According to Kozlowski, Kramer & Pallardy (1991) the thermal death point of most active plant cells varies from 50 to 65C. This is dependent on species, age of tissue and duration of exposure to high temperature. However, high temperatures below the thermal death point can often damage woody plants. Symptoms of this type of damage include scorching of leaves and fruits, abscission of leaves, sun scald or burn to bark and growth inhibition.
Often the temperatures of the surfaces adjacent to trees can exceed 65C, for example radiated heat from road surfaces, which create high evaporative potential.
The symptoms of heat stress are often interrelated or similar to those caused by water stress.
The mechanism of heat injury can be complex, however during January and early February 2009 Melbourne’s trees experienced direct ‘heat shock’ injury, this followed extended periods of excessively high ambient air and radiated temperatures, in combination with existing dehydrated plant conditions. Plants are often injured by dehydration associated with high transpiration rates caused by high temperatures and low relative humidity.
What lead to the rapid defoliation of many trees in Melbourne was a combination of high temperatures and hot, dry winds. Wind burnt leaves first wilt then dehydrate rapidly becoming brittle within a day and dependent on degree of dehydration and period of high temperatures, can be shed within a few days (Kozlowski, Kramer & Pallardy, 1991).
Melbourne’s trees were already experiencing water stress with low soil moisture levels and water absorption not matching water loss through transpiration.
Water stress is usually attributable to drought, however it develops whenever water loss exceeds absorption long enough to cause a decrease in plant water content and sufficient loss Photographs on left show the effects of high heat and water stress over one week in January 2009 on a London Plane (Platanus x acerifolia) located in Melbourne’s eastern suburbs. of turgor, which causes a decrease in cell enlargement and disturbance of various essential physiological processes.
The plant water balance is analogous to a bank balance that relies on a series of deposits and withdrawals. Therefore, either rapid transpiration (excessive withdrawals) or slow absorption (inadequate deposits) can cause plant water stress. In hot, dry weather it can be a combination of the two.
In moist soil, water absorption is controlled largely by the rate of transpiration, but in drying soil it is gradually reduced by the decreasing difference in water potential between roots and soil and the increasing resistance to water movement toward roots through drying soil.
Plant tissues become dehydrated with the initial response of wilting leaves and shoots (loss of turgor) (Costello, et al., 2003). Strong, hot winds can exacerbate water deficits and increase plant water loss by up to 30% (Costello, et al., 2003).
If water loss is severe, leaves develop marginal scorching leading to further necrosis and premature leaf senescence and leaf shedding, either by early abscission or withering. Leaf shedding could be seen as a beneficial adaptation that reduces water stress.
Plant adaptation to water stress
Plants have evolved as a result of morphological and physiological adjustments that suit their native habitats and facilitate survival of moisture stress. In general terms, droughts are avoided or tolerated to various degrees by plants with the help of structural or physiological adaptations that postpone dehydration or that enable plants to tolerate dehydration without serious injury.
Drought avoiders/evaders are generally annuals that complete their life cycles within a few weeks after seasonal rains. Evaders can also consist of perennial herbs and bulbous species, which die down or dormant during hot dry periods. They can also consist of seed banks or vegetative perennating organs.
Drought avoidance does not generally apply to trees as they are long-term components of the landscape and cannot ‘avoid’ drought conditions. There are some woody species that can exhibit this strategy, for example summer deciduous trees, such as Jacaranda.
The most important type of drought tolerance involves postponement of dehydration by effective root systems, very efficient water transport systems, control of water loss, or a combination of all three.
A good safeguard against drought injury is a deep, extensively branched root system that can absorb moisture from a large volume of soil. In times of low rainfall soils generally dry from the surface down due to evaporation and shallow root systems of plants. Therefore shallow rooted plants suffer water stress earlier than deep-rooted ones. One role of mycorrhizae may include the improvement of water absorption as well as nutrients and therefore improve drought tolerance. Large root systems are important for seedling establishment. However large root systems are not always desirable in horticultural activities, for example photosynthate is channelled away from stem and fruit production, and excessive root systems can cause damage in urban areas.
Some plants have developed mechanisms to control of transpiration. These generally comprise species that have evolved in environments where drought stress is a regular occurrence. Morphological and physiological characteristics include the development of thick, heavily cutinised, sclerophyllous leaves with low cuticular transpiration and stomata that close promptly in dry air or when leaves are water stressed. Low cuticular transpiration is probably more a result of the thickness of the wax deposited on it rather than the thickness of the cuticle. Accumulation of wax in stomatal antechambers is particularly effective. Stomatal response to water stress is more important than the number and size of them. The leaf surface exposed to radiation can be reduced by changes in orientation and angle of exposure, which is the case for many eucalypts. Leaf shedding as discussed earlier aids in decreasing water loss.
Anti-transpirants are used to reduce transpiration, either by causing closure of stomata or by coating the foliage with a film that is more or less impermeable to water.
Water storage is another way of postponing dehydration. The use of stored water to replace that lost through transpiration can supply from one-third to several days of water requirement dependent on the species. Water is stored in sapwood and to a lesser extent in bark. Water storage is an important characteristic for cacti and succulents.
Dehydration tolerance is the final step in the ability for plants to avoid desiccation and death. Different species have different dehydration tolerances, for example resurrection plants such as the genus Selaginella, which are of less importance in horticultural practices compared to postponement of dehydration.
These strategies have developed in species within their natural distribution and research into application within horticulture is very thin.
Plants can sometimes build up tolerances to heat with specific proteins called ‘heat shock’ proteins. These proteins can protect certain cell components and enzymes from inactivation (Kozlowski, Kramer & Pallardy, 1991). Some genus like Eucalyptus and Quercus can have species with both conserver and tolerator characteristics.
All species, irrespective of inherent moisture-stress tolerance, are most sensitive in the first months after transplanting in the landscape, and secondly, in the maturity to senescence phases of their life cycle.
In the first case, it is primarily due to the shoot: root ratio resulting from nursery production methods. Mature or senescent plants present a problem of size and the plants ability to supply water to its most distal parts. Furthermore as woody plants grow there is a greater percentage of non-photosynthetic parts. This reduces photosynthate (carbohydrates) that assists in osmotic adjustment and other physiological processes, including root growth.
Success in the urban landscape will be dependent on the soil moisture content and climate of the particular area and the species ability to tolerate pervading climatic conditions.
Costello, L. R., Perry, E. J., Matheny, N. P., Henry, J. M. & Geisel P. M. (2003) Abiotic disorders of landscape plants. A diagnostic guide. University of California. Agriculture and Natural Resources. Publication 3420. Kozlowski, T. T., Kramer, P. J. & Pallardy, S. G. (1991) The physiological ecology of woody plants. Academic Press.