Temporal dynamics of SPAD readings over leaf lifespan fitted to a piecewise function.
Leaf lifespan was divided into three growth stages: growth, function, and senescence. The leaf age difference is the difference in leaf physiological age between upper and lower leaves. The dynamics of leaf SPAD readings of upper and lower leaves were assumed to follow the same trajectory (piecewise function). Angle β served as an index of the SPAD reading distribution gradient.