| Drought stress is the main factor limiting crop growth and development.Along with the global warming and the decrease of precipitation,the elevated vapor pressure deficit(VPD,linked to a high air temperature or low air relative humidity or their combination)and soil water deficits have become the main abiotic stresses restricting crop yield.Considering the differences of stomatal regulation and physiological response between monocotyledons and dicotyledons crops,in the present study,barley and potato plants were used to investigate the effects of varied VPD on the response of leaf gas exchange rates,plant water relations and water use efficiency to soil water deficits in both greenhouses and field conditions.The crop water consumption under combined soil and atmospheric drought was quantitatively characterized.Meanwhile,Daisy model was used to simulate the main physiological and growth responses of potato to varied VPD levels and soil water deficits.The main results are as follows:(1)Plants leaf gas exchange rates were significantly depressed by soil water deficits and elevated VPD conditions,though the underlying mechanisms were different.Soil water deficits could trigger an increased biosynthesis of abscisic acid(ABA)in root tips,which acts as root-to-shoot chemical signal inducing decreases of stomatal conductance,then depressing the net photosynthetic rate and transpiration rate.While high VPD(caused by high temperature)could reduce net photosynthetic rate by reducing Rubisco enzyme activities.On the other hand,high temperature could enhance transpiration rate by increasing evaporation demand,which leads to a rapid water consumption in crops.Whereas low air relative humidity would also lead to an increase of plant transpiration rate but a decrease of stomatal conductance,resulting in the acceleration of crop water loss and the decrease of carbon assimilation rate,hence a low water use efficiency.The damage caused by the combined stress of high VPD and soil water deficits was much greater than that caused by a single stress.(2)A Linear-plateau model was used for describing the dynamic change of the daily transpiration per leaf area(DT)of barley plants and leaf gas exchange rates of potato plants in response to soil drying,i.e.,decrease of the fraction of transpirable soil water(FTSW).The results showed that there was a positive correlation between the FTSW threshold at which the daily transpiration of per leaf area of barley plants or transpiration rate of potato plants started to decrease and VPD.For barley plants,the FTSW threshold increased 130%under high VPD(high temperature and low humidity,0.92)than that under low VPD(high humidity at room temperature,0.40);and FTSW threshold of at which transpiration rate of potato plants grown under high VPD(0.78)increased 90%than that under low VPD(0.41).Meanwhile,compared with the FTSW threshold(0.43)at which stomatal conductance of potato plants under low VPD condition(normal air temperature and high relative humidity),high VPD(high temperature and low humidity)significantly increased the FTSW threshold(0.80),and the change of the FTSW threshold was related to the decrease of ABA concentration caused by the high temperature.The results indicating that high VPD sensitized the response of crops to soil water deficits.(3)The leaf area of plants was significantly limited when grown under the combined stress of elevated VPD and soil drying.For well-watered and drought-stressed plants,Leaf area of barley plants grown under high VPD was decreased 48 and 58%,as compared with those grown under low VPD.The trendency of potato leaf area was consistent with that of barley.In addition,relative leaf water content(barley)and leaf water potential(potato)of plants were also significantly lowered by the combination of elevated VPD and soil drying,which also caused stomatal closure and inhibited the photosynthetic rate,thereby resulting in a lower dry matter accumulation.The specific leaf area of plants is an indicator of water use efficiency.The specific leaf area of barley and potato and leaf carbon isotope discrimination for potato)plants grown under soil drying was lowered,but the water use efficiency of plants was increased.(4)In both barley and potato plants,shoot dry matter and water use efficiency were significantly inhibited by elevated VPD and soil drying.Compared with barley and potato plants grown in normal temperature and high humidity environment,the shoot dry matter of plants under high temperature and high humidity environment decreased by more than 50%,especially the plants under drought stress decreased by 63.2 and 53.4%,respectively.Disregarding VPD treatments,water use efficiency of barley and potato plants were increased under drought-stressed treatment.Regardless of irrigation regimes,the water use efficiency of the two crops decreased with increasing VPD,especially under the high temperature and low humidity condition.Compared with the well-watered and drought-stressed plants growing under the normal temperature and high humidity environment,the water use efficiency of barley plants decreased by 64%and 18%,and that of potato plants decreased by 59%and 48%,respectively.In addition,the plants growing in high temperature environment have lower leaf ABA concentration and higher transpiration rate,which lead to a lower water use efficiency.The interaction of high VPD and soil drought led to the further decrease of net photosynthetic rate,stomatal conductance,plant water relations,dry matter,and water use efficiency,which indicated that the dual stress of high VPD and soil drying aggravated the inhibition of crop growth.(5)The Daisy model was used to simulate the responses of net photosynthetic rate,stomatal conductance,leaf ABA concentration,yield,and water use efficiency of potato to soil water deficit under elevated VPD conditions in the field condition.The results showed that Daisy model could accurately simulate the dynamic change process of soil water under different irrigation conditions(for well-watered treatment:R~2=0.52,MAE=0.005,RMSE=0.007;for drought-stressed treatment:R~2=0.88,MAE=0.013,RMSE=0.016).The model could explain more than 70 and 85%of the observed variation in stomatal conductance and leaf ABA concentration for all irrigation regimes.In addition,the water use efficiency of potato plants under different irrigation treatments could be accurately simulated,although the MAE(2.6)and RMSE(2.4)were higher under drought stress treatment.The results of this study will be helpful to evaluate and predict crop water use efficiency in the future environment of increasing temperature and decreasing precipitation,which provide theoretical understanding of how multiple environmental factors coordinately control the crop water demand and water use efficiency.This knowledge will be essential for scheduling irrigation in order to enhance crop water productivity in a future drier and warmer climate. |
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