Responses of Rubisco activity in vivo to light intensity and CO2 in rice leaves

Abstract

The light response of photosynthesis and Rubisco activity at 36 Pa CO2 was first examined in leaves of the wild-type and rbcS antisense rice with 40% wild type Rubisco. Whereas light-saturated photosynthesis was about 2-fold greater in the wild-type plants than in the rbcS antisense plants, the initial slope of the light response curve was not different between the plants. This was caused by higher activation state of Rubisco in the antisense plants under low light. Rubisco inhibition was not different between the plants, and it decreased with increasing light intensity. No significant inhibition was found around light-saturation point. The activation state of NADP-malate dehydrogenase (MDH) was not correlated with Rubisco activation, but correlated with the electron transport rate at PS II measured by Chl fluorescence. The CO2 response curve at 1500 µmol quanta m-2 s-1 was next examined. While the initial slope was about 2-fold greater in the wild-type plants than in the antisense plants, the difference decreased with increasing CO2 partial pressure. Rubisco activation maintained at high levels (more than 80%) in both plants irrespective of CO2 partial pressure (5 to 120 Pa). In spite of the high light conditions, a significant Rubisco inhibition was observed around 100 Pa CO2 for both plants. This inhibition was greater in the antisense plants (about 30% of maximal activity) than in the wild type plants (20%). The NADP-MDH activation was not correlated with Rubisco activation, but correlated with the quantum yield of PS II.