Development and dry matter distribution in glasshouse tomato : a quantitative approach

In the glasshouse cultivation of a long-season tomato crop, maximum fruit production is obtained when there is a proper balance between the demand and the supply of assimilate, and an optimum proportion of vegetative growth throughout the season in order to sustain the crop photosynthetic capacity. These aspects of crop growth are mainly affected by the fruit load, defined as the assimilate demand of all fruits together. In practice fruit load is controlled by plant density, fruit thinning and temperature. These measures for crop control can be more precise and effective if their effects are known in quantitative terms. An explanatory dynamic growth model was developed that simulates assimilate demand and dry matter distribution in an indeterminate tomato crop. Number of growing organs was evaluated through prediction of initiation, abortion and harvest of individual organs. Assimilate demand was based on potential organ growth rates (growth at nonlimiting assimilate supply). Dry matter distribution in the model was in proportion to the potential growth rates of the organs.

In total 11 glasshouse experiments were conducted, six of which included temperature treatments. Truss formation rate increased with temperature (17-27°C) and declined with plant age. Truss formation rate was found to depend on the genotype, while fruit load, plant density, season and electrical conductivity of the root environment (EC: 0.3-0.9 S m ^-1) had no effect. The number of fruit that develop per truss was positively correlated with the vegetative growth of the top of the plants. The duration of the fruit growth period (time between anthesis and start of colouring) was shortened with increasing temperature, young and old fruits being the most sensitive. At the same air temperature the fruit growth period in summer was shorter than in spring. Fruits of old plants had slightly longer growth period than fruits of young plants. Potential weight of the fruits at harvest was negatively correlated with temperature, mainly due to the shorter fruit growth period. Further, the potential size increased with ontogeny, which effect was more pronounced in early than in late spring. The course of potential weight in time was described by a Gompertz growth curve exhibiting the maximum growth rate at about 40% of the fruit growth period. When during fruit development a fruit changed from limiting to nonlimiting assimilate supply, it did not immediately reach the same growth rate as fruits grown constantly at nonlimiting assimilate supply., A mechanism is proposed that explains this phenomenon. The fraction of dry matter distributed to vegetative growth declined substantially with temperature. The (apparent) potential growth rate of a vegetative unit at 24°C was estimated to be as much as 50% lower than at 19°C. The dry matter-content of fruits was negatively correlated with temperature and EC of the root environment and was higher in summer than in spring and autumn.

The model was tested with data from five commercial crops. Truss formation rate, fruit growth period and dry matter distribution were predicted reasonably well. The modelling of the number of fruits per truss requires more investigation. Simulated assimilate demand of a mature tomato crop reached values of 10 and 60g CH ₂ O m ^-2d ^-1for maintenance respiration and growth respectively. The potential growth rate (as defined by the sinks) appeared to be about twice the actual growth rate.

A simulation study indicated that maximum fruit production of tomato is probably obtained at a fairly low leaf area index (2-3 m ²m ^-2). At supra-optimum leaf area index additional leaf area for extra light interception requires more assimilate than it would produce. Computations showed that in spring and early summer the optimum plant density is determined by the required number of fruits (sink capacity) whereas in summer a combination of high plant density and fruit thinning seems required for sufficient leaf area. The results are discussed with respect to the crop sink-source system and temperature control in the glasshouse. Prospects for practical applications of the model are presented.