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 CO2 assimilation (AN) 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 (WUEp, WUEL) 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 WUEp. Contrastingly, WUEL 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 WUEL 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 WUEp. (C) 2016 Elsevier GmbH. All rights reserved.