The complex chemical reactions which occur during the formation of carbohydrates from water and carbon dioxide require the presence of chemicals called enzymes to accelerate the rate of reactions. Without these enzymes, little chemical activity would occur. Enzyme activity in living things increases with temperature from 0°C to 36°C, and ceases at 40°C. This pattern is mirrored by the effect of air temperature on the rate of photosynthesis. But here, the optimum temperature varies with plant species from 25°C to 36°C as optimum. It should be borne in mind that at very low light levels, the increase in photo-synthetic rate with increased temperature is only limited. This means that any input of heating into the growing situation during cold weather will be largely wasted if the light levels are low.
Integrated environmental control in a greenhouse is a form of computerized system developed to maintain near-optimum levels of the main environmental factors (light, temperature and carbon dioxide) necessary for plant growth. It achieves this by frequent monitoring of the greenhouse using carefully positioned sensors. Such a system is able to avoid the low temperature/light interaction described above. The beneficial effects to plant growth of lower night temperatures compared with day are well known in many species, e.g. tomato. The explanation is inconclusive, but the accumulation of sugars during the night appears to be greater, suggesting a relationship between photosynthesis and respiration rates. Such responses are shown to be related to temperature regimes experienced in the areas of origin of the species.
Adaptation to extremes in temperature can be found in a number of species; for example resistance to high temperatures above 40°C in thermophiles; resistance to chilling injury is brought about by lowering the freezing point of cell constituents. Both depend on the stage of development of the plant, e.g. a seed is relatively resistant,
but the hypocotyl of a young seedling is particularly vulnerable. Resistance to chilling injury is imparted by the cell membrane, which can also allow the accumulation of substances to prevent freezing of the cell contents. Hardening off of plants by gradual exposure to cold temperatures can develop a change in the cell membrane, as in bedding plants and peas. Examples of plant hardiness are found in Table 4.3.
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