Effects of micro-sprinkler irrigation combined with drip irrigation on greenhouse high temperature environment and crop growth physiological characteristics

Micro-sprinkler irrigation combined with drip irrigation(MSDI) refers to the use of crop canopy micro-sprinkler irrigation to improve crop growth environment on the basis of drip irrigation. The differences in greenhouse high temperature environment and crop growth physiological characteristics under two kinds of irrigation modes, MSDI and surface drip irrigation (SDI), were compared and analysed. The field observations of greenhouse high temperature environment and growth physiological characteristics of cucumber under two irrigation modes were conducted in a Venlo-type glass greenhouse in Jiangsu China from February to June 2017. The results show that greenhouse environmental variables like temperature and humidity of the canopy at different heights (0.8, 1.3 and 1.8 m) during night hours (18:00-06:00) are basically the same. However, with the increase of solar radiation during the daytime hours, the air temperature rises whereas the relative humidity falls. The pattern of fluctuation is as follows: The highest air temperature and the lowest relative humidity all occur at 1.8 m level; the highest relative humidity and the lowest air temperature also occur at 0.8 m level; and the air temperature and relative humidity levels at height 1.3 m appeared to be intermediate between that of the 0.8 and 1.8 m. The application of MSDI can increase the relative humidity of the greenhouse, reduce the air temperature and the leaf temperature to about 4 ℃. In terms of crop growth physiology characteristics, MSDI can increase plant height and stem diameterand promote cucumber plant growth. Correlation analysis results showed that the sap flow rate of cucumber plant was significantly positively correlated with air temperature, solar radiation and net radiation (P<0.01), and significantly negatively correlated with relative humidity (P<0.01). Applying MSDI can reduce the sap flow rate by decreasing the air temperature and increasing the relative humidity. The maximum photosynthetic efficiencies of cucumber under the two irrigation modes were almost the same, with the values of 0.74 (MSDI) and 0.77 (SDI), respectively. However, the daily average photosynthetic efficiencies were significantly different between the two irrigation modes, with the values of 0.57 and 0.47, respectively. This phenomenon indicates that the use of MSDI irrigation method can alleviate the stress of greenhouse high temperature on the actual light energy conversion efficiency of the PSII and improve the photosynthetic capacity of mesophyll cells. Under the two irrigation methods (MSDI and SDI), the stomatal conductance of cucumber leaves increased initially, and then decreased rapidly to a minimum. The photosynthetic rate behaved similarly, but slightly lagged behind the stomatal conductance. The effect of MSDI irrigation on the stomatal conductance and photosynthetic rate of greenhouse cucumber leaves was obvious during the study. The average daily stomatal conductance and photosynthetic rate of cucumber leaves under MSDI mode were higher than SDI mode with 182.8% and 92.4%, respectively. The results have significant importance in reasonable regulation of greenhouse high temperature environment, and in improving greenhouse crop yield.