Seasonal Changes in the Photosynthetic Capacity of Spur Leaves in Japanese Pear

Abstract

Seasonal changes in the light-saturated photosynthetic rate (photosynthetic capacity) were investigated for spur leaves of Japanese pear, using a portable open-chamber instrument. Preliminary tests demonstrated that the chamber should be ventilated at a rate of 500 μmol·s⁻¹, and the photosynthetic photon flux density should be kept at 1,500 μmol·m⁻²·s⁻¹, to evaluate the photosynthetic capacity. Moreover, all the measurements were conducted before 9:00 a.m., because the photosynthetic rate was found to decrease thereafter, the photosynthetic capacity of spur leaves reached maximum (15-20 μmolCO₂·m⁻²·s⁻¹) 30-60 days after flowering, then maintained that level until at, or just before harvest. The photosynthetic capacity gradually decreased after harvest, and then rapidly declined after October. The seasonal change in the mesophyll conductance was very similar to that in the photosynthetic capacity. Therefore, it is suggested that the photosynthetic capacity is mainly regulated by mesophyll activity. Our experiments demonstrated that the photosynthetic capacity of spur leaves of Japanese pear is maintained at a high level throughout the fruit-bearing period.