(a) What are enzymes ? Describe "Key-lock theory" with respect to mode of enzyme action. 15 (b) What is plant physiological…

(a) Enzymes and the lock-and-key theory

Enzymes are biological catalysts, mostly proteins, that speed up biochemical reactions by lowering activation energy without being consumed. Each has an active site, a pocket that binds the substrate; many need a cofactor or coenzyme (the holoenzyme is the apoenzyme plus cofactor). They act with high specificity at low concentration, are sensitive to temperature and pH, and are regulated by inhibitors and activators. Examples are amylase, catalase and nitrate reductase.

The lock-and-key theory (Emil Fischer, 1894) explains the mode of action. The active site of the enzyme is a rigid, pre-formed structure whose shape is complementary to that of its substrate, as a particular key fits only its lock. The substrate fits into the active site to form an enzyme-substrate complex, in which bonds are strained or formed; the complex then breaks into the product or products and the free enzyme, which is reused. The theory explains substrate specificity well. Its limit is the assumption of a rigid active site: it cannot explain how enzymes act on several related substrates or how allosteric regulation changes shape. Koshland's induced-fit model (1958) corrects this, with the active site changing shape as the substrate binds.

(b) Plant physiological stress and drought responses

Plant physiological stress is any external factor that causes a deviation from optimal metabolism and growth, lowering yield. Drought is water deficit that reduces tissue water potential and turgor.

Morphological changes: wilting and loss of turgor; leaf rolling and folding to reduce exposed area; reduced leaf area and shoot growth; early leaf senescence and shedding; thicker cuticle and more leaf hairs in some species; deeper, more extensive roots and a higher root-to-shoot ratio; and shortened crop duration or poor grain set when drought coincides with flowering.

Physicochemical changes: abscisic acid (ABA) accumulates and closes stomata, cutting transpiration; osmotic adjustment through accumulation of compatible solutes such as proline, glycine betaine and sugars; reduced photosynthesis from stomatal closure and damage to the photosynthetic machinery, with chlorophyll degradation; production of reactive oxygen species and lipid peroxidation of membranes, lowering the membrane stability index; and protective responses such as antioxidant enzymes (superoxide dismutase, catalase), dehydrins and heat-shock proteins.

(c) Vernalization and its practical application in crops

Vernalization is the promotion of flowering by exposure of seeds or growing plants to a low temperature, typically 0-10°C for several weeks, as in winter cereals and biennials. It is distinct from chilling injury, and from photoperiodism, which is a response to day length; the two often interact, as in winter wheat, which needs cold and then long days. The requirement is obligate in some types and facultative in others, and the effect is reversed by high temperature (devernalization).

Mechanism: in Arabidopsis, the flowering repressor FLC is epigenetically silenced during cold through VIN3 and Polycomb-based histone methylation, which allows the integrator FT, the mobile flowering signal ('florigen'), to be expressed and flowering to begin.

Applications: pre-treating germinated seed of winter wheat and rye at 0-5°C for 30-50 days lets them be sown in spring, converting winter types for spring planting; the chilling of bulbs and seeds speeds up breeding by allowing winter types to flower in a single season; seed production of temperate vegetables such as cabbage, cauliflower, carrot and radish is done in cool hill areas such as Kashmir, Himachal Pradesh and the Nilgiris, where winter cold meets the requirement; and bolting, the premature flowering of crops such as cabbage and onion after cold spells, is avoided by choosing sowing dates and varieties. Most wheat grown in the plains of Punjab and Haryana is spring type with little or no vernalization requirement, while winter and facultative types are used at higher altitudes.