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The plant water or nutrient requirement under greenhouse conditions is mainly governed by crop transpiration in a linear relation to vapour pressure deficit (VPD), particularly in soilless production systems. Being a cooling process in plants, transpiration governs the plant water requirement in relation to the subjected microclimatic conditions and VPD can be used as a tool to maintain greenhouse crop transpiration to optimal range for obtaining desired crop yields. Thus, the present investigation was undertaken to study the integrated effect of temperature (Tapc), relative humidity (Eapc) and solar radiation (Irad) on VPD and crop transpiration. A computer module was developed to monitor the behavior of the VPD and thereby the crop transpiration through charts under greenhouse conditions. The VPD indicated a linear increasing trend with Tapc (R2≥0.84) and decreasing trend with Eapc (R2=0.99), demonstrating a strong correlation in both cases. The increasing crop transpiration with VPD, particularly under hot climatic conditions significantly increased the input water and nutrient requirements of the crop. Thus, transpiration in relation to VPD should be considered as a factor for fertigation scheduling and improving irrigation control in soilless systems. Moreover, maintaining Tapc, Eapc and VPD within range of 22-27°C, 60-80% and 0.53-1.10 kPa may help to control the crop transpiration, manage the greenhouse irrigation and fertigation, and thus improve the cucumber yield.

Vapour pressure deficit (VPD) inside protective structures under cropped conditions significantly affects the plant growth and productivity through its direct relationship with crop transpiration or irrigation management. Thus, monitoring VPD inside greenhouse during crop growth period becomes essential to limit it to a desired range. The present study was undertaken to develop mathematical models for predicting SVP, AVP and VPD inside a greenhouse independently using internal and external climatic parameters as inputs. The root mean square error (RMSE) was obtained in the range of 0.03-0.10 kPa and 0.27-1.03 kPa respectively for the models developed from internal and external climatic parameters as model inputs. The average model efficiency (neff) was computed to be 98.7 per cent, 92.2 per cent and 100.0 per cent respectively for SVP, AVP and VPD when predictions were made using internal climate as input. Similarly, for the models developed from external climate as model input, neff was worked out to be 96.7, 86.1 and 93.0 per cent for SVP, AVP and VPD respectively. The developed models presented a high degree of precision in predicting SVP, AVP and VPD with both internal and external climatic conditions as model inputs inside a naturally ventilated greenhouse under cucumber crop in soilless media.