The relationship between shoot weight, plant density and time during the propagation of four vegetable species

Lettuce, cauliflower, leeks and celery were grown from seed in a glasshouse at four widely-differing densities on several occasions over a 15-month period, under conditions of ample water and nutrients. For each sowing there were four early harvests. The reciprocal of mean shoot dry weight was found to be linearly related to plant density at any particular harvest as in the Shinozaki—Kira equation. The reciprocal of the slope of this relationship, representing the ‘ceiling yield m−2’ was roughly proportional to mean daily radiation during the period of 14 d before harvest: the reciprocal of the intercept was logarithmically related to accumulated heat units, modified for daily radiation. Hence mean plant weight could be expressed as the following function of environmental variables and plant density: w−2 = w0−pe−p.ΣεDD + d.q−1 MDR−1 where w = mean shoot dry weight (g); w0 = initial value of w; p = relative growth rate of an isolated plant per ‘effective day-degree’ (g g−1 EDD−1); MDR = mean daily photosynthetically active radiation (PAR) for 14 d prior to harvest (MJ m−2); d = plant density, (plants m−2); q = slope of ceiling yield vs MDR relation (g MJ−1 d−1); and ΣEDD is cumulative ‘effective day-degrees’ calculated from: EDDi−1 = DDi−1 + f.Ri−1where DD1 is the number of day-degrees (above a base temperature, tbase) for the ith day; EDD1 is the corresponding number of ‘effective day-degrees’ (to the same base temperature); R is the total PAR for the day concerned; and f is a factor denoting the importance of light vis-a-vis temperature. By setting w0 equal to the observed mean shoot weight for the widest-spaced plants at the first harvest of each experiment, and allowing tbase, p, q, and f to vary with species, it was possible to account for 96.4–97.8 per cent of the sums of squares of In w from the 204 combinations of species, density, and harvest date. The parameter f appeared to vary very little with species. Compared with the broad-leaved species, leek had a low value of p and a high value of q, in keeping with accepted views about the effect of leaf angle on photosynthesis.