Effect of photoperiod on some stomatal characteristics of in vitro cultured fruit tree shoots

The effect of the photoperiod on some stomatal characteristics in various leaf lamina zones and in leaves of different age was studied on in vitro grown shoots of Prunus cerasifera clone Mr.S. 2/5, Malus pumila Mill clone M9, and peach x almond hybrid clone GF 677. Stomatal density was highest in leaves exposed to continuous light and lowest with continuous dark. Photoperiod treatments supplying the same quantity of daily radiation but distributed over different cycles (4 h light and 2 h dark, 16 h light and 8 h dark) led to differing stomatal densities intermediate between those of the above light treatments. The light regime with the shortest light and dark periods was found to be most favorable to stomatal differentiation. Regardless of light treatment, stomatal density was found to be lower in the leaf lamina basal zone as compared to the median and apical zones, and decreasing from the 1st to 3rd to 5th leaf counting down from the apex. The photoperiod effect was also confirmed by a stomatal index. The stomatal axis ratio showed no interaction with the photoperiod but did highlight a tendency to rounder stomatal shape with increasing stomatal age.