Role of exogenous glycinebetaine and humic acid in mitigating drought stress-induced adverse effects in Malus robusta seedlings

Glycinebetaine (GB) and humic acid (HA) are 2 commercial growth regulators that are being used worldwide to enhance the tolerance of most crops, including fruit trees, to various stresses, including drought stress (DS). Malus robusta Rehder is considered as one of the most important rootstocks for apple in China, but its growth and productivity is believed to be adversely affected by DS. The effects of different concentrations and combinations of GB (0, 100, and 200 mg L-1) and HA (0, 500, 1000, and 1500 mg L-1) on M. robusta seedling growth, photosynthesis characteristics, osmolyte accumulation, and antioxidant responses were evaluated under DS and non-DS conditions. GB and HA applied exogenously to drought-stressed M. robusta seedlings increased total dry matter, net photosynthetic rate, free proline content, endogenous glycinebetaine content, soluble sugar content, and potassium content as well as the activities of superoxide dismutase, peroxidase, and catalase. However, GB and HA decreased stomatal conductance and malondialdehyde content. The above-mentioned responses were greater for combined application of GB and HA as compared to application of only GB or HA. The best responses for most of the above parameters were with the application of 100 mg L-1 GB and 1500 mg L-1 HA. These results suggest that application of GB and HA could mitigate the deleterious effects of DS on Malus seedlings and offer an efficient, economical, and simple means to enhance DS tolerance of the apple rootstock.