Greenhouse Tomato Production with High Saline Nutrient Solution Simulating a Re- circulating Irrigation System without Environmental Discharge

Increased tomato fruit quality in response to increased salinity is well documented (Li, 2000). However, the increased fruit quality is achieved at the expenses of severe yield losses mostly due to reduced fruit size (fresh weight) and increased incidence of physiological disorders such as blossom end rot (BER). The ability to minimize the negative effects of high salinity at the root level, while increasing fruit quality, through manipulation of environmental factors, which affect potential transpiration, was demonstrated. Two greenhouse canopy environments were selected for inducing transpiration [high vapor pressure deficit (VPD) (2 kPa), and suppressing transpiration [low VPD (0.8 kPa). Fruit quality parameters from the treatment environments were contrasted with those from a standard commercial greenhouse environment in which VPD was not controlled and day-night temperature were 24oC / 19oC. All environmental treatments were associated with two electrical conductivities (EC) of the nutrient solution: 2.5 dS m -1 (EC standard) and 8 dS m -1 (EC high). Maximum total fresh yield and marketable yield was obtained in the standard environment under EC standard. The major factor in changing fresh weight and BER was EC, while canopy environment affected the magnitude of the change. High EC significantly increased % Brix, acidity, and firmness, while transpiration regime seemed to have no effect. Compared with the standard canopy and root environment, an increase of 5.5 dS m -1 in salinity at the root level resulted in a marketable yield decreased of 59%, 61% and 87% on the standard, LET0 and HET0, respectively. This decrease was due to a reduction fruit size and an increase in BER incidence. Under high salinity, suppressed transpiration regime reduced yield losses, especially by reducing BER incidence. This study suggests that, the decrease in marketable yield observed under high salinity conditions (whether resulting from a closed irrigation system or simply increased EC in the nutrient solution), could be minimized once associated with a canopy environment, which reduces potential transpiration. This combination, significantly reduced BER incidence. Fruit quality was improved with high salinity, regardless of the transpiration regime provided. Introduction Assimilate distribution between vegetative organs and fruits, is dependent on the environmental conditions that govern plant water status, nutrient uptake and fruit development. Plant transpiration, reduces leaf water potential, which in turn increases the gradient of water potential between roots and leaves, thus stimulating water and nutrient uptake. Therefore transpiration is a very important physiological process for plant growth. It serves as the driving force for water transport, as well as distribution of nutrients. In Hydroponic systems nutrients are supplied diluted in irrigation water at certain target concentration. However it is common practice to irrigate with 30% more nutrient solution [or ca 10 to 25% more salts] (van de Vooren et al.,