(a) Biological significance of mineral elements in plants 10 (b) Photo-oxidation of water by photosystem II through…

(a) Biological significance of mineral elements in plants

Seventeen elements are essential for plants, of which the mineral ones are taken from the soil. Macronutrients (N, P, K, Ca, Mg, S) are needed in large amounts, and micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni) in traces.

• Structural: N in amino acids and nucleic acids; Ca in the middle lamella as calcium pectate; Mg in chlorophyll; S in cysteine and methionine; P in phospholipids, ATP and nucleic acids.
• Enzymatic and regulatory: K activates enzymes and regulates stomata; Mo is a cofactor of nitrate reductase and nitrogenase; Zn of carbonic anhydrase; Mn in water splitting; Fe in cytochromes and ferredoxin.
• Osmotic and other roles: K and Cl maintain turgor; B assists pollen-tube growth and sugar transport.

Deficiency gives specific symptoms, such as chlorosis in Mg or Fe shortage. Mobile elements (N, P, K, Mg) show symptoms in older leaves first, and immobile ones (Ca, Fe, B) in younger leaves.

(b) Photo-oxidation of water by photosystem II

PSII, in the appressed thylakoid membrane, splits water to replace electrons lost by the reaction-centre chlorophyll P680 after light excitation, releasing oxygen.

The oxygen-evolving complex (OEC) on the lumen side holds a Mn₄CaO₅ cluster and needs Cl⁻ and Ca²⁺. Excited P680 passes an electron to pheophytin and then to plastoquinone, and the P680⁺ formed is reduced by a tyrosine residue (Yz) of the D1 protein, which takes electrons from the OEC. According to the Kok S-state cycle, the OEC is stepped through states S₀ to S₄ by four successive photons, and on reaching S₄ it takes electrons from two water molecules and releases one O₂ and returns to S₀:

2H₂O → O₂ + 4H⁺ + 4e⁻.

Protons are released into the lumen, adding to the proton gradient that drives ATP synthesis.

(c) Photoperiodic induction and the site of perception

Photoperiodism is the response of plants, especially flowering, to the relative length of day and night. Short-day plants (e.g. Xanthium, soybean) flower when the night exceeds a critical length; long-day plants (e.g. wheat, spinach) when it is shorter than a critical length; day-neutral plants flower regardless.

Induction: a required number of inductive photoperiodic cycles given to the plant is enough, afterwards flowering proceeds even in non-inductive conditions. A short flash of red light in the night breaks a long night, and far-red reverses the effect, showing the involvement of phytochrome (Pr/Pfr) and a clock.

Site of perception: the leaf, not the shoot apex. Experiments with Xanthium showed that a single leaf exposed to short days could induce flowering in the whole plant, while exposing only the bud was ineffective. The leaf produces a mobile flowering stimulus (florigen, now identified as FT protein), which passes through the phloem to the shoot apex.

(d) Role of light as a limiting factor in an ecosystem

A limiting factor is an environmental factor whose scarcity most restricts growth or distribution (Blackman's law of limiting factors, extending Liebig's law of the minimum).

Light is the source of energy for photosynthesis and is therefore a key limiting factor for primary production.

• In forests, the canopy intercepts most light, so understorey plants get only a few per cent of it; this gives stratification and shade-adapted plants with low light compensation points.
• In water, light is absorbed with depth, so photosynthesis occurs only in the photic (euphotic) zone above the compensation depth; below it is the aphotic zone.
• Light quality, intensity and duration affect plant growth, flowering, germination and animal behaviour.
• Plants are grouped into heliophytes (sun plants) and sciophytes (shade plants).
• Light also influences species succession, since pioneer species are light-demanding.

(e) Forests are an important wealth of the country

Forests, about a quarter of India's land area as forest and tree cover (ISFR 2023), are renewable natural capital.

• Economic: timber, fuelwood, bamboo, and non-timber forest products such as lac, gums, honey, tendu leaves and medicinal plants, supporting tribal livelihoods.
• Ecological: carbon sinks, regulation of climate and rainfall, soil and water conservation, protection of catchments and habitat for biodiversity, as in the Western Ghats and Himalaya.
• Social and cultural: livelihoods of forest-dependent communities, recreation, sacred groves and ecotourism.
• Policy: the National Forest Policy, 1988 aims at 33% forest and tree cover; the Forest (Conservation) Act, 1980 (as amended) restricts diversion of forest land; and the Forest Rights Act, 2006 recognises community rights.

Forest management must balance use with conservation.