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SCREENING OF CROPS ON THE EFFICIENCY OF GROWTH AND NET PHOTOSYNTHESIS IN RELATION TO ENVIRONMENTAL CONDITIONS
This relationship, according to Thornley (Mathematical models in Plant Physiology) can be described as follows:
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in which Pn is net photosynthesis, 
a constant for the photochemical efficiency, 
the efficiency for CO2 absorption (plant conductance), 
a constant for photorespiration and Rd the dark respiration.
The above mentioned constants can be obtained in a series of experiments in which light intensity (I) and carbondioxide concentration (C) are varied, e.g. in relation to ambient air temperature and other treatments.
Especially at low light intensities during winter energy saving by the application of low ambient temperatures should be analysed by screening crops and cultivars on:
- a high value of
- a high value of a low value of and Rd
In such experiments the light and CO2 compensation points can be obtained, which are also important parameters for an efficient CO2 utilisation.
The maximal value (Pmax) of net photosynthesis is an important parameter as well.
At present systematic research in net photosynthesis of horticultural crops in relation to raising of the plants (pre-treatment before planting out), short and long days, graft and root stock effects, senescence, forcing of bulbs, the presence or absence of fruits, diurnal effects are scanty.
It should be realised that photosynthesis measurements of single leaves or whole plants give an indication of the efficiency at a certain time.
The increase in dry weight material per unit time
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is used by the plant for leaf, root, fruit and stem formation.
The distribution of the produced dry weight material on these various plant parts is important for subsequent growth.
Therefore, a growth analysis study is also important in which plant parameters are measured such as leaf area ratio, leaf weight ratio, specific leaf area, net assimilation rate and relative growth rate.