Nutrient and genotypic effects on CO2-responsiveness:: Photosynthetic regulation in Leucadendron species of a nutrient-poor environment

Four South African Leucadendron congenerics with divergent soil N and P preferences were grown as juveniles at contrasting nutrient concentrations at ambient (350 mu mol mol(-1)) and elevated (700 mu mol mol(-1)) atmospheric CO2 levels. Photosynthetic parameters were related to leaf nutrient and carbohydrate status to reveal controls of carbon uptake rate. In ail species, elevated CO2 depressed both the maximum Rubisco catalytic activity (V-c,V-max, by 19-44%) and maximum electron transport rate (J(max) by 13-39%), indicating significant photosynthetic acclimation of both measures. Even so, all species had increased maximum light-saturated rate of net CO2 uptake (A(max)) at the elevated growth CO2 level, due to higher intercellular CO2 concentration (c(i)). Leaf nitrogen concentration was central to photosynthetic performance, correlating with A(max), V-c,V-max and J(max). V-c,V-max and J(max) were linearly cc-correlated, revealing a relatively invariable J(max):V-c,V-max ratio, probably due to N resource optimization between light harvesting (RuBP regeneration) and carboxylation. Leaf total non-structural carbohydrate concentration (primarily starch) increased in high CO2, and was correlated with the reduction in V-c,V-max and J(max). Apparent feedback control of V-c,V-max and J(max) was thus surprisingly consistent across all species, and may regulate carbon exchange in response to end-product fluctuation. If so, elevated CO2 may have emulated an excess end-product condition, triggering both V-c,V-max and J(max) down-regulation. In Leucadendron, a general physiological mechanism seems to control excess carbohydrate formation, and photosynthetic responsiveness to elevated CO2, independently of genotype and nutrient concentration. This mechanism may underlie photosynthetic acclimation to source:sink imbalances resulting from such diverse conditions as elevated CO2, low sink strength, low carbohydrate export, and nutrient limitation.