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

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 CO assimilation (A) during photosynthesis. Whether this reduction originates from limitations either to photochemical energy conversion or biochemical CO fixation or from a limitation to CO 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, WUE) were evaluated. Results show that neither maximum quantum use efficiency (F/F) nor in-vivo RubisCo activity were directly affected by K deficiency and that the observed impairment of A was primarily due to decreased CO mesophyll conductance (g). K deficiency additionally impaired leaf area development which, together with reduced A, resulted in inhibition of plant growth and a reduction of WUE. Contrastingly, WUE was not affected by K supply which indicated no inhibition of stomatal control. PEG-stress further impeded A by stomatal closure and resulted in enhanced WUE and high oxidative stress. It can be concluded from this study that reduction of g is a major response of leaves to K deficiency, possibly due to changes in leaf anatomy, which negatively affects A and contributes to the typical symptoms like oxidative stress, growth inhibition and reduced WUE.