(a) Define biodiversity. Explain the various ex situ and in situ methods of conserving biodiversity. (20 marks) (b) What are…

(a) Biodiversity and Its Conservation: In Situ and Ex Situ Methods

Biodiversity (biological diversity; the term was coined by W.G. Rosen in 1985 and popularised by E.O. Wilson) is, in the words of the Convention on Biological Diversity (1992), "the variability among living organisms from all sources including terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems". It has three levels:

1. Genetic diversity: variation in genes and alleles within a species, e.g., thousands of rice and mango varieties in India.
2. Species diversity: the number (richness) and relative abundance (evenness) of species in an area, measured as alpha, beta and gamma diversity.
3. Ecosystem diversity: the variety of habitats and communities, such as forests, grasslands, wetlands, deserts, mangroves and coral reefs. India is one of 17 megadiverse countries, with about 7–8% of recorded species on 2.4% of the world's land, and parts of four biodiversity hotspots (the Himalaya, Indo-Burma, the Western Ghats–Sri Lanka and Sundaland, which includes the Nicobar Islands).

In situ conservation protects species in their natural habitats, together with the ecological and evolutionary processes that sustain them. It is the preferred approach for whole ecosystems and wild populations.

1. Protected areas under the Wild Life (Protection) Act, 1972:

• National parks, e.g., Kaziranga (one-horned rhinoceros), Silent Valley (tropical evergreen forest) and the Valley of Flowers (alpine flora).
• Wildlife sanctuaries.
• Conservation reserves and community reserves.

2. Biosphere reserves (under UNESCO's Man and the Biosphere programme) are zoned into a core, a buffer and a transition area, so that conservation is combined with sustainable use by local people. India has 18, e.g., Nilgiri (the first, 1986), Nanda Devi, the Sundarbans and the Gulf of Mannar; many are in UNESCO's World Network of Biosphere Reserves.
3. Species- and habitat-focused programmes: Project Tiger (1973), Project Elephant, the Indian rhino and snow leopard programmes, Ramsar wetlands, and gene sanctuaries, such as the citrus gene sanctuary in the Garo Hills (Nokrek) for wild Citrus.
4. Biodiversity hotspots and Key Biodiversity Areas guide global priorities.
5. Community conservation: sacred groves (e.g., devrai in Maharashtra, kavu in Kerala, law kyntang in Meghalaya), the Bishnoi tradition, joint forest management, and People's Biodiversity Registers under the Biological Diversity Act, 2002 (amended 2023).
6. On-farm conservation: farmers maintain land races, supported by the Protection of Plant Varieties and Farmers' Rights Act, 2001.

Ex situ conservation keeps species or their germplasm outside their natural habitats. It is a back-up for species whose habitats are destroyed, for breeding and for reintroduction.

1. Botanical gardens and arboreta: there are over 3,000 worldwide, e.g., the Royal Botanic Gardens, Kew; the Acharya Jagadish Chandra Bose Indian Botanic Garden, Howrah; and the Lloyd Botanic Garden, Darjeeling. Zoos and captive breeding programmes do the same for animals, e.g., the gharial and the great Indian bustard.
2. Seed banks: orthodox seeds are dried to 3–7% moisture and stored at −18 to −20°C for decades. Examples are the National Gene Bank at ICAR-NBPGR, New Delhi (one of the world's largest, with about half a million accessions), the Millennium Seed Bank at Kew, and the Svalbard Global Seed Vault (Norway) as a safety back-up.
3. Field gene banks: living collections of recalcitrant-seeded and vegetatively propagated crops that cannot be seed-banked, such as mango, coconut, cocoa, banana and tuber crops.
4. ***In vitro* conservation:** slow-growth cultures of shoot tips kept at low temperature or with growth retardants.
5. Cryopreservation: meristems, embryos, pollen or seeds are stored in liquid nitrogen (−196°C), where metabolism stops, after vitrification or encapsulation–dehydration. This is used for recalcitrant and endangered species (NBPGR cryobank).
6. Pollen banks, DNA banks and herbaria. Ex situ collections support reintroduction, e.g., rare orchids and Nepenthes khasiana propagated by tissue culture.

Comparison: in situ conservation is cheaper per species, protects entire communities and allows continued evolution and adaptation, but it cannot save species whose habitat is already lost. Ex situ conservation gives security and easy access for research and breeding, but it keeps a limited sample of genetic diversity, stops adaptation to changing conditions, and needs continuous funding. The two are complementary, as recognised by the CBD and the Kunming–Montreal Global Biodiversity Framework (2022), which set the "30×30" target of protecting 30% of land and sea by 2030.

(b) Phytohormones; Role and Mechanism of Action of Auxin

Phytohormones (plant hormones) are naturally occurring organic compounds, other than nutrients, that are active at very low concentrations (often below micromolar), are produced in one tissue and act in the same or another tissue, and regulate growth, development and responses to the environment. The classical groups are auxins, gibberellins, cytokinins, abscisic acid and ethylene. Newer groups include brassinosteroids, jasmonates, salicylic acid, strigolactones and peptide hormones. Synthetic compounds with similar effects are called plant growth regulators.

Auxin: the natural auxin is indole-3-acetic acid (IAA), made mainly in shoot apices and young leaves from tryptophan by TAA1 and YUCCA enzymes. Its existence was shown by Darwin's canary grass experiments, Boysen-Jensen and Paál, and F.W. Went (1928), who used the Avena curvature test. Other natural auxins are indole-3-butyric acid (IBA) and 4-chloro-IAA; synthetic auxins include NAA, 2,4-D and 2,4,5-T.

Roles in growth and development:

1. Cell elongation in stems and coleoptiles; at higher concentrations, auxin inhibits root elongation.
2. Tropisms: lateral redistribution of auxin causes phototropic and gravitropic bending.
3. Apical dominance: auxin from the apex suppresses axillary buds, partly through strigolactones and by lowering cytokinin; decapitation releases the buds.
4. Root initiation: auxin induces lateral and adventitious roots, so IBA and NAA are used for rooting cuttings.
5. Vascular differentiation: it induces xylem and phloem differentiation and cambial activity, and patterns leaf veins.
6. Fruit set and growth: auxin causes parthenocarpy (tomato, brinjal), and auxin from the achenes drives growth of the strawberry receptacle.
7. Abscission: it delays leaf and fruit drop (NAA is used to prevent pre-harvest drop in apple and mango); at high doses, it thins fruits.
8. Tissue culture: a high auxin-to-cytokinin ratio favours roots and a low ratio favours shoots (Skoog and Miller).
9. Herbicides: 2,4-D selectively kills broad-leaved weeds in cereals.
10. Phyllotaxy: auxin maxima at the shoot apex position new leaf primordia; it also promotes embryo patterning and the formation of female flowers in cucurbits (through ethylene).

Mechanism of action:

1. Polar auxin transport: IAA moves from cell to cell in a basipetal direction in shoots. According to the chemiosmotic model, IAA enters cells as protonated IAAH or through AUX1/LAX influx carriers, becomes trapped as the anion IAA⁻ in the neutral cytosol, and leaves through PIN-FORMED (PIN) efflux carriers located at one end of the cell. ABCB transporters also export it. Relocation of PINs creates auxin gradients.
2. Acid-growth hypothesis (rapid response, within minutes): auxin activates plasma membrane H⁺-ATPase. Cell-surface TMK receptor kinases and SAUR proteins, which inhibit the PP2C-D phosphatases, keep the pump phosphorylated and active. Protons pumped into the wall lower the pH to about 4.5–5.5, which activates expansins. These loosen hydrogen bonds between cellulose microfibrils and xyloglucans, so the wall yields to turgor and the cell expands. Continued growth needs uptake of K⁺ and water and synthesis of new wall material.
3. Gene regulation (nuclear pathway): in the nucleus, auxin acts as a "molecular glue" between the F-box receptor TIR1/AFB, part of the SCF^TIR1 ubiquitin ligase, and Aux/IAA repressor proteins. The Aux/IAA proteins are polyubiquitinated and destroyed by the 26S proteasome. This frees auxin response factors (ARFs), which bind auxin-response elements (TGTCTC) in promoters and switch on early genes (Aux/IAA, GH3 and SAUR families), which lead to growth and developmental responses.

(c) Tropic Movements in Plants and Their Mechanism

Tropic movements (tropisms) are directional growth curvatures of plant organs in response to a stimulus coming from one direction. Growth towards the stimulus is positive and away from it is negative. They differ from nastic movements (e.g., the leaf folding of Mimosa pudica, flower opening), whose direction does not depend on the direction of the stimulus.

Types:

1. Phototropism (response to light): shoots are positively phototropic, roots are negatively or weakly phototropic, and leaves are diaphototropic (lying perpendicular to light).
2. Gravitropism (geotropism) (response to gravity): primary roots are positively gravitropic, shoots are negatively gravitropic, and lateral roots and branches are plagiotropic.
3. Thigmotropism (response to contact): tendrils of pea, cucurbits and Passiflora, and twining stems, coil around supports.
4. Chemotropism (response to chemicals): the pollen tube grows through the style towards the ovule, and fungal hyphae grow towards nutrients.
5. Hydrotropism (response to moisture): roots grow towards moist soil.
6. Others: thermotropism, aerotropism, electrotropism and skototropism (e.g., seedlings of Monstera grow towards dark tree trunks).

Mechanism: the common basis is differential growth, where cells on one side of the organ elongate faster than those on the opposite side. It usually follows three steps: perception of the stimulus, signal transduction, and a growth response. In most tropisms, lateral redistribution of auxin mediates the response (the Cholodny–Went hypothesis, 1926–28).

1. Phototropism:

• Perception: blue light (about 450 nm) is perceived by phototropins (phot1 and phot2), which are plasma-membrane flavoprotein kinases with LOV domains; they autophosphorylate and act through NPH3.
• Signalling: the PIN3 auxin efflux carriers relocate to the shaded side, so auxin moves laterally from the lit side to the shaded side, as Went showed with agar blocks and later work confirmed with isotopes.
• Response: the shaded side elongates faster, by acid growth, and the shoot bends towards light.

2. Gravitropism:

• Perception: according to the starch–statolith hypothesis (Haberlandt and Němec, 1900), dense starch-filled amyloplasts (statoliths) in statocytes settle to the lower side of the cell. In roots, statocytes are the columella cells of the root cap (removing the cap abolishes the response); in shoots, they are the endodermis (starch sheath). Starchless mutants respond weakly, which supports the hypothesis.
• Signalling: sedimentation leads to relocation of LAZY proteins and PIN3/PIN7 to the new lower face, together with Ca²⁺ and pH changes. Auxin flows towards the lower side, and in roots it is carried back to the elongation zone by AUX1 and PIN2.
• Response: in roots, which are very sensitive to auxin, the high auxin concentration on the lower side inhibits elongation, while the upper side grows, so the root bends downward. In shoots, higher auxin stimulates elongation of the lower side, so the shoot bends upward.

3. Thigmotropism: contact is perceived by mechanosensitive ion channels, producing rapid Ca²⁺ influx and action potentials. Growth slows on the contact side and increases on the outer side, aided by auxin and ethylene (and jasmonates in tendril coiling), so the tendril coils within minutes to hours; later lignification makes the coil firm.
4. Chemotropism of pollen tubes: the synergids secrete cysteine-rich LURE peptides (shown in Torenia), which are perceived by receptor kinases on the pollen tube tip (e.g., PRK6). In lily, the stylar protein chemocyanin also guides tubes. The tip-focused Ca²⁺ gradient reorients to steer the growing tip.
5. Hydrotropism: root caps sense the moisture gradient. In Arabidopsis it requires the gene MIZ1 in the cortex and ABA signalling, and it largely does not depend on auxin redistribution, unlike gravitropism. It can override gravitropism in dry soil.