Factors responsible for the sex expression of Japanese cucumber. X

Abstract

In this report, the dark response of Japanese cucumber plant, as related to the differentiation of the flowers, is dealt with. Short day plants, responding to the dark treatment, differentiate flower buds. In cucumber plant, at the first step, differentiate the staminate flowers and later the bisexual and further the pistillate flowers. It seems that a flower forming substance, at lower level, induces the staminate flower differentiation and at higher level bisexual and further the pistillate flower differentiation in the cucumber plant. Seedlings tend to respond to dark treatment much more sensitively as they grow older. Since the green leaves are the organs which perceive the photoperiodic stimulus, it is due mainly to the increase of the foliage leaves both in number and area. The minimum length of the dark period needed for inducing pistillate flower differentiation in the cucumber plant varies from 8 hours to 12 hours relating to the repeated photoperiodic cycles. For the pistillate flower differentiation, nine replications of 8 hours dark period are needed and three replications of 12 hours dark period suffice in its turn. Exposed to long-day and under higher temperature, there are formed fewer staminate flowers per cluster, and the buds are liable to drop. Less production and transfer of the flower forming substance seems to suppress, in this cace, the development of the staminate flowers. The number of pistillate flowers increses with lengthening the dark period to 18 hours but decreases when dark period exceeds 20 hours. Few pistillate flowers are formed under 22 hours dark period and no pistillate flowers under 24 hours dark period. Pistillate flowers are induced when the dark period is adequately long but not too long. Light and dark periods have to follow cyclically and more repetitions of the cyclic treatments induce more abundant pistillate flower differentiation. Dark period interrupted by one hour lighting at the middle of 16-hour dark period showed the same effect to induce the pistillate flower differention as the identical length of dark treatment uninterrupted by the interposed light period. With an adequate length of dark period, light irradiated through blue cellophane and double cellophane, blue and red, induced no pistillate flower differentiation. Light filtered through red cellophane induced pistillate flower differentiation. Light intensity and wave length, to same extent, are the factors in this process. The influence of the day length is modified and sometimes counteracted by temperature (including diurnal alterations in temperature) before, during, and after the dark period. Seedlings brought up under medium temperature form more pistillate flowers when they were exposed to adequate dark period than those brought up under higher temperature. The number of pistillate flowers formed on the seedlings are in the following descending order: 24°_??_17°C (day versus night temperature during the nursery-bed), 24°_??_24°C and 24°_??_32°C. The optimal temperature during the photoperiodic treatment is 17°C and the optimal diurnal alteration is 24_??_17°C. At lower and higher temperatures, pistillate flower formation is reduced or absent. Above 32°C, no pistillate flowers were induced even after twelve repetitions of the photoperiodic treatment. In general, four to six repetitions are adequate under the optimal temperature. The optimal temperature (day and night) during the light period extended by the electric lamp is 17°C. The higher night temperatures (17°C, 24°C and 30°C) reduced the pistillate flower differentiation. Temperature during the photoperiodic treatment showed a profound influence on the pistillate flower differentiation. Below 17°C, plants even under continuous illumination, formed pistillate flowers.