Abstract
The photosynthetic apparatus must cope with the excess energy when light exceeds what plant can use. Under these conditions, plants, including oaks, can activate an array of "photoprotection mechanisms", which are crucial to understand the relationships between plants and their environment. First, this chapter gives a general description of the different photoprotection mechanisms that operate at several levels: (i) the reduction of light collection by chlorophylls, (ii) the enhancement of the metabolic use of light energy absorbed, (iii) the enhancement of the dissipation of the absorbed energy as heat, and (iv) the mechanisms for preventing and repairing oxidative damage (Sect. 11.1). These photoprotection mechanisms are subsequently analyzed in detail for evergreen oaks exposed to winter stress (Sect. 11.2) and for both deciduous and evergreen oaks under drought-stress conditions (Sect. 11.3), with particular emphasis on the role of free and enzymatic antioxidants, xanthophyll cycles and sustained engagement of dissipation. Afterwards, the chapter addresses with the need of photoprotection in deciduous oaks during autumn senescence associated to the risks of chlorophyll degradation and reactive oxygen species (ROS) generation (Sect. 11.4).