Salinity, Transpiration and Greenhouse Tomato Production
Yan Huiqiao
In soilless cultivation, fertilizers are dissolved in the irrigation water (nutrient solution). The total concentration of solutes in the nutrient solution is characterized by the electrical conductivity (EC,dSm-1). Usually EC in the commercial tomato production is in the range of 2 to 5 dSm-1. Too low a concentration causes mineral deficiency and restricts plant growth, whereas, there is no negative effect of an overabundant supply of nutrients, at least within a broad range. To avoid deficiencies and to control the quality of harvestable product, large amounts of nutrients are added to the irrigation water, with little attention to the actual uptake by the crop.
Re-use of drain water enables economic use of water and fertilizers combined with an ample water supply to the crop. The Dutch government also stimulates reuse of drain water in order to reduce the emission of nutrients to the environment. However, long-term re-circulation of drain water results in accumulation of the salts that may come with re-fill water and are not taken up and the fertilizers that may be injected in excess of actual needs.
It is known that salinity (high EC) reduces yield. Uptake of water into the fruits is reduced by a high osmotic pressure of the irrigation water and as a result the fruit size is smaller, although the accumulation of dry matter per fruit is unaffected. Increased salinity also increases the incidence of blossom-end rot (BER), a disorder that is associated with low calcium concentration in the fruit especially in the distal end, On the benefit side, mild saline irrigation water may improve the quality of horticultural products by increasing dry matter content and sugar concentration in the fruit.
However, the salt concentration in the root environment is not only factor that influences the water status of the plant. Normally, more than 90% of the water absorbed by the root is lost by transpiration. The water content of the plant is the resultant of the balance between inflow (water uptake) and outflow (transpiration). The water content (water potential) in turn affects plant growth and yield. Bruggink et al. (1987) proposed that it should be possible to enhance plant growth by adapting the salinity level to the rate of transpiration. Hoffman and Rawlins (1971) reported that high humidity (and thus low potential transpiration) significantly raised the salinity level at which the yield was reduced to 50% of the non-saline yield for onion and
radish.
The aim of the research “effect of EC and transpiration on production of greenhouse tomato”was to investigate that manipulating water outflow of a plant. The practical implication of this study was to ascertain whether lowering potential transpiration could mitigate the negative effects caused by high EC in closed loop growing systems. The approach of the study was to combine different concentrations (EC) of the nutrient solution factorially with two climate treatments, “low” and “high”potential transpiration. The experiments were mainly focused on long-term effects and therefore concentrated on the production phase of the crops. For this reason the experiments were started the treatments after the plants were well developed.
