Factors responsible for the sex expression of Japanese cucumber. XI

Role of the leaves

Abstract

There can be little question that the green leaves are the organs which perceive the photoperiodic stimulus. The pistillate flower formation of the cucumber plant being remarkably influenced by the photoperiodic stimulus, the role of the green leaves played in the sex determination was dealt with in this report.
The sensitivity seems to depend somewhat on the age of the leaves. The youngest fully-expanded leaves are the most sensitive, the young expanding and the oldest mature leaves being less sensitive.
When four foliage leaves developed, they were divided into five sets and the sensitivity of the different portions of leaves to photoperiodic treatment was examined. The sensitivity of the leaf portions is in the following descending order: 1) whole, 2) basal half, 3) longitudinal half, 4) apical half and 5) one quarter.
Greater foliage area or/and greater number of foliage leaves induce pistillate flower formation more remarkably.
If plants receive three or four short days and are then defoliated, no pistillate flower results. If plants receive three or four short days and are defoliated two days after the treatment, pistillate flower formation results. It seems that a certain substance(s) is produced in the leaves as the result of stimulus to induce the pistillate flower differentiation, proportionately to the foliage area and the duration of the stimulus.
Provided any leaves and/or a growing point were continuously exposed to long day, no pistillate flower formation results. A few pistillate flowers appeared, however, on the plants identically treated, provided the room temperature was maintained at 17°C.
The successive defoliation of the apical leaves just unfolding results in hastening of the production of the pistillate flowers at the early growth stage and also in the production of the pistillate flowers, later, in the staminate flower clusters at the basal portion of the main stem. The existence of unfolding young leaves seems to restrict the pistillate flower differentiation. In case of the experiment conducted in the greenhouse, however, the defoliation of the young unfolding leaves failed to induce the production of the more vigorous pistillate flower differentiation. It seems that in this case the mature leaves failed to produce and accumulate enough substances for the favorable sex differentiation because of the rather high temperatures (25°C_??_30°C) prevailed in the greenhouse.
The receptor branch (maintained on long day) of a two branched cucumber plant, one branch (receptor branch) continuously exposed to long day and the other (donor branch) on short day, formed one pistillate flower. The photoperiodic stimulus was transmitted to the receptor branch maintained on long day. When mature leaves were removed from the receptor branch, the pistillate flower production tended to increase and when the young unfolding leaves were removed, the pistillate flower production tended to decrease. Provided the donor branch (maintained on short day) was pinched away, and the mature leaves on the receptor branch (maintained on long day) were removed, the receptor branch showed the most abundant pistillate flower production.
It seems that the photosynthetic substances produced in the receptor branch, flowing through the same path in the plant, inhibit the flow of the flower inducing substance(s) into the receptor branch, and the apical portion of the donor branch (and/or the donor branch) tends to dominate the flow of the flower inducing substances.
It may be reasonably explained that the flower inducing substance in the lower concentrations induces the staminate flower production and in the higher concentrations induces the bisexual and further the pistillate flower production.
Individual plants become more sensitive in forming the pistillate flowers as they grow older.