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.
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