Effects Of Potassium Deficiency On Leaf Area Establishment And Photosynthesis Characteristics Of Brassica Napus

The Yangtze River Basin is the largest planting area for winter oilseed rape?Brassica napus L.?in China,whose critical level of soil available K for oilseed rape increased with the altered cultivars and cultivation patterns.Potassium deficiency decreased leaf photosynthesis capacity and then plant growth and yield.Since potassium is involved in many physiological progresses related to photosynthesis,it could influence leaf photosynthesis through many means.Mounting evidence has focused on the relationship between potassium and photosynthesis per unit leaf area?Pn?.But when plant is objected to potassium deficiency leaf expansion decreases prior to the net photosynthesis rate in view of the role of potassium in physiological progresses and the distribution of potassium in the plant cell.However,little is known about the effects of potassium deficiency on leaf area establishment.So further research on the role of K on leaf area establishment is conducted in the present study.And then,a method to accurately quantify photosynthetic limitations by separating the relative controls on Pn resulting from stomatal conductance?SL?,mesophyll conductance?MCL?and biochemical limitations?BL?was used to reveal the relative contributions of these three factors to photosynthesis decline induced by potassium deficiency.Finally,we further estimated the effect of potassium deficiency on photosynthetic nitrogen allocation and carbohydrate accumulation,aiming to explain the relationship between potassium deficiency and BL.In the present study,field experiments were conducted to explain the effects of K deficiency on leaf area establishment,gas exchange and fluorescence,CO2 diffusion,photosynthetic limitations and carbohydrate accumulation.The main results are as follows:?1?The leaf appearance rate was decreased under potassium starvation(phyllochron: severe potassium deficiency treatment(K1,0 kg K₂ O hm^-2),33.5 ?d/leaf;mild potassium deficiency treatment(K₂,60 kg K₂ O hm^-2),27.9 ?d/leaf,sufficient potassium supply treatment(K3,120 kg K₂ O hm^-2),26.8 ?d/leaf),and leaf senescence was accelerated and the single leaf area was reduced.The reduction of GLAI was mainly caused by reduced single leaf area and accelerated leaf senescence,and the delayed leaf appearance rate had minor effect.The accumulated radiation interception?RIacc?and radiation use efficiency?RUE?during the seedling stage decreased due to potassium deficiency,which caused a 61.7% and 48.2% reduction of final harvested aboveground biomass in K1 and K₂ treatment.When separating RIacc and RUE into three stages,it was found that when aboveground biomass decreased by less than 23.8%,RIacc was the main limiting factor;when the decrement was more than 23.8%,RUE mainly contributed to this decrease,In conclusion,the sensitivity of leaf area establishment components to potassium deficiency was different.And the relative contribution of RIacc and RUE to aboveground biomass decline was related to the degree of potassium deficiency.?2?The results showed that K deprivation significantly decreased the net photosynthetic rate?Pn?of oilseed rape leaves.Although stomotal conductance?gs?was lower in the K deficiency treatment than in the K sufficient treatment,intercellular CO2 concentration?Ci?of the K deficiency treatment was improved,inducing a lower limiting value of stomata?Ls?in the treatment of K deficiency.Apparently,the stomatal factors were not the dominant limiting factors of the photosynthetic rate?Pn?during this period.Further evidences of chlorophyll fluorescence measurement showed that both photosystem II?PSII?maximum photochemical quantum yield?Fv/Fm?and PSII actual photochemical quantum yield??PSII?were declined under the K deficiency treatment.In addition,K deficiency affected the chloroplast ultrastructure of winter oilseed rape leaves.In K deficient plants,the length of chloroplast decreased significantly,with the average of 15.6%;the thickness of chloroplast and the distance from chloroplast to cell wall increased by 26.9% and 56.1%,respectively.In conclusion,the non-stomal factors,e.g.damaged PSII reaction center and poor chloroplast ultrastructure inhibited the photosynthetic rate?Pn?of winter oilseed rape leaves under K deficiency,which further resulted in poor crop growth during over-wintering period.?3?The potassium concentration,net photosynthesis rate?Pn?,maximum rate of carboxylation?Vc,max?,maximum rate of electron transport?Jmax?decreased due to potassium deficiency.The quantitative limitation analysis of K deficiency on photosynthesis revealed that stomatal limitation?SL?,mesophyll conductance limitation?MCL?and biochemical limitation?BL?contributed to 18.2%,17.5%,3.4% decrease in the net photosynthesis rate.The fraction of the carboxylation?PC?reduced by 30.4%,and the fraction of the bioenergetics?PB?decreased by 13.6% when compared with that of the +K treatment.In conclusion,photosynthesis decline was mainly caused by mesophyll conductance limitation?MCL?and maximum rate of carboxylation decrease was partially because of reduced fraction of the carboxylation.?4?The low sink demand?LS?induced a lower photosynthetic rate?Pn?,especially in the-K treatment during the first week.A negative relationship between Pn and carbohydrate concentration was observed in the-K treatment but not in the +K treatment,suggesting that the decrease in Pn in the-K treatment was the result of sink feedback regulation under low sink demand.Longer sink removal duration increased carbohydrate concentration,but the enhanced assimilate did not influence Pn.On the contrary,low sink demand resulted in a high K concentration,slower chloroplast degradation rate and better PSII activity,inducing a higher Pn compared with high sink demand?HS?.Consequently,low sink demand decreased leaf photosynthesis over the short term due to sink feedback regulation,and potassium deficiency enhanced the photosynthetic decrease through carbohydrate accumulation and a lower carbohydrate concentration threshold for initiating photosynthesis depression.A longer duration of limited sink demand and sufficient potassium supply resulted in a higher photosynthesis rate because of delayed chloroplast degradation.This finding indicates that the nutritional status plays a role in leaf photosynthesis variations due to sink-source manipulation.