Development of Reproductive Organs and Initial Fruit Set in Peach and Nectarine in the Northern Highlands of Thailand

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1 Jpn. J. Trop. Agr. 49(1): 30-37, 2005 Development of Reproductive Organs and Initial Fruit Set in Peach and Nectarine in the Northern Highlands of Thailand Naoko KOZAI1, Kenji BEPPU1, Sutasinee MANEETHON2, Unaroj BOONPRAKOB3, Suranant SUBHADARABANDHU3 and Ikuo KATAOKA1 1 Faculty of Agriculture, Kagawa University, Miki, Kagawa , Japan 2 The Royal Project Foundation, Chiang Mai 50200, Thailand 3 Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand * Corresponding author: I. KATAOKA (kataoka@ag.kagawa-u.ac.jp) Abstract To analyze the development of reproductive organs and initial fruit set, the pattern of bud burst and flowering, morphology of flower organs, development of embryo sac, and fruit set were investigated by using lowand mid-chill peach and nectarine cultivars in the northern highlands of Thailand. The observation was carried out at Angkhang (19 45' North, 99 10' East, 1400 m elevation) and Khunwang (18 40' North, 98 40' East, 1300 m elevation) Research Stations within the framework of the Royal Project Foundation in Chiang Mai Province. From mid-november, the decrease of the minimum daily temperature was larger in Angkhang than in Khunwang. By early March, 450 Richardson Chilling Units (CU) had accumulated in Angkhang, whereas no CU accumulation was recorded in Khunwang. In Angkhang, flower bud burst of the low-chill cultivars started in early November and the final percentage reached about 70%. Flower bud burst in Khunwang occurred earlier than in Angkhang, but the bursting rate remained below 40% even in the low-chill cultivars. Although the rate of leaf bud burst was very high at both locations, the leaf bud burst was considerably delayed in Khunwang. The flowers in Khunwang were significantly smaller than those in Angkhang. The development of the embryo sac was also less satisfactory in Khunwang than in Angkhang. The low percentage of flower bud burst and inadequate development of the reproductive organs may account for the low fruit set rate in Khunwang. Key Words: bud burst, chilling, embryo sac, Prunus persica, temperature Introduction Received Aug. 19, 2004 Accepted Dec. 18, 2004 In northern Thailand, the introduction of temperate fruit crops to the highlands has been attempted, because it was expected that they might become efficient cash crops for the ethnic minorities (Subhadrabandhu, 1995). Peach is one of the most promising crops, and many cultivars bred in other countries have been introduced for test planting. However, researchers have been confronted with the problem of yield fluctuations in this region, presumably due to the marginal climatic conditions characterized by insufficient chilling, followed by excessively high temperatures (Subhadrabandhu and Punsri, 1987; Subhadrabandhu, 1995). Since insufficient chilling usually delays and decreases the bud burst rate (Erez, 2000), the yield and quality of the products are reduced. On the other hand, it was reported that high temperatures during flower development and anthesis caused abnormal development of the embryo sac and reduced the fruit set rate under controlled conditions ozai et al., 2004). To stabilize peach production in this area by selecting suitable growing sites and adaptable cultivars, it is essential to elucidate the relationship between the meteorological conditions and reproductive growth. This study was carried out to analyze the development of the reproductive organs and initial fruit set in peach and nectarine which differ in chilling requirement at two experimental sites in the highlands of northern Thailand. Location Materials and Methods Angkhang (19 45' North, 99 10' East, 1400 m elevation) and Khunwang (18 40' North, 98 40' East, 1300 m elevation) Research Stations in Chiang Mai Province, within the framework of the Royal Project Foundation, were selected as experimental sites. Fig. 1 shows the average monthly maximum and minimum temperatures

2 Kozai et al.: Peach growth in northern Thailand 31 Fig. 1 Monthly mean maximum and minimum temperatures ( Ž) and precipitation recorded in Angkhang and Khunwang from 1998 to and precipitation from 1998 to 2001 at these sites. Average monthly maximum temperatures changed similarly at both sites, while the average minimum temperature in Khunwang was considerably higher (Noppakoonwong et al.,1998). Temperature conditions Temperature during the experimental period from October 1, 2001 to March 31, 2002 was recorded hourly at both sites. Then, chilling hours (CH) accumulation below 7.2 Ž was calculated. Chilling Units (CU) were also calculated according to the Richardson CU model (Richardson et al., 1974) assigning one unit between 2.5 and 9.1 Ž, and a 0.5 unit between 1.5 and 2.4 Ž, and 9.2 and 12.4 Ž, no unit for temperatures below 1.4 Ž and for the range between 12.5 and 15.9 Ž, and subtracting a 0.5 unit between 16 and 18 Ž, and one unit above 18 Ž. Plant materials Four four-year-old trees of'tropic Beauty' peach (Chilling requirement; 150 CU) and ' Sunblaze' nectarine (250 CU) as low chill cultivars, and 'Flordacrest' peach (400 CU) and 'Mayglo' nectarine (400 CU) as mid-chill cultivars, were selected at each site. Bud bursting and flowering Ten one-year-old shoots, about 30 cm long, were labeled in the first week of October, Flower and leaf bud burst, and flowering were observed weekly until March 1, Development of reproductive organs At anthesis, five flowers per tree were collected, and the weight of the flower, length of pistil, length and width of petal were measured. To evaluate the embryo sac development, five flowers per tree were collected at, and seven days after anthesis. The ovaries were fixed in the Copenhagen mixture (ethanol: glycerol: distilled water = 10:1:8). After dehydration, they were embedded in paraffin wax. Longitudinal serial sections, 16ƒÊm thick, were cut, and stained with Mayer's acid-haemalun. The development of the embryo sac of the primary ovule was observed under an optical microscope. The developmental stages were classified into 6 categories; before or at the time of embryo sac cell differentiation, two-nucleate, four-nucleate, eight-nucleate, eight nucleate with fused polar nuclei stages, and degeneration of embryo sac or nucellus. Pollen germinability Pollen samples were collected from five flowers at anthesis. Pollen germinability was determined on the medium containing 1% agar and 15% sucrose. Fruit set The fruit set percentage was determined by dividing the flower number on the tagged shoot at anthesis by the fruit set number on March 1, Temperature conditions Results The average daily temperature gradually decreased from mid-november to December

3 32 Jpn. J. Trop. Agr. 49 (1) 2005 (Fig. 2). From mid January, it increased again by March. The diurnal temperature differences in Angkhang became larger than those in Khunwang after mid-november, mainly due to the larger decrease in the minimum daily temperature. The maximum temperature changed similarly in Angkhang and Khunwang. In Angkhang, the chilling level exceeded 1,000 CH and 450 CU by late March (Fig. 3). In Khunwang, although only 28 CH of chilling accumulated, accumulation of CU was not recorded. Bud burst and flowering In Khunwang, some flower buds had already started to burst at the beginning of the observation in early October and bursting lasted until late November in all the cultivars (Fig. 4). Flower buds of the nectarine cultivars burst again in January. The rate of bursting was below 40% even in the low-chill cultivars. On the other hand, in Angkhang, flower buds of the low-chill cultivars rapidly burst in November and December, whereas in the mid-chill cultivars, the bursting occurred from early to mid January. The rate of flower bud bursting eventually exceeded 70% in the low-chill cultivars, whereas values of 40% and 20% were recorded in the mid-chill 'Flordacrest' and 'Mayglo' cultivars, respectively. In Khunwang, the rate of leaf bud burst, which was low from the beginning of the experiment until mid January, subsequently increased rapidly (Fig. 5). In Angkhang, the leaf buds of the low-chill cultivars started to burst in mid-november, followed by a rapid increase of the rate in mid-december. The leaf bud burst in the mid-chill cultivars was delayed by almost one month, compared with that of the low-chill cultivars. The rate of leaf bud bursting reached a value of 80% eventually, except for `Sunblaze' in Khunwang. Generally, flowering started within two weeks after bud bursting (Figs. 4, 6). In Angkhang, Fig. 2 Changes in daily temperatures during the experimental period in Angkhang and Khunwang. Fig. 3 Comparison of cumulative recorded Chill Hours (CH) (<7.2 Ž) and calculated Richardson Chilling Units (CU) during the experimental period in Angkhang and Khunwang. CU in Khunwang was not illustrated because of the 0 value of CU. Fig. 4 Flower bud burst in each cultivar in Angkhang and Khunwang. Values are the means of four trees of each cultivar.

4 Kozai et al.: Peach growth in northern Thailand 33 Fig. 5 Leaf bud burst in each cultivar in Angkhang and Khunwang. Values are the means of four trees of each cultivar. flowering lasted about one month in all the cultivars. On the other hand, in Khunwang, the flowering time extended to two months in the peach cultivars and four months in the nectarine cultivars. Development of reproductive organs The morphological characteristics of the flowers at anthesis are shown in Table 1. The flowers in Khunwang were significantly smaller than those in Angkhang, as indicated by the weight, pistil length, ovary length, and petal size. The non-showy flowers of `Sunblaze' were significantly smaller than those of the other cultivars. Pollen germinability ranged from 18.1 to 22.1% among the cultivars, but there were no significant differences among the values. At anthesis, more than 70 % of the embryo sacs remained at the stage before or at the stage of embryo sac cell differentiation at both sites (Table 2). Even at that time, the flowers in Fig. 6 Flowering percentage in each cultivar in Angkahang and Khunwang. Values are the means of four trees of each cultivar. Khunwang contained degenerated embryo sacs, with a particularly high percentage (27.3%) in ' Sunblaze'. Seven days after anthesis, embryo sacs at the eight nucleate stage were observed in all the cultivars. 'Tropic Beauty' showed the highest percentage at the eight-nucleate stage at both locations. In Khunwang, 11.8 to 50.0% of the embryo sacs in the flowers degenerated, while only 0 to 5.3% in Angkhang. Fruit set The fruit set rate was significantly lower in Khunwang than in Angkhang (Table 3). In ' Mayglo', the fruit set percentage was the lowest in all the cultivars, but the values were not significant. Pattern of bud burst Discussion The pattern of bud burst was considerably different between the experimental sites. In

5 34 Jpn. J. Trop. Agr. 49 (1) 2005 Table 1 Morphological characteristics of flower organs in each cultivar Values are indicated as means and analyzed by ANOVA. *,**,***,or NS denote significance levels at P=0.05, 0.01, 0.001, or non-significance, respectively. Different letters denote significant difference. Table 2 Development of embryo sac at, and seven days after anthesis in each cultivar Twenty primary ovules were observed for each cultivar.

6 Kozai et al.: Peach growth in northern Thailand 35 Table 3 Fruit set percentage in each cultivar Values are indicated as means and analyzed by ANOVA. ** and NS denote significance level at P =0.01 and nonsignificance, respectively. Different letters denote significant difference. Khunwang, although flower buds started to burst as early as at the beginning of October, the rate of bursting remained below 40% even in the lowchill cultivars. Apparently, chilling accumulation in Khunwang was insufficient for complete release from dormancy. The prolonged flowering period of two months in peach and four months in nectarine also suggests the lack of chilling. In Khunwang, some of the flower buds burst before they were exposed to chilling. Erez (2000) reported that flower buds of low-chill cultivars entered into endodormancy very slowly and remained in a shallow dormant state under high temperature conditions. The flower buds in such a state in Khunwang might burst by stimuli such as defoliation. On the other hand, in Angkahang, although flower bud burst started one month later than in Khunwang, the rate eventually reached a value of 70% in the low-chill cultivars. In Angkhang, the rapid increase in the rate of bud burst clearly coincided with the increase of chilling accumulation, and the chilling requirement of the cultivars was reflected in the bursting rate. Additionally, the relatively larger amount of precipitation in Angkhang might contribute to the breaking of rest. Westwood and Bjornstad (1978) suggested that rainfall shortened the rest period of apple and pear by the effects of leaching inhibitors and evaporative chilling. Comparison of the bud burst between peach and nectarine with a similar chilling requirement showed that there was no constant difference in the rate even at the same location. This suggests that the difference in the response was due to individual characteristics of the cultivars rather than to differences between the crops. In Angkhang, the rapid increase in leaf bud burst followed the flower bud burst, whereas in Khunwang, the leaf bud burst mainly occurred in late January, almost two months later than in Angkhang. Generally, since leaf buds display a higher chilling requirement than flower buds (Erez, 2000; Gilreath and Buchanan, 1981), the leaf bud burst occurred earlier after exposure to a higher level of chilling than that of the flower buds. In Khunwang, however, a relatively high rate of leaf bud burst was observed with only a low level of chilling (28 CH). Rageau et al. (1998) reported that vegetative bud burst of peach trees occurred without exposure to chilling under controlled conditions. They demonstrated that leaf buds could burst when exposed to high temperatures above 2TC under a long photoperiod or a high light intensity. In Khunwang, the leaf bud burst was induced by a high temperature and high light intensity rather than by chilling. Development of reproductive organs and fruit set rate In Khunwang, the fruit set rate was significantly lower than that in Angkhang. Generally, the initial fruit set critically depends on the success of fertilization, which is affected by pollen germination and the function of the embryo sac (Westwood, 1988; Herrero, 1992). In the present study, although the flower size was significantly smaller in Khunwang than in Angkhang, the rate of pollen germination was hardly different between the experimental sites. On the other hand, the flowers in Khunwang displayed a higher rate of degenerated embryo sacs compared with the flowers in Angkhang. As for the development of the ovule and embryo sac, it is generally recognized that environmental and nutritional factors affect their longevity and are involved in the determination of the effective pollination period (Sanzol and Herrero, 2001; Williams, 1965). Temperature is one of the critical environmental factors, and the adverse effect of high temperature on the function of the embryo sac has been demonstrated in peach (Kozai et al., 2004), apricot (Rodrigo and Herrero, 2002), sweet cherry (Beppu et al., 1997) and prune (Thompson and Liu, 1973). As for peach,

7 36 Jpn. J. Trop. Agr. 49 (1) 2005 Kozai et al. (2004) examined the effect of the temperature on the development of the reproductive organs in ehakuho funder controlled conditions in growth chambers and found that high temperatures above 25 Ž interfered with the normal development of the reproductive organs and increased the rate of degenerated embryo sacs, resulting in a low fruit set rate. In the present study, when the temperature was compared between the experimental sites, although the maximum temperature during the day changed similarly at both sites, the night temperature in Khunwang remained as high as 12.8 Ž during flowering from October to December. George et al. (1988) observed that the fruit set rate in peach was remarkably reduced when the mean night temperature was kept at above 12 Ž. The high rate of degeneration of the embryo sac found in Khunwang may be caused by the high night temperature. Additionally, the late leaf bud burst observed in Khunwang may prolong the period during which the development of the reproductive organs depends on the reserve substances, and delay the nutritional conversion from reserved stocks to assimilation products from new leaves, hence adversely affecting the development of the reproductive organs. Among the cultivars, emayglo f showed a relatively low percentage of fruit set, although the values were not significant. However, the relationship between the poor fruit set and the development of the reproductive organs or degeneration of the embryo sac was not clear. Considering the phenological pattern of bud bursting, the development of reproductive organs and eventual fruit set, it is concluded that the lowchill and some of the mid-chill cultivars could become adapted to the conditions in Angkhang, whereas in Khunwang, even for the low-chill cultivars, additional methods of cultivation such as breaking of endodormancy by chemicals and appropriate management to maintain the nutritional conditions of the trees, should be implemented to achieve a sufficient yield. References Beppu, K., S. Okamoto, A. Sugiyama and I. Kataoka Effects of temperature on flower development and fruit set of esatohnishiki fsweet cherry. J. Japan. Soc. Hort. Sci. 65: * Erez, A Bud dormancy; Phenomenon, problems and solutions in the tropics and subtropics. In: Temperate fruit crops in warm climates. (Erez, A. eds.) Kluwer Academic Publishers (Netherlands) George, A. P., R. J. Nissen and W. B. Sherman Overlapping double and early single cropping of lowchill peaches in Australia. Fruit Var. J. 42: Gilreath, P. R. and D. W. Buchanan Floral and vegetative bud development of esungold fand esunlite f nectarine as influenced by evaporative cooling by overhead sprinkling during rest. J. Amer. Soc. Hort. Sci. 106: Herrero, M From pollination to fertilization in fruit trees. Plant Growth Regul. 11: Kozai, N., K. Beppu, R. Mochioka, U. Boonprakob, S. Subhadrabandhu and I. Kataoka Adverse effects of high temperature on the development of reproductive organs in ehakuho f peach trees. J. Hort. Sci. Biotech. 79: Noppakoonwong, U., P. Sripinta, A. George, R. Nissen, J. Campbell and G. Ward Preliminary findings and observations from the ACIAR Project eadaptation of low-chill temperate fruits to Australia and Thailand f. The Royal Project Symposium: Highland Agriculture, An Overview Rageau, R., M. Bonhomme, J. P. Richard and A. Erez The climatic determinism of vegetative bud break on peach trees with no exposure to chilling: Some experimental results. Acta Hort Richardson E. A., S. D. Seeley and D. R. Walker A model for estimating the completion of rest for e Redhaven f and eelberta f peach trees. Hortscience. 9: Rodrigo, J. and M. Herrero Effects of pre-blossom temperature on flower development and fruit set in apricot. Sci. Hort. 92: Sanzol, J. and M. Herrero The geffective pollination period h in fruit trees. Sci. Hort. 90:1-17. Subhadrabandhu, Problems in growing deciduous fruits in warm tropics. Acta Hort. 395: Subhadrabandhu, S. and P. Phunsri Deciduous fruit trees as an alternative to opium poppy in Northern Thailand. Acta Hort.199: Thompson M. M. and L. J. Liu Temperature, fruit set, and embryo sac development in eitalian f prune. J. Amer. Soc. Hort. Sci. 98: Westwood, M. N Pollination and Fruit Set. In: Temperate-zone pomology. Revised edition. Timber Press (Oregon) Westwood, M. N. and H. O. Bjornstad Winter rainfall reduces rest period of apple and pear. J. Amer. Soc. Hort. Sci. 103: Williams, R. R The effect of summer nitrogen applications on the quality of apple blossom. J. Hort. Sci. 40: (*: in Japanese with English summary)

8 Kozai et al.: Peach growth in northern Thailand 2 The Royal Project Foundation, Chiang Mai 50200, Thailand 3 Faculty of Agriculture, Kasetsart University, Bangkok 10900, Thailand