A theoretical model for the influence of plant size and age on plant protein content was tested against the results of 13 field experiments on vegetable crops grown with high rates of application of N, P and K fertilizers.

For each crop the protein contents of individual plants of similar age (but of different sizes) harvested at regular intervals during growth were found to bear a linear ln-ln relationship to their contents of nitrogen-free organic matter. The slopes of the lines of best fit were almost the same at each harvest date. The intercepts were similar irrespective of harvest date when growth was exponential but showed an approximately linear decline with length of growing period when it was not. These features are in agreement with the theory.

Maintenance respiration rates were deduced by interpreting the data in terms of the theoretical model. The values determined in this way were between 8 and 21 mg of glucose/g of protein/day for the above ground parts of the plant compared with values of between 8 and 87 mg that have been obtained by entirely different methods.

It is concluded that during the exponential phase of growth the critical protein and, thus, nitrogen contents depend primarily on plant size and on the chemical composition of the nitrogen-free organic matter, but that when growth is not exponential plant age and maintenance respiration are also of importance. The models should facilitate the determination of the critical nitrogen contents for crops grown under different conditions.