Why does photosynthesis decrease with needle age in Pinus pinaster?

Rates of photosynthesis vary with foliage age and typically decline from full-leaf expansion until senescence occurs. This age-related decline in photosynthesis is especially important in species that retain foliage for several years, yet it is not known whether the internal conductance to CO2 movement (g(i)) plays any role. More generally, g(i) has been measured in only a few conifers and has never been measured in leaves or needles older than 1 year. The effect of ageing on g(i) was investigated in Pinus pinaster, a species that retains needle for 4 or more years. Measurements were made in autumn when trees were not water limited and after leaf expansion was complete. Rates of net photosynthesis decreased with needle age, from 8 mu mol m(-2) s(-1) in fully expanded current-year needles to 4.4 mu mol m(-2) s(-1) in 3-year-old needles. The relative limitation due to internal conductance (0.24-0.35 out of 1) was in all cases larger than that due to stomatal conductance (0.13-0.19 out of 1). Internal conductance and stomatal conductance approximately scaled with rates of photosynthesis. Hence, there was no difference among year-classes in the relative limitations posed by internal and stomatal conductance or evidence that they cause the age-related decline in photosynthesis. There was little evidence that the age-related decline in photosynthesis was due to decreases in contents of N or Rubisco. The decrease in rates of photosynthesis from current-year to older needles was instead related to a twofold decrease in rates of photosynthesis per unit nitrogen and V-cmax/Rubisco (i.e., in vivo specific activity).