Lemna Gibba belongs to the family Lemnaceae which includes the smallest plants among the Spermatophyta. The plants grow on
the surface of fresh, still water and multiply vegetatively. There are almost 1000 known species in the world.
The species Lemna Gibba v. Hurfesh is grown in Kibbutz Tzora. This species grows wild near Hurfesh in the upper Galilee. Cultivating Lemna as an edible, agricultural crop is a novel experiment which has not previously been attempted anywhere else in the world. The plants are raised at Kibbutz Tzora in large covered ponds of 250 square meters each under daylight condition with supervised mineral fertilization. The ideal growing conditions are not yet fully optimized especially during the seasons when development is naturally slow.
Most of the experiments done to now on light optimization were preformed under laboratory condition using the American strain Lemna G3, and in Israel, a local desert strain.
In this assignment, the light dependency for the strain Lemna Gibba v. Hurfesh was examined throughout the year. Lemna grows on the surface of the water as a single layer (and consequently all fronds receive more or less the same amount of light). Furthermore all plants are subjected to the same ideal water status and controlled mineral nutrition, providing a highly homogenous system. For examining the effects of different factors which affect the growth rate. The rapid development of the plants enables us to examine physiological
changes within a period of week.
The growth rate was examined during the course of a year and a strong correlation was found between the light intensity and the yield received. With the intensification of the light in the spring, a clear increase in growth rate was observed. In the autumn the yield declined with the lowering of light intensity and
temperature. In an early experiment, in a phytotron, to examine the effect of temperature changes on the yield, it was found that by lengthening daylight to 12 hours and applying optimal temperatures of 22-23 degree during the day and 17-22 degree at night, higher yield were achieved then at higher temperature ranges and plant appeared to be of better "quality".
In further experiment, to discover whether different light intensities caused other changes in the plants, weekly fluorescence tests were carried out on plant specimens taken from the ponds. The purpose of these experiments was to examine changes in photosynthetic antenna size in the different seasons and to calculate the photochemical efficiency of the photosynthetic system. It was found that there were slight but consistent changes in the antenna size index during the year. In high light intensities the antenna size was smaller then in plants grown in lower light. In the calculation of photochemical yield it was found that the light utilization efficiency was higher for plants grown in low light intensities then for high-light plant. The conclusion from these findings is that the plant adapts itself to different light intensities during the course of the year and in this way can make the optimal use of available light for its growth and development.
The findings in this assignment show the practical possibility of controlling the growth rate of Lemna Gibba v. Hurfesh by varying the light condition during the period of growth. These plants need high light intensity in order to achieve rapid multiplication but at the same time they need optimal temperature, so the best solution is to use ponds with covering that transmits the maximum amount of light but is sufficiently ventilated to prevent high temperature at midday when radiation is at its peak.