Thermal Energy Dissipation in Plants Under Unfavorable Soil Conditions

Abstract

Unfavorable soil conditions in crops and natural habitats include limited availability of water and nutrients, presence of salts, as well as an excess of essential nutrients and heavy metals. When plants are exposed to such stresses, rates of photosynthetic carbon fixation decrease for a variety of reasons, while plants continue gathering sunlight. As a consequence of the resulting imbalance between light absorption and energy utilization, plants experience what the research community has termed photoinhibition, which is considered to be a reflection of either photoprotection mechanisms or photodamage. Data reported to date suggest that under unfavorable soil conditions, photoprotection mechanisms are far more important than photodamage. Plants under stress generally dissipate thermally, i.e., as heat, a large part of the light absorbed by photosystem II in a process mediated by ∆pH, xanthophyll pigments (particularly zeaxanthin and antheraxanthin), and the photosystem II subunit S (PsbS) protein. Changes in thermal energy dissipation under unfavorable soil conditions are summarized here. Very high levels of thermal energy dissipation are frequently, but not always, accompanied by decreases in leaf chlorophyll concentration, such as those found under N and Fe deficiency, excess Al, or water stress in some species. The mechanisms of photoprotection remain largely unexplored for some of the stress situations reported here. In this chapter, we review changes in thermal energy dissipation in response to water stress, salinity, macronutrient (N, P, and K) deficiencies, micronutrient (Fe, Mn, Cu, and Zn) deficiencies and toxicities, and other metal (Cd, Pb, Al, and Hg) toxicities.