Photosynthetic Plasticity in Flaveria brownii

Abstract

Photosynthesis was examined in leaves of Flaveria brownii A. M. Powell, grown under either 14% or 100% full sunlight. In leaves of high light grown plants, the CO₂ compensation point and the inhibition of photosynthesis by 21% O₂ were significantly lower, while activities of ribulose 1,5-bisphosphate carboxylase/oxygenase and various C₄ cycle enzymes were considerably higher than those in leaves grown in low light. Both the CO₂ compensation point and the degree of O₂ inhibition of apparent photosynthesis were relatively insensitive to the light intensity used during measurements with plants from either growth conditions. Partitioning of atmospheric CO₂ between Rubisco of the C₃ pathway and phosphoenolpyruvate carboxylase of the C₄ cycle was determined by exposing leaves to ¹⁴CO₂ for 3 to 16 seconds, and extrapolating the labeling curves of initial products to zero time. Results indicated that ∼94% of the CO₂ was fixed by the C₄ cycle in high light grown plants, versus ∼78% in low light grown plants. Thus, growth of F. brownii in high light increased the expressed level of C₄ photosynthesis. Consistent with the carbon partitioning patterns, photosynthetic enzyme activities (on a chlorophyll basis) in protoplasts from leaves of high light grown plants showed a more C₄-like pattern of compartmentation. Pyruvate, Pi dikinase and phosphoenolpyruvate carboxylase were more enriched in the mesophyll cells, while NADP-malic enzyme and ribulose 1,5-bisphosphate carboxylase/oxygenase were relatively more abundant in the bundle sheath cells of high light than of low light grown plants. Thus, these results indicate that F. brownii has plasticity in its utilization of different pathways of carbon assimilation, depending on the light conditions during growth.