The contribution of drought-related decreases in foliar nitrogen concentration to decreases in photosynthetic capacity during and after drought in prairie grasses

While stomatal closure usually limits photosynthesis during drought, our previous results suggest that drought-related decreases in foliar nitrogen concentration (N(L)) limit photosynthesis during recovery from drought in prairie grasses. Here we estimate the importance of decreases in N(L) to decreased photosynthetic capacity (PS(cap)) during drought and a subsequent recovery period in three perennial C₄ prairie grasses, PS(cap) (O₂ evolution at light and CO₂ saturation) decreased 69 to 78% during drought in these grasses, and full recovery of PS(cap) required 8 to 12 days, until younger leaves were expanded or older leaves were repaired, depending on species. Decreases in N(L) explained 38 to 51% of the loss of PS(cap) during drought and accounted for 51 to 69% of the total loss of PS(cap) integrated over the post-drought recovery period. N-related loss of PS(cap) appeared to result more from decreases in ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39), phosphoenolpyruvate carboxylase (4.1.1.31), and other soluble photosynthetic enzymes, than from decreases in thylakoid N-containing compounds. Decreases in quantum yield of O₂ evolution and F(v)/F(m) (variable-to-maximum fluorescence of dark-adapted leaves) during drought were small, so we assumed that little damage to photsystem II (PSII) and thylakoid membrane function occurred. Further, F(o) (minimum F) decreased or remained unchanged, dark F(o) was greater than light F(o), and decreases in photochemical quenching (the fraction of oxidized PSII) were reversed within 1-3 days after drought. Therefore, prolonged increases in nonphotochemical quenching (q(n); thermal dissipation of excess light energy) during and after drought were indicative of protective downregulation and were likely associated with disproportionate loss of soluble photosynthetic proteins during drought. In support of this, post-drought recovery of q(n) paralleled recovery of N(L) and PS(cap). Thus, in C₄ prairie grasses, loss of PS(cap) during drought is largely the result of decreases in shoot N(L) and of associated protective downregulation, decreasing carbon assimilation for 1-2 weeks after drought.