The full bloom of ‘Maciel’ ranged from July 6^th to 9^th according to the rootstock, while the ‘Chimarrita’ ranged from July 19^th to 24^th (Comiotto et al. 2013). These results emphasize that rootstocks can be a strategy to either advance or delay the full bloom season, according to the peach growers’ interests.

Table 5
Number of days from autumn equinox to full bloom stage (number of days E3) among the own-rooted trees and 17 genotypes tested as clonal rootstocks for ‘BRS Kampai’ (2016 to 2018) and ‘BRS Rubimel’ (2016 and 2017) peach, respectively, in Jarinu and Jundiaí, São Paulo state, Brazil<tfn>*</tfn>.

Rootstock or own-rooted trees	‘BRS Kampai’				‘BRS Rubimel’
	2016	2017	2018ns		2016	2017
Own-rooted trees	113.07 abcd	108.25 a	125.75		102.63 b	149.00 ab
‘Barrier’	117.26 a	103.33 ab	127.14		117.50 ab	153.50 a
‘Cadaman’	110.04 cd	105.54 ab	127.24		116.56 ab	144.22 ab
‘Capdeboscq’	112.84 abcd	102.37 ab	126.60		100.50 b	160.80 a
Clone 15	108.79 d	105.80 ab	126.95		107.50 ab	149.92 ab
‘Flordaguard’	111.91 abcd	105.45 ab	127.23		114.81 ab	149.46 ab
‘Genovesa’	113.71 abcd	107.18 ab	126.48		118.30 ab	157.00 a
GxN9	115.85 abc	101.00 ab	127.29		93.50 b	135.25 ab
I-67-52-4	112.52 abcd	99.10 ab	127.92		104.14 ab	163.25 a
‘Ishtara’	111.08 bcd	101.20 ab	126.91		120.00 ab	154.93 a
México Fila-1	113.50 abcd	107.00 ab	127.53		106.10 ab	147.50 ab
‘Nemared’	117.13 ab	98.81 b	126.93		112.92 ab	155.67 a
‘Okinawa’	110.94 bcd	101.83 ab	127.44		110.38 ab	144.31 ab
‘Rigitano’	115.12 abc	105.46 ab	127.14		116.29 ab	148.50 ab
‘Santa Rosa’	116.23 ab	104.52 ab	127.85		116.50 ab	156.44 a
‘Tsukuba-1’	113.39 abcd	102.29 ab	126.62		105.50 ab	147.31 ab
‘Tsukuba-2’	111.57 bcd	101.28 ab	127.47		111.60 ab	127.67 b
‘Tsukuba-3’	113.00 abcd	105.00 ab	127.00		122.73 a	147.46 ab
Mean	113.17	103.84	127.06		111.21	149.73

• ^*
  Means followed by the same letter, in the column, do not differ statistically by Tukey’s test (p ≥ 0.05);
• ^ns
  p ≥ 0.05.

A trial conducted by Jimenes (2017), which evaluated ‘Sunraycer’ nectarine scion on 13 clonal rootstocks and own-rooted scion trees in Piracicaba, SP, Brazil (22°42’30”S, 47°38’30”W; 546 m a.s.l.), found no differences in full bloom dates (July 1^st; DOY = 182) among different treatments. Ghrab et al. (2014) evaluated peach scions ‘Early May Crest’ and ‘Royal Glory’ grafted on ‘GF677’ and ‘Cadaman’ rootstocks Mornag region (36°41’ N, 10°15’ E) in northern Tunisia, and reported differences in number of days to full bloom depending on the rootstock.

According to the authors, flowering period ranged from 14 to 17 days for trees with different scion-rootstock combinations in years with typical winter conditions. In years with low chill accumulation, this flowering duration ranges only from three to five days. These relatively small differences, which provide limited practical benefits, are expected to become increasingly common under projected climate change scenarios. These differences in full bloom stage suggest variations in chilling requirements based on the scion/rootstock combination (Beckman et al. 1992), which demands research at microregional level, such as that carried out in this paper. The mean GDH 30 values estimated for ‘BRS Kampai’ were 6,499.16, 5,905.53, and 6,425.82 in 2016, 2017, and 2018, respectively; and for ‘BRS Rubimel’, 6,677.65 in 2016 and 7,474.94 in 2017. No statistical differences were observed among the 17 rootstocks and the own-rooted trees (Tables 2 and 6). In other words, although rootstocks influenced number of days E3, they did not significantly affect GHD 30 heat requirement.

Table 6
Growing degree hours accumulated 30 days (GDH 30) after the full bloom stage among 17 rootstocks and the scions ‘BRS Kampai’ (2016 to 2018) and ‘BRS Rubimel’ (2016 and 2017) own rooted, respectively in Jarinu and Jundiaí, São Paulo state, Brazil*.

Rootstock cultivar	‘BRS Kampai’				‘BRS Rubimel’
	2016ns	2017ns	2018ns		2016ns	2017ns
Scion own rooted	6,483.65	6,020.48	6,428.93		6,417.86	7,534.76
‘Barrier’	6,508.63	5,898.05	6,422.54		6,793.13	7,433.78
‘Cadaman’	6,464.02	5,988.33	6,434.67		6,823.15	7,325.84
‘Capdeboscq’	6,511.63	5,872.74	6,424.42		6,465.27	7,541.66
‘Clone 15’	6,541.80	5,929.76	6,423.82		6,558.84	7,358.45
‘Flordaguard’	6,488.02	5,917.45	6,422.80		6,743.51	7,429.03
‘Genovesa’	6,589.85	5,701.30	6,421.29		6,322.75	7,114.25
‘GxN9’	6,438.26	6,004.76	6,425.63		6,935.48	7,731.13
‘I-67-52-4’	6,470.54	5,766.07	6,440.61		6,471.91	7,680.63
‘Ishtara’	6,478.19	5,822.74	6,423.42		6,886.11	7,646.36
‘México Fila-1’	6,507.73	5,968.24	6,420.19		6,521.21	7,401.08
‘Nemared’	6,479.53	5,782.55	6,424.52		6,835.44	7,487.17
‘Okinawa’	6,511.86	5,860.63	6,422.61		6,586.01	7,343.26
‘Rigitano’	6,517.68	5,955.46	6,421.82		6,859.72	7,367.98
‘Santa Rosa’	6,548.55	5,931.89	6,417.10		6,815.73	7,790.80
‘Tsukuba-1’	6,509.07	5,884.36	6,425.04		6,471.23	7,367.09
‘Tsukuba-2’	6,491.61	5,864.45	6,421.08		6,683.34	6,992.04
‘Tsukuba-3’	6,461.20	5,912.25	6,442.16		7,015.74	7,387.70
Mean	6,499.16	5,905.53	6,425.82		6,677.65	7,449.94

• ^*
  Means followed by the same letter, in the column, do not differ statistically by Tukey’s test (p ≥ 0.05);
• ^ns
  p ≥ 0.05.

The ANOVA results showed significant differences in effects of 17 rootstocks and the own-rooted scion trees on the FDP across all evaluated seasons for both scion cultivars (Table 2). However, no statistical differences were observed in the number of days E10 in 2017 (‘BRS Kampai’ trial) and in 2016 (‘BRS Rubimel’ trial). Tukey’s test for FDP revealed a wide variation in the effects of rootstocks (Table 7). For the scion ‘BRS Kampai’, ‘Okinawa’ was the most stable rootstock, consistently remaining in group of the less precocious genotypes across all seasons, whereas for ‘BRS Rubimel’, the rootstock ‘Genovesa’ required the greatest number of days for fruit development. In contrast, none of the most precocious rootstocks showed stable behavior.

Table 7
Fruit development period (FDP) among trees budded on 17 rootstocks and ‘BRS Kampai’ (2016 to 2018) and ‘BRS Rubimel’ (2016 and 2017) own rooted trees, respectively in Jarinu and Jundiaí, São Paulo state, Brazil*.

Rootstock cultivar	‘BRS Kampai’				‘BRS Rubimel’
	2016	2017	2018		2016	2017
Scion own rooted	112.93 ab	97.08 c	74.50 c		114.05 ab	86.85 ab
‘Barrier’	107.84 bc	105.29 abc	92.77 a		106.40 ab	82.78 ab
‘Cadaman’	115.26 ab	103.21 abc	87.75 ab		102.44 ab	89.78 ab
‘Capdeboscq’	112.53 ab	106.39 abc	85.93 ab		118.50 ab	70.95 b
‘Clone 15’	116.44 ab	106.20 abc	80.85 bc		111.50 ab	84.45 ab
‘Flordaguard’	113.45 ab	86.55 c	89.63 ab		100.03 ab	86.31 ab
‘Genovesa’	109.15 abc	107.76 abc	70.46 c		125.50 a	110.50 a
‘GxN9’	119.24 a	104.82 abc	88.02 ab		100.70 ab	79.52 ab
‘I-67-52-4’	112.67 ab	112.90 a	84.82 abc		114.86 ab	75.50 ab
‘Ishtara’	114.56 ab	106.69 abc	80.98 bc		99.00 ab	81.04 ab
‘México Fila-1’	111.71 ab	105.00 abc	83.52 abc		105.62 ab	79.69 ab
‘Nemared’	109.18 abc	109.95 ab	84.13 abc		106.08 ab	81.36 ab
‘Okinawa’	114.53 ab	110.17 ab	89.60 ab		112.44 ab	87.68 ab
‘Rigitano’	96.88 c	106.54 abc	85.69 ab		106.71 ab	81.50 ab
‘Santa Rosa’	109.20 ab	107.48 abc	85.15 abc		102.50 ab	84.19 ab
‘Tsukuba-1’	112.10 ab	109.71 ab	84.22 abc		115.83 ab	87.35 ab
‘Tsukuba-2’	113.75 ab	102.72 bc	86.05 ab		104.60 ab	99.83 ab
‘Tsukuba-3’	112.41 ab	103.67 abc	91.67 ab		95.48 b	85.16 ab
Mean	111.90	105.38	85.06		107.35	84.94

• ^*
  Means followed by the same letter, in the column, do not differ statistically by Tukey’s test (p ≥ 0.05);
• ^ns
  p ≥ 0.05.

For the scion ‘BRS Kampai’, the number of days E10 ranged from 212 to 226.14 DOY in 2016, with ‘Rigitano’ rootstock representing the most precocious and ‘Nemared’ rootstock the latest harvest (Table 8). In 2017, harvest dates ranged from 192 to 212 DOY among rootstocks, including ‘Rigitano’, ‘GxN9’, ‘I-67-52-4’, ‘México Fila-1’, ‘Okinawa’, ‘Santa Rosa’, and ‘Tsukuba-1’, with no significant differences observed. In 2018, harvest dates ranged from 197.75 to 221.54 DOY, representing the most and least precocious rootstocks, respectively. In the ‘BRS Rubimel’ trial, harvest dates in 2016 ranged from 213.40 to 222.11 DOY, with ‘México Fila-1’ as the most precocious and ‘Okinawa’ as the latest, with no statistical differences. In 2017, rootstocks budded with ‘BRS Rubimel’ formed two groups: the most precocious ranged from 250.50 to 312 DOY, and the latest from 312 to 327.33 DOY.

Table 8
Harvest date (number of days E10) among trees budded on 17 rootstocks and ‘BRS Kampai’ (2016 to 2018) and ‘BRS Rubimel’ (2016 and 2017) own rooted trees, respectively in Jarinu and Jundiaí, São Paulo state, Brazil<tfn>*</tfn>.

Rootstock cultivar	‘BRS Kampai’				‘BRS Rubimel’
	2016	2017ns	2018		2016ns	2017
Scion own rooted	226.00 a	205.33	200.25 c		217.00	242.00 a
‘Barrier’	225.06 b	208.67	221.54 a		221.80	237.30 a
‘Cadaman’	225.21 b	208.47	213.86 ab		219.00	238.36 a
‘Capdeboscq’	225.25 b	208.47	212.53 abc		219.00	240.33 a
‘Clone 15’	225.17 b	212.00	206.70 bc		219.00	239.89 a
‘Flordaguard’	225.27 b	192.00	216.86 ab		215.50	240.38 a
‘Genovesa’	225.00 b	208.47	197.75 c		219.00	245.75 a
‘GxN9’	225.17 b	212.00	214.52 ab		219.00	242.20 a
‘I-67-52-4’	225.12 b	212.00	212.74 abc		219.00	240.00 a
‘Ishtara’	225.57 ab	208.00	208.61 bc		219.00	250.50 a
‘México Fila-1’	225.13 b	212.00	211.95 abc		213.40	312.40 b
‘Nemared’	226.14 a	208.47	209.43 abc		219.00	323.89 b
‘Okinawa’	225.31 b	212.00	216.76 ab		222.11	316.29 b
‘Rigitano’	212.00 c	212.00	212.43 abc		221.80	320.00 b
‘Santa Rosa’	225.33 b	212.00	213.39 ab		219.00	327.33 b
‘Tsukuba-1’	225.36 b	212.00	212.64 abc		219.00	320.29 b
‘Tsukuba-2’	225.25 b	204.00	212.91 abc		217.25	312.00 b
‘Tsukuba-3’	225.30 b	208.67	216.22 ab		220.75	320.00 b
Mean	224.67	208.47	212.14		219.00	272.85

• ^*
  Means followed by the same letter, in the column, do not differ statistically by Tukey’s test (p ≥ 0.05);
• ^ns
  p ≥ 0.05.

Figures 3 and 4 depict the minimum, mean, and maximum daily temperatures, including results of the Tukey’s test among years, over the period from full bloom to 30 days after this phenological stage, for the locations of Jarinu (cv. ‘BRS Kampai’) and Jundiaí (cv. ‘BRS Rubimel’), respectively. These figures also indicate the range of the full bloom stage. It can be observed that, regardless of air temperature range after full bloom, this did not bias the behavior of rootstocks or the estimation of GDH 30. Only the FDP showed significant differences among rootstocks in all seasons for both scion cultivars.

The Pearson correlation between GDH 30 and FDP showed moderate to high correlations for ‘BRS Kampai’ and ‘BRS Rubimel’ scions, except in 2018 for ‘BRS Kampai’, which showed a low and non-significant correlation (Table 9). Even in years with significant correlations, the strength of the correlation varied. The highest correlation for ‘BRS Kampai’ occurred in 2017, when minimum and maximum air temperatures were significantly lower than in other years (Fig. 3). This suggests that the average minimum air temperature may have a greater influence on GDH 30 calculation and, consequently, on the correlation with FDP. Conversely, in 2018, no linear correlation was observed between GDH 30 and FDP, and the average minimum and maximum air temperatures were higher than in the other evaluated years. A similar pattern was observed for ‘BRS Rubimel’: 2016, the year with the highest correlation, also had the lowest average air temperatures (Fig. 4). These results suggest that the instability of the correlations may be due to variable climatic conditions, which are common in tropical regions.

Table 9
Pearson correlation (ρ) between growing degree hours accumulated 30 days after the full bloom stage and the fruit development period among trees budded on 17 rootstocks and ‘BRS Kampai’ and ‘BRS Rubimel’ own rooted trees, respectively in Jarinu and Jundiaí, São Paulo state, Brazil, 2016–2018.

Correlation	2016	2017	2018
	‘BRS Kampai’
ρ	-0.2415*	-0.7864*	-0.0325ns
	‘BRS Rubimel’
ρ	-0.9099*	-0.7738*	N.E.

• ^ns
  p ≥ 0.05;
• ^*
  p < 0.05.
• ^N.E.
  not evaluated.

Figure 3
Minimum, mean, and maximum air temperatures (°C) during full bloom stage and 30 days after this phenological stage in Jarinu, São Paulo state, Brazil, from 2016 to 2018. F test, coefficient of variation and Tukey’s test (air temperatures between full bloom and 30 days after this phenological stage) among years: Tmin = 5.11*, 20.11%; Tmed = 0.02ns, 15.71%; Tmax = 1.77*, 17.54%.

Figure 4
Minimum, mean, and maximum air temperatures (°C) during full bloom stage and 30 days after this phenological stage in Jundiaí, São Paulo state, Brazil, from 2016 to 2017. F test, coefficient of variation and Tukey’s test (air temperatures between full bloom and 30 days after this phenological stage) between years: Tmin = 31.12**, 21.87%; Tmed = 41.89**, 16.08%; Tmax = 43.58**, 14.33%.

CONCLUSION

Under the edaphoclimatic conditions of this study, different Prunus spp. genotypes used as clonal rootstocks significantly influenced bud dormancy release, flowering phenology, and harvest timing of ‘BRS Kampai’ and ‘BRS Rubimel’ peach scions. These effects provide relevant information for orchard establishment in areas with limited chilling accumulation. The results could possibly be applied to frost risk management during flowering and to harvest scheduling by either advancing or delaying the flowering season.

Correlations between growing degree hours accumulated during the 30 days after full bloom and length of fruit development period, evaluated across 17 rootstocks and two own-rooted peach scions, ranged from weak and non-significant to strong and statistically significant, depending on the year. This variability indicates that the reliability of predicting harvest date based solely on post-bloom heat accumulation may be negatively affected by the genotype and seasonal climatic conditions.

ACKNOWLEDGMENTS

To the Irmãos Parise Hortifruti for supporting the field trial in Jarinu.

• How to cite:
  Sobierajski, G. R., Blain, G. C. and Mayer, N. A. (2026). Estimating chill and heat requirement for peach flowering and harvesting season in tropical production areas. Bragantia, 85, e20250195. https://doi.org/10.1590/1678-4499.20250195
• FUNDING
  Conselho Nacional de Desenvolvimento Científico e Tecnológico
  Grant No.: 304609/2022-6
• DECLARATION OF USE OF ARTIFICIAL INTELLIGENCE TOOLS
  The authors declare that no artificial intelligence tools were used in the preparation, writing, data analysis, or review of this manuscript.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available on request from the corresponding author.

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Edited by

• Section Editor:
  Patrícia Cia https://orcid.org/0000-0002-3499-7725

Publication Dates

• Publication in this collection
  20 Mar 2026
• Date of issue
  2026

History

• Received
  22 Sept 2025
• Accepted
  05 Jan 2026