By analyzing the correlations between plant physiological indexes (such as net photosynthetic rate(Pn), transpiration rate (E), stomatal conductance (Gs) and, intercellular CO₂ concentration (Cint)), and environmental factors (such as photon flux density, atmospheric CO₂ concentration, atmospheric temperature, air humidity etc.), and by computing absolute value of differential of the correlation equation, it is found that the correlation between physiologic indexes and environmental factors varies with nitrogen rate, which in turn affects the sensitivity of the physiologic indexes to the environmental factors.