Characterization of aluminium-induced citrate secretion in aluminium-tolerant soybean (Glycine max) plants

Recently, we showed that secretion of citrate in an aluminium (Al) tolerant cultivar soybean (Glycine max) (cv. Suzunari) is a specific response to Al stress [Yang et al. (2000) Physiol Plant 110: 72–77]. Here we investigated the intrinsic mechanisms behind the secretion of citrate induced by Al. The amount of citrate secreted during the 24‐h Al treatment period increased with increasing concentration of Al (0–70 μM). We analysed citrate secretion basically under 3 conditions: (1) by varying light‐exposure, (2) with intact or excised shoots and (3) by using a divided chamber technique. Further, the content of organic acids in the tissue and the activity of enzymes involved in organic acid metabolism were analysed and evaluated. The results indicate that high rate of citrate secretion in soybean requires a 4‐h induction period. Al had a continuous effect on the citrate secretion when Al was removed from the treatment solution. Citrate secretion increased steadily under exposure to continuous light. However, when the shoots were excised the citrate secretion rate dropped to 3–6 times that of their control counterparts. Results of root manipulation experiments revealed that citrate secretion required the direct contact of Al. In other words, only the Al‐treated root portions secreted citrate. All these observations suggest that the shoots play a role in Al‐induced citrate secretion. Although shoots may not supply citrate for the secretion upon Al treatment, it seems that they may provide the carbon source and/or energy for citrate synthesis in the root. On the other hand, the root organic acid content (1‐cm apex) indicated that malate might contribute to citrate secretion by keeping the balance between citrate synthesis and release in the root apices. Quantification of enzymes involved in organic acid metabolism showed only a 16% increase in citrate synthase activity upon Al treatments (6 h) with no differences in other enzymes. Hence, we could not rule out completely the potential contribution of citrate from shoots and the results are discussed in the light of shoots contributing either energy or citrate itself for enhanced citrate secretion in the Al‐tolerant plant roots.