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.