ANTICIPATED CASH VALUE OF PHOTOSYNTHATE IN THE GLASSHOUSE TOMATO

Tomato fruits take some 8 weeks to grow from anthesis to ripening. The objective of this paper is to review information relevant to CO₂ stimulated photosynthesis in the glasshouse tomato, and to derive a mechanistically-based factor for converting the rate of carbon assimilation into an anticipated financial return from the future sale of fruit.

Photosynthate is stored in fruits and vegetative organs. The fraction allocated to fruits of different ages will be partitioned between them in proportion to their sink demands. A Gompertz model can be used to predict the potential growth rates, as a function of age, and hence the sink demands.

The fruits will progressively ripen for harvest and marketing according to their ages. Since growth and maturation rates depend on temperature, a model of the glasshouse temperature climate is required to relate physical and physiological times. The distribution function of photosynthate over fruit ages and the estimate of the expected delays before harvesting can be combined with a prediction for the future market prices to give a first estimate of the anticipated cash worth per unit of photosynthate for each week after first anthesis. Allowance is required for labour, down-grading, and other proportional costs.

Prior to first anthesis, there is benefit to be gained from the investment of photosynthate into leaves and other vegetative structures. Hence the estimate of worth should be revised because of the marginal increase in Leaf Area Index associated with additional photosynthesis: the resulting increment in future intercepted light will contribute a deferred benefit. This revision ensures a financial worth for photosynthate even if no fruit is yet set. The adjustment should be continued after first anthesis, but to a progressively reducing extent as the fruit demand builds up.