Rape plants were grown in solutions of 10-6, 10-5, 10e4 and 10-3 M phosphate in a controlled environment that gave near optimum climatic conditions for growth. Uptake and growth were followed by replicate harvests taken every five days. The relation between the mean root absorbing power, and the concentration of P in solution was derived. The relations between the o/0 P in the shoot dry matter and the other parameters of the growth model described in paper I were also determined.
Growth rates were exceptionally high, with RGR values above 0.5 g/g/d in solutions of concentration lo-5 M and more during the early stages of growth. RGR was reduced to about half this value in 10-G M P. The range of response to solution concentration in these conditions therefore lay between 10-e and 10-5 M P. In solutions of 10-C and 10-5 M P root hairs were abundant but in solutions of 10-4 and 10-3 M P, they were absent. Rape had a high UAR for P as a result of its high RGR, but it had a correspondingly large root surface area per unit plant weight. Onions (see Paper II of this series) had an inherently lower RGR and UAR for P, but had a comparatively low root surface area per unit plant weight. It appears that these contrasting features of rape and onions broadly compensated for each other so that the P concentration range over which the two species responded was much the same.
Fodder rape, Brassica napus, is a rapidly growing crop with fine roots and an abundance of root hairs. The importance of root hairs in enhancing the phosphorus uptake of roots in soil has been predicted from theoretical considerations 7 and also demonstrated ex-