A data base was constructed of the % N and plant d. wts (W) in t ha" 1 of C3 and C4 crops that had been grown with sufficient nitrogen to permit maximum growth rate. The % N of all crops declined sharply with increase in Wbut this decline differed between C3 and C4 crops. When Wwas greater than I t ha" 1 , 86% of the variance in In % N was removed by the model % N = aW' b with b =-05 for all crops, and a = 5-7 % for C3 crops and 4-1 % for C4 crops. The same model gave a good description of data on C3 and C4 crops entirely independent of that used for developing the model. According to this relationship the fractional decline in % N with increase in plant mass was the same for both types of crops, but C4 crops contained about 72% of the nitrogen in C3 crops at equivalent d. wts. As approx. 32% more dry matter was produced per unit of intercepted radiation for C4 and C3 crops, the N uptake (or weight of plant protein produced) per unit of intercepted radiation was approximately the same for both types of crops. A small improvement in the degree of fit to % N = aW~" was obtained by allowing both a and b to vary with the crop. Values of b obtained in this way for tall fescue, lucerne and winter wheat, but not for potato and sorghum, were consistent with Hardwick's 'skin core' hypothesis (Annals of Botany, 1989,60,439-46). The entire data set was, however, consistent with Caloin and Yu's model (Annals of Botany, 1984, 54, 69-76) in which there is a conceptual N pool for photosynthesis and another N pool for the other processes.

SummaryMeasurements were made at intervals during the growth of seven different vegetable crops grown on the same soil to find how far root development and crop growth could be described by simply derived equations and to find how far the parameter values varied from crop to crop.For each crop K1 In W + W, (where W is total plant dry weight, t ha-t and K 1 is equal to 1 t ha -1) was linearly related to time from emergence, as in past experiments.The derived equationwhere L is total root length per unit area, t is time from emergence, cj and bj are coefficients that depend on the crop (j) and m is a coefficient having the same value for all crops, removed 89.4% of the total variance in In L. The best fit was obtained with a value of m that implied that about 3% of the root carbon was mineralized each day. Generally the logarithm of root density declined linearly with increasing depth. Most of the variation between the gradients of these relations for the different crops was removed by a single regression against logarithm of total root length.The main discernible differences between species in their rooting patterns were that root length for a given top weight of legumes was about half that of non legumes, that the development of storage roots was associated with a less steep decline in root density with depth than for other crops and that onions were exceptional in that the depth to which their roots penetrated did not change appreciably during much of the growing season.A single linear relationship between root depth and top weight (r 2 = 0.85) covered all nonleguminous crops except onions and another relationship (r 2 = 0.80) covered the legumes.

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