Cycling of nitrogen and potassium between shoot and roots in maize as affected by shoot and root growth

Abstract

In nitrate-fed plants cycling of nitrogen (N) and potassium (K) may serve several functions including supply of the roots with nutrients needed for growth, signalling of the growth-related shoot demand for nutrients to the roots, and removal of excess K from the shoot. In the present study, cycling and recycling of N and K were estimated in plants showing different rates of shoot and root growth. To induce these variations in growth, the plants were cultured with the same optimal nutrient supply but with the root zone temperature (RZT) at 12°C or 24°C. Additionally at both RZT, the plants were grown with their shoot base including apical shoot meristem at high or low temperature (SBT), Decreasing the RZT to 12°C drastically diminished root growth and accumulation of N and K in the roots. Cycling of N and K were less reduced by low RZT. At both RZT, N and K cycling were markedly reduced at low in comparison to high SBT although root growth was not affected by the SBT. Obviously, N and K cycling from shoot to roots were more affected by shoot growth than by the growth related demand of the roots for nutrients. At both RZT, N and K cycling exceeded accumulation in the roots. It was estimated that at least 20-33% of the N, and 24-51% of the K translocated from the roots to the shoot in the xylem is not directly derived from root uptake but from cycling. Plant culture at low shoot base temperature (SBT) drastically diminished shoot growth, and the accumulation of N and K in the shoot to less than 50% of the values measured in plants grown at high SBT. The low SBT-induced decrease of N accumulation in the shoot, at both RZT was associated with a reduction of K circulation and recirculation rates to less than 50% of those found in plants grown at high SBT. These findings are in accordance with the suggested role of K + for charge balance facilitating the transport of NO 3 - in the xylem and disposal of the negatively charged products of NO 3 - assimilation from shoot to roots in the phloem. In plants cultured at low SBT, net uptake and translocation rates of N and K were diminished to less than 50% of those measured in plants grown at high SBT. This repression was associated with reduced rates of N and K cycling from the shoot to the roots. Obviously, low rates of N and K cycling from the shoot to the roots are not necessarily signals to increase uptake in the roots. It is suggested that for plants adequately supplied with N, high rates of N cycling and recycling might be the consequence of an apparent lack in control of phloem loading of amino acids in the leaves.