A Day in the Life of a Tree Leaf: Dynamic Acclimation of Leaf Performance to Short-Term Environmental Changes

The effect of sunflecks on the photosynthetic performance of leaves (e.g. of a tree crown) has been discussed controversially with respect to the question as to whether a frequent change in the light intensity enhances or decreases their photosynthetic efficiency. The basis of such considerations is the observation that once a leaf is in a photosynthetically active state, the increase in the rate of photosynthesis is fast (occurring in a few seconds), while the return to the “shade state” is in the range of minutes, resulting in overall elevated efficiency (Fig. 3.16c).

Fig. 3.16. Photosynthetic CO₂ net uptake by leaves of young Podocarpus trees (shade leaves) under the canopy of an old evergreen tropical forest. a Light response curve. The data show the mean values for 2-5 saplings per site with three repetitions per leaf (± standard error (S.E.)). b, c Analysis of the sunfleck effect on photosynthetic CO₂ uptake by two artificial light conditions, providing the same amount of photo-synthetically active radiation (PAR) over an identical time span. b PAR provided as a constant photon flux density of 83 µmol m^-2 s^-1. c PAR provided as intermittent lightflecks lasting 30 s at an intensity of 200 or 400 µmol m^-2 s^-1 superimposed on a basic intensity of 40 µmol m^-2 s^-1. (Modified from Strobl et al. (2011))

On the other hand, such a consideration must also take note of the nonlinear quantum efficiency of varying light intensities, which is described by the light response curve of net CO₂ uptake (Figs. 3.10 and 3.16a). Sunflecks from direct sunlight commonly surpass the range of the linear or nearly linear relation between light intensity and photosynthetic CO₂ uptake. In the examples shown in Figs. 3.16a and 3.17a for the tropical gymnosperm Podocarpus falcatus, the increase in photosynthetic CO₂ uptake was minimal beyond a photosynthetically active radiation (PAR) intensity of about 400 pmol quanta m^-2 s^-1.

Fig. 3.17. A day under the canopy in the natural forest. Daily courses of a CO₂ net uptake and ambient photosynthetically active radiation (PAR), and b stomatal conductance (g_s) and transpiration (E) of young Podocarpus leaves and of air temperature on a sunny day (24 November 2006). (Modified from Strobl et al. (2011))

The sunflecks, however, reached up to 1500 pmol quanta m^-2 s^-1 (Fig. 3.17a). Therefore, the sunflecks were of lower photosynthetic quantum efficiency than low light. When the same amount of PAR was supplied to a Podocarpus leaf over the same time period as continuous radiation of low intensity (Fig. 3.16b) or as artificial sunflecks (termed lightflecks; Fig. 3.16c), the photosynthetic gain was obviously higher under continuous low light. Under natural conditions, not only the light intensity varies (Fig. 3.17) but also the conductivity (g_s) of the stomata responding to the water status of the leaves.

When stomatal conductivity is low, as it generally is in the afternoon, the photosynthetic efficiency of sunflecks is even lower because of a low internal CO₂ concentration. This in turn indicates the importance of energy dissipation by the chloroplasts. The actual quantum efficiency of CO₂ assimilation over an entire day was only 72% of the (theoretical) quantum efficiency of the same amount of quanta administered over the same time period, when provided as continuous low light (Table 3.2).

Table 3.2. Effect of sunflecks on the photosynthetic gain of a tree. Photosynthetic efficiency of the subcanopy light climate compared with virtual constant illumination of the same PAR magnitude applied over the same time period to leaves of Podocarpus falcatus (Modified from Strobl et al. (2011))

This difference is, of course, not a constant. However, it indicates the extent to which the real light climate is less effective than artificial illumination. Continuous artificial low light mimics, to some extent, the diffuse radiation in the shade of a tree canopy which, for an entire forest canopy, is photosynthetically more efficient than direct irradiation (Mercado et al. 2009). In comparison with the mentioned minimal quantum requirement for photosynthetic CO₂ assimilation of 9.2 µmol quanta per µmol CO₂, the data in Table 3.2 suggest a sixfold lower quantum efficiency (54 mole quanta per mole of CO₂). Under adverse environmental conditions the apparent quantum efficiency can further decrease tremendously—for example, when stomata are closed under drought and merely 1-2% of the incident visible light can be used for photosynthesis (Osmond et al. 1997).

It is important to note that quantum efficiency has a different meaning in the ecological context than it does in the photosynthetic light reaction, where only those quanta that conduct photochemistry are counted. The difference is due to the energy of a high proportion of absorbed quanta being dissipated as heat under natural conditions.