Light saturation curves of photosynthesis for plants of Sinapis alba grown either under strong illumination, 'light (sun) plants' (dashed lines), or weak illumination, 'shade plants' (solid lines). The rate of photosynthesis changes similarly with change of irradiance whether expressed per unit of leaf area or unit of chlorophyll, showing that the differences in rates between sun and shade plants do not just result from a difference in total chlorophyll per unit area of leaf. Where the curves cut the x-axis is the light compensation point, below which respiration exceeds photosynthesis (negative CO2 uptake = CO2 output); this point lies at a lower irradiance for the shade plants. From Grahl &Wild (1972).
C02 h"1 dm"2
harmlessly (ultimately to heat) by reactions involving carotenoids in the thylakoids and preventing the buildup of ROS (Bartley & Scolnick 1995). When the PFD is too high to be counteracted by this process, then damage does occur, though it may still be repairable if not too extreme.
The irradiance level at which saturation occurs depends on a number of factors. If the temperature is very low, for instance, light saturation is reached at a low PFD: the rate of thermochemical reactions soon becomes limiting. Similarly, at low CO2 concentrations, light saturation is reached once CO2 has become limiting. Conversely, at higher temperatures and higher levels of CO2, light saturation is reached at a higher PFD. However, other conditions being equal, significant differences in light saturation values are shown by individual photosynthesizing systems. Some species, e.g. plants of forest floors, are obligate shade plants, able to live only at low irradiance levels; examples include dog's mercury (Mercurialis perennis) and the enchanter's nightshade (Circaea luteti-ana). There are also obligate sun plants, such as the aptly named sunflower (Helianthus annuus) and the daisy (Bellis perennis), plants of open habitats. Such species are genetically adapted for extremes of sun or shade. But for many species individuals can adjust appreciably to the light levels to which they are exposed during growth, as shown in Fig. 2.5. Trees commonly produce 'sun leaves' on the outside of the canopy, and 'shade leaves' within it. Shade plants (or leaves) become light-saturated at a much lower PFD than sun plants (or leaves). At low levels of light flux they have higher rates of photosynthesis than sun plants/leaves, whether the rate is measured per unit leaf area or per unit weight of chlorophyll. Shade plants have a low light compensation point, the value of irradiance at which photosynthesis exactly equals respiration, and below which respiration exceeds photosynthesis, leading to a net loss of organic matter. Numerical values of PFD at the light compensation point have been quoted as 20 mmol m-2 s-1 for shade plants, 80 mmol m-2 s-1 for sun plants. The shade plants can therefore survive at levels of light too low to support the growth of sun plants. In deep shade the PFD can fall below 50 mmol m-2 s-1. Adaptations to growth in the shade include thin leaves (see Fig. 9.8) and very pigment-rich chloroplasts; there is a high proportion of LHPC to reaction centres, which increases the efficiency of light capture. In bright light the shade plants are relatively inefficient with respect to photosynthesis (Fig. 2.5) because of their low density of reaction centres, and therefore they are likely to be outcompeted by sun species. The shade plants are also highly susceptible to photochemical damage by bright light. The capacity for energy dissipation in shade plants is limited, whereas adaptable plants grown at a high PFD show increased levels of carotenoid pigments.
For whole plants, light saturation requires much higher levels of irradiance than for single leaves, because in an intact plant outer and upper leaves shade inner and lower ones. This shading is kept to a minimum by the arrangement of leaves in 'leaf mosaics', leaves arranging themselves so as to shade each other minimally (seen easily by looking up through the foliage of a tree!). Nevertheless, whereas a single leaf may be light-saturated with c. 25% of full sunlight, an entire plant may not reach light saturation even with the PFD of the full midsummer sun. Heavy clouding may bring a plant as a whole to its light compensation point.
2.4 I The fixation of carbon dioxide
2.4.1 The absorption of carbon dioxide Gaseous diffusion
In the atmosphere CO2 is present at an average concentration of about 370 mmol mol-1 (see Box 2.2). The leaf provides a large absorbing surface, and in this surface the stomata provide pores for entry. Within the leaf, the abundant air spaces permit gaseous diffusion between the cells and the large internal surface of the leaf is the main area for absorption of the gas into cells. The internal CO2 concentration is kept below the atmospheric by photosynthesis.
The driving force for the inward movement of CO2 is the concentration gradient, ACO2, between the sites of fixation and the external atmosphere:
The ACO2 is equivalent to a gradient of free energy: higher concentration is equivalent to higher free energy. The steepness of the gradient depends on both the external and internal concentrations. Under field conditions, photosynthesizing plants do not deplete the CO2 supply in their vicinity greatly, for air mixes rapidly (though see below on boundary layers); however, CO2 concentrations of about 270 mmol mol-1 have been measured within a crop.
During diffusion to the photosynthetic sites, the CO2 molecules encounter resistances at various points: at the boundary air layer just outside the leaf; at the cuticle; at the stomata; and in the mesophyll. Since CO2 concentration is often the limiting factor in photosynthesis, these resistances can determine the photosynthetic rate.
The boundary-layer resistance is the result of a layer of relatively still air, also known as the unstirred layer, immediately adjacent to the outside of the leaf. In this layer the CO2 concentration is lower than in the bulk atmosphere owing to depletion by the leaf. Its presence has the effect of decreasing the effective ACO2. In still air, a relatively thick boundary layer builds up over a plant surface and this slows down the rate of diffusion of CO2 into the leaf; but usually there is sufficient air movement to keep the boundary-layer resistance low.
The cuticle, which forms a continuous layer over the epidermis, presents a very high resistance to CO2 diffusion. As long as the stomata are open at all, the proportion of CO2 entering through the cuticle is very small.
The mesophyll resistance is a combination of all the resistances that a CO2 molecule meets while diffusing through the mesophyll air spaces, the cell walls, the plasma membrane, the cytosol and the chloroplast envelope, until it finally reaches the carboxylation sites within the chloroplast. This resistance accordingly depends on leaf structure and is more or less fixed once growth of the leaf has ceased.
The stomatal resistance depends on stomatal density (number per unit area) and the size of the stomatal pores. The stomatal density, like mesophyll structure, is fixed during leaf development. But the size ofthe pore is variable: stomata respond to several stimuli by 'stomatal movements', i.e. by opening or closing (partly or fully). The stomatal resistance is under physiological control.
There are several ways of expressing the atmospheric concentration of CO2 or other gases. The SI unit is used here, mmol mol 1, micromoles per mole. Another unit in frequent use is ppm, parts per million, numerically equal to mmol mol 1 (since 1 mmol = 1 millionth of a mole). Other alternative units are % (per cent, parts per 100), or partial pressure as Pa, Pascals. Thus 370 mmol mol 1 = 37 Pa = 370 ppm = 0.0370%.
The rate of diffusion is inversely proportionalto the resistance, R. If we denote the rate of entry of CO2, which equals the rate of photosynthesis, by P, then
R comprises components contributed respectively by the stomata, Rs; the cuticle, Rc; the boundary air layer, Ra; and by the mesophylltissue, Rm. (Some authors refer to conductances rather than resistances; conductance is the reciprocalof resistance, 1/R.) Because the stomataland cuticular resistances act in parallelrather than in series, the mathematicalrelationship between them is
But since values of Rc are 500-1000 times higher than values of Rs, l/Rc Is negligible compared with l/Rs and is consequently often omitted in calculations. The boundary layer, stomatal and mesophyll resistances all act in senes and consequently can be added up to make R. If cuticular resistance is ignored, we can now expand Equation 2.2:
Usually there is sufficient air movement to keep Ra low relative to Rs and Rm, and variation in wind speed, once above a minimum, does not have much effect on CO2 uptake. As stated in the text, mesophyllresistance Rm does not vary once growth has ceased. Hence Rs, the stomatalresistance, becomes the criticalone.
Most of the entry of CO2 into photosynthetic tissues occurs through the stomata (singular: stoma). These are minute structures in the epidermis, consisting of two highly specialized elongate guard cells enclosing a pore between them (Fig. 2.6). The guard cells are often flanked by a few subsidiary (accessory) cells differing morphologically from the remaining epidermal cells. The shape of the guard cells and the arrangement of their cell-wall thickenings ensure that when the guard cells are more turgid than the subsidiary cells, the guard cells bulge outwards into the subsidiary cells and separate in the middle, opening the pore. When the guard cell turgor equals or is less than that of the adjacent cells, the guard cells shrink together and the pore closes. All intermediate stages between maximal opening, as permitted by the elasticity of the walls, and complete closure are possible. At full opening, the stomatal apertures of Phaseolus vulgaris measure only 3x7 mm, while fully open stomata of Zebrina pendula reach pore sizes of 12x31 mm. In the grass family, Poaceae, stomata are very elongate; a fully open stomatal pore of Avena sativa (oat) measures 8 x 38 mm. The stomatal frequency per cm2 of leaf surface usually ranges from c. 1000 to 200 000. The apertures are so small that at the most 3% of the total leaf surface is occupied by the pores. Yet an illuminated leaf absorbs CO2 from the atmosphere with great efficiency. A leaf can maintain a steep diffusion gradient for the gas, and many small pores have a large amount of edge in relation to their surface area. Gas diffusion through a hole is more rapid round
The structure ofstomata. The drawings show surface views of stomata. (A) Open stoma in a leaf of mung bean (Vigna radiata) and (B) closed stoma from a grass leaf. G = guard cell; S = subsidiary cell. The diagrams at the bottom of each drawing indicate the direction of cellulose microfibrils in the guard cell walls. When the guard cells are sufficiently turgid, they bulge apart (A), since the microfibrils cannot stretch, and open the pore; in the case of the grass stoma, only the bulbous cell ends are able to expand, again forcing the cells apart in the centre. See also Fig. 3.15.
the edges, where the molecules can fan out into the region of lower concentration.
The stomatal resistance Rs is nevertheless appreciable and the rate of photosynthesis depends very much on stomatal aperture; stomatal closure can reduce photosynthesis almost to zero. It is generally agreed that stomata have evolved in response to the need of land plants to permit the entry of CO2 without excessive water loss. The high surface: volume ratio of a leaf, which makes it an efficient absorber of CO2 and light, is equally conducive to water vapour loss. The cuticle protects against water loss, but, as stated, is also highly impermeable to CO2. The stomatal pores which provide for CO2 entry obviously act also as channels for the exit of water vapour, but they can open or close according to circumstances. Water stress has an overriding effect over all other stimuli, causing stomatal closure. This occurs at quite moderate levels of water stress, before wilting, i.e. the stomata do not close because of flaccidity of the guard cells, but the decline in the plant's water status acts as a specific stimulus for closure. A hormonal signal appears to be involved: the concentration of the hormone ABA (abscisic acid; see Section 7.2.7) rises rapidly in response to water stress and ABA causes stomata to close. The associated inhibition of photosynthesis is less detrimental than desiccation would be.
In many species stomata show a diurnal rhythm, opening by day and closing by night, light stimulating stomatal opening with blue wavelengths being most effective. Accordingly the pores are open during the period when light is available for photosynthesis; in the dark, when CO2 cannot be assimilated, water loss is minimized. Stomatal opening is also promoted by low concentrations of CO2 within the leaf - a feedback type of control.
Complex physiological mechanisms underlie the changes in guard cell turgor. The current view is that, basically, increases in guard cell turgidity follow from an active pumping of K+ (potassium) ions into the guard cells, water then entering by osmosis. Decreases in guard cell turgor are attributed to an outward leakage of K+ because of an opening of K+ channels (Chapter 4) in the plasma membranes. This opening is promoted by increases in cellular Ca2+ concentrations caused by the closing stimulus. Chloride and malate anions accompany the K+ cations so that ionic balance is maintained.
The mechanisms of energy transduction and the synthesis of ATP and NADPH are essentially the same not only in all flowering plants, but in all land plants, algae and cyanobacteria. The mechanism of CO2 fixation, however, shows variations. The flowering plants can be divided into three categories according to their strategies for CO2 fixation: the C3 plants, the C4 plants and the CAM (crassulacean acid metabolism) plants.
'Phosphoglycerate' is the anion of phosphoglyceric acid, which can dissociate to phosphoglycerate ions and H+. The organic acids found in cells are weak acids and at the cytosolic pH they are largely dissociated. Hence they are often referred to by their anionic names - phosphoglycerate, pyruvate, glutamate, etc., although in formulae the acid form is commonly given; this terminology is followed in the present text.
The C3 cycle is the universal CO2-fixing cycle present in all CO2-fixing photosynthetic organisms (Fig. 2.7). It is located in the chloroplast stroma. The CO2-fixing enzyme of this cycle is Rubisco (ribulose-1, 5-bisphosphate carboxylase-oxygenase). Rubisco catalyses the reaction of CO2 with the 5-carbon (5-C) phosphorylated sugar, ribulose-1, 5-bisphosphate (RuBP), the CO2 acceptor. The resulting 6-C compound immediately splits into two molecules of phosphoglycerate, PGA (see Box 2.4) and the PGA is reduced to triose (3-C, or C3) sugar in reactions utilizing the products of the light reactions, ATP and NADPH:
The name of the cycle derives from the fact that the first stable products are three-carbon compounds. Some of the sugar formed is withdrawn as C gain from the cycle and may be further processed to starch; the rest is recycled to replenish the acceptor RuBP, using one more molecule of ATP for every molecule of CO2 fixed. The overall stoichiometry is therefore
A plant which fixes CO2 exclusively by the C3 cycle is known as a C3 plant. The great majority of flowering plant species are C3 plants. Export of carbohydrate from the chloroplast is mainly as triose phosphate, which is exchanged across the inner envelope membrane
I 12 ATP 12ADP
HCOH COOH COO®
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