Abstract:Water transport through soil-plant-atmosphere continuum was a complex process, which was regulated at a variety of organizational and time scales. Quantification of the relationship between environmental factors, biophysical regulation and transpiration was critical for improving water use efficiency. Taking soil moisture, air temperature, relative humidity and photosynthetic active radiation as experimental factors, a composite quadratic orthogonal regressive rotation design of four factors and five levels was applied to characterize the coordination between environmental factors that driving and regulating transpiration of muskmelon. Soil moisture was maintained by weighing method and environmental factors were controlled by growth chambers. Transpiration and physiological parameters were determined under different combinations of environmental factors. According to stomatal behavior and vapour diffusion, a transpiration rate model was developed. Stomatal conductance was simulated by using Jarvis model with multiple environmental factors as model inputs. The proposed transpiration model had satisfactory performance with only meteorological input requirements, and thus it was an effective approach for calculating crop transpiration for greenhouse grown muskmelon. Characterization of main effect, single effect and marginal effect of environmental factors was determined. All of the environmental factors were positively correlated with transpiration rate except relative humidity. Correlation between transpiration and soil moisture, temperature can be described in linear functions with positive slopes. Whereas photosynthetic active radiation exhibited parabolic functions with transpiration rate. Relative humidity was negatively linked with transpiration. The present results demonstrated that environmental factors were tightly coupled with water transport. Coupling effects between two environmental factors were quantified by regression model, with other factors were uniformity maintained at a constant level. Individual contribution of soil moisture and air temperature in improving transpiration was enhanced by their coupling effects, which may be attributed to the larger driving force for water transport caused by the enhanced water potential gradients between soil and atmosphere. Therefore, transpiration was maximized, with increased soil moisture and air temperature. Relative humidity was negatively linked with vapor pressure deficit, thus the driving force of water flow at leafatmosphere interface was suppressed with the relative humidity. Physiological roles of soil moisture or air temperature in promoting water flow were significantly suppressed when coupled with relative humidity. Photosynthetic active radiation was not significantly correlated with other environmental factors in mediating water transport through soil-plant-atmosphere continuum. Coordination between hydraulic conductivity and stomatal sensitivity performed significant roles in maintaining a balance between vapour and liquid phase water transport. Hydraulic conductivity and stomatal conductance followed similar patterns with the increase of soil moisture, temperature and photosynthetic active radiation. Hydraulic conductivity and stomatal conductance were at maximum values when leaves were exposed to optimal ranges of environmental factors. It was demonstrated that the response of plant transpiration to environmental factors was not only determined by its individual function, coupling effects between environmental factors and physiological regulatory systems also performed significant roles in modulating water driving force. These observations provided novel information for improving water use efficiency of greenhouse grown muskmelon. Mechanism of environmental factors in regulating plant transpiration was explored under controlled environment by using growth chamber, which needed a further examination under greenhouse growth condition.
