Studies On The Mechanism Of Photosynthetic Area And Rate Of Brassica Napus Under Regulation Of Potassium Nutrition

Winter oilseed rape（Brassica napus L.）is an important source of edible oil in China.Insufficient potassium（K）application significantly reduces the oilseed rape yield which mainly depends on the photosynthetic products.Photosynthetic capacity of crops is determined by photosynthetic area and rate.As important osmotic regulator,K has a relatively clear regulation in stomatal function and CO₂ mesophyll conductance.However,the effects of K on coordination of leaf area and photosynthetic rate and the physiological mechanism of photosynthetic carboxylation were often ignored.In order to solve the above problems,both field experiments and hydroponic experiments were conducted to systematically study the effects of K nutrition on mesophyll morphological characteristics in time and space,and to explore the regulation mechanism of K on enzyme activities and central carbon metabolism.Finally,a production strategy was proposed to improve the photosynthetic potential by the application of nitrogen（N）and K fertilizer to coordinate leaf area,CO₂ conductance and carboxylation capacity.These results would provide a further understanding of the mechanism underlying the effect of K on photosynthetic potential of oilseed rape.The main results are as following:（1）Under light K deficiency stress,the decrease in leaf area preceded that in net photosynthetic rate（A）.In the upper leaves under K deficiency,increased mesophyll cell size and palisade tissue thickness triggered significant enlargement of mesophyll cell area per transverse section width（S/W）.The increased investment in longitudinal inhibits leaf expansion.As K deficiency stress increased with leaves growing,decreased mesophyll surface exposed to intercellular space（S_m/S）and longer distances between neighbouring chloroplasts(D_chl-chl)induced by decreased chloroplast density led to the reduction in the chloroplast surface area exposed to intercellular space（S_c/S）;conversely,this induced a greater limitation imposed by the cytosol on CO₂ transport,further reducing the A.During the process of increasing K deficiency,changes in S/W earlier than changes in S_c/S and D_chl-chl,inducing a decrease in leaf area prior to photosynthetic rate reduction.（2）During a leaf growth under K deficiency,the leaf area decreases fiestly;and with the increase of K deficiency stress,the A and leaf H₂O conductance capacity(K_leaf)decreased simultaneously.At the different growth stage,the volume fraction of intercellular air space(f_ias)of leaves decreaed with K deficiency,and the cell investment in longitudinal increased obviously,inhibiting leaf area expansion.However,the decreased f_ias caused by K deficiency had a negative impact on S_m/S which further affects the S_c/S and the mesophyll cell-to-cell connectivity.Consequently,the CO₂ mesophyll conductance（g_m）and the hydraulic conductance in extra-xylem pathways(K_ox)were affected by K deficiency stress.The changes of K_ox dominated the changes of K_leaf,which was coordinated with the changes in g_m.Therefore,A and K_leaf decrease simultaneously under K deficiency.K nutrition promoted loose cell arrangement and increased f_ias,which promoted S_c/S and H₂O transmission in gas phase,thereby simultaneously increasing A and K_leaf accompanied by leaf expansion.（3）Under severe K deficiency,the maximum carboxylation rate(V_cmax)significantly affected the A.After the leaf K concentration was lower than 1.2–1.3%,the A and enzyme activity were significantly reduced.The in vitro Rubisco activity of oilseed rape in response to pH indicated that Rubisco activity was relatively stronger in a mild alkaline environment（pH 8–9）.However,when the leaf K concentration was lower than 1.4–1.5%,the pH in cytoplasm began to decrease,which affected the function of chloroplast and reduced the Rubisco activity.Under severe K deficiency,acidification of subcellular reduces Rubisco activity and affects photosynthetic carboxylation rate.（4）The central C metabolism were significantly related to A,CO₂ conductance abilities and carboxylation rate under different K deficiency stresses.Light K deficiency under prevailing stomatal limitation had negligible effects on metabolism compared with sufficient K supply.However,with increased K deficiency stress when mesophyll conductance limitation predominated,the metabolism of organic acids was up-regulated.Moreover,acidification of subcellular compartments reduced the activity of sedoheptulose-1,7-bisphosphatase,inducing downregulation of sedoheptulose-7-bisphosphate and hindrance of ribulose bisphosphate regeneration,which affects the carbon fixation rate.Under severe K deficiency,insufficient CO₂ induced a shift of increased citric acid to amino acid synthesis,resulting in excessive accumulation of amino acids.In addition,the reduced serine level indicated impaired photorespiration.These triggered more serious reduction in photosynthetic biochemical capacity,which induced the biochemical limitation to be the main limiting factors for A.（5）Adequate nitrogen（N）promotes V_cmax,but K deficiency leads to a decrease in total CO₂ diffusion conductance(g_tot),making CO₂ at carboxylation site insufficient to maintain a high carboxylation.Under insufficient N nutrition,increased K supply contributed to increased g_tot and improved the proportion of N partitioning in carboxylation,promoting g_tot/V_cmax.When the leaf N/K value was kept at 1.7–3.1,the g_tot/V_cmax was about 0.72–0.75,and the A reached the maximum capacity.The synchronous increments of N and K supply ensured the appropriate N/K ratio in leaves,which facilitates the g_tot and V_cmax,and simultaneously,guarantees the coordination of CO₂ transmission and carboxylation,improving the photosynthetic capacity and leaf area.Based on the above results,it is clear that adequate K nutrition regulates mesophyll cell morphology and chloroplast morphology to coordinate leaf area expansion,leaf CO₂and H₂O conductance;the subcellular pH was reduced under K severe deficiency,which affected activities of enzymes and central C metabolites,thereby reducing C carboxylation rate.When the leaf N/K is maintained at 1.7–3.1 by reasonable combination of N and K fertilizers,leaf area and photosynthetic rate are coordinated to promote the photosynthetic potential maximized.These results provide a theoretical basis and production guidance for the application of K fertilizer to promote the yield of oilseed rape.