Analysis of microclimate and cucumber fruit yield in a screenhouse and an evaporatively cooled greenhouse in a semi-arid location

The microclimate and cucumber crop response in a screenhouse and in an evaporatively cooled greenhouse were studied in Oman during winter/spring and spring/summer cultivation periods. Measurements were carried out in two similarly shaped structures: (1) a greenhouse equipped with a pad-and-fan system for evaporative cooling of the greenhouse environment and (2) a screenhouse with no cooling system. Analysis of the spring/summer period climate data showed that the evaporative cooling in the greenhouse reduced the mean air temperature by about 4.5°C compared to outside and maintained the leaf temperature close to the greenhouse air temperature. The 24 h mean leaf and air temperatures in the greenhouse reached 25.8°C ±1.3°C and 25.9°C ±0.8°C, respectively. On the other hand, the 24 h mean leaf and air temperatures in the screenhouse were higher by 1.0°C and 1.3°C, respectively, compared to outside. The 24 h mean leaf and air temperatures in the screenhouse reached 32.8°C ±1.2°C and 31.8°C ±1.5°C, respectively. Furthermore, the evaporative cooling in the greenhouse maintained the 24 h mean air vapor pressure deficit (VPD) values at levels lower than 1.1 kPa, while the 24 h mean air VPD in the screenhouse reached values up to 4.5 kPa. These differences resulted in a 50% decrease in crop fruit yield during the spring/summer period. The radiation and water use efficiency (WUE) values observed in the two structures were similar during the winter/spring period but were higher in the greenhouse during the spring/summer period. However, for the greenhouse, when the water evaporated in the wet pad was also considered, the overall WUE was at the same level in both structures during summer. Furthermore, the evaporative cooling applied in the greenhouse enhanced the mean values of fruit quality characteristics measured during the spring/summer, such as fruit dry matter content (5.6%), fruit firmness (5.0 kg cm^-2), and chroma (18.6), compared to that of the screenhouse (5.0%, 4.9 kg cm^-2 and 16.3, respectively), but did not significantly affect other fruit quality characteristics, such as mean fruit weight (128 g for greenhouse and 123 g for screenhouse), total soluble solids content (3.9 °Brix for both structures), and juice pH (5.7 for greenhouse and 5.6 for screenhouse). Overall, it can be concluded that under the weather conditions of Oman, although greenhouses are still needed during spring/summer, screenhouses can be used during winter without jeopardizing crop production quantity and quality.