explain the structure of a seed and the process of germination with diagrams.
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Respiration:
Imbibition of water causes the resumption of metabolic activity in the rehydrated seed. Initially their respiration may be anaerobic (due to the energy provided by glycolysis) but it soon becomes aerobic as oxygen begins entering the seed. The seeds of water plants, as also rice, can germinate under water by utilizing dissolved oxygen.
The seeds of plants adapted to life on land cannot germinate under water as they require more oxygen. Such seeds obtain the oxygen from the air contained in the soil. It is for this reason that most seeds are sown in the loose soil near the surface. Ploughing and hoeing aerate the soil and facilitate seed germination. Thus the seeds planted deeper in the soil in water-logged soils often fail to germinate due to insufficient oxygen.
(iii) Effect of Light on Seed Germination:
Plants vary greatly in response to light with respect to seed germination. The seeds which respond to light for their germination are named as photoblastic. Three categories of photoblastic seeds are recognized: Positive photoblastic, negative photoblastic and non-photoblastic. Positive photoblastic seeds (lettuce, tobacco, mistletoe, etc.) do not germinate in darkness but require exposure to sunlight (may be for a brief period) for germination.
Negative photoblastic seeds (onion, lily, Amaranthus, Nigella, etc.) do not germinate if exposed to sunlight. Non-photoblastic seeds germinate irrespective of the presence (exposure) or absence (non-exposure) of light.
In these light sensitive seeds, the red region of the visible spectrum is most effective for germination. The far-red region (the region immediately after the visible red region) reverses the effect of red light and makes the seed dormant. The red and far-red sensitivity of the seeds is due to the presence of a blue-coloured photoreceptor pigment, the phytochrome. It is a phycobiloprotein and is widely distributed in plants.
Phytochrome is a regulatory pigment which controls many light-dependent development processes in plants besides germination in light- sensitive seeds. These include photo-morphogenesis (light-regulated developmental process) and flowering in a variety of plants.
Phytochrome and Reversible Red-Far-red Control of Germination:
The pigment phytochrome that absorbs light occurs in two inter-convertible forms Pr and Pfr. Pr is metabolically inactive. It absorbs red light (660 nm.) and gets transformed into metabolically active Pfr (Fig. 4.10). The latter promotes germination and other phytochrome-controlled processes in plants. Pfr reverts back to Pr after absorbing far-red (730 nm.).
In darkness too, Pfr slowly changes to Pr. Owing to this oscillation of phytochrome between Pr and Pfr status, the system has been named as “reversible red—far-red pigment system” or in brief phytochrome system. Treatment with Red light (R) stimulates seed germination, whereas far-red light (FR) treatment, on the contrary, has an inhibitory effect.

Let US examine seed germination in positive photoblastic seeds e.g. lettuce (Lactuca sativa). When brief exposure of red (R, 660 nm.) and far-red (FR, 730, nm.) wave lengths of light are given to soaked seeds in close succession, the nature of the light provided in the last exposure determines the response of seeds. Exposure to red light (R) stimulates seed germination. If exposure to Red light (R) is followed by exposure to far-red light (FR), the stimulatory effect of Red light (R) is annulled.
This trick can be repeated a number of times. What is crucial for seed germination is the quality of light to which the seeds are exposed last. This also indicates that responses induced by red light (R) are reversed by far-red light (FR).

Light requirement for seed germination may be replaced by hormones such as gibberellins or cytokinins. Several development processes of plants controlled by phytochrome may be mimicked by appropriate hormones given singly or in combination with other hormones at the correct time.
Imbibition of water causes the resumption of metabolic activity in the rehydrated seed. Initially their respiration may be anaerobic (due to the energy provided by glycolysis) but it soon becomes aerobic as oxygen begins entering the seed. The seeds of water plants, as also rice, can germinate under water by utilizing dissolved oxygen.
The seeds of plants adapted to life on land cannot germinate under water as they require more oxygen. Such seeds obtain the oxygen from the air contained in the soil. It is for this reason that most seeds are sown in the loose soil near the surface. Ploughing and hoeing aerate the soil and facilitate seed germination. Thus the seeds planted deeper in the soil in water-logged soils often fail to germinate due to insufficient oxygen.
(iii) Effect of Light on Seed Germination:
Plants vary greatly in response to light with respect to seed germination. The seeds which respond to light for their germination are named as photoblastic. Three categories of photoblastic seeds are recognized: Positive photoblastic, negative photoblastic and non-photoblastic. Positive photoblastic seeds (lettuce, tobacco, mistletoe, etc.) do not germinate in darkness but require exposure to sunlight (may be for a brief period) for germination.
Negative photoblastic seeds (onion, lily, Amaranthus, Nigella, etc.) do not germinate if exposed to sunlight. Non-photoblastic seeds germinate irrespective of the presence (exposure) or absence (non-exposure) of light.
In these light sensitive seeds, the red region of the visible spectrum is most effective for germination. The far-red region (the region immediately after the visible red region) reverses the effect of red light and makes the seed dormant. The red and far-red sensitivity of the seeds is due to the presence of a blue-coloured photoreceptor pigment, the phytochrome. It is a phycobiloprotein and is widely distributed in plants.
Phytochrome is a regulatory pigment which controls many light-dependent development processes in plants besides germination in light- sensitive seeds. These include photo-morphogenesis (light-regulated developmental process) and flowering in a variety of plants.
Phytochrome and Reversible Red-Far-red Control of Germination:
The pigment phytochrome that absorbs light occurs in two inter-convertible forms Pr and Pfr. Pr is metabolically inactive. It absorbs red light (660 nm.) and gets transformed into metabolically active Pfr (Fig. 4.10). The latter promotes germination and other phytochrome-controlled processes in plants. Pfr reverts back to Pr after absorbing far-red (730 nm.).
In darkness too, Pfr slowly changes to Pr. Owing to this oscillation of phytochrome between Pr and Pfr status, the system has been named as “reversible red—far-red pigment system” or in brief phytochrome system. Treatment with Red light (R) stimulates seed germination, whereas far-red light (FR) treatment, on the contrary, has an inhibitory effect.

Let US examine seed germination in positive photoblastic seeds e.g. lettuce (Lactuca sativa). When brief exposure of red (R, 660 nm.) and far-red (FR, 730, nm.) wave lengths of light are given to soaked seeds in close succession, the nature of the light provided in the last exposure determines the response of seeds. Exposure to red light (R) stimulates seed germination. If exposure to Red light (R) is followed by exposure to far-red light (FR), the stimulatory effect of Red light (R) is annulled.
This trick can be repeated a number of times. What is crucial for seed germination is the quality of light to which the seeds are exposed last. This also indicates that responses induced by red light (R) are reversed by far-red light (FR).

Light requirement for seed germination may be replaced by hormones such as gibberellins or cytokinins. Several development processes of plants controlled by phytochrome may be mimicked by appropriate hormones given singly or in combination with other hormones at the correct time.
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