Effects of Plant Growth Regulator S-Y on Diurnal Changes in Photosynthetic Parameters and Yield of Stevia Rebaudina Bertoni

Selecting the variety of QingTian 2, the effects of plant growth regulator S-Y(PGR-S-Y) on diurnal changes in photosynthetic parameters and yield of Stevia rebaudina Bertoni were studied in the open field. The results indicated that the appropriate concentration of PGR-S-Y (T2) significantly increased the SPAD, the utilization rate of CO2 and the Photosynthetic rate (Pn) respectively, and restricted the midday depression. The effect on the vegetative growth stage was not significant. It significantly increased the fresh leaf weight, the dry leaf weight and the stevioside content in T2 than CK by 10.24%, 11.09% and 4.74% respectivly. But when the PGR-S-Y content was over high (T3), the utilization rate of CO2, the photosynthetic rate (Pn) and photosynthate decreased. At the same time, the vegetative growth stage shortened seriously, the yield and the stevioside content decreased significantly.