A study of some factors associated with aluminium uptake by three plant species

Growth of many plant species may be limited in acid soils by aluminium excess which may be alleviated by applications of lime (calcitic or dolomitic) and phosphate fertilizers. The nature of the aluminium response is not fully understood because the factors associated with low pH-aluminium excess on plant growth and the processes involved in aluminium uptake are not completely documented. The aim of this project was to examine these factors and provide evidence which would account for aluminium uptake and translocation using three plant species, cabbage (Brassica oleracea var. capitata (L.) Alef. cv. Ballhead hybrid), lettuce (Lactuca sativa L., cv. Pennlake) and kikuyu (Pennisetum clandestinum Chiov. cv. Whittet). Aluminium uptake by excised roots consisted of two phases, rapid adsorption where most of the calcium was exchanged, followed by a slow accumulation phase that was pronounced for cabbage and lettuce and almost absent for kikuyu. Aluminium uptake in Phase I was considerably higher at pH 4.2 than at 4.0; this could have resulted from a decrease in net charge per aluminium atom, which could be expected at the higher pH. Greater dissociation of carboxyl groups at the higher pH may have also contributed to higher aluminium uptake. The effect of temperature and a metabolic inhibitor indicated that the entire uptake process was non-metabolic. Succinic-tartaric acid buffer desorbed most of the aluminium from roots. The small amount remaining was either associated with the cytoplasm and/or irreversibly bound to exchange sites. EDX-analyses (cell wall region) of freeze-fractured, dried roots from all species demonstrated that aluminium was present in all tissues throughout the epidermis, cortex and stele and along the entire length of roots. The highest concentrations were recorded in the epidermis followed by the cortex. Aluminium was also recorded in the stele and in the protoplasm of cortical cells for all species. The distribution was consistent with transport in the symplasm where aluminium was present in the radial wall(cytoplasm) of the endodermis and also with passive movement through meristematic cells hence pypassing the barrier at the endodermis. High calcium application reduced aluminium levels in the protoplasm of some xylem parenchyma and cortical cells. There was a poor correlation between aluminium and phosphorus levels in the cell walls of all tissues. Th~ yield of roots and tops of kikuyu, in contrast to cabbage and lettuce, was relatively unaffected by low pH (4.0 vs. 4.6) and aluminium compared with the yield of control plants. The control treatment level of calcium was markedly lower and the magnesium level markedly higher for kikuyu compared with cabbage and lettuce. The tolerance of kikuyu to aluminium was not associated with lower alu~inium levels of roots than cabbage and lettuce but was associated with significantly lower levels of tops. Aluminium levels of roots were higher at pH 4.6 than 4.0 which was consistent with the excised root results. Results for tops were also consistent for all species where levels were lower at the higher pH. High calcium application had no effect on aluminium levels of roots but reduced levels of tops. This supports the previous results where calcium had little effect on aluminium adsorption during Phase I, but reduced accumulation during Phase II where passive movement into the cytoplasm and transport to the stele occurs. High calcium increased the root yield of cabbage and lettuce and reduced top yield of kikuyu. This treatment overcame the inhibitory effect of aluminium on the root and top yield of cabbage and the root yield of lettuce. The magnesium levels of roots and tops were reduced by high calcium for all species. Aluminium increased phosphate levels of roots for cabbage and kikuyu, but had no consistent effect on levels of tops.