Quantitative limitations to photosynthesis in K deficient sunflower and their implications on water-use efficiency

Abstract

Potassium (K) is crucial for crop growth and is strongly related to stress tolerance and water-use efficiency (WUE). A major physiological effect of K deficiency is the inhibition of net CO₂ assimilation (A_N) during photosynthesis. Whether this reduction originates from limitations either to photochemical energy conversion or biochemical CO₂ fixation or from a limitation to CO₂ diffusion through stomata and the leaf mesophyll is debated. In this study, limitations to photosynthetic carbon gain of sunflower (Helianthus annuus L.) under K deficiency and PEG- induced water deficit were quantified and their implications on plant- and leaf-scale WUE (WUE_P, WUE_L) were evaluated. Results show that neither maximum quantum use efficiency (F_v/F_m) nor in-vivo RubisCo activity were directly affected by K deficiency and that the observed impairment of A_N was primarily due to decreased CO₂ mesophyll conductance (g_m). K deficiency additionally impaired leaf area development which, together with reduced A_N, resulted in inhibition of plant growth and a reduction of WUE_P. Contrastingly, WUE_L was not affected by K supply which indicated no inhibition of stomatal control. PEG-stress further impeded A_N by stomatal closure and resulted in enhanced WUE_L and high oxidative stress. It can be concluded from this study that reduction of g_m is a major response of leaves to K deficiency, possibly due to changes in leaf anatomy, which negatively affects A_N and contributes to the typical symptoms like oxidative stress, growth inhibition and reduced WUE_P.

Keywords: Chlorophyll fluorescence; Leaf gas exchange; Mesophyll conductance; Photosynthesis; Potassium nutrition; Water use efficiency.