14.1.2 Plant temperatures

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Most field studies on the relation between growth (and yield) and temperature are based on meteorological data collected in the vicinity of the crop or ecological area under examination. Although these data have proved to be of considerable value in assessing the response of plants to temperature, actual plant temperatures can differ considerably from air temperature and temperatures can also vary from organ to organ within a plant.

Plant temperatures will be influenced by plant form (erect or prostrate), leaf area, aspect (e.g. north- or south-facing slope), irradiance (sun angle, cloud cover and altitude) and air flow (wind). There are also plant characteristics such as reflection from the surface of the leaf, leaf angle and leaf cooling by transpiration which will reduce plant temperature and may minimise the damage associated with above-optimal air temperatures. At the other end of the scale, where there is moisture on leaves, the latent heat produced during external ice formation (a white frost) may provide some initial protection of tissues against frost damage.

The main temperature gradient within a plant is from leaves to roots. At sunrise with a clear sky there is a rapid rise in air temperature and leaf temperature, with a slower and lower rise in root and surface soil temperature. This relationship is altered considerably by cloud cover and at night root temperatures will be higher than leaf temperatures. Unfortunately a lack of direct measurement of tissue temperatures in field situations can make it difficult to extrapolate from laboratory to field (Figure 14.4). For example, organ volume (thickness) can also have a marked influence on temperature and this can be seen, for example, in the apple-growing regions around Auckland in New Zealand. On a windless sunny day with an air temperature of 26°C, while leaf temperature in an apple orchard may reach 29°C, the peripheral flesh of the fruit on the sunny side of a tree can reach 45°C.



Figure 14.4 Plant temperatures generally follow the diurnal pattern of air temperatures, but may be higher (sometimes 10°C or more) due to strong irradiance from full sunlight, or cooler (often by 2-3°C) due to transpirational cooling. Tissues and organs of a plant are not necessarily at a uniform temperature. Root temperatures are generally lower than those of leaves during daytime and higher at night-time, reflecting the differences between air and soil temperatures (Based on Davidson 1969a)