14.1 - Thermal environment and plant heat budgets

Temperatures vary with latitude, altitude, size of land mass (and position within that land mass), atmospheric conditions (cloud cover and air movement) and local topography. Atmospheric temperatures decrease by about 1°C for each 2° increase in latitude, or for each 100 m increase in altitude. Temperatures in high altitudes near the equator can be similar to low-land temperatures further from the equator. The potato, which has its origin in tropical high altitudes, is now grown widely in low-altitude regions of the world where temperature conditions are similar. However, temperature is not the only concern in this expansion of a crop plant to new growing regions, as a change in latitude and altitude may require some adaptation to changing photoperiod and radiation levels.

Seasonal variation in temperature is greatest near the poles, and small near the equator. At high altitudes the increased solar radiation which can result in rapid local heating is balanced by greater night radiation losses. The most stable temperatures occur under oceanic conditions in the tropics.

Survival outside the active growth range of all plants is nevertheless dependent on developmental stage, duration of temperature stress and degree of acclimation. Terrestrial plants have evolved life cycles that match seasonal necessity, which at low temperatures necessitates winter dormancy. Within the dynamic range of plant–temperature relations, where short-term responses are readily reversible, phenology and productivity have been successfully linked with various measures of accumulated heat (thermal time), and predictive models validated.

14.1.1 - Temperature means and extremes

Fig14.3.png

Figure 14.3. Wheat-growing areas of Australia have been classified according to the temperature conditions normally prevailing during crop growing seasons (Nix 1975).Temperature increase following heading is common in all regions and plants frequently suffer periods of heat and drought stress during grain filling. Avoiding these late stresses by earlier planting can be difficult, particularly in those regions subject to spring frosts which may damage the developing florets of the inflorescence (ear) (Based on H A Nix (1975) plus other unpublished sources)

Average monthly temperatures are more relevant to plant growth than mean annual values. Timing cardinal stages in plant development (phenology) can often be predicted from temperature measurements taken during the growing season. This integration of time and temperature is expressed in terms of degree-days (see later comments). However, when considering the effect of temperature on yield it is necessary to take extreme conditions into account, particularly at sensitive stages of growth.

Although unseasonally warm growing conditions late in winter are not necessarily harmful to fruit trees, this will often result in early bud break and flower development, increasing the possibility that these more sensitive stages of development will be damaged by a late frost.

For annual crops some degree of adjustment is possible to avoid a particular environmental stress by varying the cultivar or time of planting. However, even this increased flexibility may not be adequate to cope with the irregular nature of many stresses. In the wheat-growing areas of Australia, temperatures increase from the time of heading through to crop maturity so that grain ﬁlling often occurs at above-optimum temperatures. Added to this is the possibility of extreme high temperatures (heat shock) or drought. In warmer growing regions these temperature extremes may be avoided in part by earlier planting, but this approach will depend on suitable temperature conditions and the availability of water for germination and seedling establishment early in the year. In cooler wheat-growing regions earlier planting may expose the developing infloresence to possible frost damage resulting in reduced fertility and grain set (Figure 14.3).

14.1.2 - Plant temperatures

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.

Leaf temperatures generally follow the diurnal changes in air temperatures (Fig. 14.4). 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. 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.

Fig14.4.png

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 (1969) Ann Bot 33, 561-569.

Under normal ﬁeld conditions mean day temperatures can commonly be 5°C to 10°C higher than mean night temperatures, with a much greater amplitude between maximum and minimum temperatures. The importance of this day/night differential has received considerable attention in relation to the growth and yield of glasshouse crops where there is the possibility of some degree of temperature control. However, it is also of interest in ﬁeld crops in relation to the development of models for use in predicting the effect of weather conditions on crop development.

Total plant growth would be favoured by low night temperatures as this would reduce respiratory losses at a time when the supply of carbohydrate might become limiting. However, dry matter production for a wide range of crops grown under constant but optimal temperature is equal to and often greater than dry matter production by the same crops grown under differential day/night temperatures with the same mean value. Where a day/night temperature differential is imposed, low night temperatures rather than low day temperatures favour growth. The amplitude of the day/night temperature differential is also important and increasing this from 10°C to 20°C can reduce growth.

14.1.3 - Plant energy budgets

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Figure 14.5. A notional energy budget for a mesophytic leaf on a well-watered plant. Taking an incident irradiance of 400 J m^-2 s^-1 as representative of midday conditions, 30 units are reflected, leaving 370 J m^-2 s^-1 absorbed. If air temperature is taken to be 20°C, relative humidity 50%, wind speed 0.8 m s^-1and leaf area 0.01 m² (i.e. 100 cm²), then transpiration will be about 4 mmol H₂0 cm^-2 s^-1, and latent heat exchange will account for about 180 J m^-2s^-1. Sensible heat exchange will account for the remainder (about 190 J m^-2 s^-1) (Based on Nobel 1983)

All organisms must balance energy inputs and outputs in order to maintain tissue temperatures within a given range. Plants have evolved a range of adaptations that allow them to balance energy gain and loss and so avoid becoming too hot or too cold. Energy budget equations quantify the energy gained and lost through different processes. More complex energy budget models include leaf and canopy properties that influence the rate of energy transfer (see Lambers et al. 2008).

In a leaf energy budget, energy is gained through radiation absorbed, while energy is dissipated as radiant heat loss by emission of longwave radiation, sensible heat loss by convection, and latent heat loss by transpiration (Figure 14.5). For a leaf to maintain a constant temperature the energy exchange must be balanced such that:

Radiation absorbed = radiant heat loss + sensible heat loss + latent heat loss

Energy gain comes primarily from the sun in the form of shortwave radiation. Leaves absorb a large proportion of incident solar shortwave radiation, particularly in the visible wavebands (Figure 14.6). As any object with a temperature above absolute zero emits some radiation, leaves also absorb longwave radiation from their surroundings. A plant can reduce the amount of radiation absorbed by reducing the amount of leaf area exposed to the sun, e.g. by changing leaf orientation or leaf dimensions. Leaf properties such as surface wax, hairs and scales will also alter the amount of incident radiation that is reflected and transmitted (see section 14.7.2).

Fig14.6.png

Figure 14.6. Solar radiation striking a leaf is either reflected, transmitted or absorbed. Photosynthetically active radiation (0.4 to 0.7 µm) is strongly absorbed, whereas near-infrared radiation which would heat leaves but is useless for photosynthesis (between about 0.8 and 2 µm) is only weakly absorbed. Far-infrared radiation is strongly absorbed, but energy per quantum is greatly diminished at these long wavelengths, so that consequences for leaf heating are greatly diminished. Based on Gates (1965) Meteor Monogr 6, 1-26.

During the day, thermal energy that is absorbed by a leaf must be dissipated to prevent the leaf from overheating. A large amount of heat is lost through the emission of longwave radiation, but this is normally balanced by inputs of longwave radiation from the surrounding environment. The rate of radiant heat loss is determined by leaf temperature and by leaf emissivity. Emissivity does not vary greatly between leaves; generally darker, smoother leaves have a higher emissivity (as well as a high absorbance) (Lambers et al., 2008). At night time, radiant heat loss can cause leaf temperature to drop well below that of the surrounding air. On long, cloudless nights, when there is little radiant input from the atmosphere, dew can condense on the leaf surface and, in extreme cases, leaves may cool below zero resulting in frost damage.

During the day a leaf typically loses sensible heat to the surrounding air through convection. At night the situation is usually reversed with leaves gaining heat from the surrounding air. The rate at which heat is gained or lost is proportional to the difference in temperature and to the amount of resistance between a leaf and the surrounding air. Characteristics that increase the size of the boundary-layer, e.g. large leaf dimensions or presence of leaf hairs, will reduce the rate of heat exchange. Similarly, reduced wind speed will reduce the rate of sensible heat exchange.

For transpiration to occur, water contained in cells within a leaf must change into water vapour and escape through stomatal pores. For the water to move from a cell to the inter-cellular air space it must change from a liquid to a gas (water vapour). This phase change from liquid to gas consumes a large amount of energy and in doing so cools the leaf. This loss of energy is referred to as latent heat loss. The rate at which transpiration occurs is proportional to the difference in humidity and to the amount of resistance between the inside of the leaf and the surrounding air. As well as wind speed and boundary layer properties, transpiration is strongly influenced by stomatal resistance. When the stomata are closed, heat loss through transpiration is negligible. Although stomatal resistance has a large impact on leaf temperature, it is generally accepted that stomatal opening does not respond directly to changes in temperature. In hot, dry conditions stomata will typically remain closed even when a leaf is overheating (Lambers et al. 2008).