Cold-tolerant crop species have greater temperature homeostasis of leaf respiration and photosynthesis than cold-sensitive species.

Some plant species show constant rates of respiration and photosynthesis measured at their respective growth temperatures (temperature homeostasis), whereas others do not. However, it is unclear what species show such temperature homeostasis and what factors affect the temperature homeostasis. To analyze the inherent ability of plants to acclimate respiration and photosynthesis to different growth temperatures, we examined 11 herbace-ous crops with different cold tolerance. Leaf respiration (R(area)) and photosynthetic rate (P(area)) under high light at 360 microl l(-1) CO(2) concentrations were measured in plants grown at 15 and 30 degrees C. Cold-tolerant species showed a greater extent of temperature homeostasis of both R(area) and P(area) than cold-sensitive species. The underlying mechanisms which caused differences in the extent of temperature homeostasis were examined. The extent of temperature homeostasis of P(area) was not determined by differences in leaf mass and nitrogen content per leaf area, but by differences in photosynthetic nitrogen use efficiency (PNUE). Moreover, differences in PNUE were due to differences in the maximum catalytic rate of Rubisco, Rubisco contents and amounts of nitrogen invested in Rubisco. These findings indicated that the temperature homeostasis of photosynthesis was regulated by various parameters. On the other hand, the extent of temperature homeostasis of R(area) was unrelated to the maximum activity of the respiratory enzyme (NAD-malic enzyme). The R(area)/P(area) ratio was maintained irrespective of the growth temperatures in all the species, suggesting that the extent of temperature homeostasis of R(area) interacted with the photosynthetic rate and/or the homeostasis of photosynthesis.