Response of tomato crop growth and development to a vertical temperature gradient in a semi-closed greenhouse

Semi-closed greenhouses have been developed over the last decade to conserve energy. Energy consumption is reduced by collecting the excess solar energy in Summer, storing it in aquifers and re-using it in Winter to heat the greenhouse. Cooling systems placed in the lower part of the greenhouse, can cause vertical temperature gradients, which can be more than 5ºC at high levels of solar radiation. Given the substantial effect that air temperature has on a number of plant physiological processes, we expected to observe effects on plant growth and fruit production. Tomato plants were grown in semi-closed greenhouses with or without a vertical temperature gradient. The gradient was most pronounced from June to September when, for 55% of the time, the temperature difference between the top and bottom of the canopy was > 2ºC, and for 20% of the time the difference was > 5ºC. Despite these large vertical temperature gradients, plant growth and fruit yields were mostly unaffected. Leaf and truss initiation rates did not differ between treatments, since air temperatures at the top of the canopy were comparable. The only observed response of plants to the vertical temperature gradient was a reduced rate of fruit development in the lower part of the canopy. This resulted in a longer time between anthesis and fruit harvest in the treatment with a vertical temperature gradient, and an increase in the average fruit weight in Summer. However, total fruit production over the whole season was not affected. These results are important when designing greenhouses, as well as heating and cooling systems for greenhouses