Effect of foliar spray of growth regulators on survival and growth of Paulownia fortunei seedlings under drought stress

Paulownia fortunei (Seem.) Hemsl is one of the fastest growing tree species of Paulownia genus, which is a suitable option for wood farming and agroforestry in north of Iran. Drought is one of the most important factors limiting the growth and survival of this species, which can cause severe damage to its performance in the nursery and planting area. This study intends while to investigate the growth and survival response of this species seedling to changes in soil moisture, to consider the possibility of using foliar spray of salicylic acid and ascorbic acid to increase drought resistance of P. fortunei in seedling stage.

In this study, 225 seedlings were selected, and were transplanted in 3 liter pots filled with suitable soil, and were examined after establishment under 3 levels of soil moisture (90%, 60% and 30% of field capacity) and 5 levels of foliar spray (control, 1 and 10 mM ascorbic acid, 100 and 150 mg.l-1 salicylic acid), using two-factor factorial experiment with 3 replications for 4 month. Survival of seedlings and growth characteristics, including stem height, collar root diameter, chlorophyll, and specific area of leaves, and also length and number of main and lateral roots for each seedling were measured at the end of the study. Collected data were analyzed using two-way analysis of variance, Duncan multiple comparison of means, and simple linear regression method at 95% confidence level.

The results showed that in all foliar spraying treatments, a decreasing and significant linear relationship was observed between soil moisture and seedling survival. So that at 30% moisture level (severe drought) salicylic acid foliar spray treatment increased the survival of seedlings by 7 times. Also, lessening of soil moisture from 90% to 60% caused a significant reduction in growth traits of stems and leaves of seedlings, The results showed that the effects of foliar spray on seedling root properties are strongly affected by soil moisture, so that at 90% moisture level, foliar spray reduces root area and number of first order lateral roots, nevertheless, at 30% moisture level, seedlings were treated with foliar spray of salicylic acid at a concentration of 150 mg.l-1 and ascorbic acid at a concentration of 10 mM, showed the highest mean values of root area, total root length and number of first order lateral roots.

P. fortunei seedlings are very sensitive to changes in soil moisture. Also, the use of foliar spray, especially in the treatment of salicylic acid with a concentration of 150 mg.l-1 improves the survival and growth characteristics of this species seedling under water scarcity conditions.