The Physiology of Flowering Plants - KHAM PHA MOI

18 FLOW OF ENERGY AND CARBON THROUGH THE PLANTshown in Fig. 2.5. Trees commonly produce ‘sun leaves’ on theoutside of the canopy, and ‘shade leaves’ within it. Shade plants(or leaves) become light-saturated at a much lower PFD than sunplants (or leaves). At low levels of light flux they have higher ratesof photosynthesis than sun plants/leaves, whether the rate is measuredper unit leaf area or per unit weight of chlorophyll. Shadeplantshavealowlight compensation point, the value of irradiance atwhich photosynthesis exactly equals respiration, and below whichrespiration exceeds photosynthesis, leading to a net loss of organicmatter. Numerical values of PFD at the light compensation pointhave been quoted as 20 mmol m –2 s –1 for shade plants, 80 mmol m –2s –1 for sun plants. The shade plants can therefore survive at levelsof light too low to support the growth of sun plants. In deep shadethe PFD can fall below 50 mmol m –2 s –1 . Adaptations to growth inthe shade include thin leaves (see Fig. 9.8) and very pigment-richchloroplasts; there is a high proportion of LHPC to reaction centres,which increases the efficiency of light capture. In bright light theshade plants are relatively inefficient with respect to photosynthesis(Fig. 2.5) because of their low density of reaction centres, and thereforethey are likely to be outcompeted by sun species. The shadeplants are also highly susceptible to photochemical damage bybright light. The capacity for energy dissipation in shade plants islimited, whereas adaptable plants grown at a high PFD showincreased levels of carotenoid pigments.For whole plants, light saturation requires much higher levels ofirradiance than for single leaves, because in an intact plant outerand upper leaves shade inner and lower ones. This shading is kept toa minimum by the arrangement of leaves in ‘leaf mosaics’, leavesarranging themselves so as to shade each other minimally (seeneasily by looking up through the foliage of a tree!). Nevertheless,whereas a single leaf may be light-saturated with c. 25% of fullsunlight, an entire plant may not reach light saturation even withthe PFD of the full midsummer sun. Heavy clouding may bring aplant as a whole to its light compensation point.2.4 The fixation of carbon dioxide2.4.1 The absorption of carbon dioxideGaseous diffusionIn the atmosphere CO 2 is present at an average concentration ofabout 370 mmol mol –1 (see Box 2.2). The leaf provides a large absorbingsurface, and in this surface the stomata provide pores for entry.Within the leaf, the abundant air spaces permit gaseous diffusionbetween the cells and the large internal surface of the leaf is themain area for absorption of the gas into cells. The internal CO 2concentration is kept below the atmospheric by photosynthesis.