AMINO-ACID INFLUX AT THE SOIL-ROOT INTERFACE OF ZEA-MAYS L AND ITS IMPLICATIONS IN THE RHIZOSPHERE

The aim of the study was to investigate the ability of intact Zea mays . L. roots to regulate the amount of free amino-acids present in the r hizosphere. Using metabolic inhibitors it was demonstrated that the re lease of aminoacids from the root occurred by passive diffusion, whils t free amino-acids outside the root could be re-captured by an active transport mechanism. The influx of amino-acids into the root was shown to be relatively independent of spatial location along the root and w as little affected by the presence of other organic compounds in solut ion. It was deduced from root concentration gradients that the main si te of amino-acid exudation was at root tips. Amino-acid uptake by the root was shown to be independent of both inorganic-N concentration and the presence of other organic solutes in solution. A computer simulat ion model was constructed to assess the contribution of organic-N upta ke (acidic, basic and neutral amino-acids) to the plant's N budget, in comparison to the inorganic solutes NO3 and NH4. Simulations of N upt ake from a 0.5 mm radius rhizosphere indicated that when inorganic-N c oncentrations in soil were limiting (less than or equal to 0.1 mu mole s cm(-3) soil), the uptake of amino-N accounted for up to 90% the tota l N taken up by the roots. In situations where fertilizer inputs are h igh, and levels of organic matter in soil are low, the contribution of amino-N might still be expected to form < 30% of the total N taken up by the root system. It was concluded that the uptake of amino-acids f rom the rhizosphere may be important in both N nutrition and in the mi nimization of root C and N losses to the soil. Consequently this may b e important in governing the size of the rhizosphere microbial populat ion.