EXTERNAL GREENHOUSE MOBILE SHADING: EFFECT ON MICROCLIMATE, WATER USE EFFICIENCY AND YIELD OF A TOMATO CROP GROWN UNDER DIFFERENT SALINITY LEVELS OF THE NUTRIENT SOLUTION
season generates in greenhouses microclimates characterized by their severe temperature regime and high evaporative demand. These stressful conditions for the growth and development of crops reduce yields significantly, specially when only high-EC irrigation water is available. In this study an evaluation of greenhouse climate, potential transpiration, water and radiation use efficiency (WUE and RUE) and yield of a tomato crop has been conducted in a greenhouse equiped with external mobile shade screen compared with a reference greenhouse. These parameters were determined under two different salinity levels of the nutrient solution, 3.1 and 5.1 dS m-1 EC. The use of mobile shade screen improved greenhouse climate: vapour pressure deficit (VPD) and air temperature. This improvement implied a 36% reduction of the total integral of the incident global radiation during the cropping cycle. Marketable yield of the shaded - EC 3.1 dS m-1 treatment was significantly higher, 12.1 kg m-2, than the 11.1 kg m-2 obtained in the control greenhouse. No differences were found between the EC 5.1 dS m-1 treatments with 8.7 kg m.-2 of marketable yield both. The incidence of BER was remarkably lower in the shaded treatments under both ECs. Total water uptake of the crop in the shaded greenhouse was 32% lower than that in the control greenhouse. The water use efficiency in terms of marketable fruits were 19.3 g L-1 and 28.6 g L-1 under saline conditions in control and shade treatments respectively (48% increase). Low EC treatments showed water use efficiencies of 23 and 37.2 g L-1 in control and shade greenhouses respectively (62% increase). The shade treatments increased Radiation Use Efficiency from 0.8 g mol-1 in the control treatments to 1.1 g mol-1. The results suggest that the use of external mobile shade screen during the spring-summer growing season contributes to increase water and radiation use efficiencies and to improve the quality of tomato fruits irrigated with the two salinity levels of the nutrient solution considered in this study.