(a) Define leaf senescence. Describe important physiological and biochemical changes taking place during this process. Comment…

(a) Leaf senescence: changes and hormonal regulation

Definition. Leaf senescence is the final stage of leaf development. It is a genetically programmed, active and orderly process in which the leaf's cells are dismantled and their nutrients remobilised to growing organs (seeds, fruits, young leaves, storage organs), ending in cell death and abscission. It is not simple decay. Thousands of senescence-associated genes (SAGs) are switched on, and the nucleus and mitochondria stay functional until late. It can be triggered by age, by reproductive development (monocarpic senescence in wheat and rice), or early by darkness, drought, nitrogen shortage, shading or pathogens. Early on it is reversible: cytokinin or re-greening can reverse it.

Physiological changes

• Decline in photosynthesis: thylakoids and Rubisco are dismantled first, and the chloroplast becomes a gerontoplast full of plastoglobuli. Photosynthesis falls before visible yellowing.
• Colour change: chlorophyll is lost faster than carotenoids, so leaves turn yellow; some species make red anthocyanins in autumn.
• Nutrient remobilisation: in cereals up to 60–90% of grain nitrogen comes from remobilisation of leaf N. Phosphorus, potassium, magnesium and sulphur are also exported. Immobile calcium stays behind.
• Other changes: respiration may rise briefly, water content falls, membranes become leaky, and finally an abscission layer forms.

Biochemical changes

Chlorophyll breakdown (the PAO/phyllobilin pathway):

• Chlorophyll b is reduced to chlorophyll a (NYC1/NOL).
• The Mg-dechelatase STAY-GREEN (SGR) removes Mg²⁺ to give pheophytin.
• Pheophytinase (PPH) removes phytol.
• Pheophorbide a oxygenase (PAO) opens the ring, and RCC reductase gives a colourless catabolite.
• This is exported and stored in the vacuole as non-fluorescent chlorophyll catabolites.
• Removing the phototoxic pigment first prevents photodamage.
• SGR loss gives "stay-green" mutants; Mendel's green-cotyledon pea is one.
• Protein breakdown: Rubisco, which holds about half of leaf protein, is degraded by chloroplast proteases, senescence-associated vacuoles, autophagy and the proteasome. The cysteine protease SAG12 is a marker. The amino acids are exported through the phloem mainly as glutamine and asparagine (GS1 and asparagine synthetase are induced).
• Nucleic acids: RNA falls sharply as RNases and nucleases rise, and phosphate is recovered.
• Lipids: galactolipids and phospholipids are hydrolysed. Fatty acids are β-oxidised, and the glyoxylate cycle converts them to sugars for export.
• Oxidative changes: ROS and lipid peroxidation increase late, as antioxidants decline.
• Gene regulation: transcription factors of the NAC (for example ORE1, NAP) and WRKY (WRKY53) families switch the programme on. Delayed-senescence mutants (ore) identified many of these genes.

Hormonal regulation

Promoters:

• Ethylene: the ethylene-insensitive mutant ein2 (= ore3) stays green longer. Ethylene acts through EIN3, which induces ORE1.
• Abscisic acid: raised by drought; acts through ABF transcription factors and SAGs.
• Jasmonates and salicylic acid: raised during defence and old age.
• Strigolactones and brassinosteroids.

Delayers:

• Cytokinins are the strongest; their level falls in ageing leaves. In the classic experiment, tobacco carrying the cytokinin-synthesis gene IPT under the senescence-activated SAG12 promoter (Gan and Amasino, 1995) stayed green and productive. Cytokinin acts through the receptor AHK3 and by strengthening sink activity.
• Gibberellins and auxin (partially), and polyamines.
• Balance: the outcome depends on the balance of these hormones, on sugar signalling and on leaf age.

Practical importance

• Stay-green crops: "functional stay-green" sorghum (from line B35; Stg QTLs), maize and wheat continue photosynthesising during grain filling and resist terminal drought.
• Post-harvest: 1-MCP (an ethylene-action blocker) and cytokinin treatments extend the shelf life of leafy vegetables and cut flowers.
• Genetic engineering: SAG12::IPT constructs have been used to delay senescence in lettuce, cassava and rice.

(b) Chloroplast ATP synthase: molecular organisation and mechanism

(i) Molecular organisation

The chloroplast ATP synthase (CF₁CF₀, about 560 kDa) is an F-type ATPase in the stroma thylakoids and grana margins. Its CF₁ head faces the stroma.

CF₁ (hydrophilic, extrinsic): α₃β₃γδε.

• Three α (atpA) and three β (atpB) subunits alternate to form a hexamer. The three catalytic sites lie mainly on the β subunits; non-catalytic nucleotide sites are on α.
• γ (atpC) is the central shaft running into the hexamer. It is plant-specific in having a redox switch: a disulfide bridge is reduced by thioredoxin in light (active) and re-oxidised in the dark, so the enzyme does not waste ATP by hydrolysis at night.
• ε (atpE) connects γ to the c-ring and inhibits ATP hydrolysis.
• δ (atpD) sits on top of the hexamer and anchors the peripheral stalk.

CF₀ (membrane-embedded): subunits a, b, b′ and c₁₄.

• c (atpH): 14 copies form the c₁₄ ring (rotor). Each c subunit has a conserved glutamate that binds one proton.
• a (atpI): the stator subunit next to the ring, with two half-channels, one open to the lumen and one to the stroma.
• b (atpF) and b′ (atpG): form the peripheral stalk with δ, holding α₃β₃ stationary.
• Rotor and stator: the rotor is c₁₄ + γ + ε; the stator is a + b + b′ + δ + α₃β₃.
• Genetic origin: six subunits (atpA, B, E, F, H, I) are encoded in plastid DNA and three (atpC, D, G) in the nucleus, so assembly needs import and co-ordination.

(ii) Mechanism of action

1. Proton-motive force (Mitchell's chemiosmotic theory): light-driven electron transport and water splitting pump protons into the lumen (lumen pH about 5, stroma about 8). In chloroplasts most of the proton-motive force is the ΔpH, whereas in mitochondria the membrane potential dominates. Jagendorf's acid-bath experiment (1966) showed that an artificial pH gradient alone makes thylakoids synthesise ATP in the dark.
2. Proton flow and rotation: a proton enters the lumen half-channel of subunit a and neutralises the glutamate of a c subunit. The ring rotates so that the protonated subunit moves through the lipid. After nearly a full turn, the proton is released into the stroma through the second half-channel. Proton flow therefore turns the c₁₄ ring together with γ and ε. Rotation was seen directly in F₁ by Noji and colleagues (1997).
3. Binding-change mechanism (Boyer; structure by Walker; Nobel Prize 1997): the asymmetric γ turns inside the fixed α₃β₃ hexamer and forces each β site through three states:

• Loose (L), which binds ADP and Pi;
• Tight (T), where ATP forms spontaneously;
• Open (O), which releases ATP. Each 120° step releases one ATP, so one full turn makes 3 ATP. The energy is used mainly to release the tightly bound ATP, not to form the bond.

4. Stoichiometry: 14 H⁺ per turn and 3 ATP per turn give about 4.7 H⁺ per ATP. Cryo-EM of spinach CF₁CF₀ (Hahn and colleagues, 2018) confirmed the c₁₄ ring and showed how the γ redox switch blocks rotation in the dark.

The ATP produced, together with NADPH, drives the Calvin cycle in the stroma.

(c)(i) Eutrophication: causes, consequences and control

Eutrophication is the enrichment of a water body with nutrients, chiefly phosphorus and nitrogen, which leads to excessive growth of algae and plants. Natural eutrophication is the slow ageing of lakes over thousands of years. Cultural (anthropogenic) eutrophication speeds this up to decades. Phosphorus usually limits growth in fresh water (the Redfield ratio and Liebig's law), and nitrogen in many coastal waters.

Causes

• Point sources: untreated or partly treated sewage, which contains phosphate from detergents; industrial effluents from dairies, sugar mills and fertiliser plants.
• Non-point sources: fertiliser and manure runoff from farms, soil erosion, and livestock waste.
• Other sources: urban stormwater, atmospheric nitrogen deposition, and aquaculture.
• Natural factors: catchment geology, internal loading (phosphorus released from anoxic sediments), and warm, still water.

Consequences

• Algal blooms: dense blooms, often of cyanobacteria (Microcystis, Anabaena). Some make toxins such as microcystins, which harm livestock and people.
• Oxygen depletion: blooms shade out submerged plants. When they die, decomposition uses up oxygen, causing hypoxia and fish kills and releasing H₂S, and in the sea "dead zones" (for example the Gulf of Mexico).
• Loss of biodiversity: food webs change to favour tolerant species.
• Weed infestation: floating weeds such as water hyacinth spread.
• Economic and health costs: higher treatment costs for drinking water, losses to fisheries and tourism, and mosquito breeding.
• Indian examples: Dal Lake (Srinagar), Hussain Sagar (Hyderabad), Bellandur Lake (Bengaluru, with toxic foam and fires), and Loktak Lake (Manipur, with proliferating phumdis).

Control

Reduce inputs at source:

• sewage treatment with tertiary phosphorus and nitrogen removal (chemical precipitation, biological nutrient removal);
• phosphate-free detergents;
• balanced, precise fertiliser use (soil health cards, neem-coated urea);
• buffer strips and constructed wetlands along streams;
• diverting sewage away from lakes.

In-lake restoration:

• dredging nutrient-rich sediment;
• alum or lanthanum-clay treatment to lock up phosphorus;
• aeration of the hypolimnion;
• harvesting weeds;
• biomanipulation (stocking fish-eating fish so that zooplankton can graze algae).

Policy:

• the National Plan for Conservation of Aquatic Ecosystems (NPCA, 2013);
• the Wetlands (Conservation and Management) Rules 2017;
• Namami Gange, for river water quality;
• the Water (Prevention and Control of Pollution) Act 1974, with effluent standards.

(c)(ii) Importance of biosphere reserves in biodiversity conservation in India today

Biosphere reserves (BRs) are large areas of terrestrial or coastal ecosystems recognised under UNESCO's Man and the Biosphere (MAB) programme (1971). They are meant to reconcile conservation with sustainable development.

Zonation

• Core zone: legally protected, minimally disturbed; often a national park or sanctuary.
• Buffer zone: research, education and regulated low-impact use.
• Transition zone: settlements and sustainable economic activities, managed with local people.

Indian status

• India began its national BR programme in 1986. Nilgiri was the first reserve, and there are now 18, the most recent designated being Panna (2011).
• 13 of them are in UNESCO's World Network: Nilgiri, Gulf of Mannar, Sundarban, Nanda Devi, Nokrek, Pachmarhi, Similipal, Achanakmar–Amarkantak, Great Nicobar, Agasthyamala, Khangchendzonga, Panna and, most recently, the Cold Desert Biosphere Reserve of Himachal Pradesh (September 2025).

Importance today

Landscape-scale in-situ conservation:

• BRs protect whole ecosystems and representative biogeographic zones, from the high-altitude cold desert (snow leopard) to mangroves (Sundarban tigers) and coral reefs (Gulf of Mannar dugongs).
• They also conserve wild relatives of crops, for example wild rice and citrus in Nokrek, and pepper and cardamom relatives in Agasthyamala.

People-centred conservation:

• Unlike national parks, BRs include local communities.
• The transition zone supports livelihoods (eco-tourism, sustainable harvesting of non-timber forest products, traditional agriculture).
• This reduces conflict and draws on traditional knowledge, for example of the Todas and Kurumbas in the Nilgiris and the Garos in Nokrek.
• Research and monitoring: BRs are long-term sites for studying climate change, which is urgent in Himalayan and coastal reserves, and for testing restoration.
• Connectivity: the Nilgiri BR links Mudumalai, Wayanad, Bandipur, Nagarhole and Silent Valley for elephants and tigers.
• Meeting commitments: BRs help India meet its Convention on Biological Diversity and Kunming–Montreal targets ("30 by 30"), complementing protected areas, community reserves and the Biological Diversity Act 2002.

Challenges: the buffer zones often lack legal backing, funding and co-ordination with state forest departments. Development pressures such as the Great Nicobar project, tourism and invasive species persist. Stronger community participation is needed.