Dynamics of the photosynthetic capacity during the lifetime of leaves in three subtropical tree species.

Two important functional traits of plants, the photosynthetic capacity and leaf lifespan, are usually negatively correlated. To compare the lifetime pattern of photosynthetic activity in leaves of Bischofia javanica, Gordonia axillaris, and Machilus japonica var. kusanoi, we monitored the dynamics of the photosynthetic capacity of leaves from a young age to full expansion then abscission. The mean leaf lifespans of B. javanica, G. axillaris, and M. japonica were 224±7, 345±21, and 515±28 d, respectively. Leaves of B. javanica had fully expanded at 27 d of leaf age and had reached a stage of optimal photosynthesis (see below), while leaves of G. axillaris and M. japonica had fully expanded at around 20 d but did not reach the optimal photosynthetic stage until 32 and 59 d, respectively. No significant differences in the maximum photosynthetic capacity (A(subscript max)) were observed among the 3 species, and all were in the range of 12.7 to 13.8 μmol m^(-2) s^(-1). The photosynthetic capacity was maintained above 90% of A(subscript max) (referred to as a stage of optimal photosynthesis) for only 40~50 d in the 3 species. The photosynthetic capacity linearly declined as leaves aged in B. javanica. However, leaves of both G. axillaris and M. japonica maintained their photosynthetic capacity steadily at 30~40% of A(subscript max) for about 5 mo at the leaf ages of 200~350 d. Our study showed that leaves with a longer lifespan did not always have a significantly lower photosynthetic capacity than leaves with a shorter lifespan in either interspecific or intraspecific comparisons, a result which differs from findings of other researchers. Regression analysis of the photosynthetic capacity vs. leaf age of B. javanica (with a shorter leaf lifespan) yielded a slope steeper than that of G. axillaris and M. japonica (with longer leaf lifespans). Compared within each species, leaves with a shorter lifespan having steeper slopes was only found in leaves of G. axillaris, but not in those of the other 2 species.