After reading this article you will learn about the effects of global warming on plants.
Some of the greenhouse gases like tropospheric CO2 have direct effects on vegetation while CFCs indirectly affect plant growth by enhancing the flux of UV-B radiation. Others, such as CH4 and N2O, appear to have no direct effects on plants, although they form significant microbial parts of the carbon and nitrogen cycle which also involve plants, soils and the atmosphere.
However, it is CO2 as a source of photosynthetic that dominates thought on likely effects of global warming on plant growth. This, however, is too simplistic because other features of global warming, apart from CO2, affect plant growth.
Temperature, rainfall (water availability), the length and quality of incident sunlight, and the availability of other nutrients (e.g., P, K, N) are all important determinants of growth and all are likely to change as a result of global warming.
Much is already known about plant growth at high levels of atmospheric CO2 because CO2 enrichment is often used during commercial horticulture inside glasshouses. Originally, oil-fired burners were used to enrich glasshouses with CO2 but, after the costs for heating soared the practice of redirecting fuel gases directly into the glasshouses from heating systems (which burn either propane or natural gas) has become more common.
The possible benefits in growth that are obtained during commercial horticulture from such CO2 enrichment are shown by Fig. 14.7. Although there are variations in growth even between cultivators of the same crop species as a general rule an additional 30% of growth is obtained by raising glasshouse atmospheres to 1000 µl-1 CO2 thereafter, the gains are not significant. The additional nitrogen oxide pollution problems, which often abolish all these possible gains, have been covered elsewhere.
Significant reductions in growth due to deprivation of CO2 are frequently experienced by plants outdoors. In crops growing close together, such as in tropical and subtropical forest canopies, levels of CO2 are often reduced well below ambient.
Over long periods of time competition for these reduced amounts of CO2 between species within canopies favours those species that are better able to capture CO2 than their neighbours. In subtropical climates, this evolutionary pressure has caused the emergence of so-called C4 plants with more efficient CO2 concentrating mechanisms.
All plants trap atmospheric CO2 by means of the enzyme RubisCO with the assistance of other enzymes of the Calvin (C3) Cycle. Such a process appears to have existed since the evolution of photosynthesis (about 2.5 billion years ago) but, occasionally since then, a significant modification to conventional C3 photosynthesis involving four carbon (C4) acids has occurred in a range of plant species which include some very important crops such as maize, sorghum, sugarcane, and millet.
Modern understanding of this modification now would be to regard C4 photosynthesis not as a replacement for C3 photosynthesis but as a beneficial adjunct to this basic process under certain environmental conditions. Perhaps the best way to appreciate this modification to photosynthesis is to consider it is being similar to the improvement to the performance of a combustion engine one would gain by adding a turbocharger.
All types of C4 plants discovered so far have a different arrangement of leaf cells as compared to normal C3 plants. Outer mesophyll cells completely surround inner bundle sheath cells which in turn enclose the vascular elements leading from the leaves down to the roots.
The basic feature of C4 photosynthesis is the primary assimilation of CO2 by the carboxylation of phospho-enol-pyru-vate (PEP) catalysed by PEP carboxylase (a more efficient CO2 trapping enzyme than Rubis CO). The oxaloacetate so formed (a C4 compound) is then reduced to malate or trasaminated to aspartate and then transpored from mesophyll to bundle sheath cells through cell-to-cell connections (plasmodesmata).
Subsequent metabolic steps in different C4 variants differ, but the CO2 released by decarboxylation is then re-fixed by RubisCO in the plastids of the bundle sheath cells. The C3 compounds, pyruvate or alanine, left over are then transferred back into mesophyll cells where they are converted back into the C3 precursor (i.e., PEP), which completes the cycle of C4 photosynthesis across the two types of cell.
The essence of the C4 process is that advantage is taken of the higher affinity for CO2 of the enzyme PEP carboxylase over that of RubisCO, an enzyme which is unusual in that it can either carboxylate or oxidise RuBP. The latter alternative is favoured at low internal CO2 concentrations and is the starting point for a process known as photorespiration which consumes O2 and releases CO2.
In C4 plants, the oxygenase activity of RubisCO is suppressed by the CO2 pumping action of PEP carboxylase which raises the effective concentration of CO in the bundle sheath plastids and favours carboxylation activity by RubisCO over that of oxidation.
Any photo respiratory CO2 that escapes from the bundle sheath cells is then immediately recaptured by PEP carboxylase. As a consequence, C4 plants show lower internal concentrations of CO2 but are still more effective than C3 plants in being able to reduce CO2 concentrations around a leaf.
Within stands of vegetation, where amounts of CO2 are limiting this property of C4 plants is of considerable advantage over C3 plants in warm climates. Theoretically, this should mean that when CO2 levels are raised this advantage of C4 plants diminishes.
This, in turn, is of significance to global crop productivity because many of the major tropical crops are C4 plants (maize, sorghum, millet and sugarcane) but most C3 plants (wheat, rice, barley, oats) grow in temperate regions. This also implies that extra CO2 may benefit temperate crops in the future and switches from maize to wheat and rice in areas where all three are grown may follow.
Moreover, 14 of the world’s major weeds that affect C3 plants are C4 plants. This means C3 crops should compete better against some of their weeds at higher CO, concentrations. However, atmospheric CO2 concentrations cannot be considered in isolation. Other determinants of plant growth such as temperature must also be taken into account.
In the majority of C3 species, rates of photorespiration increase faster than the equivalent rates of photosynthesis as temperature rises. Consequently, C3 plants show a fattened range of temperature optima between 10 and 28°C.
In C4 plants, however, where high internal CO2 levels preclude the oxygenase activity of RubisCO (i.e. no respiration), the temperature optima of C4 plants are higher (35-40°C) and more sharply defined. This means that the raising of temperature alone favours plants with C4 photosynthesis over those using just C3 mechanisms—exactly the opposite effect of increasing atmospheric CO2 levels. However, the situation changes as CO2 levels rise and photorespiration becomes less likely C3 plants then tend to behave more like C4 plants with elevated and sharper temperature optima.
Consequently, only C3 crops growing beyond their temperature optima are hampered relative to C4 plants. For most crops, this limitation does not apply temperature is the major limiting factor for productivity. It controls, for example, the length of the growing season (the time between spring and autumn frosts) which affects a host of growth parameters (e.g., leaf expansion and development, flowering, and fruiting).
As a rough guide, a rise in temperature of 1°C lengthens the growing season by 10 days, which means the areas devoted to crops could be extended to higher latitudes and altitudes of the chances of late frosts in spring and early frosts in autumn are diminished. Minimum temperatures are also important for trees and shrubs, especially if they retain their leaves or needles through winter.
A general rise in global temperature will again allow the range of the less hardy to extend to higher latitudes and altitudes but this has large consequences for biodiversity. If temperate plants move towards higher latitudes and at the same time, those plants that currently grow there take advantage of raised temperature and CO2 then there will be a crushing effect on certain species less able to compete and they may be eliminated by natural selection.
By and large, the consequences of increases in temperature and CO2 levels taken in isolation appear to be rather encouraging but, unfortunately, water evaporation rates from land surfaces will also increase and cause larger areas to become more arid. Some plants are already adapted to water deprivation, but this major limitation to growth will become more important as the full global consequence of global warming materializes.
Certain succulents and cacti show an alternative type of photosynthesis called crassulaccan acid metabolism (CAM) which has many similarities to that found in C4 plants. The principal differences between them are that mallic or aspartic acids are decarboxylated immediately in C4 plants while in CAM plants they are accumulated during darkness and decarboxylated later the next day.
In other words, there is a temporal rather than a spatial separation of the main CO2-fixing activities of PEP carboxylase and RubisCO. As a consequence, CAM plants lack the double cell cooperation of C4 plants but have water storing (succulent) tissues instead. This is possible because stomata in CAM plants are normally open at night and close during the day and, consequently, their water use efficiency (i.e. weight of H2O transpired per weight of CO2 assimilated) is very high compared with C, plants.
Usually, CAM photosynthesis is associated with plants that grow in hot, dry habitats with unpredictable rainfall while C4 plants tend to be midway between CAM plants and C3 plants in terms of their water use efficiency. This is because the intercellular levels of CO2 tend to be lower in C4 plants because their stomata do not have to open quite as wide which means less H2O is lost by transpiration.
A similar situation occurs in C3 plants when CO2 levels are raised. Stomata as a consequence do not have to open quite as wide to let an equivalent amount of CO2 in. As a result, less H2O is lost by transpiration and the water use efficiency in C3 plants also rises.