The plant response: stress in the daily environment.

Abstract

Like animals, plants have evolved to survive in almost every climatic and environmental niche available. They have, however, evolved more sophisticated and varied methods to enable them to survive environmental changes in light, temperature, atmosphere composition, water and nutrients and salinity. This, in part, is necessary because of the sessile nature of plants; they do not have the ability to move to more favourable environments. Stress conditions that plants encounter are not always as rare or unusual as we might at first think. The most common environmental variables, necessary for growth, can impose significant stresses on the plant. But should we think of these as unusual and extreme or just part of the normal diurnal responses experienced by the plant? One example is light. While requiring light for photosynthesis and developmental processes, many plants are damaged by excess light, and mechanisms have evolved to dissipate light energy, for example, using metabolites such as flavonoids, which divert electron flow and avoid harmful build up of free radicals. This sensitivity to excess light, sometimes manifest as photobleaching, is not uncommon in plants which have evolved to live in more shaded environments such as the forest understorey, but which have been developed as crop plants in new more open environments. Even plants that continue to live in such shaded conditions have to adapt to sudden high light; sun flecks which occur as upper canopy trees move in the wind can be damaging to plants which are adapted to low light conditions. Again, pigments, such as anthocyanins, a common feature of understorey leaves, are used to deflect damaging energy. The important issue to understand here is that these conditions are not what we might have thought of as extreme, but rather those which occur regularly in any climate. Another good example is high temperature. Leaves generally do not attain temperatures much above 35 oC. They are able to control this by the fact that they can control internal water and air flow, and are not bulky tissues. However, organs such as fruit do not have this same control capacity, and even in temperate climates can attain high temperatures. For instance, an apple growing in an air temperature of not more than 25 oC, as experienced in temperate climates, can reach temperatures of 40 oC−50 oC in the flesh when in direct sunlight (Ferguson et al., 1998). These are temperatures that would be sufficient to inactivate many enzymes in vitro. The flesh temperature of the same fruit may drop to 10 oC −15 oC overnight, representing a possible 30 oC change in temperature in less than 12 h. The process is cyclic, rising and falling each day under normal, temperate, growing conditions. We think of these high temperatures as extreme and the response in terms of stress physiology. However, we should recognize that experiencing and reacting to such conditions is normal, often happens in diurnal cycles, and is part of regular cellular homeostasis. Journal of Zhejiang University SCIENCE ISSN 1009-3095 http://www.zju.edu.cn/jzus E-mail: jzus@zju.edu.cn