Co-ordination of root salinity and shoot zinc level with rhizosphere organic acid secretion in maize

Zinc deficiency and salinity stress often limits crop production in many calcareous soils. It has been suggested that root organic acids are a response of plants to both conditions. The aim of this hydroponic experiment was to investigate the effect of salinity on organic acids (malic, citric and acetic acids) release by roots in maize plant, under Zinc deficiency conditions. Treatments consisted of three zinc levels (0, 0.5 and 1 μM) and three salinity levels (0, 30 and 60 mM NaCl and CaCl2 in 1:1 ratio). Results showed that mean shoot and root dry matter yields significantly increased with increasing zinc concentration in nutrient solution, but increase in salinity levels significantly decreased mean shoot and root dry matter yields. Salinity had higher detrimental effect on maize shoot dry matter yield compared to those of zinc-deficiency treatments. By increasing zinc concentration and salinity levels in nutrient solution, the rate of malic, citric and acetic acids exudation decreased. The highest rate of malic, citric and acetic acids exudation was 6.61, 1.06 and 0.66 mg g RDW−1 respectively, that observed to the lowest level of zinc (Zinc0) and salinity (0 mM). The highest rate of zinc transport occurred at 60 mM salinity level which had the lowest rate of organic acids exudation. The rate of malic acid was significantly higher than citric acid (4 times higher) and acetic acic (15 times higher). In conclusion, it seems that zinc concentration in maize shoot control the release of root exudation of organic acids. However, it is not clear that the elevated organic acids release under saline condition is due to the lower zinc transport from root to shoot or higher roots membrane permeability or a combination of both mechanisms.