Aluminum inhibition of shoot lateral branches ofGlycine max and reversal by exogenous cytokinin

Abstract

Aluminum effects on the morphological development of soybean (Glycine max (L.) Merr.) were characterized in greenhouse and growth chamber experiments. An Al-sensitive cultivar, 'Ransom', was grown in an acid soil (Aeric Paleudult) adjusted to 3 levels of exchangeable A1. Lateral shoot development at the nodes of the main stem was extensive in the limed soil containing 0.06 cmol(+)A1 kg -~. However, lateral shoot length and weight were severely inhibited in the unlimed soil containing 2.19 cmol(+)A1 kg -I, and in the unlimed soil amended to 2.63 cmol(+) A1 kg-l with A1C13. This inhibition by the high Al/low pH condition was reversed by the exogenous application of a synthetic cytokinin 6-benzylaminopurine (BA). The daily application of 20 pg mL-1 BA applied locally to the lateral meristems of plants grown in the unlimed soil stimulated lateral shoot growth substantially, such that it was either comparable to or greater than that observed in the limed treatment without BA. Accumulation of K, Ca, and Mg in lateral shoot branches was also stimulated by the local application of BA. The inhibitory effects of A1 on lateral shoot development were confirmed in solution culture. In addition, differential sensitivity to A1 was evident among the primary root, first order lateral roots, and second order lateral roots. The length of the primary root was only slightly decreased by increasing concentrations of A1 up to 30 #M. In contrast, the length of basipetally located first order lateral roots was restricted to greater extent; up to 50~ by 30 pM AI. Second order lateral lengths were inhibited even more severely; up to 86% by 30/~M AI. Substantial evidence in the literature indicates that the root apex is a major site for the biosynthesis of cytokinin that is supplied to shoots, and cellular function and development in this region of the root are impaired during A1 toxic conditions. This suggests that one mode of action by which A1 may affect shoot growth is by inhibiting the synthesis and subsequent translocation of cytokinin to the meristematic regions of the shoot. The present observation of a reversal of Al-inhibited lateral shoot development by exogenously applied cytokinin supports this hypothesis. However, the inability of applied cytokinin to counter the restriction imposed by A1 on total shoot dry matter production implies the impairment by A1 toxicity of other root functions, such as ion and water transport, also played an important role in altering shoot morphology.

Nutrient uptake of soybean genotypes under aluminum toxicity

“…However, when liming exceeded 0.5×Al(ec), Mg uptake in the leaves did not increase (Table 3), but it kept increasing in the roots (Table 2). Increasing Mg uptake in the leaves with increasing Al(ec) was also reported by Pan et al (1989). Increasing Mg uptake in the roots and leaves was triggered by increasing Mg in the soil due to liming.…”

Mechanisms of adaptation of plants to acid soils

“…When the stress applied to the roots of a sensitive genotype is very severe, however, (e.g. at high levels of exchangeable A1) and root regeneration is impaired, shoot development may be inhibited not only by a deficiency in water or mineral nutrients but also by limited root-supply of CYT (Pan et al, 1989). Because of the wide range of stress factors to which roots may be subjected in acid soils plants have developed different strategies (avoidance and tolerance) in adaptation to these soils.…”

The aluminium signal: New dimensions to mechanisms of aluminium tolerance

“…Aluminium-induced changes in concentration have been demonstrated thus far only in the case of ABA (Klimashevskii, 1983) and cytokinins, although it seems probable that the involvement of cytokinins may be directed through altered shoot growth (Pan et al, 1989).…”