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

Zhifeng Lu, Yonghui Pan, Wenshi Hu, Rihuan Cong, Tao Ren, Shiwei Guo, Jianwei Lu

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. 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. 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.