Effects of Foliar Application of Nano Zinc Chelate and Zinc Sulfate on Zinc Content, Pigments and Photosynthetic Indices of Holy Basil (Ocimum sanctum(

Introduction: Holy basil is a perennial plant belongs to Lamiaceae family. The plant is a perennial and thrives well in the hot and humid climate. Its aerial parts have been in use for food, pharmaceuticals, cosmetics and perfumery industries. Leaves contain 0.5-1.5% essential oil and main oil components are eugenol, methyl eugenol, carvacrol, methyl chavicol and1,8-cineole. A balanced fertilization program with macro and micronutrients is very important in producing high quality yield. Zinc is involved in IAA production, chlorophyll biosynthesis, carbon assimilation, saccharids accumulation, reactive oxygen radicals scavenging and finally carbon utilization in volatile oil biosynthesis. Material and methods: In order to evaluate the effect on zinc foliar application on zinc content of leaves, photosynthetic indices and pigments of holy basil, an experiment was carried out in 2013 at a research farm of Horticultural Science, Shahid Chamran University (31°20'N latitude and 48°40'E longitude and 22.5 m mean sea level), Ahvaz (Iran), a region characterized by semi-dry climate. The experiment was arranged based on Randomized Complete Block Design (RCBD) with six treatments and three replications. The treatments were nano zinc chelate (0, 0.5, 1 and 1.5 g.l-1) and zinc sulfate (1 and 1.5 g.l-1) fertilizers. Land preparation includes disking and the formation of raising beds (15cm high and 45cm wide across the top) using a press-pan-type bed shaper. Holy basil seeds were sown on two rows on each bed, with 15 cm in-row and 40 cm between-row spacing. The plants were irrigated weekly as needed. Foliar application of zinc fertilizers was done at six-eight leaf stage and were repeated with interval 15 days until full bloom stage. Zinc content, stomata conductance (gs), CO2 under stomata (Ci), transpiration rate (E), net photosynthesis (Pn), light use efficiency (LUE), water use efficiency (WUE) and also chlorophyll a, chlorophyll b, chlorophyll a+b and carotenoid contents were measured at beginning of flowering stage. Photosynthetic parameters were measured by Infra-red gas analyzer (LCA4, ADC Co. Ltd., Hoddesdon, UK). Instantaneous water use efficiency (WUEinst) was calculated as Pn/E ratio. Light use efficiency was calculated as Pn/PPFD ratio. Result and discussion: The results showed that the effect of foliar application of zinc fertilizers on all measured traits except Ci and WUE was significant (p≥0.01). The highest values of zinc content (110.53 mg.kg-1), chlorophyll a (0.99 mg.kg-1 fresh weight), chlorophyll b (0.30 mg.kg-1 fresh weight), chlorophyll a+b (1.29 mg.kg-1 fresh weight) and carotenoid (0.18 mg.kg-1 fresh weight) traits were obtained in plants sprayed with 1.5 g.l-1 nano zinc chelate. The lowest amount of zinc content (21.37 mg.kg-1), chlorophyll a (0.58 mg.kg-1 fresh weight), chlorophyll b (0.14 mg.kg-1 fresh weight), chlorophyll a+b (0.72 mg.kg-1 fresh weight) and carotenoid (0.13 mg.kg-1 fresh weight) traits were obtained in control plants. Foliar application of holy basil with 1.5 g.l-1 nano zinc chelate led to increase in stomata conductance (322.22 mm H2O.m-2.s-1), transpiration rate (2.86 mm H2O.m-2.s-1), net photosynthesis (11.75 μm CO2.m-2.s-1) and light use efficiency (6.10 μm CO2. μm photon-1). The minimum amount of stomata conductance (172.00 mm H2O.m-2.s-1), transpiration rate (2.16 mm H2O.m-2.s-1), net photosynthesis (8.23 μm CO2.m-2.s-1) and light use efficiency (4.46 μm CO2. μm photon-1) were observed in control plants. There were positive correlation (p≥0.01) between zinc content and chlorophyll a, chlorophyll b, chlorophyll a+b and carotenoid. Zinc content also had positive and significant correlation (p≥0.01) with stomata conductance, CO2 under stomata, transpiration rate, net photosynthesis and light use efficiency. So, providing zinc by foliar application with 1 and 1.5 g.l-1 nano zinc chelate and 1.5 g.l-1 zinc sulfate can lead to increase in chlorophyll and carotenoid contents. Increase in net photosynthesis may be due to higher photosynthesis pigments and also stomata conductance and CO2 under stomata. In the other hand, zinc is an essential micronutrient that acts either as a metal component of various enzymes or as a functional, structural, or regulatory cofactor such as carbonic anhydrase, ribulose 1, 5-bisphosphate carboxylase/oxygenase and fructose-1, 6-bisphosphate, and is thus associated with saccharide metabolism and photosynthesis. Conclusion: Although the highest amount of most measured traits was obtained in plants that treated with 1.5 g.l-1 nano zinc chelate, there were no significant difference between 1 and 1.5 g.l-1 nano zinc chelate and 1.5 g.l-1 zinc sulfate treatments for zinc content, stomata conductance, CO2 under stomata, transpiration rate, net photosynthesis, water use efficiency and light use efficiency. Therefore, in order to increase zinc content and photosynthetic traits of holy basil, foliar application of with 1.5 g.l-1 zinc sulfate is recommended.