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 Kataoka
1Faculty 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).
1Tel.:81-87-891-3066; fax:81-87-891-3066/3021.
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
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
Table 1
Onset of heating, ®rst bloom and harvest under various culture conditions
Culture condition Onset of heating First bloom First harvest Early forcing 31 January 6 March 1 May Late forcing 4 March 27 March 26 May
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 Bract Late forcing 7.7 15.4 7.7 46.2 23.1 ± Non-forcing ± 66.7 33.3 ± ± ± 26 July Early forcing ± ± ± ± 23.1 76.9
Late forcing ± ± ± 7.1 35.7 57.1 Non-forcing ± 20.7 42.0 33.6 3.7 ± 5 August Early forcing ± ± ± 7.1 21.4 71.4
Late forcing ± ± ± ± 9.1 90.9 Non-forcing ± 6.7 20.0 40.0 33.3 ± 15 August Early forcing ± ± ± ± ± 100.0
Progression of ¯ower bud formation at the onset and at the end of the high temperature treatment in `Satohnishiki' sweet cherry Period of high temperature
treatment
Sampling date
Percentage of flowers that had differentiated Bract
primordia
Flower primordia
Sepal primordia
Petal primordia
Stamen primordia
Pistil primordia
21 June±5 July 21 June 100.0 ± ± ± ± ±
5 July 67.3 19.2 5.8 7.7 ± ±
6±20 July 6 July 28.6 42.9 12.2 16.3 ± ±
20 July 6.5 21.7 47.8 23.9 ± ±
21 July±4 August 21 July 6.4 21.3 55.3 17.0 ± ±
4 August ± ± 12.8 25.5 46.8 14.9
5±19 August 5 August ± ± 21.2 21.2 34.6 23.1
19 August ± ± 4.4 11.1 33.3 51.1
20 August±3 September 20 August ± ± ± ± 24.0 76.0
3 September ± ± ± ± 7.5 92.5
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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 late-July, 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.
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 con-siderably. 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.
trees during the sensitive period of the buds may be effective (Beppu and Kataoka, 2000).
References
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