Arti®cial shading reduces the occurrence of double
pistils in `Satohnishiki' sweet cherry
Kenji Beppu
1, Ikuo Kataoka
*Faculty of Agriculture, Kagawa University, Miki, Kagawa 761-0795, Japan
Accepted 23 August 1999
Abstract
The effect of arti®cial shading in summer and early autumn on the occurrence of double pistils in `Satohnishiki' sweet cherry was studied. Potted trees were grown in steel frame structures covered with woven shade cloth made of silvered polyethylene and providing 53% and 78% light reduction, from 23rd July to 4th October in 1995 and from 16th July to 16th September in 1997. In 1995, shading levels of 53% and 78% reduced the daily maximum air temperature by 1.88C and 3.28C, respectively, compared to the unshaded control. Forty-seven percent of the buds on nonshaded trees formed double pistil primordia, and 51% of them developed into ¯owers with double pistils the following spring. Reducing light 78% reduced the frequency of double pistils to 24% in buds and 1.5% at anthesis, whereas 53% shade reduced the frequency of double pistils only slightly. Doubling was classi®ed into three groups based upon relative length of the two pistils: (1) one pistil much shorter than the other (S), (2) one pistil about half the length of the other (M), and (3) pistils of approximately equal length (L). The frequency of ¯ower primordia with equally developed pistils in autumn buds corresponded to that of ¯owers with double pistils at anthesis. A 78% level of shading markedly reduced the rate of ¯ower primordia with two equally developed pistils. Because of the relatively cool summer in 1997, the frequency of double pistils in autumn buds, even under unshaded conditions, was about half of that in 1995, but shadings still reduced the frequency of double pistils to less than 3%. These results suggest the possibility of applying arti®cial shading to reduce the occurrence of double pistils in `Satohnishiki' sweet cherry grown in regions with hot summers.#2000 Elsevier Science B.V. All rights reserved.
Keywords: Prunus avium; Flower morphology; Temperature; Abnormal differentiation
*Corresponding author. Tel.:81-87-891-3066; fax:81-87-891-3021.
E-mail addresses: beppuk@ag.kagawa-u.ac.jp (K. Beppu), kataoka@ag.kagawa-u.ac.jp (I. Kataoka).
1Tel.:81-87-891-3075; fax:81-87-891-3066.
1. Introduction
Occurrence of double fruits sometimes becomes a serious problem in sweet cherry production in warm regions (Ryugo, 1988). The malformation results from abnormal differentiation of pistil primordia the previous summer (Tucker, 1935; Micke et al., 1983).
We demonstrated, under controlled conditions, that the occurrence of double pistils in the `Satohnishiki' cultivar increased markedly when the trees were exposed to temperatures above 308C (Beppu and Kataoka, 1999).
In ®eld-grown trees, double fruits reportedly occur more frequently on the sunny than on the shaded portion of the canopy (Philp, 1933; Tucker, 1934). These facts imply that high solar radiation increases bud temperature which in turn enhances double pistil formation, and that conversely, arti®cial shading may prevent the occurrence of the disorder.
In this study, we examined the effect of arti®cial shading in summer on the occurrence of double pistils in `Satohnishiki' sweet cherry grown in a warm region.
2. Materials and methods
2.1. Experiment 1 (1995)
`Satohnishiki' sweet cherry (Prunus avium L.) grafted on `Aobazakura' (Prunus lannesiana Wils.), a common rootstock for sweet cherry in Japan, was used. Five-year-old trees planted in 7 l pots, ®lled with a two granite soil:one bark compost (v/v) were grown under ®eld conditions in the experimental ®eld of Kagawa University prior to the shading treatments. On 23rd July 1996, the trees were placed into steel frame structures covered with woven shade cloth made of silvered polyethylene with two different levels of light transmission. The photosynthetic photon ¯ux density (PPFD) under these shadings, as measured with a LI-190SA quantum sensor (LI-COR, Lincoln, NE, USA) positioned horizontally at the level of the upper leaves, was 53% and 78% of that under full sunlight. Control trees were grown without shading. Three trees were used per treatment. When the soil moisture tension reached 13 kPa, 1 l of water was supplied.
During the treatment period, air temperatures in a ventilated case (Fig. 1), and inside and outside the shaded structures (Fig. 2) were measured with thermocouples placed 1.5 m above the ground. Mid-day leaf water potential in each tree was measured using a pressure chamber.
Fig. 1. Daily average, maximum and minimum temperatures in the research ®eld of Kagawa University in 1995 and 1997.
were classi®ed into three groups based upon relative length of the two pistils: (1) one pistil much shorter than the other (S) (Fig. 3b), (2) one pistil about half the length of the other (M) (Fig. 3c), and (3) pistils of approximately equal length (L) (Fig. 3d). Then all the trees were transferred to the ®eld. At anthesis in the spring of 1996, the percentage of ¯owers with double pistils was determined.
2.2. Experiment 2 (1997)
Nine-year-old `Satohnishiki' trees on `Aobazakura' rootstock in 36 l containers ®lled with the same medium as in Experiment 1 were grown under ®eld conditions. The trees were subjected to the same levels of arti®cial shading as in Experiment 1 from 16th July to 16th September 1997. Control trees were grown without shading. Three trees were used per treatment. When the soil moisture tension reached 13 kPa, 2 l of water was supplied. Field temperatures were recorded during the treatment period (Fig. 1). After the termination of the treatment, all the trees were transferred to the ®eld. On 13th October, 20 spur
buds per tree were collected and the occurrence of double pistils was evaluated based on the same classi®cation as that described in Experiment 1. In the spring of 1998, the percentage of ¯owers with double pistils was determined.
3. Results
3.1. Experiment 1 (1995)
Daily maximum temperature from mid-July to August averaged 33.68C, and daily maximum temperature reached 358C for 14 days (Fig. 1). Shading lowered the average daily maximum air temperatures between 23 July and 31 August in the structures by 1.88C (53% shade) and 3.28C (78% shade), and the average temperature during the day (0600±1800 h) by 1.28C and 2.18C, respectively. Shading had little effect on mid-day leaf water potential (data not shown).
In the control, a total of 47% of the ¯ower buds contained double pistils (Table 1), and the percentage was not affected by 53% shading. A 78% level of shading tended to reduce the total rate of double pistils, but not signi®cantly. Twenty-®ve percent of the control ¯ower buds had equally developed double pistils. Although a 53% level of shading did not change the rate signi®cantly, a 78% level of shading markedly reduced it.
Table 1
Effect of shading on the occurrence of double pistils in `Satohnishiki' sweet cherry
Shading Percentage of flowers with double pistilsSE
In the bud At anthesis
One pistil much shorter than the other (S), one pistil about half the length of the other (M), and pistils of approximately equal length (L).
b
Nonsigni®cant.
cSigni®cant at
The percentage of ¯owers with double pistils observed at anthesis the following spring coincided with the rate of ¯ower buds with equally developed pistils, but not with the total rate including the buds with unequally developed pistils (Table 1).
3.2. Experiment 2 (1997)
The average of the daily maximum temperature from mid-July to August was 31.28C, and the daily maximum temperature reached 358C for only two days (Fig. 1).
Generally, the frequency of occurrence of double pistils in Experiment 2 was considerably lower than in Experiment 1 in 1995 (Table 1). In the control buds, the total rate of double pistils was 21%, and half of the buds showed equally developed pistils. Shading reduced the rate, only 2.4% of the buds had double pistils even under a 53% level of shade. The percentage of ¯owers with double pistils the following spring was as low as 5.5% in the control. Shading tended to reduce the rate, but not signi®cantly.
4. Discussion
In both experiments in this study, the frequency of double pistils was considerably higher in the autumn buds than in the ¯owers at anthesis. We obtained similar results in our previous study when we examined the effect of the temperature on doubling (Beppu and Kataoka, 1999). The present study showed that most of the ¯owers with double pistils developed from ¯ower primordia with equally developed pistils. This ®nding suggests that the weakest of the two pistils degenerates during bud break, probably because of competition with the stronger one. Shading treatments markedly reduced the percentage of ¯owers with equally developed pistils, and hence reduced the percentage with double pistils.
The shading treatments reduced maximum air temperature by 1.8±3.28C. In a previous study under controlled environment, we showed that the formation of double pistils occurred in `Satohnishiki' when trees were exposed to a high day temperature (>308C) during ¯ower initiation (Beppu and Kataoka, 1999).
The reason for the reduction in the percentage of ¯ower buds with equally developed double pistils by shading remains unclear. Shading may delay the initiation of pistils, possibly through nutritional or hormonal factors.
Our data suggest that arti®cial shading could be a practical method for reducing the occurrence of double pistils, especially in unusually hot summers. In `Bing' and `Mazzard', the light saturation for photosynthesis is as low as 500mmol mÿ2sÿ1 (DeJong, 1983; Roper and Kennedy, 1986). The reduction of PPFD by 1400mmol mÿ2sÿ1 caused by the 78% shading level in this experiment would have little effect on the photosynthetic rate, at least in the outer leaves of the canopy. Considering the cost of materials and labor, the suitability of shading treatment for commercial production should be examined in detail.
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