Differences on photosynthetic limitations between leaf margins and leaf centers under potassium deficiency for Brassica napus L.

Analyzing the proportions of stomatal (S(L)), mesophyll conductance (MC(L)) and biochemical limitations (B(L)) imposed by potassium (K) deficit, and evaluating their relationships to leaf K status will be helpful to understand the mechanism underlying the inhibition of K deficiency on photosynthesis (A). A quantitative limitation analysis of K deficiency on photosynthesis was performed on leaf margins and centers under K deficiency and sufficient K supply treatments of Brassica napus L. Potassium deficiency decreased A, stomatal (g(s)) and mesophyll conductance (g(m)) of margins, S(L), MC(L) and B(L) accounted for 23.9%, 33.0% and 43.1% of the total limitations. While for leaf centers, relatively low limitations occurred. Nonlinear curve fitting analysis indicated that each limiting factor generated at same leaf K status (1.07%). Although MC(L) was the main component of limitations when A began to fall, B(L) replaced it at a leaf K concentration below 0.78%. Up-regulated MC(L) was related to lower surface area of chloroplasts exposed to intercellular airspaces (S(c)/S) and larger cytosol diffusion resistance but not the cell wall thickness. Our results highlighted that photosynthetic limitations appear simultaneously under K deficiency and vary with increasing K deficiency intensity.