Changes that occur during the leaf senescence process

The senescence process is beneficial to plants as it allows nutrients to be relocated in the plant to places of need. It is an age dependent process that is under the control of both internal and external stimuli. The senescence process is a coordinated event and can act as either an organ level process, such as the decay of leaves on a tree in autumn, or at a whole plant level such is seen at the end of monocarpic plant lifespans. The senescence of the leaf is controlled by a process known as programmed cell death. This section takes a look at the biochemical and structural changes that occur during leaf senescence.

Leaf structure changes during senescence

One of the main structural changes that occur in leaves during the in senescence process is an ordered change in intracellular organelle disintegration. The first process in leaf senescence involves changes in the structure of the grana located in the chloroplast; this leads to a formation of lipid plastoglobuli. At this stage the mitochondria and nucleus of the leaf plant cells are left alone; this allows for them to function correctly until the final stages of programmed cell death occur.

During the later stages of programmed cell death in senescent leaves vacuolar collapse, condensation of chromatin and the cleaving of DNA occurs; this in turn leads to the death of the cell. In turn this leads to the disintegration of the cell plasma and membranes and the end of cellular homoeostasis.

Biochemical changes during senescence

As can be expected the structural changes that occur during leaf senescence are underlaid by biochemical mechanisms.

Initially anabolism is reduced during the senescence process; this results in a lowering of polysomes and ribosomes. This indicates that one of the early process of senescence involves the reduction in the synthesis of proteins, and a lowering of rRNA and tRNA. Chloroplast degeneration lowers the levels of proteins such as Rubisco and CAB.

The next biochemical changes are geared towards the removal of nutrients from the cell. These include hydrolysis and mobilisation of macromolecules. The proteins are broken down to amino acids by peptidases. Enzymes such as phospholipase D are involved in the break down of lipids in leaf senescent cell membranes. Another biochemical change that occurs during the leaf senescence process is a reduction in the amount of nucleic acids such as RNA, which is mediated by RNAase. The fatty acids that are present in the cell are oxidises and used as a source of energy in the break down of the cell.

It is seen that many biochemical and structural changes occur during the programmed termination of cells during leaf senescence.