photosynthetic rates per unit area at those levels than do leaves of plants restricted or
acclimated to low irradiance levels, and vice versa (Bjorkman et al. 1972a, 1972b; Jurik
et al. 1979; Bjorkman 1981; but see analysis to contrary below). The second assumption
seems more questionable. For example, Bjdrkman (1981) argues that because
leaves with higher concentrations of Rubisco (RuP2 carboxylase-oxygenase) have higher
photosynthetic rates at high irradiance levels-where carboxylation is likely to limit
photosynthesis-the high levels of Rubisco in sun-adapted leaves are adaptive. Con-versely, he argues that low Rubisco levels are adaptive under shady conditions because
Rubisco content is correlated with rates of dark respiration, and hence inversely correlated
with net photosynthesis at low irradiance levels. However, the problem with such