ALUMINUM TOLERANCE IN WHEAT (TRITICUM-AESTIVUM L) .1. UPTAKE AND DISTRIBUTION OF ALUMINUM IN ROOT APICES

We investigated the uptake and distribution of Al in root apices of ne ar-isogenic wheat (Triticum aestivum L.) lines differing in Al toleran ce at a single locus (Alt1: aluminum tolerance). Seedlings were grown in nutrient solution that contained 100 mu M Al, and the roots were su bsequently stained with hematoxylin, a compound that binds Al in vitro to form a colored complex. Root apices of Al-sensitive genotypes stai ned after short exposures to Al (10 min and 1 h), whereas apices of Al -tolerant seedlings showed less intense staining after equivalent expo sures. Differential staining preceded differences observed in either r oot elongation or total Al concentrations of root apices (terminal 2-3 mm of root). After 4 h or exposure to 100 mu M Al in nutrient solutio n, Al-sensitive genotypes accumulated more total Al in root apices tha n Al-tolerant genotypes, and the differences became more marked with t ime. Analysis of freeze-dried root apices by x-ray microanalysis showe d that Al entered root apices of Al-sensitive plants and accumulated i n the epidermal layer and in the cortical layer immediately below the epidermis. Long-term exposure of sensitive apices to Al (24 h) resulte d in a distribution of Al coinciding with the absence of K. Quantitati on of Al in the cortical layer showed that sensitive apices accumulate d 5- to 10-fold more Al than tolerant apices exposed to Al solutions f or equivalent times. These data are consistent with the hypothesis tha t Alt1 encodes a mechanism that excludes Al from root apices.