Interaction of Endogenous and Environmental Periods in Plant Growth

Abstract

In its natural environment the plant is exposed to alternating periods of light and dark, the combined lengths of which are always 24 hours. Thus we may speak of plants in nature as growing on a 24 hour cycle length. Garner and Allard (2) investigated the effect of light-dark cycles shorter than 24 hours on growth and on flowering. They found that in general such cycles cause reduction in growth. Highkin and Hanson (4) have reported that tomato plants grown under 12 hour and 48 hour cycles become abnormal and that growth is markedly reduced as compared with that of plants grown on a 24 hour cycle. Verkerk (8) has shown that tomatoes require a daily dark period for normal development. Hillman (5) also found that leaf injury appears very quickly when tomatoes are grown in continuous light or in cycle lengths which deviate too greatly from 24 hours, e.g., 18 hours or shorter. Hillman found in addition that the requirement of tomatoes for a daily dark period can be replaced by a period, in light, at a temperature much lower than that during the balance of the cycle. It may be concluded that normal development of the tomato requires that each cvcle include a dark period (or a period at low temperature) and that the length of the cycle must be close to 24 hours. This suggests that the plant possesses some endogenously rhythmic process with a period of 24 hours, which is involved in the growth process, and which must be properly in register with the environmental cycle. Endogenous rhythms are known to participate in and regulate other phases of plant behavior. Thus the leaf movements of beans possess an endogenous rhythmicity. Buinning (1) has shown that the period of such leaf movements of Phaseolus is but slightly temperature dependent. The period, which is 24 hours at 260 C increases to approximately 30 hours at 150 C. A comparable lengthening of the period of the photoperiodic response in the soybean has been found by Nanda and Hamner (6). Plant growth rate is also temperature sensitive (Went. (9)) and decreases in growth rate under unfavorable temperature conditions might be due in principle to effects of temperature on the period of such an internal rhythm. It has in fact been shown by \Vent (9) that the African Violet (Saintpaulia ionantha Wendl.) grows but poorly at 100 C on a 24 hour cycle, but grows more vigorously at the same temperature if the cycle length is increased to 32 hours. Went obtained similar results with Baeria chrysostoma Fisch. and Mey. as well as with tomatoes. The present paper deals with further studies in this field. It will be shown that the environmental cycle length which yields maximal dry matter production becomes longer as the temperature is lowered and the reverse. Symptoms of temperature injury can be lessened or prevented by adjusting the length of the environmental cycle and without alteration of the temperature.