Previous studies on the photosynthesis of crops indicated that the yield of the crops depended mainly on
the photosynthetic area, the photosynthetic rate, the photosynthetic time and the distribution and
translation of the photosynthates. It was a law there was positive correlation not negative correlation
between the photosynthetic rate and the crop yield, but the law performed only on the condition that other
excellent physiological function and economic characters combined with various stress resistance [13]. In
this study, the improvement of yield and stevioside content in T2 was significant, because the vegetative
growth stage was enough long and the Pn in T2 was the highest of all treatments. The results in this study
coincided with the former studies.