Analysis of greenhouse tomato production in relation to salinity and shoot environment

This work deals with the yield loss caused by saline irrigation water in greenhouse tomato cultivation, and the way climate manipulation may be used to limit damage. The hypothesis is that by "controlling" the evaporative demand of the ambient, it is possible to manipulate plant water status, in order to restore the balance distorted by high salinity (osmotic pressure) in the root environment. The scientific aim of this work is to explain the interaction between water inflow (root environment) and water outflow (shoot environment) in determining crop fresh yield.The results are presented of a series of long-term experiments with commercially-grown greenhouse tomato. A constant ratio was maintained between a "high" and a "low" potential transpiration (ET ₀ ), combined with a "high" and a "low" concentration of salt in the nutrient solution (EC), in the range between 2 and 9.5 dS m ^-1 . Neither EC nor ET ₀ had effect on the number of harvested fruits or on dry weight of individual fruits, whereas EC affected water content of fruits. Marketable fresh-yield production-efficiency decreased linearly with increasing EC of the nutrient solution. Yield decrease (5.1% per dS m ^-1 ) at high transpiration resulted from reduced weight of individual fruits (3.8% per dS m ^-1 ) and an increased fraction of unmarketable fruits (mainly caused by blossom-end rot). For the low transpiration treatment, however, yield loss was 3.4 % per dS m ^-1 , fully accounted for by the reduction in fruit weight. The lack of effect of either treatment upon dry matter production is explained by the observation that individual leaf area was reduced (8% per dS m ^-1 ) only at EC exceeding 6.5 dS m ^-1 ; and that the number of leaves produced by each plant was increased by EC (2% per dS m ^-1 ). The reduction of fruit size at high EC was mainly caused by a smaller fruit growth rate, in particular during the cell-expansion phase, and a slightly shorter duration of growth. There was no effect of transpiration on fruit growth rate at 2 dS m ^-1 , but a significant negative effect at higher EC. The limitation of growth by high EC was shown to be reversible in an experiment whereby the EC of the nutrient solution was lowered after a 5-month 9 dS m ^-1 treatment. Size of harvested fruits increased gradually, until fruits that had developed fully under the new EC condition, reached a "normal" volume. Similarly, leaves that were already expanded at the moment of lowering EC did not respond to the change in EC, but "new" leaves did.Water uptake per unit leaf area was not affected by osmotic potential of the nutrient solution. The hydraulic resistance within the plant (deduced from measurements of the water potential of the leaf and the stem) was independent of the transpiration flow and was not affected by the osmotic potential of the nutrient solution. The stem water potential was shown to react similarly to a decrease of water potential of the nutrient solution (higher salt concentration) and to an increase in water outflow (transpiration) from the leaves. Since the water potential of the fruits responded weakly to variations in both root and shoot environment, water transport into fruits was regulated mainly by the water potential of the stem.It is concluded that modern greenhouse management offers an opportunity to optimise environmental conditions in relation to root zone salinity, and that the principle of transpiration control developed here, gives a blueprint for climate control when dealing with high EC in the nutrient solution.<strong>Key words</strong> : salinity, EC, osmotic potential, soilless cultivation, potential evaporation, ET ₀ , transpiration, humidity, greenhouse climate, water uptake, water potential, water relation, tomato, Lycopersicon esculentum , fruit weight, fruit size, growth rate, maturation, leaf area index (LAI), vegetative growth, yield, quality, crackin