Effect Of Water Deficit On Water Use Efficiency And Photosynthetic Production In Soybean Under Low Irradiance And Its Mechanism

In China, half of the total grain yield is produced with multiple cropping, as the benefits of intercropping facilitation, yield advantage, high utilization efficiency of light and water. Corn/soybean relay strip intercropping system is popular in Southwest of China. Plants usually experience several stresses simultaneously under field conditions, limit of light and water availability are main environmental factors affecting relay strip intercropping soybean. Pot experiments and field experiments were carried out (1) to determine the soil water content at which non-hydraulic root-sourced signal (nHRS) and hydraulic root-sourced signal (HRS) occurred in the two species under two different light conditions during soil drying;(2) to characterize the differences in photosynthesis characters, water use efficiency, root morphology characters, biomass accumulation and yield stability responses of the two irradiance conditions under water stress conditions. The results showed as follow:1Water use efficiency was changed under low irradiance and water deficit1.1Shade of maize promoted the outset of non-hydraulic root-sourced signalUnder shade of maize, nHRS appeared1-2days earlier as compared to the treatment of normal irradiance; temporarily wilt (TW) and permanent wilt (PW) appeared1-3days later as compared to the treatment of normal irradiance. In response to soil drying, nHRS appeared earlier in Gongxuan No.1(GX) at V5stage than Gongqiudou (GQ) under two irradiance treatment, but it disappeared earlier for GX than GQ under normal irradiance. GX cultivar under low irradiance had the widest soil moisture threshold range (53.3-65.2.1%FWC, that is,11.9%), GQ cultivar under low irradiance had the narrowest range (56.4-62.3%FWC, that is,5.9%), but GQ cultivar under high irradiance had wider range between51.8and61.6%FWC (that is,9.8%).1.2Water consumption of soybean during whole growth period was reduced under shade of maizeSoybean canopy intensity and temperature under shade of maize were lower than normal irradiance, and relative humidity was opposite. Drought promoted soybean root to have a thicker root and bigger root volume under shade of maize. Soybean root of14days drought treated and then rewatering, had a higher root length, had a higher root length/root volume ratio under shade of maize as compared to that of normal light. There were smaller soybean root surface area, smaller root diameter, smaller root volume, under shade of maize than that of normal light. The ratio of root length and root volume was higher under shade of maize than that of normal light. Under shade conditions, there existed longer total root length, smaller transpiration amount, higher instantaneous water use efficiency, lower water consumption. Water use efficiency(WUE) increased as water deficit increased, observations reached maximum values under RH and nHRS for GX and GQ, respectively. WUE was notably higher in GX than GQ under drought conditions in both irradiance.2Physiological characteristics of soybean were changed under low irradiance and water deficit2.1Shade of maize promoted soybean to maintain a high actual photochemical efficiency of photosystem IIWater availability affected the minimum fluorescence (Fo), but no significant effects were detected between exposed soybean and shaded soybean. Drought also caused decreases in Fv/Fm, OPSII and qP; and an increase of non-photochemical quenching (NPQ). Chlorophyll (Chl) a, Chl b, total Chl (a+b) contents were significantly increased under mild drought, decreased under moderate and high drought stress. Shaded soybean have an effective control of water loss, and may have coped with stress by upregulating protective mechanisms such as increasing qP, ΦPSII, Fv/Fm, leaf area per plant, decreasing Fo, Chl a/b ratio(bigger antenna size) compare to exposed soybean.2.2Endogenous hormones content were changed under low irradiance and water deficitUnder shade stress, seedling height and first internode length increased, stem diameter decreased; abscisic acid (ABA) and zeatin (ZT) concentration decreased, while indoleacetic acid (IAA) and gibberellins3(GA3) concentration increased; branches numbers, pod number of branches, seed number of branches increased. Branch yield did not reduce significantly under shade stress, which was related to the decrease of ABA and IAA. Based on the results, soybean yield decrease under shade and drought stresses was mainly due to the yield reduction of main-stem. Shaded soybean had higher seedling height and higher IAA concentration. Seedling height and IAA concentration had positive correlation,Y=0.001XL+0.067X+44.02, r=0.8944>r0.001=0.8721(n=8). IAA concentrations appeared to significantly affect soybean seedling height, it decreased under drought stress which was benefit to inhibit vertical growth and avoid lodging for shade soybean.3Low irradiance and water deficit affected soybean morphological traitsThere existed interaction effects of shade and soil drought on soybean leaf physiology, plant morphology, yield and yield components. These two stress had interaction effects on leaf relative water content, root activity, specific leaf weight, the leaf area, chlorophyll b, total chlorophyll, chlorophyll a/b, photosynthetic rate, transpiration rate, water use efficiency, photochemical quenching coefficient, oxidation property enzymes, malondialdehyde, main stem height, branch number, stem diameter, stem dry weight, whole plant dry matter accumulation. The effect levels were different, drought had greater impact on physiology than that of morphology, but irradiance had greater impact on morphology than that of physiology. Soybean under low irradiance maintain photosynthetic capacity and respiration through responses of physiology, to minimize the impact of drought.4Low irradiance and water deficit affected soybean yieldThe branch pod and seed number did not reduce significantly under drought and shade stress; they increased under drought stress oppositely. Yield decrease of shade and drought stresses was mainly due to yield reduction of main-stem, so soybean genotype which has better main-stem yield stability may be suggested under the environment of low irradiance and soil moisture.