A Review on Effect of Senescence in Plants and Role of Phytohormones in Delaying Senescence
Senescence is the final stage of organ development in plants. It is an oxidative process which involves a general deterioration of cellular metabolism in plants. The symptoms of senescence include loss in chlorophyll, carotenoids, proteins and increase in lipid peroxidation and membrane permeability which results in injury and leads to decrease in the photosynthetic output. During the period of senescence, a multiple factors such as hormones, environmental factors and senescence associated genes (SAGs) are involved. Plant hormones or phytohormones acts as chemical messengers which are involved in plant growth and developmental process and play a major role in regulation of senescence. The phytohormones such as salicylic acid, abscisic acid, jasmonic acid and ethylene promote senescence, where as others like cytokinins, gibberellins, and auxins delay this process. During the process of senescence generation of reactive oxygen species (ROS) which includes superoxide, singlet oxygen, hydroxyl radical and hydrogen peroxide radicals increased in thylakoids takes place. These free radicals are capable of inducing cellular damage by oxidation of proteins, inactivation of enzymes, alterations in the gene expression, and decomposition of biomembranes. Thus, plants have evolved a defense mechanism which scavange ROS by antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT). Thus in the present review all factors that are responsible for causing or accelerating senescence have discussed, antioxidant activity which protects plants from ROS and role of phytohormones such as auxins, gibberillic acid and cytokinins have participated significantly in delaying or to control senescence have been discussed.