Studies On Effect Of Soil Water Change On Leaf Photosynthesis And Root Physiological Of Cotton Under-Mulch-Drip Irrigation In Xinjiang

Soil water deficit is one of the mainly environmental factors that limiting crop productivity.Cotton (Gossipium hirsutum L.)is one of the most important fiber-producing plants throughout the world.In Xinjiang-China a precision irrigation technology,created by combining drip irrigation and mulched with plastic film,hence called Under-Mulch-Drip irrigation,has been applied,by which increased high yield and water use efficiency has been achieved.In order to explore the acclimation effect of soil water change on leaf photosynthesis and root physiological,and the possible avenue to achieve high yield and high water use efficiency,three experiments were conducted under Under-Mulch-Drip irrigation at Xinjiang.Firstly, we controlled the different lower limit of soil relative water content under field in order to study the effect of different soil water on leaf photosynthesis and photosynthate distribution.Secondly,we adopted PVP tubes cultured method to research the regulative effect of water from deep soil on the leaf photosynthetic characteristic,root growth and physiological function.Lastly,we studied on the acclimation of leaf photosynthesis to water deficit and rewatering at different growth stages and its relationship with root physiological function under field.The main results are given bellow.â… .Studies on the effect of different soil water on leaf photosynthesis and photosynthate distribution1 In order to develop water saving irrigation stratagem and increase cotton yield in Xinjiang,we did the irrigation experiment in the field,controlled the lower limit of soil relative moisture content in layer of 0-60 cm at 45%,60%,and 75%(the control)respectively of field water-holding capacity,which was the upper irrigation limit,and studied the effects of rewatering after drought on photosynthesis and yield during flowering-boll stage of cotton under-mulch-drip irrigation.The result showed that drought reduced the parameters of gas exchange.Photochemical quenching coefficient(qP)and PSâ…¡photochemical quantum yield(φ_PSâ…¡)were reduced but non-photochemical quenching coefficient(NPQ)was increased significantly under moderate drought.Net photosynthetic rate(P_n)and stomatal conductance(G_s)of three water treatments could be rapidly recovered within three days after rewatering,and the recovery of mild drought treatment was the most rapid.The recovery ofφ_PSâ…¡and qP in three water treatments was similar to that of Pn which achieved the maximum in 2-3 days after rewatering.The NPQ of three water treatments decreased significantly in 1-2 days after rewatering.From early flowering stage to early full boll stage, accumulation of single plant photosynthate was not much different from that of mild drought treatment after rewatering and the control,but the accumulation of single plant photosynthate was decreased significantly from later full boll stage to boll opening stage that let the seed cotton yield in mild drought treatment after rewatering be lower than that in control.Accumulation of single plant photosynthate and seed cotton yield in moderate drought treatment after rewatering was always lower than that in mild drought treatment and control.2 Gas exchange,modulated and direct(OJIP-transient;dark-adapted state)fluorescence measurements were employed to probe the photosynthetic activity of cotton plants grown in field conditions with different soil water contents(SWC;75,60,45%of soil capacity)regulated by Under-Mulch-Drip irrigation.The result showed that at light intensities above 1200μmol·m^-2·s^-1,the CO₂ assimilation rate(P_n)at 60 and 45%SWC was lower than at 75%SWC by～25%and～55%,but the electron transport rate(ETR)was higher by～5%and lower by～10%respectively.This discrepancy implies that SWC lowering affects processes not detectable by the fluorescence quenching analysis.On the contrary,the analysis of OJIP by the JIP-test,though applied at the dark-adapted state,revealed differential effects at different sites of the photosynthetic primary processes:(â…°)a decrease of the photosystem(PS)â…¡active reaction centres per chlorophyll,an increase of the functional PSII-antenna size and a decrease of energetic connectivity among PSII units followed the SWC lowering;(â…±)the electron flow from plastoquinol to the PSI end electron acceptors pool decreased only at 45%SWC,in association with a decrease of the pool capacity.A very interesting finding was that the potential of the whole photosynthetic electron transport chain,expressed by the "performance index" at the dark-adapted state,was affected by the SWC lowering in the same way as P_n,which quantifies how the potential drives,under local mid-day light conditions.3 With the cultivars of Xinluzao 10 and Xinluzao 13,we did the irrigation experiment in the field,and respectively controlled the lower limit of soil relative moisture content in layer of 0-60cm were 60%,75% and 90%of the field capacity,the upper limit was field capacity.Leaf gas exchange and isotopic tracer techniques were used to investigate the change of photosynthesis,production and distribution of photosynthetic assimilate and yield during flowering and boll-setting stage under different soil moisture content.When the lower limit of soil relative moisture content was 60%,leaf photosynthetic rate and photosynthesis products accumulation of aerial part were decreased,the speed of export percentage ¹⁴C-assimilates were accelerated and distribution percentage ¹⁴C-assimilates in bud and boll was increased. When the lower limit was 75%and 90%,leaf photosynthetic rate and photosynthesis products accumulation of aerial part were enhanced,but distribution percentage ¹⁴C-assimilates in stem and leaf significant was increased in 90%treatment.So the seed cotton yield of 75%treatment was the highest,90% treatment was inferior to 75%treatment,60%treatment was the lowest,however seed cotton yield water use efficiency was 60%＞75%＞90%.The differences of response to drip irrigation among varieties were great,the seed cotton yield and water use efficiency of Xinluzao 10 were significant lower than Xinluzao 13 in 60%and 75%treatment,but that of Xinluzao 10 were higher than Xinluzao 13 in 90%treatment. These results suggest that different soil moisture content had significant influence on the production and distribution of photosynthetic assimilate and seed cotton yield.When the lower limit of soil relative moisture content was 75%～90%during flowering and boll-setting stage would work better in achieving high yield.If the lower limit of soil relative moisture content range between 60%and 75%,cotton plant could develop the ability of adaptation to increase water use efficiency. â…¡Studies on the effect of water supply from deep soil on leaf photosynthetic characteristic and root physiological function4.The results in PVP tubes cultured cotton under Under-Mulch-Drip irrigation revealed that the abundance of water from deep soil had advantage of maintenance root size appropriately,furthermore,it increased root biomass,root length,root surface area,root SOD activity and virgour of 40-120 cm soil layer,consequently,it improved leaf photosynthetic rate,leaf water potential,chlorophyll content and plant photosynthate.Under the abundance of water from deep soil,the treatment that controlled the lower limited of soil water content in 0-60 cm soil layer at 60%that had higher consumption ratio of water from deep soil was due to accelerating the root growth and increasing root physiology function in middle and lower layer, so it improved the photosynthesis function of leaf.However,the photosynthesis matter accumulation of aerial part was significantly decreased that led to the decrease of cotton seed yield.It indicated that cotton root could be mainly through the increasing the total root length and root surface area rather than root biomass,i.e.,increasing the unit weight on the roots and root surface area to adapt to water deficit.The change of spatial distribution under water deficit indicated that,under the abundance of water from deep soil,cotton root mainly increased small root length and surface area in 40-100 cm soil layer in order to fully absorb the water from soil deep,but under the shortage of water from deep soil,cotton root mainly increased the small root length and surface area in 0-40 cm soil layer to furthest capture water resources.â…¢Studies on the effects of fluctuated soil moisture on leaf photosynthetic characteristic and root physiological function of cotton at different growth stage5.Our experiment showed that leaf water potential,stomata conductance and net photosynthetic rate decreased with water deficit at different growth stages.Meanwhile the content of ABA in root increased significantly,that of ZRs decreased.However,root SOD activity and virgour were inhibited,the extent of which with the different growth stages and soil layer.After rawatering,leaf water potential,stomata conductance and photosynthetic rate of limited irrigation(which supplies 2/5 of water quantity which was widely employed in Xinjiang cotton's cultivation)recovered rapidly,even exceeded that of convention irrigation.The content of ABA in root did not reduce with the soil water supply.The content of root ZRs, root SOD activity and root virgour recovered more rapidly than leaf photosynthesis.The analysis of relationship between root physiology and leave photosynthesis of cotton under fluctuated soil moisture at different growth stage indicated that after rewatering,the opening stomata of cotton leaves was accelerated as to accelerate the synthesis of ZRs in root.Simultaneously,the rapid recovery of root virgour ensured the water and mineral elements requested of photosynthesis,which made photosynthetic rate recovered rapidly.Compared to convention irrigation,decrease water supply in flowering stage decreased yield and water use efficiency greatly,nevertheless,decrease water supply in bud stage and in boll opening stage decreased yield and water use efficiency less,but achieved higher water use efficiency.Therefore,basis on the responds of different cultivars to soil water change,the water consumption of Xinluzao 13 and Biaoza A₁ are 540～554 mm and 590～623 mm,respectively.Under which, through reducing the quantity of drip irrigation in bud stage and boll-opening stage,which brought compensate effect of rewatering after water deficit,the high yield and water use efficiency will be attained.