THE CONTRIBUTION OF FRUIT PHOTOSYNTHESIS TO THE CARBON REQUIREMENT OFCUCUMBER FRUITS AS AFFECTED BY IRRADIANCE, TEMPERATURE AND ONTOGENY

The photosynthetic contribution of a fruit to its carbon requirement t hroughout ontogeny and under different growing conditions was quantifi ed in cucumber (Cucumis sativus L. cv. Corona). In addition, the effec ts of shading on fruit dry matter accumulation and the diurnal course of the elongation rate were studied. Fruit darkening had no photomorph ogenic effect on fruit growth, while the cumulative photosynthetic con tribution of a fruit to its own carbon requirement ranged from 1 to 5% . During the day there was always a net CO2 efflux. The photosynthetic rate per fruit, calculated as the difference between rates of CO2 exc hange in light and dark, increased during fruit ontogeny, while the ph otosynthetic rate per unit fruit surface area declined. The latter was not dependent on fruit size. The photosynthetic activity per unit sur face area of fruits was estimated to be about 20-30% as efficient as t hat of leaves. The rate of calculated photosynthesis was reduced by 60 -65% when the photosynthetically active radiation incident on the frui t decreased from 200 to 50 mu mol m(-2) s(-1). Temperature (70-30 degr ees C had no pronounced effect on the rate of calculated fruit photosy nthesis when fruits of the same developmental stage (temperature sum) were compared. However, the relative photosynthetic contribution of a fruit to its carbon requirement increased when temperature decreased. Moreover, this contribution increased when irradiance increased or fru it growth was reduced by competing fruits. During fruit ontogeny the d aily photosynthetic contribution was highest (up to 15%) in young and old fruits, with a small growth rate.