14.2.4 Growth and development

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The response of both vegetative and reproductive growth to a temperature change is often associated directly with the response of a growing organ to that change. This conclusion is reached by comparing whole-plant responses to temperature with the response obtained when temperature changes are restricted to just part of the plant, or by measuring the growth response of isolated parts to temperature change. The optimum temperature for leaf growth in intact maize plants is about 33°C and this is also true when the variation in temperature is restricted to the meristematic regions of the leaf. However, detached growing organs do not always show an identical response to temperature as the same organs still attached to the plant.

Growth response to temperature may be controlled by enzyme activity and there are examples where specific enzymes associated with sugar metabolism or starch synthesis have been linked directly with growth of both vegetative and reproductive tissues. Temperature effects on expansion growth may also be regulated by cell wall properties and any control mechanism is likely to involve a number of interacting factors.

Temperature effects on growth can be viewed in terms of rate times duration of growth where individual components have different temperature optima (Figure 14.11). As temperature increases within a plant’s dynamic range, duration of growth decreases but rate of growth increases. As a consequence, organ size at maturity may change very little in response to temperature despite variation in growth rate. As temperatures are raised further, an increased rate of growth is no longer able to compensate for a reduction in duration, and the final mass (or volume) of a given organ at maturity is reduced. This response can be seen in a range of tissues including leaves, stems and fruits (and seeds). A smaller organ size at maturity due to high temperature is associated with smaller cells rather than a change in cell number. This implies that cell enlargement is more sensitive to temperature than is cell division (Section 6.2.2).



Figure 14.11 Temperature effects on growth can be viewed in relation to either rate of organ production or the final size attained. Size results from both rate and duration of growth, and there are many examples where organ size is reduced at high temperature because rate of growth cannot compensate for a reduced duration of growth. This is illustrated for wheat (a temperate cereal) and rice (a subtropical cereal). At low temperature wheat has a much larger grain than rice, but rice has a much more stable grain size than wheat in relation to temperature and at 30°C grain size in the two species is very similar. (Based on Tashiro and Wardlaw 1989)