The strawberry cv Ai-Berry produces larger fruits with good taste than other cultlvars and has a high productivity However, malformed fruits, which are accompanied by undeveloped, seedless achenes on the distal region of the receptacles, occur frequently in forcing culture This presents a serious problem in commercial production and its disadvantage in the use as a breeding material The object~ves of these studies were to clarify the mechanism of fruit malformation and to establish practical methods to reduce fruit malformation
1. Factors affecting fruit malformation
The primary or secondary flowers developed into malformed fruits which were accompanied by undeveloped achenes on the distal region of receptacles more frequently than the tertiary or quaternary flowers within inflorescence T h e planting of large plants and high nitrogen nutrition induced fruit malformation Young and small plants raised in the field whithout transplanting produced less malformed fruits even with high nitrogen nutrition There were negative correlation between the number of flowers in the 1st inflorescence and a) large fruit (L30g) yield and b) normal large fruit yield
The pistils on the distal region of receptacles of the primary flower which developed into severely malformed fruits were immature and smaller compared with those on the proximal region T h e stigma of distal pistils showed low respiration rates and developed into seedless achenes On the contrary, in the 5th flower of 'Ai-Berry' and the primary flower. of 'Nyoho', the differences in size and maturity between the distal and proximal pistils were small And whole pistils were equally fertilized and fully developed Fruit malformation might be reduced by extending the duration of pollination up to 8 days However, growth of distal pistils after anthesis will be suppressed by the competition of photosynthate with proximal pistils, as the proximal pistils showed rapid growth within 8 days after anthesis
The fertility of pistils on the distal region of receptacles increased with dela,yed pollination In some primary flowers, however, the distal pistils were still immature and could not fertilized even 10 days after anthesis Moreover, the basal pistils lost their fertility 4 days after anthesis Thus, the flowers pollinated from 4 or more da,ys after anthesis developed into malformed fruits which had undeveloped, seedless achenes on the proximal region of receptacles This indicates that fruit malformation in 'Ai-Berry' can not be prevented by controlling the timing or the duration of pollination period when there is a wide range of pistil maturity
from the apex to the base of a receptacle
From these results, there is a wide range of pistil maturity from the apex to the base of receptacle In the primary or secondary flowers in 'Ai-Berry', and the distal pistils are Immature and fall to set seeds These flowers developed into malformed fruits accompanied by undeveloped, seedless achenes To overcome this problem of malformation in 'Ai-Berry', cultural conditions which might narrow the range of pistil maturity from the apex to the base of receptacle should be investigated
2. Differences i n t h e differentiation a n d development of pistils, a n d i n fruit malformation in some strawberry cultivars
In the primary flowers of 'Ai-Berry', the number of rows of pistils on the receptacles were particularly larger than in the other 5 cultivars: Hokowase, Reiko, Toyonoka, Nyoho, and Morioka No 16 At anthesis, the diameter of ovaries on the apex of receptacle and the T/B ratio (the ratio of ovary diameter on the apex to the base of receptacle), which indicates the differences in the maturlty from the apex to the base of receptacles, were also smaller than the others There were significant correlations between the degree of fruit malformation, with a) the number of rows of pistils, b) the T/B ratio, and c) the ovary diameter on the apex of receptacle Thus, the pa~ticulary wlde range of pistil maturity from the apex to the base is the cause of fruit malformation due to the seedless achenes on the distal region of receptacle in 'Ai-Berry'
The difference in the time of differentiation of pistils from the base to the apex on the receptacle was larger in 'Ai-Berry' than in the other cultivars Moreovrer, the basal pistils grew more rapidly than apical pistils, and the days from differentiation of apical pistils to anthesis was smaller compared with the other cultivars As 'Ai-Berry' has these features, the difference of the pistil maturity from the apex to the base of receptacle becomes large, and the pistils on the distal regions are immature at anthesis
In 'Ai-Berry', there was a negative correlation between the number of rows of pistils and the diameter of ovaries on the apex of receptacle A similar correlation existed in 'Hokowase' which is the pollen parent of 'Ai-Berry' Therefore, 'Ai-Berry' may have inherited the wide range of pistil maturity within the receptacles at anthesis from 'Hokowase'
3 Effects of nitrogen nutrition, temperature and daylength on t h e differentiation and development of pistils, a n d o n fruit malformation
(1) Effect of nitrogen nutrition
The number of rows of pistils differentiating on the receptacle, and the difference in the time of pistil differentation from the base to the apex increased up to 30 days with an increase in nitrogen application As the result, T/B ratio was small and severely malformed fruits occurred with high nitrogen nutrition However, if high rate of nitrogen was applied after the differentiation of sepals, the differences in pistil maturity from the apex to the base of receptacles decreased and fruit malformation was reduced Thus, to reduce fruit malformation, the nitrogen levels in 'Ai-Berry' should be controlled lower than the usual
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practices during flower development
(2) Effect of temperature
At temperatures of 15C or lower, the development of flower buds was delayed, and the number of rows of pistils on the receptacles increased As the result, T/B ratio was small and the occurrence of malformed fruits was high a t low temperatures Temperatures lower than 15C after the end of pistil differentiation delayed flowering, but the T/B ratio increased and fruit malformation was reduced At the average of 20°C. fruit malformations were reduced but fruit weight decreased considerably Thus, the optimum average temperatures during flower development should be 16 to 18T. in the commercial production of 'Ai-Berry'
(3) Effect of daylength
Daylength had no significant influence in advancing of flower development However, the number of rows of pistils decreased and the T/B ratio increased with long daylength (LD:16hr) Early LD treatments had greater effects than when LD was started after the beginning of pistil differentiation Thus, LD may be effective in reducing fruit malfo~mation Since LD prevents flower bud differentiation on the axillary bud, this should be started in late October when the inflorescences initiate on the axillary bud