The stem of upright herbaceous plants always grows upwards, continuously producing new leaves. Once the leaves have grown, they fix in position, and soon those left behind turn yellow and wither.

Within a plant community, a shortage of leaves is caused by leaf withering, which is known as leaf aging.

This process is akin to the aging and eventual death process seen in animals. However, the aging of plant leaves does not necessarily have a direct connection to plant death; it can be seen as a strategy plants adopt to enhance their adaptability.

The Physiology of Leaf Aging

The most crucial function of leaves is to obtain carbon through the process of photosynthesis. The rate of photosynthesis increases with increasing light intensity until it reaches a saturation point.

The photosynthetic capacity at saturated light varies among the same plant species and even among different leaves of the same plant. Generally, photosynthetic capacity increases with the enlargement of the leaf surface area. When the leaf expansion is complete, the photosynthetic capacity peaks and then gradually declines over time until death.

Photosynthesis is a metabolic process that involves many proteins. The reason for the decline in photosynthetic capacity, after it peaks, is due to the gradual decrease in proteins synthesized during photosynthesis. About 16% of protein weight consists of nitrogen, and a high photosynthetic capacity requires a significant amount of nitrogen. Half of the nitrogen in leaves is contained within proteins synthesized during photosynthesis.

Therefore, the photosynthetic capacity of leaves is closely related to the amount of nitrogen they contain. The proteins that have been broken down are mainly transferred as amino acids to new organs. Since nitrogen is often a limiting element for plants, even aging leaves, still contain nitrogen.

Plants reclaim the nitrogen from old leaves, and the old leaves contribute their nitrogen to new organs for reuse. Many plants have high efficiency in nitrogen reuse.

Like the nitrogen content of leaves, which peaks when leaf expansion is complete, 80-90% of the nitrogen is transferred at this time, with the remaining 10-20% repeatedly reused to sustain life and finally transferred to fruits.

Plant aging is often likened to "programmed cell death" in animals. Programmed cell death refers to the phenomenon where cells that are no longer needed actively die during development or differentiation.

This is a widespread phenomenon in various tissues of many animals, such as the disappearance of the tadpole's tail. However, programmed cell death is also regulated by genetic factors, and it is not random cell death. Particularly in organisms like certain nematodes, the timing of cell death is predetermined.

Leaf aging is also influenced by genetic factors. The methods typically used in physiological, biochemical, and molecular biology research on leaf aging involve floating leaves cut from plants on the water surface, where they turn yellow in the dark after 2-30 days. This phenomenon suggests that if the synthesis of nuclear proteins is inhibited at this stage, leaf aging can be suppressed.

This also implies a signal for the beginning of leaf aging and suggests that the genetic factors for protein-degrading enzymes are located in the nucleus. Thus, the process of leaf aging is a manifestation of various genetic factors and is a tightly regulated process.

The synthesis of photosystem proteins is greatly increased during leaf expansion, and once expansion is complete, their synthesis is inhibited, promoting degradation. There are various theories regarding the mechanisms of protein degradation, but it has not been fully elucidated.

During leaf aging, a common morphological change is a reduction in the amount of chlorophyll or a decrease in the volume of individual chlorophylls. When rice leaves age, the amount of chlorophyll significantly decreases, but in wheat, the reduction in volume is more pronounced.

This phenomenon suggests that the structure of protein degradation may vary depending on the species. While the decrease in chlorophyll volume can be speculated to be due to protein degradation within chlorophyll, the reasons for the reduction in chlorophyll quantity are not yet clear.

Leaf aging in plants is a complex and tightly regulated process influenced by genetic factors. Further research into the genetic mechanisms and biochemical pathways involved in leaf aging promises a deeper understanding of plant physiology and development.