Stomatal, mesophyll conductance and biochemical limitations to photosynthesis as affected by drought and leaf ontogeny in ash and oak trees

Gas exchange measurements were carried out on ash and oak trees in a forest plantation during three whole growing seasons characterized by different water availability (2001, 2002 and 2003). A quantitative limitation analysis was applied to estimate the effects of drought and leaf ontogeny on stomatal (S_L) and non-stomatal limitations (NS_L) to light-saturated net photosynthesis (A_max), relative to the seasonal maximum rates obtained under conditions of optimal soil water content. Furthermore, based on combined gas exchange and chlorophyll fluorescence measurements, NS_L was partitioned into a diffusive (due to a decrease in mesophyll conductance, MC_L) and a biochemical component (due to a decrease in carboxylation capacity, B_L). During the wettest year (2002), the seasonal pattern of both A_max and stomatal conductance (g_sw) was characterized in both species by a rapid increase during spring and a slight decline over the summer. However, with a moderate (year 2001) or a severe (year 2003) water stress, the summer decline of A_max and g_sw was more pronounced and increased with drought intensity (30–40% in 2001, 60–75% in 2003). The limitation analysis showed that during the spring and the autumn periods S_L, MC_L and B_L were of similar magnitude. By contrast, from the summer data it emerged that all the limitations increased with drought intensity, but their relative contribution changed. At mild to moderate water stress (corresponding to values of g_sw > 100 mmol H₂O m⁻² s⁻¹) about two-thirds of the decline in A_max was attributable to S_L. However, with increasing drought intensity, NS_L increased more than S_L and nearly equalled it when the stress was very severe (i.e. with g_sw < 60 mmol H₂O m⁻² s⁻¹). Within NS_L, MC_L represented the main component, except at the most severe water stress levels when it was equalled by B_L. It is concluded that diffusional limitations (i.e. S_L + MC_L) largely affect net assimilation during most of the year, whereas biochemical limitations are quantitatively important only during leaf development and senescence or with severe droughts.