Response of stem diameter of vegetable crops to soil moisture in greenhouse

Based on one year experiment, variations of microclimatic factors such as light intensity, ambient temperature and relative humidity in greenhouse were analyzed. The diurnal stem diameter difference (SD) and sap flow rate (SF) of greenhouse crops, e.g, cucumber and tomato were observed during this year. Both the variation of SF as an index of transpiration and SD as an index reflecting the water balance inside the stem were influenced by weather and shading screen, the maximum of SF and minimum of SD during daytime were stronger on sunny days no matter shading screen than those on overcast days and weaker on sunny days with shading screen than those without it. This implied that the shading screen could influence the crop transpiration. Furthermore, the microclimatic factors influencing SF and SD were explored using correlating analysis methods, the results showed that the radiation in greenhouse had no significant correlation to SF and SD, whereas the correlation coefficient between the vapor pressure deficit (VPD) and SF and SD at 12∶00, 13∶00, 14∶00, 15∶00 during reproduction stages of cucumber and tomato respectively were calculated, results showed that the correlation coefficients at 14∶00 were the biggest; and there was a strong linear relationship between the SD/VPD at 14∶00 and soil moisture, and SD/VPD at 14∶00 was relatively constant in a larger value at high water content than that at low water content. so the SD/VPD at 14∶00 might be suggested as an index to wake up the irrigation system.