The Physiology of Flowering Plants - KHAM PHA MOI

16 FLOW OF ENERGY AND CARBON THROUGH THE PLANTthrough which the H + move to the stroma, and this movement, downtheir free energy gradient, is coupled with ATP synthesis.The net result of the light reactions is thus the synthesis of ATPand NADPH, which can be utilized in CO 2 fixation (but can also bechannelled into other processes, e.g. nitrate reduction). Normally allphotosynthetic reactions occur simultaneously: ATP and NADPHcannot be stored and the cessation of illumination results in a stoppageof CO 2 fixation within a second or two. Several enzymes of CO 2metabolism need light activation.2.3.3 Levels of irradiance and rates of photosynthesisBox 2.1ROS, reactive oxygen species(alternative: AOS, active oxygenspecies) are extremely unstable,reactive and potentially destructive;they attack membranes bylipid peroxidation, and degradeDNA, RNA and proteins. From thesuperoxide radical O2 *– , reactionswith cellular protons and electronsproduce further ROS: the perhydroxylradical HO2 * , the hydroxylradical OH * and hydrogen peroxide,H2O2. The symbol*denotesan unpaired electron. Smallamounts of ROS are inevitablyproduced during photosyntheticand respiratory electron transport,and continually removed.Superoxide is broken down by theenzyme superoxide dismutase,SOD, and hydrogen peroxideby catalase, two very fastactingenzymes. SOD exists inseveral forms with different metalcofactors, FeSOD, MnSOD andCu-ZnSOD, specific to subcellularlocations. Cells also producereductive antioxidants which reactwith ROS, including ascorbic acid(vitamin C) and glutathione.Numerous stresses stimulate theformation of ROS to levels whichcan be dangerous (Chapter 13).Photometric unitsIn view of the basic role of light in photosynthesis, the rate of photosynthesiswould be expected to vary with the amount of light available.Here it is appropriate to consider what exactly is meant by the‘amount’ of light.Since light is the energy source for photosynthesis, one’s firstinstinct might be to measure it in energy units, say J m –2 s –1 (energyper unit area, as joules per square metre per second). For energybalance sheets this may be appropriate. However, photochemicalreactions are energized by individual quanta, the units of lightenergy carried by individual photons of light. One pigment moleculeabsorbs one quantum of energy at a time, to undergo one photochemicalreaction. Hence, for many studies, the most meaningfulmeasure of the ‘amount’ of light is the number of photons (orquanta), this number being given in moles. One mole of quantacan energize one mole of pigment molecules. The number of molesof photons of PAR, per unit area and unit time, is called the photonflux density or PFD. It is the PFD that exhibits the most directrelationship with the rate of photosynthesis. Bright sunlight has aPFD of 2000–2300 mmol m –2 s –1 .Effects of varying the PFD: reactions of sun and shadeplantsAs the level of irradiance on a photosynthetic organ is increased,the rate of photosynthesis at first rises linearly, then levels off toa steady rate as light saturation is reached (Fig. 2.5). But theabsorption of light does not fall proportionately, so that atincreasing PFD, less CO 2 fixation takes place per photon absorbed:photoinhibition occurs. This was originally interpreted as dueto photochemical damage. Excess light-excited chlorophyll canenergize the formation of reactive oxygen species, ROS, fromO 2 : singlet oxygen 1 O 2 *andthesuperoxideradicalO 2 *– (oxygenwith an extra unpaired electron). These chemicals and their derivatives(Box 2.1) can destroy components of the photosystems.There is good evidence now that photoinhibition is in fact aprotective process, during which excess energy is dissipated