Functions of P :- Visual symptoms :- Predisposing factors:- Corrective measures :- Phosphorus is vital in plant metabolism and phosphate is not reduced like sulfate and nitrate but remains in its highest oxidized form. After uptake, phosphate remains as inorganic phosphate (Pi) or esterified with carbon chain or another phosphate ion. Most phosphate esters are intermediates in metabolic pathways of biosynthesis and degradation and their function and formation are directly linked to photosynthesis, phosphorylation, Kreb’s cycle and nitrogen metabolism. The amount of phosphorus supplied affects various fractions present in the leaf but with increase in the supply from sub optimal to optimal level, all the phosphorus fractions in the leaves increase and above this level only Pi increases reflecting its storage functions in vacuolated tissues. Because of the active involvement of phosphorus in the growth and metabolism of plants, deficiency leads to general reduction of most metabolic processes. Phosphorus improves the colour of the flue-cured tobacco leaf. Phosphorus requirement for optimal growth is in the range of 0.3- 0.5% of the plant dry weight during vegetative stages of growth. Plants suffering from phosphorus deficiency exhibit retarded growth and dark green colour. In the early stages of plant growth, when phosphorus was completely omitted in the nutrient medium, plant growth was stunted and leaves showed lesions on the surface. After attaining certain growth, phosphorus deficiency results in dark green leaves and leaves form an acute angle with the stalk. P deficiency is not seen normally in tobacco plants. High phosphate buffering capacity and high phosphate adsorption at low pH levels may cause P deficiency. Normally deficiency occurs due to inadequate P fertilization and/ or low soil pH. To avoid depletion of soil P in the long run, tobacco growing soils rich in phosphorus need a maintenance dose of P. Phosphorus deficiency can be avoided by closely following soil test recommendations.

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