Effect of rice husk biochar on growth and micronutrients concentration of holy basil (Ocimum sanctum L.) under water stress

Biochar as a soil amendment improves soil physical and chemical properties, increase nutrient availability, decrease greenhouse gases, reduce nutrient leaching and ultimately increase crop production. Drought stress is one of the most important factors that limit plants growth. Arid lands have a little organic matter, and because of high pH, micronutrients deficiency was observed in these soils. Therefore, the objective of this study was to investigate the effects of rice husk biochar and water stress on the growth and micronutrients concentration of holy basil (Ocimum sanctum L.). The experiment was conducted in a factorial 3×3 arranged in a completely randomized design with three replications under greenhouse conditions. Treatments consisted of three soil moisture levels (100 (blank), 75, and 55, % of field capacity) and three rice husk biochar levels (0, 2 and 4% W). 10 holy basil seeds were sown in each pot and thinned to six uniform seedlings per pot after 3 weeks. Then, moisture levels applied by weighting the pot each day at growing period. 12 weeks after emergence, SPAD values, shoots height, number of inflorescences, and number of branches and micronutrients concentration in aerial part of plant was determined. Results showed that 75 % of field capacity (FC) did not have a significant effect on dry weight of holy basil plant but shoot dry weight in 55% FC significantly decreased by about 39.7%. Water stresses (55 % of FC) also reduced wet weight, shoot height, number of inflorescences, and number of branches by 49.5, 29.5, 38.9 and 17.8 % as compared to the control, respectively, but increased SPAD values. In 55 % of FC, the average of copper (Cu) and zinc (Zn) concentration significantly decrease by 43.31 and 39.5%, respectively; but the average of iron (Fe) and manganese (Mn) concentration significantly increased by 29.72 and 28.2%, in plant shoots. Generally, application of rice husk biochar significantly reduced the average of shoots zinc (Zn) and copper (Cu) concentration while it increased manganese (Mn) concentration but did not have a significant effect on dry and wet weight of holly basil. Results indicated that 75 % of field capacity (FC) did not have a significant effect on dry weight of holy basil plant. Therefore, deficit irrigation can be considered as a suitable strategy in water limited conditions. Also application of rice husk biochar significantly increase the average of shoots manganese (Mn) concentration but had no effects on dry weight probably due to short term of plant growth, type of biochar, and biochor levels. More investigation using another biochar at other different levels is recommended.