Effect of GA3 and Ethephon On Photosynthesis of Wild Rhododendron Delavayi Franch

Flowering and growth of wild populations of Rhododendron delavayi Franch. could be regulated using GA₃ and ethephon. To better understand their effect, diurnal photosynthesis was monitored using portable LI-6400 photosynthesis system. The photosynthetic active radiations at the site were between 127.4-1257.3 µmol•m^-2•s^-1 and the temperatures ranged from 16.3 and 27.4C. Ambient CO2 concentrations were 307.1-390.4 µmol•mol^-1 and the relative humidity was 80.3% at 7:30am. The results indicated that both GA₃ and ethephon did not alternate the trend of diurnal photosynthesis rates. Under the treatments of 100, 200 mg•L^-1 and control (just pure water), the first peak rates were 11.60, 10.1, 8.29 µmolCO₂•m^-2•s^-1 at 11:30am and the 2^nd peak rates were 9.41, 9.42, 7.42 µmolCO₂•m^-2•s^-1 at 3:30pm, respectively. However, ethephon treatments reduced the photosynthesis rates. As ethephon concentration increased from 100 to 200 mg•L^-1, the photosynthesis rate were much lower. Transpiration rate showed similar trend as the photosynthesis rate under all treatments. But, the highest rate, 4.62 mmolH₂O•m^-2•s^-1, occurred under the treatment of GA₃ 200 mg•L^-1 at 1:30pm. Compared with the control, plants spayed with GA₃ at 100 mg•L^-1 increased water use efficiency (WUE), while reduced WUE at GA₃ 200 mg•L^-1.  Stomata conductance and stomata limitation followed the same trend as the photosynthesis rates. When spaying GA₃ on Rh. delavayi, intercellular CO₂ concentrations (Ci) reduced. If ethephon was applied, Ci was higher than that of the control. It is possible that GA₃ could improve the photosynthesis rates because GA₃ regulated stomata function. Further studies are needed to address how did application of ethephon reduce the photosynthesis rates of Rh. delavayi.