Much of the country is now experiencing winter weather. Recently, snow closed the better part of Interstate 40 in New Mexico, and Hayward, WI, was 2° F a night or two ago. But here on California’s central coast, many plants are still growing and flowering (a little slowly perhaps, given the short days). But some of them, those adapted to more temperate, northern climes, are confused. The day length tells them that it’s time to pack it in for the year, but 70° F temperatures tell them to be fruitful and multiply.
Despite mixed environmental messages, most plants here are not fooled. Common deciduous trees (sycamores, maples, alders) and some perennials (wisteria and roses, for instance) are looking a little wan, losing their color, and dropping their leaves. They are senescing.
It appears to be so passive, senescence. In the annuals, the entire plant simply shrivels and dies. Trees and shrubs may briefly flash brilliant orange, red or yellow, but then their leaves fall to the ground after a weekend of rain. Or your once vibrant perennial garden gradually fades to dull colors like a photograph left in the sun. It all seems so effortless.
But senescence is anything but passive or effortless. It is another stage in plant development and is a tightly regulated and genetically programmed developmental process. In a sense, it is opposite other major plant developmental events that mostly involve cell division, differentiation and growth—but not quite. Senescence is not only death or quiescence; senescence also ensures continued life either in the next generation or the next growing season.
Senescence is initiated by a number of factors. Among these are environmental cues such as temperature extremes, day length, drought, nutrient deficiency, pathogen infestation, wounding and shading; a plant’s age, reproductive state and phytohormone levels also play a role. Once initiated, senescence is a complex, sequential process of “programmed cell death” that leads to the destruction of leaf cells and the recycling of nutrients and the energy that was captured from the Sun during the growing season and that was incorporated into cellular building blocks of the leaves.
Senescence is a little like dismantling a house from the inside out. The valuable interior parts are taken out and stored away and finally the structural elements are removed. New gene expression is required for orderly self destruction of the cell and recycling its nutrients, so the cellular machinery of gene expression must be maintained while the cell dismantles itself and transports its valuable nutrients to a storage site in the stem, roots or wherever. The destruction begins with the chloroplast, the site of photosynthesis, where much of the nitrogen in the leaf cell is located. As the green pigment disappears, the other pigments in the leaf become visible, hence “fall color”. Amyloplasts (starch bodies), simpler carbohydrates, and various fats and proteins are broken down and translocated out of the cell. But only late in the process is the nucleus, the genetic “framework” of the cell, broken down.
Senescence may be initiated and influenced by many factors, but we know from experiments with the water plant Elodea that senescence, and chloroplast senescence specifically, is directly under the control of the nucleus. When Elodea leaves are exposed to a high-salt solution, some protoplasts split into two more or less equal parts. There are many chloroplasts in a cell but only one nucleus, so both halves have chloroplasts but only one half has a nucleus. Those halves without a nucleus remain green and continue to photosynthesize, but those with a nucleus senesce on schedule.
There are lots of good, practical economic reasons to study senescence too. Senescence has a big impact on agriculture. With the onset of senescence, brought on by drought, pathogen infestation—whatever, photosynthesis drops off sharply in the senescing leaves. And these leaves are the main photosynthesizing organ in plants, so reduced photosynthesis means reduced crop yields and reduced biomass production. Also nutritional components such as proteins and antioxidants can be degraded in fruits and vegetables produced by senescing plants, both before and after harvest, thus reducing nutritional quality. Furthermore, senescing tissues are more susceptible to pathogen infestation and postharvest decay. Each year lots of research dollars are devoted to efforts to understand and manipulate senescence better.
Remarkable, isn’t it? What seems like an afterthought to one growing season is really the prelude to the next. It is not an end but an interlude in plant life.