Elevated CO2 Helps Reduce the Negative Impacts of Low Temperatures on Plant Growth
Due to the fact that the growth-promoting effects of atmospheric CO2 enrichment are generally so much more pronounced at high temperatures than they are at low temperatures, much less experimental work has been conducted at temperatures close to the lower thermal limits of biological activity. There have been a few such studies, however; and even in this region of slow to negligible growth, the effects of elevated levels of atmospheric CO2 have often been dramatic.
In the early 1970s, for example, it was shown that atmospheric CO2 enrichment partially protects some C3 crops against chilling injury, even after prolonged low-temperature stress. Work in the 1980s with certain C4 grasses also demonstrated that atmospheric CO2 enrichment allowed photosynthetic products to be transported within plants on nights that were cold enough to totally inhibit translocation in plants growing in normal air. In one particular study from this early time period, Sionit et al. (1981) found that okra plants grown in ambient air at day/night temperatures of 17/11°C, 20/14°C and 23/17°C all died after 17, 35 and 44 days after emergence, respectively; while plants exposed to CO2-enriched air of only 450 ppm grew to maturity and produced fruit.
Recent work continues to confirm that elevated CO2 improves plant water relations during chilling and can mitigate photosynthetic depression and chilling damage. One of the ways by which these benefits may be derived is via a CO2-induced reduction in plant transpirational water loss, which may mitigate chilling-induced water stress and leaf tissue damage. Another possible mechanism is the CO2-induced enhancement of photosynthate production, which tends to lower leaf osmotic potentials and thereby increase leaf turgor. This latter suggestion has much to commend it; for it has been demonstrated that the freezing tolerance of cereals is positively correlated with rates of leaf photosynthesis; and in plants ranging from crops to trees, it has been demonstrated that atmospheric CO2 enrichment can significantly enhance rates of foliage photosynthesis, even at severely sub-optimal temperatures. Another benefit of this latter phenomenon is that the higher rates of photosynthesis may allow foliage to recover from low winter photosynthesis and enhance its carbon fixation potential to rapidly attain a higher photosynthesis rate by the time of bud-break.