Studies on the Photosynthesis of Forage Crops : III. Influence of the different temperature levels on diurnal changes in the photosynthesis of forage crops under constant conditions.

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Diurnal changes in the photosynthesis and respiration of nine forage crops were determined under two different temperature levels, 15 and 30°C, enviromental factors including light intensity being kept constant. The followin results were obtained : 1). Under both temperatures, two types were observed of the diurnal changes in photosynthetic activity, one of which did not show any change throughout day and night and the other showed some decrease during the night hours. In any species, so-called "midday depression" was not seen. 2). The changes in photosynthesic activity, if they did occur, took place at the same time, though they failed in most cases to show the ordinary, clear-cut pattern of diurnal changes. 3). Depending on species, the following three groups were distinguished of the eflect of temperature on diurnal changes in photosynthesis : (a). Those, such as Italian ryegrass, red clover, orchard grass and rye, non-sensitive both to temperature levels and to other conditions, remaining unchanged in their photosynthesis throughout day and night. (b). Those, such as Dallisgrass, Sudangrass and new sorgo, sensitive to temperature, showing little change in photosynthesis when the temperature level fell within the optimum temperature range for their photosynthesis, but showing depression in photosynthesis, when the temperature level fell beyond the optimum range. (c). Those, such as teosinte and common vetch, more sensitive to some factors other than temperature level, showing depression in photosynthesis during the night hours under both temperature levels. 4). In all species tested, respiration under both temperature levels. showed a gradual decrease as time passed by.