Whereas most high-tech tomato greenhouses focus primarily on high production yields, consumers prefer a higher quality product. Dry matter content is one of the key factors determining fruit quality, and is known to be substantially influenced by altering the salinity of the nutrient solution. While this imposed osmotic stress can improve fruit quality, this often goes hand in hand with a decrease in production due to less water accumulation in the fruit. A more thorough insight in the underlying mechanisms might contribute to a better understanding and eventually steering of this delicate balance. To achieve this deeper knowledge, we combined intensive monitoring of plant and fruit physiological variables with a model-based approach. An experiment on tomato (Solanum lycopersicum L. 'Dirk') was set up in a greenhouse, where two different water treatments were imposed by altering the salinity (Electric Conductivity, EC) of the substrate. Besides plant variables such as sap flow, stem diameter variation and stem water potential, fruit growth and quality parameters were measured as well. These data were then used in a recently developed virtual tomato plant and fruit model, which is capable of modelling both plant and fruit growth as well as fruit quality (sugars and acids) and xylem and phloem contribution to fruit growth, but which has not been tested under salt stressed conditions. Results did not only show that the model can be used to predict fruit growth during salt stress conditions, but also which model parameters and related plant traits are affected most. This is an important step towards a better understanding of the underlying mechanisms controlling fruit development under osmotic stress.

Changes in leaf thickness can be a rapid indicator of the plant's water status and can therefore serve as an alarm signal for potential water deficits. Combining the use of continuous leaf thickness measurements with a mechanistic plant model describing optimal leaf growth and diel variations, would allow growers to optimize greenhouse growing conditions by adaptation of the microclimate and applied irrigation. Recent development of new sensors offers the possibility for real time measurements of leaf thickness on small plants, including ornamentals. However, the accuracy of leaf thickness variation measurements needs to be assured. In this study, the temperature influence on 12 LeafSen (Netafim, Tel Aviv, Israel) sensors has been tested in a temperature range from 16 °C to 31 °C by installation of the sensors on aluminium plates. Temperature variations in the investigated range resulted in sensor signal differences of up to 48 µm, indicating that temperature response can exceed the expected diel leaf thickness variation. Two typical temperature responses were distinguished, pointing to the need for a sensor specific temperature correction. The practical use of leaf thickness sensors and the established temperature corrections has been demonstrated by installing the sensors on the stem and leaf of three Ficus plants (Ficus benjamina) and three pot roses (Rosa chinensis cv.) starting from cutting stage in a commercial greenhouse environment.