Fig. 2. Photosynthesis as a function of irradiance in Atriplex triangularis, expressed as daily carbon
balance per unit leaf area (a), mass (b), and soluble protein content (c) (see text). Within each graph,
arrows indicate the high, intermediate, and low irradiance levels to which leaves were acclimated. In each
graph, leaves acclimated to high irradiance have the greatest photosynthetic rates at high irradiance
levels; those acclimated to intermediate irradiance have intermediate rates; and those acclimated to low
irradiance have the lowest rates. However, leaves acclimated to a given irradiance have the greatest
photosynthetic rate at that irradiance in all cases only if photosynthesis is expressed per unit investment
in mass or soluble protein.
This suggests that expressing photosynthesis and respiration as a function of leaf
mass or protein content, and thereby indirectly incorporating leaf construction costs,
may be more useful in assessing adaptation to light level than expressing them as a
function of leaf area. Reporting photosynthesis per unit area affords ease of analysis
for such traits as quantum yield, but may bias one's perspective by ignoring the effect
of leaf construction costs on whole-plant energy capture. The reason for differences in
the results based on leaf area v. leaf mass or soluble protein is clear: leaves acclimated
to low irradiance levels have low photosynthetic rates per unit area at those levels, but
also have a much lower biomass and soluble protein content per unit area than leaves
acclimated to higher irradiance levels. When photosynthesis is expressed per unit investment
in either leaf mass or soluble protein, the higher rates of return at low irradiances
of leaves acclimated to those conditions become apparent.
Four points should be made in connection with the preceding analysis. First, it does
not incorporate differences between plants acclimated to different irradiances in root or
stem costs, leaf orientation, or total leaf area. Although data do not exist to quantify
root costs (sensu Givnish 1986d), they seem unlikely to affect this particular analysis,
given that calculated transpiration rates per unit leaf mass at different irradiances differ
little between leaves acclimated to those irradiances.
Second, the Michaelis-Menten equations used in the preceding calculations (equation
3, Table 2) provide a reasonable but inexact fit to the data of Bjijrkman et al. (1972b).
Consequently, the conclusions drawn should be viewed as tentative until confirmed
using more sophisticated modelling techniques. Third, although a comparative analysis
of photosynthesis of sun- and shade-acclimated leaves under constant low irradiance
may be warranted for plants grown in growth chambers, such as Atriplex triangularis