Influence of Zinc deficiency on root exudation of organic acids in two maize genotypes

 Zinc deficiency is aggravated mainly in arid and semi-arid regions, due to low organic matter and soil moisture as well as high levels of pH and salinity. Maize which serves as staple food is sensitive to Zn deficiency. One of the mechanisms by which plants can adapt to nutrient deficient soils has suggested producing and secreting organic substances, including aliphatic low molecular weight organic acids, into the rhizosphere for mobilization and uptake of nutrients. Under Zn deficiency, plants tend to modify rhizosphere in order to increase Zn phyto-availability. Zinc mobilization efficiency is dependent upon the amount and type of organic acids exuded by plant roots and physiochemical properties of soil. Therefore, the objectives of the present study were to investigate the influence of Zn deficiency on the shoot and root dry matter yields and the release rate of organic acids (malic, citric and acetic acids) commonly identified in root exudations of maize under Zn deficiency conditions. Seeds of maize (SC703 and SC704) were surface sterilized and germinated in perlite moistened with distilled water. After seven days, the seedlings were transferred to 5-L containers with continuously aerated nutrient solution. Three Zn levels (0, 0.5, and 1 µM) were added to nutrient solutions. Ten weeks after maize emergence, intact plants were removed from nutrient solution and after two hours of the onset of the light period, roots samples were in opaque vessels containing fresh solution.  The volume of collected solution was sufficient to submerge the whole maize roots samples. After three hours, roots samples were removed from the vessel and solution containing roots exudates was filtered and frozen at −20 ◦C until analysis of organic acids was performed. Organic acids were analyzed using high performance liquid chromatography (HPLC). Organic acids in the samples were identified by comparison with the retention time and absorption spectra of pure standards including malic, citric and acetic acid. The 1-cm washed root segments were placed in a beaker containing 10 mL deionized water and then root samples were immersed at 30◦C for three h, and then conductivity of solution was measured. The samples were boiled for 2 min, cooled to room temperature (25◦C) and then EC samples were measured. The electrolyte leakage was calculated as follows:   Where C1 and C2 are electrical conductivities measured before and after boiling, respectively.  Roots and shoot samples were ignited at 580 ◦C in an oven for 5 h and Zn concentration measured using atomic absorption spectroscopy (AAS). In both genotypes shoot dry matter yield (SDMY) was significantly improved with increasing Zn concentration in nutrient solution. The highest value of SDMY was 19.8 g and belonged to Zn-adequacy level (1 µM) in SC703 genotype which had no significant difference with SC704 under the same treatments. There was no significant difference between Zn-sufficient and Zn-deficiency (0.5 µM) in SDMY in genotype SC703 whereas, a significant difference was observed at the same treatments in genotype SC704. The lowest value of SDMY was 14.7 g and belonged to the Zn-free treatment for genotype SC704. Root dry matter yield (RDMY) significantly increased with increasing Zn concentration in nutrient solution in both genotypes. The highest value of RDMY was 9.6 g and belonged to the treatment of Zn-adequacy for SC703 genotype which had no significant difference with SC704 genotype under the same treatment. The lowest value of RDMY was 4.8 g which was observed in Zn-free treatment for SC704 genotype. Results showed that the rate of organic acid exudation in both Maize genotypes decreased with increasing Zn levels in nutrient solution. The highest rate of MA exudation (6.6 mg /g root dry weight) was observed in Zn-free (Zn0) treatment in SC703 genotype and the lowest rate (1.98 mg g RDW-1) was observed in 1µm Zn treatment in SC704 genotype. Similar to MA, the rate of citric acid (CA) exudation rate significantly decreased with increasing Zn levels in nutrient solution. The highest rate of CA exudation rate was 1.06 (mg gRDW-1) and observed in Zn-free (Zn0) SC703 genotype. The lowest rate of CA was observed in 1µm Zn treatment SC704 genotype 0.2 (mg gRDW-1). The concentration of acetic acid (AA) was below the detection limit of HPLC in Zn sufficient and Zn deficiency treatments. However, AA concentrations in Zn-free were 0.66 and 0.25 (mg gRDW-1), respectively in SC703 and SC704 genotypes.    The rate of MA was significantly higher than CA (4times) and AA (15 times higher). All organic acids exudation rate decreased with increasing Zn concentration in nutrient solution. There was a negative relationship between root and shoot Zn concentration with MA and CA exudation rate. MA, CA and AA exudation rate decreased as the concentration of Zn increased in root and shoot of maize. Roots membrane permeability decreased with increasing Zn concentration in nutrients solution which led to the reduction in root exudations. In both maize genotypes, the highest rate of root membrane permeability belonged to the Zn-free treatment (Zn0) which had the highest root exudation of organic acid and the lowest rate was observed in1µM Zn level  with the lowest rate of organic acid exudation. It seems that Zn concentration in maize shoot control the release of root exudation of organic acids. In general, based on the results it can be concluded that SC703 genotype was more tolerant to Zn-deficiency compared to SC704 genotype partly due to the higher release rate of root organic acids. Further investigation is required to fully understand the physiology of organic acids release under Zn deficiency conditions.