Effect Of Greenhouse Environmental Factors On Cucumber Water Transport And Transpiration

Plant transpiration, acting as the main driving force of greenhouse water and heat transport and the main water consumption in the process of greenhouse crop cultivating, is affected by the combined action of soil, atmospheric and plant physiological factors, so that it can be regulated by greenhouse environmental and irrigation management to enhance water use efficiency, based on the mechanism of plant transpiration responses to environmental factors under different spatial and temporal scales. For instantaneous transpiration rate at leaf scale, transpiration responses to environmental variation mainly by the regulation of plant water transport, making sense to analyze the mechanism of environmental factors affecting crop water transport for optimizing environmental management in greenhouse. For daily scale transpiration at whole plant level, it is essential to estimate crop transpiration accurately,which will reduce water waste and decrease air humidity, thus improving water use efficiency and fruit quality. This study analyzed the effects of environmental factors such as soil moisture(θ), air temperature(T), air relative humidity(RH), and photosynthetic active radiation(PAR) on stomatal conductance(Gs) and total hydraulic conductance of soil-plant system(Kt) under steady flow condition, and how these impact factors regulate plant transpiration. Greenhouse cucumber was used as the experimental material, basing on the design of composite quadratic orthogonal regressive rotation where the artificial climate box was adopted. Four relative soil moisture gradient: 80%(w1)、70%(w2)、60%(w3)、50%(w4)were set in this study to establish estimation models for daily transpiration of greenhouse cucumber in its vegetative growth period. Firstly, direct method and indirect method were compared for estimating potential daily transpiration under the condition of sufficient water supply, and then the cucumber transpiration model under the condition of water deficit was established. The models were verified by independent data. The main results are as follows:1. All environmental factors exerted positive effect on Gs and Kt, interaction effect wasalso found between θ and PAR, T and RH on Gs and Kt. The single effect of RH can be described by a parabolic function and of other impact factors were described as the linear increasing with Gs and Kt. The marginal effects of each impact factors on Gs and Kt showed that RH was the main regulating path for Gs, and all the impact factors except PAR efficiently regulated Kt.2. The regulating effects of four environmental factors, Gs and Kt on transpiration were analyzed with the paths and correlation analyses. The results showed that T, PAR and Gs mainly exerted indirect positive effect on transpiration by strengthening Kt and then the direct positive effect. θ mainly exerted indirect positive effect on transpiration by strengthening both Kt and Gs. RH exerted direct negative effect, while its main effect path is exerting indirect positive effect on transpiration by promoting Gs and Kt. Both Gs and Kt remarkably responsed to four environmental impact factors and interactively regulated the transpiration synergistically.3. Under the condition of sufficient water supply, using direct method possessed the best precision for estimating cucumber transpiration, the root mean squared error(RMSE) and relative root mean squared error(rRMSE) of which was 25.5g/d and 19.25% respectively;when using indirect methods, the transpiration model of which basal crop coefficient was based on days gave the worst precision, the RMSE, rRMSE was 48.1g/d and 36.46%,respectively; simulating basal crop coefficient with effective accumulated temperature could effectively improve the prediction accuracy of transpiration model, the RMSE, rRMSE was31.1g/d and 23.36%, respectively, this model is simple to use comparing with the direct method and has great application potential.4. The impact of water deficit on crop transpiration is complex, which can be divided into short-term impact and long-term impact. Short-term impact refers to the fact that when soil water can not support the need of crop transpiration, the leaf water potential decreases and leaf stomatal resistance rises, thus weakening the transpiration rate. Long-term impact refers to the fact that the form, structure, and physiological function of crop keep changing to adapt water deficit stress with drought continuing. Short-term impact depends on the intensity of water deficit while long-term impact depends on both the intensity and the duration of water deficit. Based on the frame of crop coefficient approach, soil water stress coefficient was divided into short-term impact factor and long-term impact factor in this model. The form of water stress coefficient recommended by FAO-56 was adopted in short-term impact factor sub-model. In long-term impact factor sub-model, effective accumulated temperature was introduced in a power function to describe the comprehensive impact of the intensity and the duration of water deficit. The model was verified by independent data, which showed that: forw2 to w4, the RMSE and r RMSE between simulated values and measured values were19.8g/d、17.7g/d、3.5g/d and 19.05%、26.12%、14.53%, respectively, suggesting that the model had high prediction accuracy on the condition of water deficit.The model established in this paper possessed pretty high mechanism owes to it comprehensively described the driving role that temperature and water played during the crop growth period and the process of drought development. Moreover, this model has simple formation and its inputted parameters are easy to obtain, so that it can offer theoretical support for water-saving irrigation of greenhouse cucumber and method for modeling transpiration of other crops.