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Physiological Responses of Gibberellin ( Role of Gibberellin)

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Physiological Responses of Gibberellin ( Role of Gibberellin) Plant Physiology Prof.(Dr.) Punam Jeswal Head B.Sc (Hons.) Part III Botany Department GIBBERELLINS Physiological Responses of Gibberellin ( Role of Gibberellin) Some important roles of gibberellin are as follows :- 1) Apical Bud Dormancy 2) Role In Sub-apical Meristem. 3) Cell Elongation 4) Fruit Growth 5) Flowering 6) Stem Elongation 7) Seed Germination 8) Mobilisation of Food In Seed Storage Cells Apical Bud Dormancy :- Vegetative regions, such as apical meristem, sub-apical meristem and the elongation zone, the activity of apical meristem is almost independent of gibberellin. But under certain environment conditions when the apical meristem become dormant and meiotic activity ceases, Gibberellin can reverse this dormancy. The cold treatment which breaks the natural bud dormancy increases the endogenous level of gibberellins. The activity of this region is regulated by gibberellin via counter-acting the effects of endogenous growth retardants (dormin). So the action of gibberellin in apical meristem is simple as to protect if from the inhibitory effect of dormins. When the level of GA increases, there is a decrease in the of dormin. Role In Sub-Apical Meristem :- Gibberellin's role in sub-apical meristem is direct. It regulate the mitotic processes in this region. Sachs (1960) studied the effect of growth retardants on the tall varieties of Chrysanthemum which has an active sub-apical meristem. Growth retardants (AMO 1618) resulted in cessation of cell division in this region and dwarfing of stem. He observed that the normal activity could be restored by subsequent addition of gibberellin. Fig :- Plant undergo stem and petiole elongation only in long days, remaining in a rosette from in short days. Treatment with GA biosynthesis inhibitors AMO- 1618 prevent stem and petiole elongation. Gibberellic acid can reverse the inhibitory effect of AMO-1618 on stem and petiole elongation. Cell Elongation :- The role of gibberellin in cell elongation is less clear. Gibberellin play a minor role in this respect. However, the interaction between light and gibberellin offers a different conclusion. In certain varieties of pea, red light induce strong inhibition of cell division, cell elongation and extensibility of cell wall. The experiment showed that red light dwarfed pea was 37 mm in height but treated with GA1 and GA5, it attained the height of 120mm and 54 mm, respectively. Fruit Growth :- The roles of gibberellin controlling fruit growth are many. Using gibberellin, Crane (1964) has produced normal looking fruits from the unfertilised flowers of tomato and from certain varieties of apple and peaches. These parthenocarpic fruits were seedless. Fig :- Gibberellin Induces Fruit Growth Flowering :- Gibberellin also play an important role in the initiation of flowering. Lang (1960) demonstrated that added gibberellin could substitute for the proper environment condition in Hyoscyamus niger which require long day treatment for flowering. Such effects of gibberellin are common among vernalised and long day plants. The synthesis of florigen is mediated through GA. Stem Elongation :- The most important effect of GA is the stem elongation, i.e. GA induce inter-nodal elongation or sub-apical elongation. It has been confirmed on several plants such as pea, bean, tomato, pepper, sweat corn, summer squash, cucumber, lettuce, cabbage etc. In these plants, a significant elongation of internodes is reported. A rosette habit showing cabbage grows 2 m tall and produces flower after treated with GA3. A) B) Fig :- Gibberellin Stimulated Stem Elongation (Growth). Seed Germination :- During seed germination, the role of GA in the induction of synthesis of ά-amylase and other hydrolytic enzymes among monocots and certain dicots is well documented. GA3 appears mainly to induce activity of the gluconeogenic enzymes during early stages of seed germination. Fig :- Mobilization of nutrients by gibberellin during seed germination Mobilisation of Food in Seed Storage Cells :- Akazawa and Miyata (1982) working on cereal grains suggested that GA stimulates conversion of storage polymers (polysaccharides, protein and fats) into sucrose or mobile amino acids or amides to facilitate their translocation via phloem into through-out the young root and shoot. The embryo in these plants surrounded by the food reserves present in the metabolically inactive cells of endosperms and then aleurone layers. Aleurone cells provide hydrolytic enzymes that digest the starch, protein, phytin, RNA and certain cell wall materials present in the endosperm cells. One of the necessary enzymes for these digestion processors is ά-amylase which is stimulated by GA synthesised in the scutellum (cotyledon) or other parts of embryo. Fig :- A Schematic illustrating Gibberellin induced release of enzymes and carbohydrates mobilization during germination of barley seed. .
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