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Effect of high temperature exposure time during flower bud formation on the occurrence of double pistils in ‘Satohnishiki’ sweet cherry
Scientia Horticulturae 87 (2001) 77±84
Effect of high temperature exposure time during ¯ower bud formation on the occurrence of double pistils in `Satohnishiki' sweet cherry Kenji Beppu*, Takayuki Ikeda, Ikuo Kataoka1 Faculty of Agriculture, Kagawa University, Miki, Kagawa 761-0795, Japan Accepted 8 May 2000 Abstract Exposure time of trees to high temperatures during ¯ower differentiation in¯uenced the occurrence of double pistils in `Satohnishiki' sweet cherry. Mature trees were grown under both early and late forcing, and under non-forcing conditions until harvest in a commercial orchard located in Kagawa, southwestern Japan. In mid-July, when the maximum temperature began to rise rapidly following the rainy season, petal and stamen primordia had been formed in the buds under early forcing (93%) and late forcing (69%) conditions, but under non-forcing conditions most of the buds were still at the stage of sepal differentiation. Pistil doubling rarely occurred under forcing conditions, whereas 10.3% of the ¯owers developed double pistils under non-forcing conditions. In another experiment, potted trees were exposed to high temperatures (358C/258C, day/night) for 15 days at intervals of 15 days during the period from late-June to early-September. High temperature induced double pistils most severely in the buds that contained sepal and petal primordia at the beginning of the treatment, and the frequency of occurrence of double pistils was slightly lower in the buds treated at the earlier stage of ¯ower differentiation. On the other hand, high temperature had little effect on pistil doubling in buds with differentiated stamen and pistil primordia. These results suggest that (1) the buds are most sensitive to the induction of double pistils at high temperatures at the transition stage from sepal to petal differentiation, and (2) forcing culture can be applied to sweet cherry production in warm areas to reduce double pistil formation by avoiding the exposure of buds to high temperatures while the buds are still in the sensitive period. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Prunus avium; Pistil doubling; Floral differentiation; Temperature condition; Forcing culture *
Corresponding author. Tel.: 81-87-891-3075; fax: 81-87-891-3021. E-mail addresses: [email protected] (K. Beppu), [email protected] (I. Kataoka). 1 Tel.: 81-87-891-3066; fax: 81-87-891-3066/3021. 0304-4238/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 0 0 ) 0 0 1 7 3 - 4
78
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
1. Introduction Recently, attempts have been made to produce sweet cherry in the southwestern part of Japan in order to harvest the fruits earlier than in the northern major production areas and to supply local markets. In this region, however, the occurrence of double fruits is a major problem (Beppu et al., 1996). The malformation is due to abnormal differentiation of pistil primordia in the previous growing season (Philp, 1933; Tucker, 1934). Under natural conditions, ¯oral initiation of sweet cherry starts in early-July in this area. Sepals, petals and stamens differentiate sequentially, and pistil primordia are initiated in mid-August (Beppu et al., 1996). Under controlled conditions, we reported that the occurrence of double pistils in `Satohnishiki' markedly increased when the trees were exposed to high temperatures (above 308C) throughout the period of ¯ower differentiation (Beppu and Kataoka, 1999). However, the stage of ¯ower formation when high temperatures are most effective in inducing pistil doubling has yet to be determined. Double fruits seldom occur in forcing culture which accelerates the growth by heating in the plastic greenhouse in order to increase marketability by earlier harvest, even in the warm region. This implies that ¯ower differentiation is hastened and is no longer sensitive to high temperatures during summer. In this study, we determined the effect of time of exposure to high temperatures on the occurrence of double pistils under controlled conditions. Furthermore, we investigated the progression of ¯ower differentiation and frequency of pistil doubling both in trees under forcing cultivation and those under natural conditions. 2. Materials and methods 2.1. Experiment 1: in¯uence of forcing on ¯ower bud formation and pistil doubling Adult trees of `Satohnishiki' sweet cherry (Prunus avium L.) were grown under early and late forcing, and non-forcing conditions in a commercial orchard located in Mannou-cho, Kagawa prefecture. In forcing culture, the temperature in the pipe frame structure covered with the polyethylene ®lm was kept at above 58C at night and at around 208C in the daytime using a petroleum heater. The onset of heating, ®rst blooming and harvest for each treatment are listed in Table 1. After harvest, the polyethylene ®lm covering the pipe frame structure was removed, and all the trees were exposed to ®eld conditions. Five trees were used per treatment. Five spur buds per tree were collected periodically from 24 June to 26 August 1996, and the progression of ¯ower differentiation was observed by stereomicro-
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
79
Table 1 Onset of heating, ®rst bloom and harvest under various culture conditions Culture condition
Onset of heating
First bloom
First harvest
Early forcing Late forcing Non-forcing
31 January 4 March ±
6 March 27 March 25 April
1 May 26 May 9 June
scopy. The developmental stage of the ¯ower buds was classi®ed as described by Diaz et al. (1981) for sour cherry. On 28 October, 20 spur buds per tree were collected, and the occurrence of double pistils was observed. 2.2. Experiment 2: effect of high temperature applied at different stages of ¯ower initiation on pistil doubling Three-year-old `Satohnishiki' sweet cherry trees were grown in 7 l pots in the research ®eld of Kagawa University. Three trees were exposed to high temperatures, 358C/258C (day/night, 9 h day) in sunlit growth chambers, for 15 days, at intervals from 21 June to 3 September 1997. In order to avoid the effect of natural high temperature as much as possible, the trees were placed in steel frame structures covered with woven shade cloth made of silver polyethylene with 22% levels of light transmission during the experimental period, except for the duration of the high temperature treatment (Beppu and Kataoka, 2000). The ambient temperature in the structure under shade during the treatment averaged 26.28C. Control trees were grown in the structures throughout the experimental period. Morphology of the ¯ower primordia was observed for ®ve spur buds per tree collected at the onset and end of each treatment. The occurrence of double pistils was evaluated on 11 September, using another 40 spur buds per tree. 3. Results 3.1. Experiment 1 The daily maximum temperature increased rapidly in mid-July, immediately after the end of the rainy season, and exceeded 308C continuously from late-July to mid-August (Fig. 1). Under all the cultural conditions, early signs of ¯oral initiation were already apparent by 24 June (Table 2). Then, under early forcing conditions, the differentiation of petal and stamen primordia progressed rapidly, and more than 70% of the buds had formed pistil primordia by 26 July. Under late forcing conditions, the differentiation process was slightly slower, but 57% of the
80
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
Fig. 1. Changes in daily average, maximum and minimum temperature in commercial orchard located in Mannou-cho, Kagawa prefecture in 1996. Table 2 In¯uence of forcing on the progression of ¯ower bud formation in `Satohnishiki' sweet cherry Sampling date
Culture condition
Percentage of flowers that had differentiated
24 June
Early forcing Late forcing Non-forcing
28.6 100.0 100.0
71.4 ± ±
± ± ±
± ± ±
± ± ±
± ± ±
4 July
Early forcing Late forcing Non-forcing
± 33.3 57.1
± 11.1 42.9
25.0 22.2 ±
62.5 33.3 ±
12.5 ± ±
± ± ±
15 July
Early forcing Late forcing Non-forcing
± 7.7 ±
± 15.4 66.7
7.1 7.7 33.3
57.1 46.2 ±
35.7 23.1 ±
± ± ±
26 July
Early forcing Late forcing Non-forcing
± ± ±
± ± 20.7
± ± 42.0
± 7.1 33.6
23.1 35.7 3.7
76.9 57.1 ±
5 August
Early forcing Late forcing Non-forcing
± ± ±
± ± 6.7
± ± 20.0
7.1 ± 40.0
21.4 9.1 33.3
71.4 90.9 ±
15 August Early forcing Late forcing Non-forcing
± ± ±
± ± ±
± ± 7.7
± ± ±
± ± 69.2
100.0 100.0 23.1
26 August Early forcing Late forcing Non-forcing
± ± ±
± ± ±
± ± ±
± ± ±
± ± ±
100.0 100.0 100.0
Bract Flower Sepal Petal Stamen Pistil primordia primordia primordia primordia primordia primordia
Period of high temperature treatment
Sampling date
Percentage of flowers that had differentiated Bract primordia
Flower primordia
21 June±5 July
21 June 5 July
100.0 67.3
± 19.2
± 5.8
± 7.7
± ±
± ±
6±20 July
6 July 20 July
28.6 6.5
42.9 21.7
12.2 47.8
16.3 23.9
± ±
± ±
21 July±4 August
21 July 4 August
6.4 ±
21.3 ±
55.3 12.8
17.0 25.5
± 46.8
± 14.9
5±19 August
5 August 19 August
± ±
± ±
21.2 4.4
21.2 11.1
34.6 33.3
23.1 51.1
20 August±3 September
20 August 3 September
± ±
± ±
± ±
± ±
24.0 7.5
76.0 92.5
Sepal primordia
Petal primordia
Stamen primordia
Pistil primordia
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
Table 3 Progression of ¯ower bud formation at the onset and at the end of the high temperature treatment in `Satohnishiki' sweet cherry
81
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K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
buds had differentiated pistils by 26 July. In contrast, under non-forcing conditions, sepal and petal primordia began to differentiate as late as mid- to lateJuly, and only 23% of the buds had differentiated pistils on 15 August. Only 0.8% of ¯owers differentiated double pistils under early forcing conditions, and pistil doubling was not observed under late forcing condition. In contrast, 10.3% of the ¯owers showed double pistils under non-forcing conditions. 3.2. Experiment 2 At the start of the high temperature treatment on 21 June, all the buds were at the stage of bract formation (Table 3). Flower initiation progressed during the 15 days period of high temperature treatment, and petal primordia were initiated in about 8% of the buds. In the trees exposed to high temperatures from 6 July, 28.5% of the buds had formed sepal and petal primordia at the start of the treatment, and this increased to more than 70% after 15 days of treatment. However, stamen and pistil initiations were never observed. At the start of exposure to high temperatures on 21 July, 72.3% of the buds had differentiated sepal and petal primordia; after 15 days, 15 and 47% of the buds had differentiated farther into pistil and stamen primordia, respectively. On 5 August, more than 23% of the buds had already formed pistil primordia before the treatment; this increased to 51% after treatment. On 20 August, 76% of the buds
Fig. 2. Effect of high temperature applied in different periods on pistil doubling in `Satohnishiki' sweet cherry. Bars indicate one S.E.
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
83
contained pistil primordia at the start of the treatment, and most had differentiated pistil primordia after the treatment. Generally, high temperature treatment slightly retarded the progression of initiation. High temperature treatment before 4 August induced double pistil formation (Fig. 2), especially when applied from 21 July to 4 August. On the other hand, exposure of the trees to high temperatures after 5 August had no appreciable effect. 4. Discussion We con®rmed that double pistils seldom occurred either under early or late forcing conditions. Forcing conditions accelerated ¯ower differentiation considerably. Therefore, in mid-July, when the maximum temperature began to rise rapidly, petal and stamen primordia had been formed in the buds under forcing conditions, but under non-forcing conditions most of the buds were still at the stage of sepal differentiation. This fact suggested that the buds of the forced trees were exposed to high temperatures (above 308C) during stamen and pistil formation, whereas the buds of non-forced trees were exposed at early stages. Although the physiological changes of the trees induced under forcing conditions may indirectly affect the occurrence of double pistils, we assume that exposure to high temperatures at the sensitive stage during the differentiation process is critical to the formation of double pistils. In experiment 2, high temperature induced the formation of double pistils most severely in buds that had formed sepal and petal primordia; the frequency of occurrence of double pistils was lower in buds treated earlier, and was very low in buds with stamen and pistil primordia. These facts suggest that the buds are most sensitive to high temperatures when they are at the transition stage from sepal to petal differentiation. Our results suggested that pistil primordia are formed in a short period of time. Conversely, after the primary pistil is formed and has developed to some extent, a second one seldom differentiates. Although this assumption needs to be con®rmed by anatomical observation, control of the temperature during the speci®c period of the ¯ower differentiation process may reduce the occurrence of double pistils. Based on the results of this study, in addition to early harvest, forcing culture can reduce the formation of double pistils by avoiding exposure to high temperatures while the buds are still in the sensitive period of differentiation. The plastic cover should be removed as early as possible after harvest, because ¯ower differentiation of sweet cherry generally begins soon after harvest (Watanabe and Umetsu, 1980; Guimond et al., 1998). Also, to maintain the temperature below the threshold required for induction of double pistils, short-term shading of the
84
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
trees during the sensitive period of the buds may be effective (Beppu and Kataoka, 2000). References Beppu, K., Kataoka, I., 1999. High temperature rather than drought stress is responsible for the occurrence of double pistils in `Satohnishiki' sweet cherry. Sci. Hort. 81, 125±134. Beppu, K., Kataoka, I., 2000. Arti®cial shading reduces the occurrence of double pistils in `Satohnishiki' sweet cherry. Sci. Hort. 83, 241±247. Beppu, K., Sasaki, N., Kataoka, I., 1996. Effects of summer temperature on pistil doubling of sweet cherry. J. Jpn. Sci. Hort. Sci. 65 (Suppl. 1), 156±157 (in Japanese). Diaz, D.H., Rasmussen, H.P., Dennis Jr., F.G., 1981. Scanning electron microscope examination of ¯ower bud differentiation in sour cherry. J. Am. Soc. Hort. Sci. 106, 513±515. Guimond, C.M., Andrews, P.K., Lang, G.A., 1998. Scanning electron microscopy of ¯oral initiation in sweet cherry. J. Am. Soc. Hort. Sci. 123, 509±512. Philp, G.L., 1933. Abnormality in sweet cherry blossoms and fruit. Bot. Gaz. 94, 815±820. Tucker, L.R., 1934. Notes on sweet cherry doubling. Proc. Am. Soc. Hort. Sci. 32, 300±302. Watanabe, S., Umetsu, K., 1980. Flower-bud differentiation and development in sweet cherry. J. Yamagata Agric. For. Soc. 36, 19±24 (in Japanese with English summary).
Effect of high temperature exposure time during ¯ower bud formation on the occurrence of double pistils in `Satohnishiki' sweet cherry Kenji Beppu*, Takayuki Ikeda, Ikuo Kataoka1 Faculty of Agriculture, Kagawa University, Miki, Kagawa 761-0795, Japan Accepted 8 May 2000 Abstract Exposure time of trees to high temperatures during ¯ower differentiation in¯uenced the occurrence of double pistils in `Satohnishiki' sweet cherry. Mature trees were grown under both early and late forcing, and under non-forcing conditions until harvest in a commercial orchard located in Kagawa, southwestern Japan. In mid-July, when the maximum temperature began to rise rapidly following the rainy season, petal and stamen primordia had been formed in the buds under early forcing (93%) and late forcing (69%) conditions, but under non-forcing conditions most of the buds were still at the stage of sepal differentiation. Pistil doubling rarely occurred under forcing conditions, whereas 10.3% of the ¯owers developed double pistils under non-forcing conditions. In another experiment, potted trees were exposed to high temperatures (358C/258C, day/night) for 15 days at intervals of 15 days during the period from late-June to early-September. High temperature induced double pistils most severely in the buds that contained sepal and petal primordia at the beginning of the treatment, and the frequency of occurrence of double pistils was slightly lower in the buds treated at the earlier stage of ¯ower differentiation. On the other hand, high temperature had little effect on pistil doubling in buds with differentiated stamen and pistil primordia. These results suggest that (1) the buds are most sensitive to the induction of double pistils at high temperatures at the transition stage from sepal to petal differentiation, and (2) forcing culture can be applied to sweet cherry production in warm areas to reduce double pistil formation by avoiding the exposure of buds to high temperatures while the buds are still in the sensitive period. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Prunus avium; Pistil doubling; Floral differentiation; Temperature condition; Forcing culture *
Corresponding author. Tel.: 81-87-891-3075; fax: 81-87-891-3021. E-mail addresses: [email protected] (K. Beppu), [email protected] (I. Kataoka). 1 Tel.: 81-87-891-3066; fax: 81-87-891-3066/3021. 0304-4238/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 3 8 ( 0 0 ) 0 0 1 7 3 - 4
78
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
1. Introduction Recently, attempts have been made to produce sweet cherry in the southwestern part of Japan in order to harvest the fruits earlier than in the northern major production areas and to supply local markets. In this region, however, the occurrence of double fruits is a major problem (Beppu et al., 1996). The malformation is due to abnormal differentiation of pistil primordia in the previous growing season (Philp, 1933; Tucker, 1934). Under natural conditions, ¯oral initiation of sweet cherry starts in early-July in this area. Sepals, petals and stamens differentiate sequentially, and pistil primordia are initiated in mid-August (Beppu et al., 1996). Under controlled conditions, we reported that the occurrence of double pistils in `Satohnishiki' markedly increased when the trees were exposed to high temperatures (above 308C) throughout the period of ¯ower differentiation (Beppu and Kataoka, 1999). However, the stage of ¯ower formation when high temperatures are most effective in inducing pistil doubling has yet to be determined. Double fruits seldom occur in forcing culture which accelerates the growth by heating in the plastic greenhouse in order to increase marketability by earlier harvest, even in the warm region. This implies that ¯ower differentiation is hastened and is no longer sensitive to high temperatures during summer. In this study, we determined the effect of time of exposure to high temperatures on the occurrence of double pistils under controlled conditions. Furthermore, we investigated the progression of ¯ower differentiation and frequency of pistil doubling both in trees under forcing cultivation and those under natural conditions. 2. Materials and methods 2.1. Experiment 1: in¯uence of forcing on ¯ower bud formation and pistil doubling Adult trees of `Satohnishiki' sweet cherry (Prunus avium L.) were grown under early and late forcing, and non-forcing conditions in a commercial orchard located in Mannou-cho, Kagawa prefecture. In forcing culture, the temperature in the pipe frame structure covered with the polyethylene ®lm was kept at above 58C at night and at around 208C in the daytime using a petroleum heater. The onset of heating, ®rst blooming and harvest for each treatment are listed in Table 1. After harvest, the polyethylene ®lm covering the pipe frame structure was removed, and all the trees were exposed to ®eld conditions. Five trees were used per treatment. Five spur buds per tree were collected periodically from 24 June to 26 August 1996, and the progression of ¯ower differentiation was observed by stereomicro-
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
79
Table 1 Onset of heating, ®rst bloom and harvest under various culture conditions Culture condition
Onset of heating
First bloom
First harvest
Early forcing Late forcing Non-forcing
31 January 4 March ±
6 March 27 March 25 April
1 May 26 May 9 June
scopy. The developmental stage of the ¯ower buds was classi®ed as described by Diaz et al. (1981) for sour cherry. On 28 October, 20 spur buds per tree were collected, and the occurrence of double pistils was observed. 2.2. Experiment 2: effect of high temperature applied at different stages of ¯ower initiation on pistil doubling Three-year-old `Satohnishiki' sweet cherry trees were grown in 7 l pots in the research ®eld of Kagawa University. Three trees were exposed to high temperatures, 358C/258C (day/night, 9 h day) in sunlit growth chambers, for 15 days, at intervals from 21 June to 3 September 1997. In order to avoid the effect of natural high temperature as much as possible, the trees were placed in steel frame structures covered with woven shade cloth made of silver polyethylene with 22% levels of light transmission during the experimental period, except for the duration of the high temperature treatment (Beppu and Kataoka, 2000). The ambient temperature in the structure under shade during the treatment averaged 26.28C. Control trees were grown in the structures throughout the experimental period. Morphology of the ¯ower primordia was observed for ®ve spur buds per tree collected at the onset and end of each treatment. The occurrence of double pistils was evaluated on 11 September, using another 40 spur buds per tree. 3. Results 3.1. Experiment 1 The daily maximum temperature increased rapidly in mid-July, immediately after the end of the rainy season, and exceeded 308C continuously from late-July to mid-August (Fig. 1). Under all the cultural conditions, early signs of ¯oral initiation were already apparent by 24 June (Table 2). Then, under early forcing conditions, the differentiation of petal and stamen primordia progressed rapidly, and more than 70% of the buds had formed pistil primordia by 26 July. Under late forcing conditions, the differentiation process was slightly slower, but 57% of the
80
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
Fig. 1. Changes in daily average, maximum and minimum temperature in commercial orchard located in Mannou-cho, Kagawa prefecture in 1996. Table 2 In¯uence of forcing on the progression of ¯ower bud formation in `Satohnishiki' sweet cherry Sampling date
Culture condition
Percentage of flowers that had differentiated
24 June
Early forcing Late forcing Non-forcing
28.6 100.0 100.0
71.4 ± ±
± ± ±
± ± ±
± ± ±
± ± ±
4 July
Early forcing Late forcing Non-forcing
± 33.3 57.1
± 11.1 42.9
25.0 22.2 ±
62.5 33.3 ±
12.5 ± ±
± ± ±
15 July
Early forcing Late forcing Non-forcing
± 7.7 ±
± 15.4 66.7
7.1 7.7 33.3
57.1 46.2 ±
35.7 23.1 ±
± ± ±
26 July
Early forcing Late forcing Non-forcing
± ± ±
± ± 20.7
± ± 42.0
± 7.1 33.6
23.1 35.7 3.7
76.9 57.1 ±
5 August
Early forcing Late forcing Non-forcing
± ± ±
± ± 6.7
± ± 20.0
7.1 ± 40.0
21.4 9.1 33.3
71.4 90.9 ±
15 August Early forcing Late forcing Non-forcing
± ± ±
± ± ±
± ± 7.7
± ± ±
± ± 69.2
100.0 100.0 23.1
26 August Early forcing Late forcing Non-forcing
± ± ±
± ± ±
± ± ±
± ± ±
± ± ±
100.0 100.0 100.0
Bract Flower Sepal Petal Stamen Pistil primordia primordia primordia primordia primordia primordia
Period of high temperature treatment
Sampling date
Percentage of flowers that had differentiated Bract primordia
Flower primordia
21 June±5 July
21 June 5 July
100.0 67.3
± 19.2
± 5.8
± 7.7
± ±
± ±
6±20 July
6 July 20 July
28.6 6.5
42.9 21.7
12.2 47.8
16.3 23.9
± ±
± ±
21 July±4 August
21 July 4 August
6.4 ±
21.3 ±
55.3 12.8
17.0 25.5
± 46.8
± 14.9
5±19 August
5 August 19 August
± ±
± ±
21.2 4.4
21.2 11.1
34.6 33.3
23.1 51.1
20 August±3 September
20 August 3 September
± ±
± ±
± ±
± ±
24.0 7.5
76.0 92.5
Sepal primordia
Petal primordia
Stamen primordia
Pistil primordia
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
Table 3 Progression of ¯ower bud formation at the onset and at the end of the high temperature treatment in `Satohnishiki' sweet cherry
81
82
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
buds had differentiated pistils by 26 July. In contrast, under non-forcing conditions, sepal and petal primordia began to differentiate as late as mid- to lateJuly, and only 23% of the buds had differentiated pistils on 15 August. Only 0.8% of ¯owers differentiated double pistils under early forcing conditions, and pistil doubling was not observed under late forcing condition. In contrast, 10.3% of the ¯owers showed double pistils under non-forcing conditions. 3.2. Experiment 2 At the start of the high temperature treatment on 21 June, all the buds were at the stage of bract formation (Table 3). Flower initiation progressed during the 15 days period of high temperature treatment, and petal primordia were initiated in about 8% of the buds. In the trees exposed to high temperatures from 6 July, 28.5% of the buds had formed sepal and petal primordia at the start of the treatment, and this increased to more than 70% after 15 days of treatment. However, stamen and pistil initiations were never observed. At the start of exposure to high temperatures on 21 July, 72.3% of the buds had differentiated sepal and petal primordia; after 15 days, 15 and 47% of the buds had differentiated farther into pistil and stamen primordia, respectively. On 5 August, more than 23% of the buds had already formed pistil primordia before the treatment; this increased to 51% after treatment. On 20 August, 76% of the buds
Fig. 2. Effect of high temperature applied in different periods on pistil doubling in `Satohnishiki' sweet cherry. Bars indicate one S.E.
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
83
contained pistil primordia at the start of the treatment, and most had differentiated pistil primordia after the treatment. Generally, high temperature treatment slightly retarded the progression of initiation. High temperature treatment before 4 August induced double pistil formation (Fig. 2), especially when applied from 21 July to 4 August. On the other hand, exposure of the trees to high temperatures after 5 August had no appreciable effect. 4. Discussion We con®rmed that double pistils seldom occurred either under early or late forcing conditions. Forcing conditions accelerated ¯ower differentiation considerably. Therefore, in mid-July, when the maximum temperature began to rise rapidly, petal and stamen primordia had been formed in the buds under forcing conditions, but under non-forcing conditions most of the buds were still at the stage of sepal differentiation. This fact suggested that the buds of the forced trees were exposed to high temperatures (above 308C) during stamen and pistil formation, whereas the buds of non-forced trees were exposed at early stages. Although the physiological changes of the trees induced under forcing conditions may indirectly affect the occurrence of double pistils, we assume that exposure to high temperatures at the sensitive stage during the differentiation process is critical to the formation of double pistils. In experiment 2, high temperature induced the formation of double pistils most severely in buds that had formed sepal and petal primordia; the frequency of occurrence of double pistils was lower in buds treated earlier, and was very low in buds with stamen and pistil primordia. These facts suggest that the buds are most sensitive to high temperatures when they are at the transition stage from sepal to petal differentiation. Our results suggested that pistil primordia are formed in a short period of time. Conversely, after the primary pistil is formed and has developed to some extent, a second one seldom differentiates. Although this assumption needs to be con®rmed by anatomical observation, control of the temperature during the speci®c period of the ¯ower differentiation process may reduce the occurrence of double pistils. Based on the results of this study, in addition to early harvest, forcing culture can reduce the formation of double pistils by avoiding exposure to high temperatures while the buds are still in the sensitive period of differentiation. The plastic cover should be removed as early as possible after harvest, because ¯ower differentiation of sweet cherry generally begins soon after harvest (Watanabe and Umetsu, 1980; Guimond et al., 1998). Also, to maintain the temperature below the threshold required for induction of double pistils, short-term shading of the
84
K. Beppu et al. / Scientia Horticulturae 87 (2001) 77±84
trees during the sensitive period of the buds may be effective (Beppu and Kataoka, 2000). References Beppu, K., Kataoka, I., 1999. High temperature rather than drought stress is responsible for the occurrence of double pistils in `Satohnishiki' sweet cherry. Sci. Hort. 81, 125±134. Beppu, K., Kataoka, I., 2000. Arti®cial shading reduces the occurrence of double pistils in `Satohnishiki' sweet cherry. Sci. Hort. 83, 241±247. Beppu, K., Sasaki, N., Kataoka, I., 1996. Effects of summer temperature on pistil doubling of sweet cherry. J. Jpn. Sci. Hort. Sci. 65 (Suppl. 1), 156±157 (in Japanese). Diaz, D.H., Rasmussen, H.P., Dennis Jr., F.G., 1981. Scanning electron microscope examination of ¯ower bud differentiation in sour cherry. J. Am. Soc. Hort. Sci. 106, 513±515. Guimond, C.M., Andrews, P.K., Lang, G.A., 1998. Scanning electron microscopy of ¯oral initiation in sweet cherry. J. Am. Soc. Hort. Sci. 123, 509±512. Philp, G.L., 1933. Abnormality in sweet cherry blossoms and fruit. Bot. Gaz. 94, 815±820. Tucker, L.R., 1934. Notes on sweet cherry doubling. Proc. Am. Soc. Hort. Sci. 32, 300±302. Watanabe, S., Umetsu, K., 1980. Flower-bud differentiation and development in sweet cherry. J. Yamagata Agric. For. Soc. 36, 19±24 (in Japanese with English summary).