PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Thermal tolerance of photosynthesis is enhanced in antisense tobacco plants with a high Rubisco activase:Rubisco ratio. Steven Crafts-Brandner^*,1, Michael Salvucci¹, ¹ USDA-ARS Western Cotton Research Laboratory, Phoenix, AZ, US

ABSTRACT- The inhibition of photosynthesis at supra-optimum leaf temperatures is closely associated with temperature-dependent decreases in the Rubisco activation state. The decrease in Rubisco activation at high leaf temperature is caused in large part by the thermal lability of Rubisco activase. We hypothesized that increasing the relative abundance of activase to Rubisco would reduce the deactivation of Rubisco, and the inhibition of photosynthesis, at high leaf temperature. Antisense tobacco rbcS plants containing 30 and 90% of the wild type levels of Rubisco and activase, respectively, were evaluated for heat tolerance. As leaf temperature was increased from 28 to 41 C under high light, the rate of photosynthesis adjusted for respiration was decreased 25 and 6% for the wild type and antisense plants, respectively. Rubisco activation state declined 27% for the wild type but remained at near 100% for antisense plants. At both 210 and 10 mbar O₂, increasing the CO₂ supply to heat-stressed leaves increased the rate of photosynthesis substantially above the rates measured at 28 C and ambient CO₂. For both wild type and antisense plants the relative inhibition of photosynthesis by heat stress under low light conditions was comparable to that measured under light saturating conditions. Although light was limiting, the rate of photosynthesis was significantly increased for both genotypes by higher levels of CO₂ at both 210 and 10 mbar O₂. For both wild type and antisense plants it appeared that the same mechanism, the thermal sensitivity of activase, was associated with high-temperature-inhibition of photosynthesis at both low and high light. However, the high activase:Rubisco ratio of the antisense plants conferred a greater thermotolerance to photosynthesis compared to the wild type plants.

KEY WORDS: rubisco, heat stress, activase