Studies On Mechanisms Underlying The Effects Of Potassium Nutrition On Leaf Photosynthesis Of Brassica Napus

Winter oilseed rape?Brassica napus L.?,one of the most important oil crops in China,is mainly distributed in the Yangtze River Basin.The contradiction between high cost of soil potassium?K?for satisfying plant growth and inadequate K fertilization has been one of the main limiting factors for high seed yield,whereas this gap can be filled by proper K management.Potassium,one of the macronutrients essential for plant growth and development,is associated with many physiological processes,such as stomata aperture,photosynthesis and assimilate transport.It is a truism that the most of the dry matter and seed yield are formed by photosynthesis of green organs which have been reported to decrease under K-starved conditions ascribing to the restraints of CO₂ diffusion and utilization.However,mechanisms of K-driven photosynthetic inhibition are not fully understood,especially under varied K deficiency severities.Therefore,the present study was conducted to uncover the leaf photosynthesis of rapeseed in response to K deficiency,also to analyse the contribution of photosynthetic limitations?stomatal conductance limitation,mesophyll conductance limitation and biochemical limitation?to the decline of leaf photosynthesis,and construct the underlying relationship between each limiting factor and leaf K concentrations to quantify the K-based critical thresholds for limitations.To date,the mechanisms underlying the regulation of K on stomatal openness have been extensively studied,nevertheless,the information about the influences of K deficiency on mesophyll conductance and biochemical traits is conflicting and less advanced.We therefore,further analysed the reasons with respect to the decrease of mesophyll conductance and biochemical capacity under K deficiency.Ultimately,the question arises,as to whether or not the basis found under steady state conditions may equally apply to the non-steady state conditions.Therefore,the analysis was performed on the responses of photosynthetic limitations to the rapid change of radiation intensity and CO₂ concentration.Overall,the above researches provide a comprehensive and in-depth understanding of the mechanisms underlying the effects of K on leaf photosynthesis.In the present study,field and hydroponic experiments were combined to learn the effects of K deficiency on leaf gas exchange and fluorescence,CO₂ diffusion,photosynthetic limitations and photorespiration,etc.and establish the K-based critical thresholds for photosynthetic limitations,as well as to reveal the regulatory mechanisms of K on each limiting factor.The main results are listed as follows:?1?Potassium deficiency significantly inhibited rapeseed growth and decreased leaf area,leaf chlorophyll and K concentrations during over-wintering stage.In comparison with K supply treatment,the leaf net CO₂ assimilation rate?A?,stomatal conductance?gs?and stomatal limitation?Ls?of K-starved leaves decreased 27.9%,42.5% and 21.4%,respectively.Conversely,K deficiency resulted in the dramatic increase in intercellular CO₂ concentration?Ci?,which revealed that non-stomatal limitation contributed to the majority of A decline.Besides,K deficiency increased leaf light and CO₂ compensation point,however,decreased Rubisco carboxylation efficiency?CE?,apparent quantum yield?AQY?,actual photochemical efficiency of PSII?Y?II??and electron transport rate?rETR?.The quantum yield of regulated energy dissipation?Y?NPQ??up-regulated in K starvation leaves,nevertheless,the quantum yield of nonregulated energy dissipation?Y?NO??was same between different K treatments.It is concluded that the inhibition of plant growth is primarily ascribed to the decrease of leaf area and photosynthesis,of which the latter is caused by declining carboxylation capacity?i.e.CE?and reductive ability?i.e.ATP and NADPH?.Down-regulated reductive ability in K-deficient leaves can be interpreted in terms of lower light absorption?AQY?,transformation?Y?II??and transfer?rETR?.?2?The A,gs,gm and J of lower leaves,especially in leaf margins,were considerably decreased under K deficiency.It was concluded that the leaf K-based critical value for A was 1.17-1.36% by establishing the relationship between relative photosynthetic parameters and leaf K concentration.The photosynthetic limitations?stomatal conductance limitation,SL;mesophyll conductance limitation,MCL;biochemical limitation,BL?were appeared simultaneously under K deficiency and down-regulated with the increasing of leaf K concentration.The predominant constraint changed along the variation of leaf K concentration,namely that 1.07?K<1.17-1.36%,0.69?K<1.07% and K<0.69% were the K-based thresholds for the dominance of SL,MCL and BL.Overall,the photosynthetic limitations were synergistic on the regulation of leaf photosynthesis,and SL,MCL and BL may be considered as the predominant constraints under mild,moderate and severe K deficiency,respectively.?3?The down-regulation of gm under moderate K deficiency contributed to one half of the decrease of leaf A.Since gm is composed of gas-and liquid-phase conductance,both of them were analysed in the present study,showing that liquid-phase resistance account for approximately 90% of total resistance.The reduced internal air space in K-starved leaves was associated with the increase of gas-phase resistance.Potassium deficiency reduced liquid-phase conductance by decreasing the exposed surface area of chloroplasts per unit leaf area?Sc/S?,and enlarging the resistance of the cytoplasm that can be interpreted by the increasing distance of chloroplast from cell wall,and between adjacent chloroplasts.These results emphasize the important role of K on the regulation of gm by enhancing Sc/S and reducing cytoplasm resistance.?4?Obstacles of biochemical process were the main restraints of leaf photosynthesis under severe K deficiency.Leaves in K-starved conditions,had limited Rubisco activity and maximum carboxylation rate.The ratio of the oxygenation to the carboxylation rate was up-regulated under K deficiency,due to the decline of Rubisco specificity factor rather than the change of the partial pressure of O2 and CO₂ at Rubisco catalytic sites.Besides,K starvation profoundly decreased leaf electron transport rate,enhanced the distribution of electron flowing to oxygenation,which therefore enlarged the carbon consumption during photorespiration.Electron flow to oxygenation increased the generation rate of O2.-and the subsequent POD activity.Taken together,the increase of biochemical limitation imposed on leaf photosynthesis under K deficiency can be primarily explained by the inhibition of Rubisco properties and enhanced electron consumption through photorespiration.?5?Leaf photosynthetic limitations imposed by K deficiency can vary significantly with the change of radiation intensity and CO₂ concentration.The gs,gm and J were improved under high light condition,however,K deficiency retarded the response of gs and gm to the rapid change of light intensity.The total limitation of leaf photosynthesis under high light was larger than that under low light,and was mainly consisted of SL and MCL,nevertheless,the proportion of BL to total limitation increased with a decreasing light intensity.The elevated CO₂ concentration considerably improved gm,while had less effects on gs,K supply enhanced the response rate of gm to the rapid change of CO₂ concentration.SL and MCL were the primary limitations of leaf photosynthesis when atmospheric CO₂ concentration below 400 ?mol mol-1,however,BL becoming the main restraint under high CO₂ concentrations.In conclusion,the restriction of leaf photosynthesis in response to K deficiency is more severe under high light and low CO₂ concentration,the regulation of K on the response rate of gs and gm are crucial for the variation of relative contribution of each limitation under non-steady conditions.