Single leaf photosynthesis of rice plants under field conditions. II. Individual variations in single-leaf photosynthesis and the physiological basis for it.

Abstract

A group of leaves, each of which was situated at the equivalent position on a stem (for instance, the second leaf counted from the uppermost fully expanded leaf of each stem), were chosen and their photosynthetic rates were estimated under field conditions. A rather large variation in single-leaf photosynthetic rates (CER) was observed among them even though the measurements were conducted at approximately the same time of a day and under the same light regime (saturation light intensity). The variation in CER amounted to 9-12 mg CO2 dm-2h-1 when it was expressed as the difference between the largest and the smallest values found in a group of leaves. This variation was named individual variation in single-leaf photosynthesis (CER); the nature and physiological basis of this variation were examined. (1) The variation in CER was closely correlated with variations in stomatal conductance (gs). (2) Leaves having large gs values at a certain time of a day also showed large gs values at other times throughout the measuring period. Therefore, it seemed that the rank in gs of a leaf among leaves was kept rather constant over a period of time. (3) In the case where the gs value of the first leaf was large, the values of the second and the third leaves on the same stem also tended to be large. (4) The rate of bleeding from the section of basal part of a leaf was measured after sunset and compared with gs values of the leaf in the daytime. The leaves which showed large gs values in the daytime tended to exhibit large rates of bleeding after sunset. (5) In the case where the bleeding rate of the first leaf was high, those for the second and the third leaves on the same stem also tended to be high. (6) The obtained results suggested that the basis for variation in CER was a variation in water supplying ability to leaves, which, in turn, varied from stem to stem, probably due to a difference in the amount and activity of roots formed on stem.