Revisiting the role of organic acids in the bicarbonate tolerance of zinc-efficient rice genotypes

It has been hypothesised that enhanced organic acid release from the roots of zinc-efficient rice (Oryza sativa L.) genotypes plays a strong role in plant tolerance to both bicarbonate excess and Zn deficiency. To address several uncertainties in the literature surrounding the tolerance of rice to bicarbonate, we initially assessed the tolerance of six rice genotypes to bicarbonate stress under field conditions and in solution culture. The landrace Jalmagna and its recombinant inbred offspring, RIL46, consistently performed better in terms of maintenance of biomass and root length under high bicarbonate concentrations. In the hydroponic experiments, increased root malate (but not citrate) accumulation and efflux were responses to high solution bicarbonate in the short-term (12 h) in all genotypes. Although both citrate and malate accumulation and efflux increased after long-term exposure (10 days) to high bicarbonate and Zn deficiency, it coincided with amino acid leakage from the roots. Partial least-squares regression showed that this leakage consistently ranked highly as an indicator of poor plant health under all stress conditions, whereas specific malate efflux (the ratio of malate to amino acid efflux) was an important predictor of good plant health. The root leakage of Zn-inefficient genotypes under bicarbonate and dual stress (bicarbonate with low Zn) was typically higher than in Zn-efficient genotypes, and coincided with higher peroxide concentrations, suggesting that bicarbonate tolerance is related to the ability of Zn-efficient genotypes to overcome oxidative stress, maintain root membrane integrity and minimise root leakage.