Effects of drought stress and humic acid application on flower yield and content of macro-elements in medical plant borage (Borago officinalis L.)

In order to evaluate effects of drought stress and humic acid foliar application on flower yield and concentration of macronutrient elements in borage, an experiment was conducted as a split plots design with three replications in 2012 at Agricultural Research Station of Zabol University. Drought stress treatments after reaching the soil moisture at fifteen centimeters depth to 90, 70 and 50 percent field capacity were used as main plots and humic acid treatment including: 0 or without any humic acid application, 1.5, 3 and 4.5 liter at 1000 L water were used as sub plots. Four foliar application of humic acid used at vegetative and early reproductive stages. Results showed that drought stress had significant effect on flower yield, biological yield, number of branches per plant and the shoot yield respectively. Drought stress increased flower yield until W2 treatment, but by increasing drought stress and rich it to 50 percent field capacity (W3), the flower yield, biological yield, number of branches per plant and shoot yield in borage plants were decreased. Drought stress had significant effect on phosphorus, potassium and calcium concentration in leaves and flower tissues. By increasing drought stress from 90 (control) to 70 percent field capacity, the content of these elements in leaves and flower tissues were increased. Humic acid application had positive effect on flower yield, biological yield, number of branches per plant, shoot yield, plant high and then the phosphorus, potassium and calcium in leaves and flower, and increased them until 1.5 liter of humic acid per thousand liter of water treatment. Based on the results of this study, drought stress until 70 percent field capacity or 30 percent decreasing in soil moisture, had not reduced flower yield and the concentration of nutrient elements in leaves and flower of borage plant. Maximum efficiency of humic acid at all levels of drought stress, obtained by application 1.5 liter of humic acid per thousand liter of water.