Crop photosynthesis (P) is a major (fast reacting) yield determining process and therefore it is a suitable criterion to evaluate short term effects of climate control in greenhouses.
For optimal climate control the relation between control actions and marginal yields and marginal costs should be settled, within the scope of the established tactical plan.
Thus it is essential to evaluate variations in daily P (ΔP) economically.
Taking a full diurnal cycle at time t as the basis for our considerations, it is clear that the dry weight (W(t+1)) at the end of that day will change by ΔW, due to ΔP. The economic value of ΔW depends on the crop phase.
In the young phase, the phase of crop establishment, ΔW primarily causes variations in time (Δtp) required to attain the production phase and hence earliness.
In the production phase, ΔW will give rise to a change in economic yield (ΔY), or a change in time till harvest (Δth). The economic value of Δth is difficult to assess in general terms since it depends strongly on the management objectives.
A change in earliness Δtp corresponds to a change in dry weight production in the production phase ΔW equivalent to Δtp x GR(tp), assuming that the growth rate at tp (GR(tp)) will not be changed by Δtp.
An important problem to be solved is the fraction of dry weight increment that is diverted to the harvestable products (Fhp). For crops like pot plants, chrysanthemum or lettuce Fhp ≈ 1. For several other crops Fhp appeared to be constant over longer periods of time (months), though appreciable short term fluctuations in Fhp occurred.
Fhp is not necessarily a fixed crop characteristic, but may depend on the growing conditions.
Within a given tactical production plan, in our opinion, it may be treated as a constant.