ROOT EXUDATION AND MICROFLORA POPULATIONS IN RHIZOSPHERE OF CROP GENOTYPES DIFFERING IN TOLERANCE TO MICRONUTRIENT DEFICIENCY

Crop genotypes differ in their tolerance to micronutrient-deficient so ils, but the underlying mechanisms are poorly understood. This paper r eviews information on mechanisms of tolerance to Zn and Mn deficiency, concentrating on plant-induced changes in chemistry and biology of rh izosphere that alter availability of Zn and Mn. When grown under condi tions of Zn deficiency, wheat genotypes more tolerant of Zn deficiency released greater amounts of phytosiderophore, 2'-deoxymugineic acid, than the sensitive genotypes. In addition, Zn deficiency increased num bers of fluorescent pseudomonads in rhizosphere of all wheat genotypes tested, but the effect was particularly obvious for genotypes toleran t of Zn deficiency. Rhizosphere of wheat genotypes contained an increa sed proportion of Mn reducers under Mn-deficiency compared to Mn-suffi ciency conditions. When grown in soils of low Mn availability, some wh eat genotypes tolerant of Mn deficiency (like cv. Aroona) had a greate r ratio of Mn-reducers to Mn-oxidisers in the rhizosphere compared to the sensitive genotypes. In contrast, microflora in the rhizosphere of other wheat genotypes tolerant of Mn deficiency (like C8MM) did not s how the same response as Aroona. It therefore appears that different m echanisms may underlie the expression of tolerance to Mn deficiency in wheat genotypes. It is concluded that wheat genotypes tolerant of Zn or Mn deficiency have a capacity to alter chemical and biological prop erties of the rhizosphere, thus increasing availability of critical mi cronutrients.