Environmental Control Parameters

Water, irradiance, and carbon dioxide (CO2) are the dominating environmental control parameters in the physiological ecology of photosynthesis and frequently stress limitations are due to water supply and high irradiance. The interaction of these factors can be assessed using Figure 3 and will be discussed below.

Figure 4 Scalar levels of structures (in a one-dimensional notation of meters) and time constants of functions (in seconds) in relation to photosynthesis and dependent processes.

Figure 4 Scalar levels of structures (in a one-dimensional notation of meters) and time constants of functions (in seconds) in relation to photosynthesis and dependent processes.

Hydraulic limitation

The most important regulatory response of photosynthe-sizing plant leaves to problematic water supply is stomatal closure by guard cell movements. This prevents or at least highly reduces the loss of water in the gaseous form by transpiration. However, it must be considered a compromise and also has negative effects. The most obvious one is that as long as stomata are closed CO2 cannot be taken up from the atmosphere and production of assimilates stalls. Stomatal closure also amplifies adverse effects of high irradiance. It prevents transpirational cooling when high insulation heats up the leaves. Blocking CO2 uptake, stomatal closure contributes much to overenergization of the photosynthetic apparatus because the remaining CO2 in the internal airspaces of leaves is rapidly fixed in photosynthesis and then CO2 is lacking as an acceptor for the reduction and energy equivalents of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) (Figure 5) produced in photosynthetic electron transport.

High irradiance

When the production of NADH and ATP is faster than their consumption by the energy-demanding photochemical work of CO2 reduction and assimilation the molecular elements ofthe photosynthetic electron transport chain rapidly get reduced and closed for further electrons. This occurs at high irradiance when not all of the excitation energy of chlorophyll can be used in photochemical work and the excited chlorophyll cannot relax in this way. The plant has evolved several protection mechanisms for this situation.

First, there is photorespiration. This is based on the dual affinity of the CO2-fixing enzyme ribulose-bis-phos-phate carboxylase/oxygenase (RubisCO) for both CO2 and oxygen (O2). When it reacts with O2, at low CO2/ O2 concentration ratios in the leaves, carbon and metabolite flow occurs through the cycle of photorespiration. This is also photochemical work, but it is futile and dissipates energy. Second, the energy absorbing and transferring photosystems of the thylakoid membranes have a very complex sophisticated structure, and there are pigments absorbing light energy in the light-harvesting antennas of photosystems and diverting the excitation away from the central reaction centers of the photosystems as well as futile cycles of oxidation (forming epoxides)/ reduction of xanthophyll-type pigments dissipating energy in the form of heat. Third, partial and reversible destruction of structural components reduces the light-absorbing activity of photosystems. This involves turnover of proteins as well as the arrangement of antennas and cores of photosystems in the thylakoid membranes. Fourth, the ROS formed from excited electrons of chlorophyll molecules notwithstanding the first three protective mechanisms can be deactivated by superoxide dismutases, and biochemical redox active mechanisms, like the glu-tathion/ascorbate cycle.

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Figure 5 Chemical structures of (a) nicotine adenine dinucleotide (phosphate) (NAD(P)) and (b) adenosine triphosphate (ATP), where the insert at the upper left corner in (a) shows the transfer of a reduction equivalent [2H] and the squiggled bonds in (b) indicate energy-rich phosphate bonds of energy equivalents.

We realize that there are cascades of measures to overcome high irradiance stress. They are related to the phenomenon called photoinhibition which may be protective even if partially destructive, when destruction is reversible. Finally, when the intensity of stress overrules the protective measures parts of the plant or the whole plants may die: they are literally burnt by the ROS.

Carbon dioxide (CO2)

Currently anxieties are nourished by dramatic anthropogenic increases of atmospheric CO2 concentrations. However, we may note that during the geological history of our planet and the evolution of photosynthesis, CO2 concentrations have fluctuated very much, for example, 230 million years ago they were about 6 times the present levels, 270-310 million years ago they were similar, and 460-535 million years ago they were much higher, that is, about 20 times more than right now. Thus, the affinity to its substrate which the key enzyme of primary photosyn-thetic CO2 fixation, RubisCO, has evolved is not so high that it could operate at saturation under the current atmospheric CO2 concentration. This problem is amplified for water plants by the low solubility of CO2 which dissolves in the form of bicarbonate (HCO-) while the actual substrate ofRubisCO is CO2. Hence, many photosynthe-sizing water plants, cyanobacteria, and algae, have developed carbon-concentrating mechanisms in which carbonic anhydrase, the enzyme catalyzing the CO2/ HCO3 - equilibrium, plays a central role.

Among the terrestrial higher plants two different mechanisms of carbon concentrating have evolved which are of eminent ecophysiological relevance, especially in relation to hydraulic and irradiance stress. These are C4 photosynthesis and Crassulacean acid metabolism (CAM). The standard photosynthetic pathway is C3 photosynthesis named after the first stable compound produced after fixation of CO2 by RubisCO, the three-carbon organic acid 3-phosphoglyceric acid (3PGS), which is then reduced to carbohydrate in the Calvin cycle. C4 photosynthesis and CAM are modifications of this pathway, where, in both cases, the primary fixation of CO2 is not by RubisCO but by phosphoenolpyruvate carboxylase (PEPC) and the first stable fixation product is the four-carbon organic acid malic acid (or malate the anion of this acid). This gave C4 photosynthesis its name. Subsequently, malate is decarboxylated again and the CO2 regenerated is refixed and assimilated via RubisCO. Why this detour via PEPC and what is the difference between C4 photosynthesis and CAM.? The CO2 affinity ofprimary fixation by PEPC is 60 times higher than that of RubisCO. This enables PEPC to operate at substrate saturation and drive CO2-concentrating mechanisms.

In C4 photosynthesis, primary CO2 fixation by PEPC occurs in an outer tissue (the so-called mesophyll tissue), the malate produced is transported into an inner tissue around the veins or bundles of the leaves (the so-called bundle sheath tissue) where decarboxylation and CO2 concentrating to about 6 times that of the ambient atmosphere for subsequent fixation by RubisCO takes place. At this elevated CO2, RubisCO works close to its saturation and can use a higher proportion of excitation energy for photochemical work. Even if stomata are partially closed to reduce transpiration, still enough CO2 can diffuse into the leaves for high-affinity fixation by PEPC. We can see that compared to C3 photosynthesis C4 photosynthesis is better suited to deal with stress due to hydraulic limitation and high irradiance.

CAM occurs in many different families of vascular plants, but the name is derived from the family of Crassulaceae (Crassulacean acid metabolism, CAM) where it was first discovered. In C3 and C4 plants, CO2 fixation and the light-dependent reactions of photosyn-thetic electron transport must run simultaneously which

includes the dilemma that during CO2 uptake water vapor is lost by transpiration through open stomata. In CAM, primary CO2 fixation occurs in darkness during the night when the driving force for transpiration, that is, warm and dry ambient air, is highly reduced and loss of water vapor is minimized. The malic acid produced is stored overnight in the central cell sap vacuoles and is remobilized and decarboxylated during the day. In this phase, stomata are closed and very high internal CO2 concentrations between twofold and 60-fold atmospheres are built up so that RubisCO works at substrate saturation while transpiratory loss of water is prevented. Again, this is an effective mechanism of dealing with hydraulic limitation and high irradiance. The principal difference between C4 photosynthesis and CAM is that the two carboxylation processes are separated in space (mesophyll and bundle sheath) in the former and in time (night and day) in the latter, that is, that CO2 concentrating is restricted in space (bundle-sheath in C4 photosynthesis) and time (light period in CAM), respectively.

The three modes of photosynthesis are constitutive in many plant species. However, there are also C3/CAM intermediate plants. Constitutive CAM itself is already quite plastic because when water stress is not severe, CAM plants can open the stomata in the later afternoon, when the nocturnally stored malic acid is consumed, and take up and fix atmospheric CO2 like C3 plants. In the truly C3/CAM intermediate species, the expression of either of the two photosynthetic phenotypes is flexible and, in many cases, reversible depending on the environmental conditions. For various intrinsic physiological and biochemical reasons the productivity of CAM is lower than that of C3 photosynthesis, and therefore it is only profitable for the plant to perform CAM when the environmental situation is stressful and it is preferable to perform C3 photosynthesis when water availability is sufficient.

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