Plant senescence is the process of aging in plants. Plants have both stress-induced and age-related developmental aging. Chlorophyll degradation during leaf senescence reveals the carotenoids, such as anthocyanin and xanthophylls, which are the cause of autumn leaf color in deciduous trees. Leaf senescence has the important function of recycling nutrients, mostly nitrogen, to growing and storage organs of the plant. Unlike animals, plants continually form new organs and older organs undergo a highly regulated senescence program to maximize nutrient export. Hormonal regulation of senescence Programmed senescence seems to be heavily influenced by plant hormones. The hormones abscisic acid, ethylene, jasmonic acid and salicylic acid are accepted by most scientists as promoters of senescence, but at least one source lists gibberellins, brassinosteroids and strigolactone as also being involved. Cytokinins help to maintain the plant cell and expression of cytokinin biosynthesis genes late in development prevents leaf senescence. A withdrawal of or inability of the cell to perceive cytokinin may cause it to undergo apoptosis or senescence. In addition, mutants that cannot perceive ethylene show delayed senescence. Genome-wide comparison of mRNAs expressed during dark-induced senescence versus those expressed during age-related developmental senescence demonstrate that jasmonic acid and ethylene are more important for dark-induced (stress-related) senescence while salicylic acid is more important for developmental senescence. Annual versus perennial benefits Some plants have evolved into annuals which die off at the end of each season and leave seeds for the next, whereas closely related plants in the same family have evolved to live as perennials. This may be a programmed "strategy" for the plants. The benefit of an annual strategy may be genetic diversity, as one set of genes does continue year after year, but a new mix is produced each year. Secondly, being annual may allow the plants a better survival strategy, since the plant can put most of its accumulated energy and resources into seed production rather than saving some for the plant to overwinter, which would limit seed production. Conversely, the perennial strategy may sometimes be the more effective survival strategy, because the plant has a head start every spring with growing points, roots, and stored energy that have survived through the winter. In trees for example, the structure can be built on year after year so that the tree and root structure can become larger, stronger, and capable of producing more fruit and seed than the year before, out-competing other