ORIGINAL ARTICLES
Chilling Requirements and Blooming Dates of Leading Peach Cultivars and a Promising Early Maturing Peach Selection, Momo Tsukuba 127
2017 Volume 86 Issue 4 Pages 426-436
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2017 Volume 86 Issue 4 Pages 426-436
A major goal of peach breeding programs in Japan is to develop cultivars with lower chilling requirements than the leading cultivars. Low-chill cultivars can be grown in subtropical as well as temperate regions. We investigated the chilling requirements (chill units; CU), heat requirements (growing degree hours; GDH), and blooming dates of 7 leading Japanese peach cultivars, 3 subtropical low-chill cultivars, and a promising new selection, Momo Tsukuba 127. In general, the CU of the 7 leading cultivars were higher than those of the 3 subtropical cultivars and Momo Tsukuba 127. The chilling and heat requirements were determined for the 3 leading high-chill cultivars (‘Akatsuki’, ‘Hikawahakuhou’, and ‘Kawanakajimahakutou’), the low-chill cultivar ‘Okinawa 1’, and Momo Tsukuba 127 during 4 seasons at a single location. The CU for ‘Okinawa 1’ and Momo Tsukuba 127 were significantly lower than those of the three high-chill cultivars. Because Momo Tsukuba 127 had lower chilling requirements than the 7 leading peach cultivars but higher chilling requirements than the subtropical cultivars, we classified this new selection as a mid-chill variety. We used the CU and GDH, along with local temperature data, to estimate the blooming dates of 4 cultivars and the new selection during 11 seasons at one location. Regression analyses showed high correlations between the calculated and actual blooming dates. We also compared calculated and actual blooming dates for the 3 leading cultivars and Momo Tsukuba 127 at between17 and 21 locations per genotype. A total of 25 locations were used, and these were widely spread over the temperate zones of Japan. The correlations between the calculated and actual blooming dates were close to 1:1. Our results indicated that our CU and GDH values, along with actual temperature data, could be used to reliably estimate the blooming dates of the genotypes. Because of its lower chilling requirements, the new selection, Momo Tsukuba 127, bloomed 7 or more days earlier than the leading peach cultivars in this study.
The peach (Prunus persica L.) is one of the most important fruits in Japan and many breeders have worked to develop desirable cultivars. The NARO Institute of Fruit Tree and Tea Science (NIFTS) has carried out peach breeding projects since 1935 and developed many canning and table-use cultivars. For example, the early ripening ‘Chiyohime’ and the mid ripening ‘Akatsuki’ were developed at our institute as table-use cultivars with good productivity, appropriate harvesting times, and excellent fruit quality. ‘Akatsuki’ is the most widely cultivated peach cultivar in Japan. NIFTS has an ongoing peach breeding program with the aim of increasing fruit productivity.
Japan consists of four main islands, Hokkaido, Honshu, Shikoku, and Kyushu, and about seven thousand smaller islands. The archipelago stretches in a 3000 km long arc from the north-east to the south-west within the latitudes 20°N and 46°N (Inden, 2006). Most of Japan has as a temperate climate, but there is a subarctic zone in the north (Hokkaido Island), a subtropical zone toward the south (Okinawa, Amami, and Ogasawara Islands), and a tropical zone in the southernmost region (Miyako and Yaeyama Islands). Based on statistics for 2013 from the Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF), 9250 ha were under peach production, mainly in six large peach growing areas: Yamanashi Prefecture (approx. 2930 ha), Fukushima Prefecture (approx. 1740 ha), Nagano Prefecture (approx. 1040 ha), Wakayama Prefecture (approx. 760 ha), Okayama Prefecture (approx. 620 ha), and Yamagata Prefecture (approx. 600 ha). All of these areas are situated on the temperate Honshu Island. Only 9.2 ha were used for peach production on the subarctic Hokkaido Island, and production in the subtropical Okinawa Prefecture was not counted by MAFF. Thus, peach production in Japan is almost completely limited to the temperate zones (Honshu, Shikoku, and Kyushu Islands between 31°N and 41°N).
The peach is thought to be native to the Tarim Basin, north of the KunLun Mountains in China (Bassi and Monet, 2008). Commercial peach cultivation in China occurs between the latitudes 23°N and 50°N, and these peach cultivation areas include northern and/or high plateau cold regions as well as subtropical southern regions (Huang et al., 2008). Therefore, the environmental adaptability of the peach species is not limited to temperate zones. In fact, wild peaches have been found in subtropical parts of Japan. The cultivar ‘Okinawa’, which has root-knot nematode resistance and a low chilling requirement, was introduced from the subtropical Okinawa Island to Florida in the United States (Sharpe, 1957).
Enhancement of environmental adaptability (i.e., adaptability to variations in climate, soil, water, and other conditions) is one of the most important goals in crop breeding. The production of crops with high environmental adaptability can expand production to new areas, leading to stable and increased productivity.
The chilling requirement of a plant variety is the amount of cold (i.e., the length of time with temperatures below a given threshold) required by flowers and leaf buds in order to complete their morphological development (Bassi and Monet, 2008). For varieties or cultivars from subtropical regions, a low chilling requirement is one of the environmental adaptations needed for flowering and fruit production. Almost all Japanese peach cultivars originated in temperate zones and may not be able to bloom in subtropical zones because those regions cannot satisfy the high chilling requirements of these cultivars.
If a low chilling requirement were added to the leading peach cultivars of Japan (‘Akatsuki’, ‘Hikawahakuhou’, and others), the production of these cultivars could be expanded to subtropical zones, leading to greater reliability and stability in peach production. This advantage is particularly important because of recent global warming. Moreover, low-chill peach cultivars are economically important because they bloom and produce fruit earlier than the high-chill cultivars. These early fruit are traded at higher prices than the more abundant fruit that are harvested later from high-chill cultivars (Topp et al., 2008). For these reasons, there are breeding programs aimed at developing low-chill peach cultivars in many locations around the world, including Sao Paulo (Brazil), Mexico State (Mexico), Florida (United States), and California (United States). In Japan, Beppu et al. (2014, 2015) developed a new low-chill peach selection by crossing the high chill domestic cultivar ‘Hakuhou’ with the low-chill cultivar ‘Flordaprince’ (HKH×FLP3). This selection is registered as ‘KU-PP1’ under the Plant Variety Protection and Seed Act of Japan.
The development of low-chill cultivars has become one of the main targets of peach breeding in Japan. At NIFTS, the low-chill peach breeding program was initiated in 1994. We have developed some low-chill selections by crossing several high-chill domestic cultivars with the low-chill genotypes ‘Coral’ and ‘Chimarrita’ from Brazil (Yamaguchi et al., 2007). In the development of these cultivars, it is important to compare their chilling requirements, heat requirements, and blooming days with those of the leading cultivars, as part of the process needed to evaluate their productivity and marketability.
In this study we compared a number of leading (high-chill) Japanese cultivars with several low-chill cultivars and a promising early maturing selection, Momo Tsukuba 127. This selection is an F3 progeny of the low-chill cultivar ‘Coral’, ripens around late June at NIFTS in Tsukuba, and is registered under the Plant Variety Protection and Seed Act of Japan. For each line we examined the chilling requirements for rest completion of the floral buds, and the heat requirements from the dates of rest completion to the dates of full bloom. We used the chilling and heat requirement data to estimate the blooming dates for several established cultivars and our selection, and compared these calculated dates with the actual blooming dates during 11 seasons at one location, and during one season at 25 different locations.
In this study we examined 7 leading Japanese peach cultivars: ‘Akatsuki’, ‘Hikawahakuhou’, ‘Hakuhou’, ‘Kawanakajimahakutou’, ‘Shimizuhakutou’, ‘Natsukko’, and ‘Misakahakuhou’; 3 low-chill cultivars: ‘Chimarrita’, ‘Coral’, and ‘Okinawa 1’; and a promising new selection; Momo Tsukuba 127. Momo Tsukuba 127 is a synonym for the low-chill selection 402-13 reported by Yamaguchi et al. (2007). Like the ‘Okinawa’ developed in Florida by Sharpe (1957), ‘Okinawa 1’ originated from the subtropical Okinawa Island and has a low chilling requirement (Yoshida, 1981). ‘Chimarrita’ and ‘Coral’ were developed in Brazil as low-chill genotypes (Topp et al., 2008; Yamaguchi et al., 2007; Yooyongwech et al., 2006). These three low-chill cultivars were used in this experiment as low-chill standards for comparison with Japan’s leading peach cultivars and our selection, Momo Tsukuba 127. The ages of the trees, rootstocks, and additional information about these cultivars are shown in Table 1. All trees were planted in orchards at NIFTS in Tsukuba, Japan. Additional Momo Tsukuba 127, ‘Akatsuki’, ‘Hikawahakuhou’, and ‘Kawanakajimahakutou’ trees were also examined at 24 other research institutes in the temperate zones of Japan (Table 2).
Information about trees used in this experiment.
Locations of institutes where four peach genotypes (Akatsuki, Hikawahakuhou, Kawanakajimahakutou, and Momo Tsukuba 127) were examined for this study, along with climate information from neighboring observatories, for September 2014 to May 2015 (data from the 9th National Trial of Peaches in Japan).
Temperature data at 60 min intervals were taken from automated meteorological data acquisition system (AMeDAS) observatories located near each research institute between September 1 of each year and May 30 of the following year. The data were collected over a period of 11 years at the AMeDAS observatory in Tsukuba and during the 2014–2015 season at the other observatories listed in Table 2. The north latitude, east longitude, and altitude of each research institute and its nearest AMeDAS observatory, along with the average temperatures at some of the institutes, the average and lowest temperatures at the neighboring observatories, and the genotypes studied at each institute, are listed in Table 2.
Sampling of shoots for analysis of chilling requirementsThe chilling hour (<7.2°C) model of Weinberger (1950) was chosen to determine the times to sample cut shoots from the experimental trees. Chilling hours (CH) were counted as the number of hours when the air temperature dropped to below 7.2°C. Based on our preliminary or past research data, we sampled the shoots of each genotype 5 to 8 times during specific CH intervals in each season. The sampling dates, CH, and chill units (CU; see below) for each season at NIFTS in Tsukuba are shown in Table 3.
Sampling dates of cut shoots, chilling hours (CH), and chill units (CU) during the September to May seasons between 2012 and 2016 at NIFTS in Tsukuba.
The dates of rest completion (endodormancy breaking), were determined as described previously (Shenghua et al., 2010; Weinberger, 1950; Werner et al., 1988; Yamane et al., 2011). Three annual shoots (approximately 30–50 cm and populated with floral buds) were taken from each tree (Table 1). Each cut shoot was placed in a tube with 2% ‘Chrysal’ (Chrysal Japan Co. Ltd., Osaka, Japan), which is a commercial preparation containing nutrients and fungicides used to prolong the life of cut flowers. During the 2012–2013, 2013–2014, and 2014–2015 seasons, the cuttings were kept in a phytotron at 20°C, >70% humidity, and natural light conditions. In the 2015–2016 season the cuttings were kept in an artificial climate room at 20–25°C, >70% humidity, and artificial lighting (10 h light and 14 h dark). The nutrient solution was replaced each week. After 3 weeks, the progression of floral bud break was evaluated for each shoot. The date of rest completion for each cultivar/selection was determined as the first sampling date of a shoot on which 50% of the floral buds clearly showed petals or green calices of >3 mm.
Calculation of chill unitsAfter the chilling requirements were evaluated based on the CH (<7.2°C) model, the chilling requirement of each genotype was recalculated as CU based on the Utah model (Richardson et al., 1974). As in the calculation of the CH, we used temperature data from the neighboring AMeDAS observatories. Under the Utah model, different hourly temperatures are assigned different weights as follows: <1.4°C, 0 CU; 1.5–2.4°C, 0.5 CU; 2.5–9.1°C, 1.0 CU; 9.2–12.4°C, 0.5 CU; 12.5–15.9°C, 0 CU; 16.0–18.0°C, −0.5 CU; >18.0°C, −1.0 CU. The sampling dates, CH, and calculated CU for the trees at NIFTS in Tsukuba are listed in Table 3.
Investigation of blooming datesThe date of full bloom for each genotype was determined as the date at which 80% of the floral buds reached the full bloom stage. To determine the dates, a whole tree of one clone of each genotype was observed every 1–3 days in the spring. These dates were identified at NIFTS in Tsukuba for all cultivars, with the exception of ‘Coral’, during the seasons 2013–2014, 2014–2015, and 2015–2016. The dates were also determined during the 8 preceding seasons at Tsukuba for ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, ‘Okinawa 1’, and Momo Tsukuba 127. In addition, the dates for ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, and Momo Tsukuba 127 were identified at between 17 and 21 other locations each for the 2014–2015 season (see Table 2). These additional investigations for the 4 genotypes were performed as part of the 9th National Trial of Peaches in Japan, which was conducted as described for the 8th National Trial of Peaches (Yaegaki et al., 2016). The blooming dates were recorded and analyzed as Julian dates (the number of days from January 1 to the date of full bloom).
Investigation of heat requirements and ANOVAThe heat requirement from the date of rest completion (endodormancy breaking) to the date of full bloom was investigated for each cultivar/selection. The heat requirement was evaluated using the growing degree hours (GDH) model developed by Richardson et al. (1975). Because no growth or development of peach buds occurs at temperatures below 4.5°C, the GDH were calculated by subtracting 4.5°C from each hourly temperature above 4.5°C between the date of rest completion and the date of full bloom. The trees gain no additional growth benefit at temperatures above 25°C; therefore, the temperatures above 25°C were treated as equal to 25°C. Thus, the greatest accumulation for 1 h was limited to 20.5 GDH. As for the CH and CU, we used temperature data from neighboring observatories to calculate the heat requirements.
The CH, CU, and GDH were obtained for ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, ‘Okinawa 1’, and Momo Tsukuba 127 during four seasons (2012–2013 to 2015–2016). These data were subjected to analysis of variance (ANOVA) and the means of each genotype were separated by the Tukey’s test using statistics software R (R Development Core Team, 2012).
Comparison of calculated and actual blooming datesTo confirm that the values of the CU and GDH for each genotype were reliable, regression analyses were conducted to examine the relationships between the actual blooming dates and calculated blooming dates based on the CU and GDH for 5 genotypes: ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, Momo Tsukuba 127, and ‘Okinawa 1’. The data were recorded at NIFTS in Tsukuba over a period of 11 years. The CU and GDH for each genotype were determined using temperature data from the neighboring AMeDAS observatory. These calculations were performed using statistics software R (R Development Core Team, 2012). A few temperature data points were absent in two seasons, and in those cases we used temperature data from the AMeDAS in Tsuchiura, which was the next nearest observatory to Tsukuba. These data were revised according to the monthly average temperatures between Tsuchiura and Tsukuba.
To confirm the adaptability of the blooming date calculations to other locations or conditions, we also compared the actual and calculated blooming dates for four genotypes at a total of 25 research institutes. The dates during the 2014–2015 season were analyzed at each location for the genotypes ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, and Momo Tsukuba 127. Between 17 and 21 of the 25 institutes were used for individual genotypes. For the blooming date calculations we used the CU and GDH data for each genotype at each location, which were determined using temperature data from the neighboring observatories. We then revised the calculated blooming dates based on the regression analyses described above for the 11 seasons at NIFTS in Tsukuba. As for the previous analysis, a few temperature data points were missing at 13 observatories, and in those cases we used temperature data from the next nearest observatories. These tempearture data were revised according to the monthly average temperatures between the nearest and next nearest observatories.
One goal of our peach breeding program at NIFTS is to develop low-chill cultivars with marketable fruit quality and good productivity. Recently, we identified a promising early maturing peach selection, Momo Tsukuba 127, and registered it under the Plant Variety Protection and Seed Act of Japan at the meeting of the 9th National Trial of Peaches. In this paper, we analyzed the relationships between chilling requirements, heat requirements, and blooming dates of the leading peach cultivars in Japan, along with those of Momo Tsukuba 127, for 11 years at NIFTS in Tsukuba and at 24 other locations within the temperate zone of Japan. Because many quantity studies (Topp et al., 2008), genetic or phonologic studies (Yamane et al., 2011; Yooyongwech et al., 2006), and QTL analyses (Fan et al., 2010) of peach chilling requirements have employed the Utah model to determine CU (Richardson et al., 1974), we used this model to determine the chilling requirements in this study.
Comparisons of chilling and heat requirementsThe chilling requirements (CH and CU) and heat requirements (GDH) were determined for each of 10 peach cultivars and Momo Tsukuba 127 during three or four seasons (2012–2015, 2013–2016, or 2012–2016). The CU of the 7 leading Japanese peach cultivars (‘Akatsuki’, ‘Hikawahakuhou’, ‘Hakuhou’, ‘Kawanakajimahakutou’, ‘Shimizuhakutou’, ‘Natsukko’, and ‘Misakahakuhou’) varied between about 880 and 1350. On the other hand, the CU of the 3 subtropical cultivars (‘Chimarrita’, ‘Coral’, and ‘Okinawa 1’) varied between about 320 and 590. In our experiment, the CU for ‘Chimarrita’ were about 430, which is consistent with the 350 CU already reported by Topp et al. (2008). Similarly, the CU that we found for ‘Akatsuki’ and ‘Shimizuhakutou’ (1065 and 1130, respectively) were consistent with those reported by Yamane et al. (2011), which were both about 1000 CU. These results indicate that our data are reliable. All of the leading (high-chill) peach cultivars had approximately 1000 CU in this experiment. The GDH of 10 peach cultivars and Momo Tsukuba 127 varied between about 4211 (‘Okinawa 1’ in 2015–2016) and 6122 (‘Hakuhou’, ‘Kawanakajimahakutou’, and ‘Misakahakuhou’ in 2015–2016).
The chilling and heat requirement data were obtained during all four seasons (2012–2013 to 2015–2016) for the genotypes ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, ‘Okinawa 1’, and Momo Tsukuba 127 (Table 4). These data were subjected to ANOVA, and the means of each genotypes were compared using the Tukey’s test. We found statistically significant differences among cultivars for the CU (P < 0.001) and GDH (P < 0.05), and among years for the CU (P < 0.01). The Tukey’s test indicated that the means of the CU for ‘Okinawa 1’ and Momo Tsukuba 127 were statistically different (P < 0.05) from those of ‘Akatsuki’, ‘Hikawahakuhou’, and ‘Kawanakajimahakutou’. This confirmed that ‘Okinawa 1’ and Momo Tsukuba 127 are low-chill types, whereas the temperate cultivars ‘Akatsuki’, ‘Hikawahakuhou’, and ‘Kawanakajimahakutou’ are high-chill types. However, in some cases, peach genotypes with CU values in the range 400 to 650 have been categorized as mid-chill cultivars (Topp et al., 2008). By this standard Momo Tsukuba 127, with CU values ranging from 507 to 869, is a mid-chill type. The Tukey’s test separated the mean GDH of ‘Okinawa 1’ from those of ‘Akatsuki’, ‘Hikawahakuhou’, and ‘Kawanakajimahakutou’ with statistical significance at P < 0.05. However, the GDH of Momo Tsukuba 127 was not statistically significant from any other cultivars.
Means of chilling requirements (CH, CU) and heat requirements (GDH) of 5 peach genotypes during 4 seasons (2012–2013 to 2015–2016) at NIFTS in Tsukuba.
We determined dates of full bloom for the genotypes ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, ‘Okinawa 1’, and Momo Tsukuba 127 over 11 seasons at NIFTS in Tsukuba. Using the CU, GDH, and temperature data from nearby observatories, we were able to estimate the dates of rest completion and full bloom for each cultivar in each of the 11 seasons. The averages of these dates, along with the averages of the actual dates of full bloom, are shown in Table 5. To confirm the reliability of these calculations, we performed regression analyses to compare the calculated and actual blooming dates of the four genotypes in each of the 11 seasons (Fig. 1). In these seasons, the average temperatures ranged from 10.6°C (2005–2006) to 12.0°C (2015–2016), and the lowest temperatures ranged from −5.0°C (February 5 in 2007) to −8.4°C (February 19 in 2012). The correlation coefficients between the calculated and observed dates of full bloom were 0.9014 for ‘Akatsuki’, 0.9319 for ‘Hikawahakuhou’, 0.8941 for ‘Kawanakajimahakutou’, 0.9360 for Momo Tsukuba 127, and 0.8738 for ‘Okinawa 1’. These high correlation coefficients indicated that the actual dates of full bloom could be explained by the recorded temperatures and the specific chilling and heat requirements for each genotype.
Average date of chilling initiation, average calculated dates of rest completion and full bloom, average revised calculated dates of full bloom, and average actual dates of full bloom for 5 peach genotypes during 11 seasons at NIFTS in Tsukuba.
Scatter plots of calculated and observed blooming dates for 5 peach genotypes during 11 seasons (from 2005–2006 to 2015–2016) at one location. The linear regression equation and regression coefficient are shown for each dataset. All lines are significant at P < 0.001. The calculated and observed dates are represented as Julian dates calculated from Jan. 1 (12:00) in each season.
Even though the correlation coefficients between the actual and calculated dates of full bloom were high, they were not 1:1. It has been reported that warm periods during pre-chilling result in reduced bud break in peaches (Nichols et al., 1974) and that extended chilling results in reductions in the heat requirements of peach buds (Citadin et al., 2001; Scalabrelli and Couvillon, 1986). Furthermore, in our experiment there was significant variance among four years in the CU values for the 5 genotypes (Table 4). For these reasons we revised the calculated dates of full bloom for each genotype in each of the 11 seasons using the regression equations shown in Figure 1. The averages of these revised dates are shown in Table 5. We also calculated the root mean square errors (RMSEs) between the revised and actual dates of full bloom (Table 5). Because the RMSEs were between 1.59 and 1.82, the revised calculated dates closely fit the actual dates of full bloom for four genotypes (‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, and Momo Tsukuba 127). The differences in full bloom dates among these four genotypes were most likely due to differences in their chilling requirements, because no significant differences in heat requirements were observed among these genotypes in this experiment (Table 4). On the other hand, the RMSE for ‘Okinawa 1’ during 11 years at Tsukuba was 3.03, a value much higher than those found for the other four genotypes (Table 5). In this experiment we used the Utah model (Richardson et al., 1974) to compare the chill units of our peach cultivars and selection. More optimal rest prediction models have been developed for low-chilling nectarines and peaches (Gilreath and Buchanan, 1981b; Lu et al., 2012). Therefore, it may be effective to use a model specific for low-chill cultivars to obtain a more accurate prediction of blooming dates for ‘Okinawa 1’.
Comparison of calculated and observed blooming dates of four peach genotypes at 17 or more locations within the temperate zones of JapanWe used the CU, GDH, and temperature data to estimate the dates of chilling initiation, rest completion, and full bloom for the genotypes ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, and Momo Tsukuba 127 at 17 or more locations per genotype. The data were collected in the 2014–2015 season. A total of 25 locations were used (Table 2), and these were scattered from northern Aomori to southern Kagoshima, covering almost all of the temperate zones of Japan. The average temperatures at the nearby AMeDAS observatories ranged from 7.7°C at Yamagata to 14.9°C at Miyazaki, and the lowest temperatures ranged from −0.3°C on February 9 in Tokushima to −10.8°C on February 16 in Gifu.
The regression equations shown in Figure 1 were used to revise the calculated dates of full bloom for each genotype at each location. Scatter plots were made of the revised and the actual dates of full bloom for each genotype (Fig. 2). The average observed dates of chilling initiation and full bloom, the average calculated dates of rest completion and full bloom, the averages of the revised calculated dates of full bloom, and the RMSEs between the revised and actual dates of full bloom are shown in Table 6. The RMSEs between the calculated and actual dates of full bloom were between 3.99 and 5.96 (data not shown), and the RMSEs between the revised and actual dates were between 3.09 and 4.58 (Table 6). Although the regression equations shown in Figure 1 improved the predicted dates of full bloom, these RMSE values were higher than those found for the same four genotypes during 11 years at Tsukuba (Table 5). The data obtained in this experiment were likely to be affected by many environmental factors at each location and during each season. For example, it has been reported that bud break of peach and nectarine may be influenced by photoperiod (Nichols et al., 1974) and water conditions (Gilreath and Buchanan, 1981a) as well as temperature. It is also possible that the high RMSE values were affected by differences in temperature between the research locations and their neighboring AMeDAS observatories. As shown in the scatter plots in Figure 2, many data points were above the 1:1 line for each genotype. We obtained average temperatures for 14 of the 25 research locations, and of these, the temperatures at 10 research locations were lower than the average temperatures at their neighboring AMeDAS observatories, with an average difference of 0.6°C for the 14 locations (Table 2). This may explain the relatively high RMSE values. In our simulation using the temperature data at the AMeDAS in Tsukuba for the 2014–2015 season, a 0.5°C decrease in the average temperature caused delays in full bloom dates of about 3.4, 4.6, 2.4, and 2.3 days for ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, and Momo Tsukuba 127, respectively. If the temperature differences had not occurred, the revised calculated dates of full bloom for each genotype would have been delayed, the RMSE values in Table 6 would have been lower, and the scatter plots shown in Figure 2 would have more closely fit the 1:1 lines. Nevertheless, the results of the analyses from the 25 locations indicated that the full bloom dates of ‘Akatsuki’, ‘Hikawahakuhou’, ‘Kawanakajimahakutou’, and Momo Tsukuba 127 could be explained by their CU, GDH, and the recorded temperatures at each location.
Scatter plots of calculated and observed blooming dates for 4 genotypes at 17 or more locations each during the 2014–2015 season. The regression equations in Figure 1 were used to revise the calculated blooming dates. The revised calculated and observed dates are represented as Julian dates calculated from Jan. 1 (12:00). The lines are drawn at y = x. The locations used for each genotype are shown in Table 2.
Average dates of chilling initiation, average calculated dates of rest completion and full bloom, average revised calculated dates of full bloom, and average actual dates of full bloom for 4 peach genotypes at 17 to 21 locations each during the 2014–2015 seasonz.
The blooming date may also be suppressed by low field temperatures after rest completion (ecodormancy). For example, as shown in Table 4 the CU value of Momo Tsukuba 127 was significantly lower than that of ‘Hikawahakuhou’, whereas the GDH values were not significantly different between these two genotypes. The difference in CU should cause approximately one month’s difference in the calculated date of rest completion (Table 6). However, only about a week’s difference was observed in the actual blooming dates for the two genotypes. In a temperate zone, ecodormancy is caused by low temperatures in the field after rest completion, and results in the suppression of bud development. Therefore, longer periods of ecodormancy will reduce the effects of early rest completion on blooming dates. The period of ecodormancy in warm temperate zones is shorter than in cold temperate zones, and thus the effects of CU on blooming dates are greater in warm zones than in cold zones. It is likely that these factors explain the earlier blooming dates of Momo Tsukuba 127 compared with those of ‘Hikawahakuhou’ in the warm temperate zones. For example, the Julian dates of Momo Tsukuba 127 were 71 in Miyazaki Prefecture and 76 in Kagoshima Prefecture, while those of ‘Hikawahakuhou’ were 82 in Miyazaki Prefecture and 85 in Kagoshima Prefecture. On the other hand, differences of only two days were observed in blooming dates between these two cultivars in cold temperate zones such as Gifu Prefecture and Aomori Prefecture.
In this experiment we showed that Momo Tsukuba 127 blooms earlier than some of the leading peach cultivars of Japan, and that the chilling requirements of this selection are significantly lower than those of the tested cultivars. Because of this, the dates of rest completion of Momo Tsukuba 127 in temperate zones of Japan were generally earlier than the dates for the leading Japanese (high-chill) cultivars (Table 6). In many areas, the lowest detected temperatures at the nearby observatories occurred after the rest completion dates for Momo Tsukuba 127 (Tables 2 and 6). There were no reports of low temperature damage of Momo Tsukuba 127 at the 9th National Trial of Peaches; however, there is a need to continuously monitor the production of this selection, especially in seasons with severe low temperatures in late winter and early spring.
We gratefully acknowledge contributors to the 9th National Trial of Peaches for providing the blooming data for 3 peach cultivars and the Momo Tsukuba 127 selection.
