The effect of root temperature on the uptake of nitrogen and the relative size of the root system in Lolium perenne. I. Solutions containing both NH+4 and NO3−
Аннотация:Abstract Lolium perenne L. cv. S23 was grown in flowing culture solution, pH 5, in which the concentrations of NH 4 + , NO 3 − and K + were frequently monitored and adjusted to set values. In a pre‐experimental period, plants were acclimatized to a regime in which roots were treated at 5°C with shoots at 25°C. The root temperature was then changed to one of the following, 3, 7, 9, 11, 13, 17 or 25°C, while air temperature remained at 25°C. When root temperature was increased from 5X, the relative growth rate of roots increased immediately while that of shoots changed much less for a period of approximately 9 d (phase 1). Thus, the root: shoot ratio increased, but eventually approached a new, temperature‐dependent, steady value (phase 2). The fresh: freeze‐dried weight ratio (i.e. water content) in shoots (and roots) increased during the first phase of morphological adjustment (phase 1). In both growth phases and at all temperatures, plants absorbed more NH 4 + than NO 4 + , the tendency being extreme at temperatures below 9° where more than 85% of the N absorbed was NH 4 + . Plants at different root temperatures, growing at markedly different rates, had very similar concentrations of total N in their tissues (cells) on a fresh weight basis, despite the fact that they derived their N with differing preference for NH 4 + . Specific absorption rates for NH 4 + , NO x − , K + and H 2 PO 4 − showed very marked dependence on root temperature in phase 1, but ceased to show this dependence once a steady state root: shoot ratio had been established in phase 2. The results indicate the importance of relative root size in determining ion fluxes at the root surface. At higher temperatures where the root system was relatively large, ‘demand’ per unit root was low, whereas at low temperatures roots were small relative to shoots and ‘demand’ was high enough to offset the inhibitory effects of low temperature on transport processes.
