Plant Photosynthetic Response to Metal(loid) Stress

Abstract

Photosynthesis is an essential energy process in all plants and algae that sustains life on the earth. Plants are capable of converting the energy of light into a biochemical form of energy, the process that occurs in the chloroplasts. Light harvesting pigments capture the energy of photons and transfer the excitation energy to the reaction centers of the photosystems. Photosystem II oxidizes water to oxygen and provides electron flow and protons for the generation of NADPH and ATP which are used in the photosynthetic carbon assimilation process, leading to the biosynthesis of organic compounds. The essential metals such as copper, iron, manganese, and zinc are redox active transition metals which can act as cofactors of metalloproteins involved in the photosynthetic electron transport, or they are integral components of enzymes involved in the stress antioxidant response. However, metal(loid)s such as arsenic, lead, chromium, and cadmium do not have physiological functions and can be toxic at low concentrations. Metal(loid)s pollution represents a great environmental concern due to human activities (mining, coal fly ash, metal industry, agriculture practice, solid waste, and traffic) and may greatly affect photosynthesis in plants. Therefore, metal deficiency or metal excess may induce changes in pigment composition, alterations in light harvesting protein complexes, photosystem core complexes, chlorophyll fluorescence, the electron transport system, gas exchange parameters, and activity of key photosynthetic enzymes. In addition, metal(loid) stress can lead to the overproduction of reactive oxygen species which may cause extensive damage in plant cells. The chapter summarizes the present knowledge on the effects of metal(loid)s on photosynthetic process and on the defence systems of plants exposed to metal(loid) stress, such as activity of enzymatic and non-enzymatic antioxidant compounds. Additional emphasis is given to the alleviation of metal(loid)s toxicity on photosynthesis by silicon.