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

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 Al. Lateral shoot development at the nodes of the main stem was extensive in the limed soil containing 0.06 cmol(+) Alkg⁻¹. However, lateral shoot length and weight were severely inhibited in the unlimed soil containing 2.19 cmol(+) Alkg⁻¹, and in the unlimed soil amended to 2.63 cmol(+) Alkg⁻¹ with AlCl₃. 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 μg 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 Al on lateral shoot development were confirmed in solution culture. In addition, differential sensitivity to Al 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 Al up to 30 μM. In contrast, the length of basipetally located first order lateral roots was restricted to greater extent; up to 50% by 30 μM Al. Second order lateral lengths were inhibited even more severely; up to 86% by 30 μM Al. 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 Al toxic conditions. This suggests that one mode of action by which Al 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 Al on total shoot dry matter production implies the impairment by Al toxicity of other root functions, such as ion and water transport, also played an important role in altering shoot morphology.