To describe the relationship between threshold SPAD
values and the growing season (DAE), we attempted to
fit the experimental data to linear, exponential, logarithmic
and quadratic equations. We observed that the quadratic
equation provided the statistically best fit to the data. The
continuous variation in trends of threshold SPAD values with
the growing season of cultivar Kexin 1 at Linxi, Chifeng City
and Sanjianfang, Wuchuan City of Inner Mongolia in 2010,
and at Xiatuhai, Wuchuan City of Inner Mongolia in 2009
are shown in Fig. 1. No signifiant difference exists among
the three equations, although they showed some variation
between the sites, indicating that the thresholds determined
by the regression equation did not differ with sites for the
same cultivar. Therefore, the data from the three sites were