The end of fall at Bard College comes along with drizzles, soon, a sense of winter sneaks in without notice. Winter catches our attention by changing our surroundings, the goldenrods are no longer bright, flowers of wild carrots no longer dancing in the wind. Without the scorching heat of the summer and the cozy wind in fall, the canopy in the forest no longer covers the blue sky, but turns yellow, then those leaves drift down like feathers. Like the picture below, they are taken at the same place at different times in the season. The leaves are still green and flourishing in fall. When winter comes, those leaves turn yellow and fall from trees. These aging leaves, also known as leaf senescence, created the last scenery for us before the pale winter. Soon, the woods will be bare and lifeless, but a new cycle of life is secretly being bred, and waiting for the coming of the next spring.
Leaf senescence is the final stage of leaf development, and it is a crucial strategy for plants to restore energy. How do they accomplish that? Usually, energy in the leaves stays in the form of chloroplast (a leaf structure that contains up to 70% of the leaf protein). In addition energy is also gained from photosynthesis (a process of using sunlight to synthesize foods for plants), which takes place in the chloroplast. To restore energy, the chloroplast breaks down, thus leading to the ending of photosynthesis. Though we can not see these fancy mechanisms that happen inside of plants with our bare eyes, we can tell the senescence is going on by simply looking at the appearance of leaves.
Take a few minutes walk into the forest near Fisher Center of Bard, hear the fallen leaves rustling as you move. Have you noticed that the edge of the leaves turn yellow before the center does? The structure of a plant supports the resources to go through the stem, and then enter the vein in each leaf. These veins are like the vascular system in the human body, diverging essential nutrients to each part of the plant. When leaves begin to age, they prioritize the protection of those veins, thus the parts closest to the veins, which lays at the center of the leaves, degrade the last.
Most importantly, leaf senescence is not simply a degenerative process, but a recycling process. Nutrients, including the energy recycled from chloroplast breakdown and saved from the resting photosynthesis that I mentioned before, are transferred to young leaves, developing seeds, or storage tissues.
There are a few factors that will trigger leaf senescence. Environmental stresses play a significant role because the mechanism of leaf senescence was developed for adaptation and to complete its life cycle even under stressful conditions. The senescence process we see near Fisher is triggered by the cooling weather at the end of fall for plants to protect themselves from the coming winter. However, when pressure from the outside environment is not applied, the age of a leaf has a major influence on the initiation of senescence. The rate of photosynthesis declines once the leaf has fully matured. There is a speculation that when the photosynthetic rate drops below a certain threshold, leaf senescence starts. The mechanism is still unclear and needs further exploration by scientists.
For modern industry, lead senescence technology is also widely applicable. Leaf senescence in crops may limit yield, therefore people can manipulate senescence for agricultural applications. If leaf senescence is delayed, then the performance of horticultural crops can be improved. For example, for vegetables with green-leaves, it is key for those leaves to stay green.