The photosynthetic and structural differences between leaves and siliques of Brassica napus exposed to potassium deficiency.

BACKGROUND: Most studies of photosynthesis in chlorenchymas under potassium (K) deficiency focus exclusively on leaves; however, little information is available on the physiological role of K on reproductive structures, which play a critical role in plant carbon gain. Brassica napus L., a natural organ-succession species, was used to compare the morphological, anatomical and photo-physiological differences between leaves and siliques exposed to K-deficiency.

RESULTS: Compared to leaves, siliques displayed considerably lower CO2 assimilation rates (A) under K-deficient (-K) or sufficient conditions (+K), limited by decreased stomatal conductance (g s ), apparent quantum yield (α) and carboxylation efficiency (CE), as well as the ratio of the maximum rate of electron transport (J max ) and the maximum rate of ribulose 1,5-bisphosphate (RuBP) carboxylation (V cmax ). The estimated J max , V cmax and α of siliques were considerably lower than the theoretical value calculated on the basis of a similar ratio between these parameters and chlorophyll concentration (i.e. J max /Chl, V cmax /Chl and α/Chl) to leaves, of which the gaps between estimated- and theoretical-J max was the largest. In addition, the average ratio of J max to V cmax was 16.1% lower than that of leaves, indicating that the weakened electron transport was insufficient to meet the requirements for carbon assimilation. Siliques contained larger but fewer stoma, tightly packed cross-section with larger cells and fewer intercellular air spaces, fewer and smaller chloroplasts and thin grana lamellae, which might be linked to the reduction in light capture and CO2 diffusion. K-deficiency significantly decreased leaf and silique A under the combination of down-regulated stomatal size and g s , chloroplast number, α, V cmax and J max , while the CO2 diffusion distance between chloroplast and cell wall (D chl-cw ) was enhanced. Siliques were more sensitive than leaves to K-starvation, exhibiting smaller reductions in tissue K and parameters such as g s , V cmax , J max and D chl-cw .

CONCLUSION: Siliques had substantially smaller A than leaves, which was attributed to less efficient functioning of the photosynthetic apparatus, especially the integrated limitations of biochemical processes (J max and V cmax ) and α; however, siliques were slightly less sensitive to K deficiency.