The Relationship Between Electron Transport Components and Photosynthetic Capacity in Pea Leaves Grown at Different Irradiances

Pea plants (Pisum sativum) were grown under different irradiances and the photosynthetic characteristics and chloroplast properties were determined on young, fully expanded leaves. The light-saturated rate of oxygen evolution for leaf discs, measured in 1% CO2, correlated closely with the uncoupled whole- chain electron transport activity by the thylakoid membranes. When expressed on a chlorophyll basis, the photosynthetic rate was proportional to both the cytochrome f content and the coupling factor activity. The ratio of coupling factor activity to cytochrome f content was constant across all the growth irradiance treatments. The content of photosystem II reaction centres, determined by 14Catrazine binding, varied to a small extent while the content of photosystem I reaction centres was unaltered by growth irradiance. Reaction centres do not seem to limit the rate of non-cyclic electron transport. Analysis of the CO2 response curves suggested that the ratio of electron transport capacity to RuP2 carboxylation capacity was greater for plants grown at higher irradiances than for plants grown at lower irradiances. Within any irradiance treatment, the ratio of the two capacities was approximately constant. The proportion of leaf nitrogen allocated to thylakoid proteins (27%) was independent of growth irradiance. Adaptation to low irradiance was associated with a reduction in the electron transport components and an increase in the light-harvesting chlorophyll a/b protein such that the amount of chlorophyll per unit of thylakoid protein nitrogen increased. By contrast, adaptation to high irradiance was associated with an increase in the electron transport capacity per unit of chlorophyll such that the electron transport rate per unit of thylakoid nitrogen was increased.