Effects of three plant growth regulators on the photosynthetic indicesindexes of giant juncao under different drought stress conditions

Using giant juncao seedlings as an experimental material, we measured the rates of net photosynthesis (P_n), transpiration (T_r), stomatal conductance (G_s), and water-use efficiency (WUE) using CIRAS-3. We analyzed the effects of different concentrations of the growth regulators methyl jasmonate (MeJA), 3-indoleacetic acid (IAA), and zeatin (ZT), applied via foliar spraying, on the photosynthetic indices of giant juncao subjected to drought stress induced by 15% PEG. The results revealed that under drought stress alone, plant growth was significantly reduced, and there were significant reductions in P_n, T_r, G_s, and WUE compared with control plants. However, spraying stressed plants with different concentrations of MeJA, IAA, and ZT resulted in significant increases in P_n, T_r, G_s, and WUE after spraying with 25 μmol·L⁻¹ MeJA, 510 μmol·L⁻¹ IAA, and 6 μmol·L⁻¹ ZT compared with stressed control, and these were established to be the optimal concentration of the respective growth promoters Furthermore, by spraying plants with 25 μmol·L⁻¹ MeJA, 510 μmol·L⁻¹ IAA, and 6 μmol·L⁻¹ ZT under different levels of drought stress and at different periods of time, we found that the optimal time for applying MeJA was during the second half of the stress period, between 8 and 24 h after stress onset. In contrast, application of IAA and ZT during the first half of the stress period, between 1 and 8 h, was found to be the most effective. In conclusion, this study revealed the effect mechanism of MeJA, IAA and ZT on photosynthetic indexes of giant juncao under drought stress, which provided a reference for studying its response mechanism to giant juncao under drought stress.