(a) What is high density orcharding ? Describe management practices for high density orcharding in citrus group. (20 marks) (b)…

(a) High density orcharding and its management in citrus (20 marks)

High density orcharding (or high density planting, HDP) means planting more trees per unit area than the conventional spacing, by using dwarfing or compact plants and controlling the canopy, so as to get early, higher and more efficient returns per hectare. In citrus, the conventional spacing of about 6 × 6 m gives around 275 trees per hectare, whereas HDP uses about 3 × 3 m (about 1,100 trees) or closer, in hedgerows.

Management practices:

1. Planting material: healthy, virus- and greening-free, certified budded plants on suitable dwarfing or semi-dwarfing rootstocks (trifoliate orange, Carrizo or Troyer citrange, Flying Dragon) and compact scion varieties.
2. Site and layout: a deep, well-drained soil, a dependable water supply and full sun; square or hedgerow layout in the correct row direction.
3. Training and pruning: training to a modified leader or open-centre shape from the first year, with regular light pruning, topping and skirting to hold the tree height at about 2.5 to 3 m, and thinning to admit light. Growth retardants such as paclobutrazol may be used to control vigour.
4. Nutrition and irrigation: drip irrigation with fertigation in split doses, in line with the crop stage, and organic manure, with foliar sprays of micronutrients (zinc, iron, manganese) where deficient.
5. Weed and soil management: mulching, and intercropping (legumes, vegetables) in the early years, before the canopies close.
6. Crop regulation: bahar (flowering season) treatment in mandarin, sweet orange and lime, by withholding water and manuring, together with fruit thinning to maintain size and to check alternate bearing.
7. Plant protection: integrated management of citrus psylla (which carries greening), leaf miner, canker, gummosis and mites, and prompt removal of infected trees.
8. Rejuvenation and replacement when orchards age, and mechanisation for spraying and harvesting.

Advantages are early bearing, higher yield and returns per hectare, more efficient use of land, water, fertiliser and light, easier operations and better fruit quality. The drawbacks are the higher initial cost and the need for skilled management.

(b) Biosynthesis of chlorophylls and carotenoids; effect of environmental factors on plant pigments (20 marks)

Chlorophyll biosynthesis (in the chloroplast):

1. Glutamate, activated as glutamyl-tRNA, is converted through glutamate-1-semialdehyde into 5-aminolevulinic acid (ALA), the first committed precursor. In plants this is the C5 pathway.
2. Two ALA molecules condense (ALA dehydratase) into porphobilinogen (PBG); four PBG join (PBG deaminase and uroporphyrinogen III synthase) to give uroporphyrinogen III.
3. Successive decarboxylation and oxidation give coproporphyrinogen III, protoporphyrinogen IX and protoporphyrin IX. Here the pathway branches: Fe insertion by ferrochelatase leads to haem, and Mg insertion by Mg-chelatase leads to chlorophyll.
4. Mg-protoporphyrin IX is methylated and cyclised to form protochlorophyllide.
5. In angiosperms, the light-dependent enzyme protochlorophyllide oxidoreductase (POR) reduces protochlorophyllide to chlorophyllide a; this is why seedlings are pale (etiolated) in the dark.
6. Chlorophyll synthase adds the phytol tail to make chlorophyll a, and chlorophyllide a oxygenase converts part of it to chlorophyll b.

Carotenoid biosynthesis (in plastids):

1. Isopentenyl diphosphate (IPP) and dimethylallyl diphosphate are formed by the plastidial MEP pathway from pyruvate and glyceraldehyde-3-phosphate.
2. Geranylgeranyl diphosphate (C20) is formed, and two molecules condense (phytoene synthase, the key regulatory step) to give phytoene (C40).
3. Desaturation and isomerisation (phytoene and ζ-carotene desaturases, isomerases) give lycopene.
4. Cyclisation: two β-rings give β-carotene, and one β- and one ε-ring give α-carotene.
5. Hydroxylation gives the xanthophylls: lutein from α-carotene, and zeaxanthin from β-carotene; zeaxanthin is epoxidised to violaxanthin and neoxanthin, and this is the xanthophyll cycle. Cleavage of these gives abscisic acid.

Environmental effects:

• Light: chlorophyll formation in angiosperms needs light. Shade leaves have more chlorophyll per unit of reaction centre, with a higher chlorophyll b ratio, while high light or UV lowers chlorophyll and raises carotenoids and xanthophylls (photoprotection) and anthocyanins.
• Temperature: chlorophyll is best formed at moderate temperature; cold or high heat reduces it and hastens degradation, and cool nights promote anthocyanin and carotenoid colour in fruit.
• Water stress and salinity: lower chlorophyll and hasten senescence.
• Nutrients: nitrogen and magnesium are constituents of chlorophyll, and iron (and manganese, sulphur) is needed for its synthesis, so their deficiency causes chlorosis.
• Others: senescence, ethylene, waterlogging and pollutants (ozone, heavy metals) break down chlorophyll and unmask carotenoids, as in autumn colours and ripening fruit.

(c) Nutrient deficiency in humans and its major causes in India (10 marks)

Nutrient deficiency in humans is a condition in which the diet or the body's absorption does not supply enough of one or more nutrients for normal growth, health and function. It includes deficiency of energy and protein (undernutrition, stunting, wasting) and of micronutrients such as iron, iodine, vitamin A, zinc, folate, and vitamins B₁₂ and D, called hidden hunger because the signs appear late. Anaemia and stunting remain widespread: NFHS-5 (2019–21) found about 57% of women aged 15–49 and 67% of children aged 6–59 months anaemic, and about 36% of children under five stunted.

Major causes in India:

• Poverty and low purchasing power, which limit food quantity and diversity.
• A cereal-dominated, monotonous diet, with too little pulses, milk, fruit, vegetables and animal foods.
• Low nutrient content of foods and crops, from soils poor in zinc, iron, iodine and selenium, and losses during milling (polished rice) and cooking.
• Poor absorption, from phytates and tannins in the diet, and infections, diarrhoea and worm infestation, with poor sanitation and unsafe water.
• Poor maternal nutrition, early marriage and pregnancy, and inadequate breastfeeding and complementary feeding of infants.
• Low awareness, gender inequity in food distribution, and limited access to health services.

Responses include fortification (iodised salt, fortified rice and oil), biofortified crops, supplementation and school and child feeding programmes, and dietary diversification.