The response of lucerne and red clover roots to aluminium/hematoxylin: how universal is the hematoxylin test for aluminium?

Abstract

Despite the long association between hematoxylin and aluminium (Al) in studies of Al rhizotoxicity, it is evident that many aspects of the hematoxylin test are not fully understood. Controlled environment studies of the responses of lucerne and red clover roots to the Al/hematoxylin system have indicated that the intensity and distribution of the stain in root tissue was influenced by, (1) the pH of the growing medium, and (2) the age of the root at the time of the treatment. In the younger (≦ 120 h), Al-treated (30 μM; 24 h) roots the highest intensity of hematoxylin stain was encountered at pH 4.1. In contrast, some roots (pH 4.5 ≦ 72 h) did not respond at all to hematoxylin, although growth studies indicated that a physiologically active Al component was present in these roots. Intermediate pH's (4.3) resulted in intermediate stain intensities and a spiral motif of root stain was present in many of these roots (72 h and 120 h). The roots' affinity for hematoxylin increased with age. At 192 h, the pH-dependent differences found in the younger roots had disappeared and Al-treated roots across the pH spectrum now showed similar intensities of root stain. Moreover, some roots in 0 Al treatments (144 h) also responded positively to hematoxylin. Root age-dependent increases in root stainability in hematoxylin for both the +AI and -Al conditions coincided with severe retardation of root growth. These unexpected findings are discussed in terms of redefining roles for Al and root growth inhibition in modulating the response of plant roots to hematoxylin.