Effects of Global Warming on Plants | Environmental Issues

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.

Temperature:

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.

Water availability:

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.