Physical Environment and Symbiotic Nitrogen Fixation III. Root Temperature Effects on Shoot and Root Development and Nitrogen Distribution In Trifolium Subterraneum

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Abstract
Root and shoot growth, and the distribution of nitrogen to the roots and shoots, were examined in five varieties of Trifolium subterraneum. The plants were grown between 5 and 30[degree]C root temperature, and received their nitrogen from root nodules, or as ammonium nitrate. At the lower root temperatures (5 and 10[degree]C), the translocation of nitrogen to the shoots was retarded in both nodulated and ammonium nitrate control plants, and nitrogen was retained in the roots. Up to 18[degree]C there was a progressive increase in the proportion of the total nitrogen assimilated and translocated to the shoots. In the nodulated treatments, there were both host variety and bacterial strain effects on the distribution of fixed nitrogen. With an increase in root temperature from 8 to 18[degree]C, the increase in the rate of shoot and root dry weight gain was similar (two varieties), or the rate of increase in the dry weight of the shoots increased more than that of the roots. Up to 18[degree]C the percentage nitrogen level in the roots of the nodulated plants was a function of the strain of nodule bacteria, whereas the percentage nitrogen level in the shoots was a function of the host variety. The strain of nodule bacteria affected the morphology of the root system of the host plant. Above 20[degree]C root temperature, changes in dry weight increase and its distribution between the roots and the shoots, were largely controlled by the effect of root temperature on symbiotic nitrogen fixation (root temperature X bacterial strain interaction) although varietal effects were also evident. Marked varietal effects were observed on the distribution of nitrogen to the shoots and roots of plants grown at higher root temperatures.

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