Physiological Changes During Senescence

                                 Physiological Changes During Senescence

(i) Photosynthesis:

It is generally observed that the net photosynthetic rate increases as leaves grow and then declines gradually at the time of maximum leaf expansion. This indicates that the rate of photosynthesis is a function of individual leaf age. During the rapid phase of whole plant senescence, young and old leaves degenerate almost together. The rate of photosynthesis also declines with equal rapidity in young and old leaves. Simultaneous senescence of leaves occurs not only in monocarpic senescence but also in autumnal senescence of polycarpic plants. Although loss of chlorophyll content is a commonly used index of senescence, it does not always measure accurately either senescence or photosynthetic activity. Chlorophyll content usually decreases well after photosynthetic activity has begun to decline, and in some cases, chlorophyll content shows an increasing trend while the photosynthetic activity is dropping. Even then, chlorophyll loss during senescence offers a visual method of estimating the degree of senescence. As with chlorophyll content, the synthesis and activities of chloroplast enzymes (e.g., RuBPcase) decline after the cessation of leaf growth which is parallel to the loss of photosynthetic activity. Batt and Woolhouse, demonstrated that the Calvin cycle enzymes that are synthesized on chloroplast ribosomes decline earlier during senescence than those synthesized primarily on cytoplasmic ribosomes. It has been shown that conformational changes and loss of active site leading to the loss of RuBPcase occur at a faster rate than the breakdown of total Fraction I. Although photorespiration does not show a uniform pattern of change in all aging plants, it increases greatly in senescing C4 plants, and the increased photorespiration, a characteristic of C3 plants, may be related to a shift to C3 type CO2 fixation. Thus, it appears that the overall decrease in photosynthetic rate in senescing leaves results from a decline in CO2 reduction reactions.

 

(ii) Respiration:

The respiratory apparatus of senescing tissue remains active until late in senescence, and then declines rapidly. Loss of respiratory capacity can be a factor in the final stage of senescence, whereas there is evidence that the initial stage of senescence is not linked with respiratory decline. The mitochondria often swell and decrease in number, but these changes occur late in senescence. Amino acids have been found to accumulate, giving rise to a change in the respiratory quotient. A climacteric rise in respiration may occur in the senescence of both intact and detached leaves. From the fact that respiration decreases only late in senescence and that the lack of oxygen supply prevents the breakdown of chlorophyll and protein, it appears that cellular energy is required during senescence, possibly for the synthesis of the degradative enzymes. The respiratory climacteric of senescing leaves is correlated with a rise in RNA levels and an increase in protease activity.

 

(iii) Nitrogen Fixation and Mineral Uptake:

Several studies have demonstrated that symbiotic nitrogen fixation in legumes directly depends upon the amount of photosynthetic available to the root nodules. It is interesting to note that depodding which increases the availability of photosynthetic can cause appreciable increase in nitrogen fixation and delay nodule senescence. In a similar way, the decline in whole plant photosynthesis parallels the decline in nitrogen fixation. Symbiotic nitrogen fixation decreases with the aging and senescence of leguminous plants. Such decline in nitrogen fixation is associated with the structural changes of individual nodules and bacteroids which is followed by their senescence. Root growth as well as root function usually slow early in the reproductive phase, lust like nitrogen fixation, mineral uptake through the roots suffers a decline during the reproductive phase.

 

(iv) Protein and Nucleic Acids:

The most basic of all events accompanying senescence is the decline in protein and nucleic acid levels. It has been shown that although there is a decline in protein, RNA and DNA of senescing tissues, these three metabolites do not decline at the same rate. There is an early gradual decline in protein and RNA about the time when vegetative growth ceases, while DNA decreases last. The initial decrease in protein and nucleic acid in the chloroplast involves a decrease in the major chloroplast enzyme. RuBP case and other enzymes which are synthesized on chloroplast ribosome. The loss of these enzymes is correlated with reduced ability of the chloroplast to synthesize protein and RNA coupled with a reduction in the number of chloroplast polysomes. The decline in protein and nucleic acids takes place in two stages. The period of gradual decline, characteristic of initial stage of senescence, is followed by more rapid senescence and rapid decline when chlorosis or leaf yellowing starts. During the rapid decline, protein and nucleic acid degradation is accelerated in both the chloroplasts and the cytoplasm. In addition, the synthetic capacity in the whole cell decreases. During this period, large quantities of hydrolytic enzymes like protease, RNase, phosphatase and chlorophyllase are synthesized on cytoplasmic ribosome. Thus, the prime candidates for central centres for a marked decline in protein and nucleic acids seem to be due to (a) release or activation of hydrolases present in the vacuoles or lysosomes, (b) a possible decrease in protease inhibitor, and (c) a decrease in protein and nucleic acid synthesis. In the whole plant, the senescing leaves before their abscission are depleted of materials like amino acids, sugars which are transported to other parts.

 

(v) Membranes and Organelles:

Of all the cell organelles, the chloroplast shows the earliest symptoms of physiological decline. Decrease in RuBP case activity is noticed first with a corresponding early loss of chloroplast ribosomes and this precedes the decrease in cytoplasmic ribosomes. In later phases of senescence, however, pronounced ultra-structural changes in the thylakoids occur. Other changes occurring at this time include swelling, vesiculation and disappearance of endoplasmic reticulum (ER) and Golgi bodies together with the loss of ER-associated ribosome. The galactolipids and sulpho-lipids, the constituents of chloroplast lipids, decrease before the phospholipids that predominate in the non-chloroplast membranes, suggesting that the biochemical decline may occur relatively early in chloroplasts. Although the changes in chloroplasts occur early, they remain intact till the time when the tonoplast ruptures. As soon as the tonoplast disintegrates, the hydrolytic enzymes, acids and toxic compounds are released in the vacuole and this triggers the cellular degradation. In the final stage of cellular degradation, the plasma lemma disintegrates and the nucleus undergoes massive alteration, whereas the mitochondria may persist with intact shape even at this stage. It is generally acknowledged that membranes seem to play an important role in plant senescence.

 

(vi) Nutrient Deficiency Syndrome:

Nutrient deficiencies may be important in the senescence programme of particular organs or the whole plant. This concept finds support in the senescence of monocarpic plants where the emergence of reproductive structures like flowers and fruits seems to impose a great demand on the vegetative body, thus causing nutrient deficiency. In such cases, the life of the plant can be spared or at least death can be delayed by removal of the reproductive structures, obviously, through avoidance of nutrient deficiency. At present, it has been possible to recognize two patterns of deficiency:

(i) Nutrient drain or withdrawal from the senescing parts

(ii) Diversion of the supply of nutrients away from the senescing organs

Thus, nutrient deficiency seems to be the primary cause of monocarpic senescence. Deficiency caused by nutrient drain, e.g., mineral withdrawal, and nutrient diversion, e.g., mineral and cytokinins from the roots and photosynthetic produced by the leaves, presumably play a role in monocarpic senescence.