Optimum cool root zone temperature positively influences the production of greenhouse vegetables grown during summer/high temperature period under hydroponics system. Hence, the effect of root-zone temperature was investigated on the growth, yield and nutrient uptake of cucumber (Cucumis sativus L.) plants grown in pots filled with perlite medium under recirculating hydroponic system in greenhouse during summer period (June-August) in two consecutive years 2016/2017 and 2017/2018 using three cooling treatments-T1 (22 ºC), T2 (25 ºC) and T3 (28 ºC) and non-cooled treatment T4 (33 ºC) as control in Randomized Complete Design (RCD). All the treatments received the same nutrient concentrations. Significant (p < 0.05) differences were observed for all the characters viz. plant height, leaf number/m2, chlorophyll content, leaf area (cm2), fruit number /m2, yield (t/gh), fresh (g) and dry matter weight (g) of shoot and root at all cooled root-zone temperatures as compared to control in both the years. Plants at cooled root-zone temperature (RZT) of 22 ºC gave high number of fruits/m2 to the extent of 180 in 2016/2017 and 220 in 2017/2018 followed by that at 25 ºC (167, 221) and 28 ºC (178, 143) as compared to those in control (33 ºC) (101,133) in both the years. Similarly, highest fruit yields were found at cooled RZT of 22 ºC (5.0 t/gh) and 28 ºC (4.7 t/gh) in the first year and 22 ºC (6.1 t/gh) and 25 ºC (6.0 t/gh) in the second year. The plants at cooled RZT responded positively and significantly (p < 0.05) in the uptake of all nutrient elements in shoots and roots in comparison with those at non-cooled RZT in both years.

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High nutrient solution temperature stress around root-zone (rhizosphere) is a major factor limiting hydroponic crop growth due to change in dissolved oxygen (DO) levels in nutrient solution during the high-temperature seasons. This suggests that easier and economical control of root-zone temperature (RZT) could be an effective solution to the temperature stress for the crop plants in hydroponics. In this respect, the present investigation was undertaken to comprehend the influence of nutrient solution temperature on its oxygen level (ambient dissolved oxygen in feeding tank), growth, yield and quality of cucumber (Cucumis sativus L.) plants. The plants were cultivated in recirculating hydroponic system in a greenhouse with dimensions of 9 m wide × 30 m long and total area of 270 m2 during three cropping periods in Oman viz. summer (June-August), fall (September-November) and spring (February-May) in two consecutive years 2016/2017 and 2017/2018. Three cooling treatments were applied viz. T1 (22 ᵒC), T2 (25 ᵒC) and T3 (28 ᵒC) through cooling nutrient solution besides non-cooled treatment T4 (33 ᵒC) as control. The plants were grown in pots filled with perlite medium in randomized complete design (RCD) with four replications. All the treatments received the same nutrient concentrations. The results showed that cooling of nutrient solution temperature influenced positively the levels of DO in the fresh nutrient solution feeding tank and drain nutrient solution for all cropping periods in both the years. Significant (p < 0.05) differences between the treatments were observed in oxygen consumption by the roots of cucumber in the cropping periods in both the years. High levels of oxygen consumptions were recorded with cooled RZTs as compared to that of control-non-cooled RZT for all cropping periods in both the years. All growth, production and quality attributes were positively influenced and greatly expressed by cooled root-zone temperature in the root-zone of cucumber with increased DO levels in nutrient solution. This study revealed that availability of optimum aeration (oxygen levels) in nutrient solution through cooling of nutrient solution temperature could be of significance for boosting cucumber yield as observed during all the growing periods especially in summer in Oman.