Difference in single leaf photosynthesis between old and new rice varieties II. A physiological basis for the difference in stomatal conductance between varieties.

Abstract

Using many rice varieties differing in the year of registration, the physiological basis for higher stomatal conductance in new varieties at the ripening stage was examined. (1) At the heading stage, no varietal difference was observed in the rate of bleeding from the section of stem after sunset; whereas at the milk ripe stage, a marked varietal difference was found in the rate. Namely, the newer the variety was, the higher was the rate. (2) At the milk ripe stage, a positive and fairly close correlation existed between the bleeding rate and stomatal conductance (gs) in most cases. This fact suggests that, in new varieties, the higher ability to supply water actively to leaves, based on the higher physiological activity of roots, resulted in the larger gs. (3) No difference was found in aerenchyma resistance (the resistance to air flow through aerenchyma extending from leaves to roots in the plant body) before heading. After heading, however, the trend that the newer varieties had the lower aerenchyma resistance appeared and became more and more marked with time. From this, it was suggested that, in new varieties, the better development of the aerenchyma enabled the plant to supply root tissues with oxygen from the aerial part better, which maintained higher physiological activity of roots at the ripening stage.