Nutrition in Plants

Photosynthesis: the most important chemical reaction on Earth

All food on Earth ultimately comes from photosynthesis. Every calorie you eat traces back to a plant (or algae/cyanobacterium) that converted sunlight into chemical energy.

Where it happens: In chloroplasts — organelles containing the green pigment chlorophyll (and accessory pigments: carotenoids = yellow/orange). Only in cells with chloroplasts (green parts of plants — mainly leaves).

Stomata: Tiny pores on leaf undersurface; CO₂ enters, O₂ exits; water vapour exits (transpiration). Guard cells control stomata opening/closing.

Why leaves are flat and thin: Maximise surface area for sunlight absorption; minimise distance CO₂ must diffuse to reach chloroplasts.

Two stages of photosynthesis:

1. Light reactions (in thylakoids): Light splits water (photolysis) → O₂ released → energy captured as ATP and NADPH
2. Calvin cycle/Dark reactions (in stroma): ATP + NADPH + CO₂ → glucose; does NOT require light directly (but depends on products of light reactions)

Significance for climate:

• Plants absorb CO₂ → reduce greenhouse effect
• Deforestation → less CO₂ absorbed → contributes to climate change
• Phytoplankton (marine algae): Responsible for ~50% of Earth's total photosynthesis; produce ~50% of Earth's oxygen

Insectivorous plants — why do they eat insects?

Insectivorous plants grow in nutrient-poor habitats (bogs, swamps, rocky terrain) where nitrogen is scarce. They supplement photosynthesis by digesting insects to get nitrogen.

Indian insectivorous plants:

• Pitcher plant (Nepenthes khasiana): Found ONLY in Meghalaya (Khasi Hills); modified leaf forms a pitcher containing digestive fluid; insects attracted → fall in → digested; Endangered (EN) per IUCN Red List (NOT Critically Endangered); CITES Appendix I (trade ban); protected under Wildlife Protection Act
• Bladderwort (Utricularia): Aquatic; tiny underwater bladders with trapdoor; sucks in microscopic organisms; common in Indian ponds

Cuscuta (dodder):

• Parasitic flowering plant; yellow/orange leafless vine
• Wraps around host plant; haustoria penetrate host stem and steal water, nutrients, sugars
• Found on many host plants including agricultural crops — a weed problem
• Has NO chlorophyll → completely dependent on host (holoparasite)

Lichens:

• Symbiosis between fungi and algae/cyanobacteria
• Algae provides food (photosynthesis); fungi provides structure, water retention, minerals
• Grow on bare rock → help start soil formation (pioneer species in succession)
• Bioindicators: Very sensitive to air pollution (especially SO₂) → if lichens grow = clean air; absence = polluted air
• Found extensively in Western Ghats, Himalayas

17 Essential Elements for Plant Growth:

• From air and water (3): Carbon (C), Hydrogen (H), Oxygen (O) — obtained through photosynthesis and water absorption; not from soil
• Primary macronutrients (3): Nitrogen (N), Phosphorus (P), Potassium (K) — needed in large amounts; the "NPK" of fertilisers
• Secondary macronutrients (3): Calcium (Ca), Magnesium (Mg), Sulphur (S)
• Micronutrients (8): Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo), Chlorine (Cl), Nickel (Ni)

Key roles and deficiency symptoms:

Diagnostic key (commonly tested in UPSC):

• Old leaves yellow first → mobile nutrient deficiency (N, P, K, Mg — plant scavenges from old tissue)
• Young leaves yellow first → immobile nutrient deficiency (Fe, Ca, B — plant cannot relocate these)

Mycorrhiza (from Greek: mykes = fungus + rhiza = root): A mutualistic symbiosis between fungi and plant roots.

The exchange:

• Plant → Fungi: Photosynthate (sugars) — 10–20% of plant's photosynthesis output goes to feed the fungal partner
• Fungi → Plant: Water and minerals — especially phosphorus, which moves very slowly in soil; fungal hyphae extend far beyond the root zone, dramatically increasing the effective absorptive surface area

Why this matters: Phosphorus (P) is essential but nearly immobile in soil — it doesn't flow toward roots with water. Fungal hyphae network reaches P far beyond what roots alone could access. Plants with healthy mycorrhiza can thrive in P-poor soils that would be fatal without the fungal partner.

Two main types:

Ecological importance:

• In healthy natural forests, trees share carbon and nutrients via mycorrhizal networks (the "Wood Wide Web") — larger trees can support smaller seedlings through fungal connections
• Mycorrhizal fungi are critical for ecosystem recovery after disturbance — deforested land often has poor mycorrhizal communities, hindering reforestation
• Biofertiliser application: Mycorrhizal inoculants added to soil or seeds can reduce phosphorus fertiliser requirement by 25–50%

India's context: ICAR and the National Centre for Organic and Natural Farming (NCONF), Ghaziabad (under Dept. of Agriculture and Farmers Welfare) develop and promote mycorrhizal biofertilisers as part of the National Mission on Natural Farming — reducing chemical fertiliser dependency while maintaining crop yields.