The Physiology of Flowering Plants - KHAM PHA MOI

LOW-TEMPERATURE STRESS 355above the ambient have sometimes been recorded, but usually, eveninside bulky organs, the temperature is close to air temperatureexcept in direct sunlight. On cold nights, leaf and bud temperatureoften falls below the ambient. Some mechanisms for keeping thetemperatures of plant organs above air temperature have beennoted. For example, at high altitudes in Africa and South America,where night temperatures may fall below 10 8C throughout theyear, some species produce leaf rosettes up to 0.5 m in diameter.The adult leaves on the outside of the rosette fold over nightly into agiant bud enclosing the apical meristem, and the temperature of themeristem remains above zero. But usually, when a plant tissue withstandsa specified external temperature, this is also the actual temperatureof the cells. Avoidance of low temperatures by sensitivetissues is achieved almost exclusively through appropriate timing ofdevelopmental stages. Actively growing tissues are the most susceptibleto cold stress, and the growth periods of many plants areconfined to warm seasons, leaving dormant, resistant, tissues tooverwinter.The geographical spread of many species is determined by theirability to survive low temperatures. Plant species native to climateswith a cold winter are more resistant than species of warm climates,as might be expected. The northernmost limit of a perennial plantspecies in the northern hemisphere frequently coincides with acertain winter minimum isotherm. Whilst frost is not responsiblefor as much famine as drought, it still has a considerable impact onagriculture, limiting the areas for growing particular crops and causinglarge losses in produce when frosts occur atypically late in thespring or early in the autumn, when even frost-hardy species are in asensitive state. The cold resistance of perennials of temperate andarctic zones undergoes large annual changes (Fig. 13.7), being verymuch greater in the winter months than in the summer. The hardeningoccurs in the autumn as a response to the falling temperatureand shortening daylength and is associated with the onset of winterdormancy. The loss of hardiness in the spring follows from the risein temperature in the spring. Artificially, hardening can beinduced at any season by chilling (see below), and resistance can bedestroyed in midwinter by warming. There is a clear correspondencebetween levels of resistance and the temperatures to which variousplant organs are exposed in their normal environment. Undergroundstructures are much less resistant than aerial parts and their resistanceis more uniform over the year, reflecting the less extreme andless fluctuating temperatures of the soil. In severe winters greaterresistance is developed than in mild ones.13.4.2 Chilling stress, freezing stress and limits of resistanceNumerous species originating in warm climates cannot endure exposureto temperatures below 5 8Cor108C, in some cases not even below15 8C. This reaction is known as chilling injury or cold shock. Theability to survive temperatures from 10 8C (or 15 8C) down to zero, is