Consequences of diurnal variation in salinity on water relations and yield of tomato

In soilless culture the EC (Electric Conductivity; mS cm <sup><font size="-2">-1</font></SUP>) is an important measure for the total solute concentration (salinity level) of the nutrient solution in the root environment. This study concentrates on the possibilities of short-term control of the total nutrient concentration (salinity level) in the root environment in relation to the greenhouse climate. A general assumption in this study is that the EC mainly influences plant functioning via its effect on the water relations of the plant: high salinity in the root environment osmotically decreases the availability of water to the roots of the plant. A chapter is included which deals with the current concepts, measures and methods related to the thermodynamic approach of plant water relations and the associated term water potential.<p>The experimental part of this study concentrates on short- and long-term responses of (changes in the) EC on plant growth and functioning. Short-term experiments were done to investigate short-term plant responses (changes in expansion growth, water status and transpiration within a day) upon changes in EC. Long-term experiments were done to investigate the effect of different day and night EC-levels on yield and quality of the fruits during a whole growth season. A simulation model was developed, which is based on the thermodynamic approach of plant water relations, to analyse and integrate the results of the short- and long- term experiments. To obtain reliable measurements of short-term changes in transpiration, water uptake and changes in plant water content (the total amount of water stored in the plant), a new method for simultaneous measurements of water uptake, transpiration and changes in plant water content on one plant was developed.<p>Short-term experiments were done in a growth-chamber and in a greenhouse. Changes in EC were applied in dark and in light, while transpiration, water uptake and plant water content were measured. In dark, a bipartite response was measured on expansion growth: a change in salinity initially changed expansion growth enormously, followed by partial adaptation which occurred in the light: this was interpreted as a direct hydraulic response followed by some adaptation (probably under metabolic control). Salinity changed transpiration clearly in the growth chamber but not in the greenhouse. In the growth-chamber no simple quantitative relationships were found between the changes in plant water status and changes in the rates of transpiration and expansion growth. In the greenhouse, however, the relationship between plant water deficit and transpiration over a day was influenced by salinity. A hysteresis effect over a day was observed on this relationship at low and high salinity level. At low salinity the slope of the relationship was lower than at high salinity and the hysteresis effect was less pronounced. Solute accumulation at the endodermis in the root was raised as a possible explanation for the hysteresis effect and the different slopes at low and high salinity. Simulations, however, showed that solute accumulation was not important enough to be the only cause for the different slopes. The hysteresis effect over a day was clearly simulated by the model. It was concluded that present simulation model needs some adaptation and should probably be combined with metabolic oriented models to be able to describe expansion growth.<p>In the long-term experiments clear positive effects were measured of the intuitive treatment (low salinity during the day and high salinity during the night) on the growth and yield of tomato: yield was increased greatly, mainly by a positive effect on average fruit size, but dry- matter distribution towards the fruits was slightly increased too. Fruit quality was influenced by the intuitive salinity treatment: dry matter percentage of the harvested fruits was decreased, while the incidence of Blossom-End-Rot was greatly decreased.<p>It is concluded that short-term control of the EC mainly influenced expansion growth of the fruits, and that restriction of the supply of macro-nutrients to the dark period did not influence plant growth and production negatively. Short-term control of the EC could probably be well used as a tool to choose between yield and quality in tomato culture