GROWTH-RATE AND PERCENT N OF FIELD-GROWN CROPS - THEORY AND EXPERIMENTS

Mechanistic equations were developed to link the % N in plant dry matter to plant mass per unit area and to various expressions of growth rate. They were tested against the results of multi-harvest, multi-N-level fertilizer field experiments on French beans, potatoes, tall fescue and winter wheat.Sub-critical values of % N measured at different times during the growing period were for each crop almost linearly related to relative growth rate, but the values of the regression coefficients varied from experiment to experiment.A single equation was developed that denned the % N in the dry matter at any time with any sub-optimal level of fertilizer-N, in terms of the plant d.wt per unit area of N-sufficient plants measured at that time, and the ratio of the growth rate coefficient (see below) of the N-deficient crop to that of the N-sufficient crop. Even though there were no other variables, the equation removed 86.5% of the variance of all the measured sub-optimal and critical values of % N for the different crops. The growth rate coefficient Kx(F) was defined by the eqn:dW(F,t)/dt=[Kx(F)×W(F,t)]/[x+W(Ft)]where W(F, t) is plant d.wt per unit area, t is time, x is a coefficient that is strongly correlated with Kx and F,t in parentheses indicates that the variable varies with the level of fertilizer and with time. Kx(F) was always almost constant for long periods of time at each level of fertilizer. This coefficient corrects for the effects of mass per se on growth rate.The apparent similarity in growth/% N relationships for different species was explained in terms of the high N-requirement for the photosynthetic apparatus, the similarity in photosynthetic rates and the low % N in storage and structural tissues. © 1991 Annals of Botany Company.