14.1.3 Variation in sensitivity

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At any given stage of development, where other growth conditions are known, it is possible to define the minimum temperature below which a plant will not grow, suboptimal temperatures where a further increase in temperature will result in increased growth, optimal temperatures when growth is at its maximum, supraoptimal temperatures where a decrease in temperature will result in increased growth and the maximum temperature above which a plant will stop growing. Sustained temperatures beyond this range will be lethal.

When considering temperature responses it is possible to divide higher plants into broad, overlapping groups. One division is based on differences in the pathway of CO2 fixation during photosynthesis, where species in which initial fixation occurs via Rubisco and the photosynthetic carbon reduction cycle are designated C3 species and species in which initial fixation occurs via phosphoenolpyruvate (PEP) carboxylase and the four-carbon dicarboxylic acid pathway are designated C4 species (Section 2.1). A second division is provided by sensitivity to low temperature: ‘chilling-sensitive’ species are unable to grow, and often suffer visible damage, below 10–15°C and ‘chilling-tolerant’ species are able to grow down to 0°C and able to survive below this temperature.

C4 species include maize and sorghum and such species have high optimum temperatures (≥30°C) for growth. Most C4 plants are also chilling sensitive (Section 14.4). C3 species, on the other hand, show considerable variation in their optimum growth temperature, which can range from close to 30°C in rice to less than 10°C in some alpine species, and they also include a wide range of both chilling-sensitive and chilling-tolerant plants (Figure 14.5).



Figure 14.5 Plant species differ widely in their response to temperature, with alpine C3 species having a low optimum temperature and subtropical C4 species a high optimum temperature. While the alpine species will survive at temperatures below zero, temperatures of 10°C to 15°C are critically low for many subtropical C4 species. Maximum temperatures for active growth are generally higher in subtropical than in alpine species (Based on Wardlaw 1979)

Temperature is an important factor in controlling changes in development from germination, through vegetative growth to floral initiation and reproductive growth (Section 7.1). Not all stages of development, or different physiological processes, are equally sensitive to temperature. For example, the stage of pollen meiosis during anther development in the major cereals is particularly sensitive to temperature extremes (low temperature is an issue for rice culture in the Murrumbidgee Irrigation Area, and high temperature a common problem in wheat).

Variation in temperature tolerance is also evident within single plants as well as between genotypes. Young actively growing tissues are generally more sensitive to temperature variation than mature tissues and dormant tissues such as buds and seeds can often survive quite extreme temperatures. Frost tolerance in apple is greatest in the stem and then follows a sequence of decreasing tolerance from the mature leaf to the young leaf and floral parts, with the ovary, style and stigma of flowers being least tolerant of frost.

Variation in temperature sensitivity between tissues and physiological processes is also seen in sorghum, a subtropical C4 species. Frost damage occurs to green tissue below –1°C, there is poor seed germination below 6°C, chlorophyll destruction occurs in mature green tissue under high light at 10°C, there is poor pollen development below 13°C and chlorophyll formation in young developing leaves is inhibited at temperatures below 16.5°C (Bagnall 1982).