Abstract
Summary Plants of C. canephora grown in pots under low nitrogen (LN) or high nitrogen (HN) applications were submitted to either cyclic water stress or daily irrigation. Water deficit led to marked decreases in net carbon assimilation rate (A) and, to a lesser extent, in stomatal conductance (gs), regardless of the N treatments. In well-watered plants, A appreciably increased in HN plants relative to LN plants without significant changes in gs. As a whole, changes in internal CO2 concentration predominantly reflected changes in A rather than in gs. Under irrigated conditions, A, but not gs, correlated with leaf N concentration in a curvilinear way. Photosynthetic nitrogen-use efficiency was considerably low, and decreased with increasing leaf N concentration. Limited N, but not water, slightly decreased the maximum photochemical efficiency of photosystem II (PSII). Under continuous irrigation, LN plants had a smaller quantum yield of electron transport (ϕPSII) through slight decreases of photochemical quenching (qp) and capture efficiency of excitation energy by open PSII reaction centres, and increases in Stern-Volmer non-photochemical quenching. Under water-stressed conditions, changes in PSII photochemistry were apparent only in HN plants, with a 25 % decrease in ϕPSII, due mainly to variations in qp. Biochemical constraints, rather than stomatal or photochemical limitations, provoked the decreases in A under limited supply of either N or water.
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