Ageing of plant organs is retarded by cytokinins. They help in protein synthesis and mobilises nutrient resources.

The condition of a seed when it' fails to germinate even in suitable environmental condition is called seed dormancy. There are two .main reasons for the development of dormancy: Imposed dormancy and innate dormancy.
Methods of breaking dormancy:
The dormancy of seeds can, be broken by different methods. These are:
1. Scarification: Mechanical and chemical treatments like cutting or chipping of hard tough seed coat and use of organic solvents to remove waxy or fatty compounds are called as Scarification.
2. Impaction: In some seeds water and oxygen are unable to penetrate micropyle due to blockage by cork cells. These seeds are shaken vigorously to remove the plug which is called Impaction
3. Stratification: Seeds of rosa ceo us plants (Apple, Plum, Peach and Cherry) will not germinate! until they have been exposed to well aerated, moist condition under low temperature (0°C to 10°C) for weeks to months. Such treatment is called Stratification.
4. Alternating temperatures: Germination of some seeds is strongly promoted by alternating daily temperatures. An alternation of low and high temperature improves the germination of seeds
5. Light: The dormancy of photoblastic seeds can be broken by exposing them to red light.

Auxin is a phytohormone - Physiological effects
1. They promote cell elongation in stem and coleoptile.
2. At higher concentrations auxins inhibit the elongation of roots but induce more lateral roots. Promotes growth of root only at extremely low concentrations.
3. Suppression of growth in lateral bud by apical 'buddue to auxin produced by apical bud is termed as apical dominance,
4. Auxin prevents abscission.
5. It is responsible for initiation and promotion of cell division in cambium, which is responsible for the secondary growth and tumor. This property of induction cell division has been exploited for tissue culture techniques and for the formation of callus.
6. Auxin stimulates respiration.
7. Auxin induces vascular differentiation.
Agricultural role:
1) It is used to eradicate weeds. Example: 2,4-D and 2, 4, 5 - T.
2) Synthetic auxins are used in the formation of seedless fruits (Parthenocarpic fruit).
3) It is used to break the dormancy in seeds.
4) Induce flowering in Pineapple by NAA & 2, 4 - D.
5) Increase the number of female flowers and fruits in cucurbits.

old age is called" senescence 'in plants,  senescence refers to all col1ective, progressive and deteriorative processes which ultimately lead to complete loss of organization and 'function.'
Four types of senescence:
I) Overall senescence
2) Top senescence
3) Deciduous senescence ,
4) Progressive senescence
Overall senescencer This kind of senescence occurs in annual plants when entire plant get 'affected and dies . Eg: Wheat and Soybean. It also occurs in few perennials also'. Eg: Agave and Bamboo.
Top senescence: It occurs in aerial parts of plants. It 'is common in perennials, underground and root system remains viable. Eg: Banana and Gladiolus.
Deciduous senescence: It is common in deciduous plants and occurs only in leaves of plants, bulk of the stein and root system remains alive. Eg: Elm and Maple.
Progressive senescence: This kind of senescence is gradual. First it occurs in old leaves followed by new leaves then stem and' finally root system. It is common in annuals.
Physiology of Senescence:
1) Cells undergo changes in structure.
2) Vacuole of the cell acts as lysosome and secretes hydrolytic enzymes.
3) The starch content is decreased in the cells.
4) Photosynthesis is reduced due to loss of chlorophyll accompanied by synthesis and accumulation of anthocyanin pigments therefore the leaf becomes red .
5) There is a marked decrease in protein content in the senescing organ.
6) RNA content of the leaf particularly rRNAlevel is decreased in the cells due to increased activity of the enzyme RNA ase.
7) DNA molecules in senescencing leaves degenerate by the increased activity of enzyme DNA ase. 
Factors affecting Senescence:
1) ABA and ethylene accelerate senescence while auxin and cytokinin retard senescence.
2) Nitrogen deficiency increases . senescence whereas nitrogen supply retards senescence.
3) High temperature. accelerates senescence but low temperature retards senescence.'
4) Senescence is rapid in dark'than iti light.
5) Water stress 'leads to accumulation of ABA leading to senescence.
Programned cell death:
Senescence is controlled by plants own genetic programme. Death of the plant or plant part consequent to senescence is called programmed cell Death. In short senescence of an individual cell is called PCD.

The physiological change on flowering due to relative length of light and darkness (photoperiod) is called Photoperiodism. The term photoperiodism was coined by Gamer and Allard.
The photoperiod required to induce flowering is called critical day length. Maryland mammoth (tobacco variety) requires 12 hours of light and cocklebur (Xanthium pensylvanicum)requires 15.05 hours of light for flowering.
Classification of plants ba ed on Photoperiodism:
Depending upon the photoperiodic responses plants are of the following types:
1. Lonz day plants: The plants that require longb critica day length for flowering are called long day plants or short night plants. Eg: Pea. Barley and Oats.
2. Short lonz day plants: These are long day plants but Shoula be exposed to short day lengths 
during early period of growth for flowering Eg: Wheat and Rye
3. Short day plants: The plants that require a short critical day length for flowering are called short day plants or long night plants Eg: Tobacco. Cocklebur, Soybean, Rice and Chrysanthemum'
4. short-day plants but they have to be exposed to long days during their early periods of growth for flowering. Eg: Some species of of Bryophyllum and Night jasmine.
5. Intermediate day plants: These require a photo period between long day and short day for flowering. Eg Sugarcane and Coleus.
6. Day neutral plants: There are a number of plants which can flower in all possible photo periods. They are also called photo neutrals or indeterminate plants. Eg: potato Rhododendron, Tomato and cotton.