essential elements.
A: If excess of Nitrogen is supplied to a healthy plant, its
Vegetative growth is more but reproductive growth is inhibited. It
results in poor yield.
Moderate increase in the concentration of micronutrients causes toxicity.
Any mineral ion concentration in tissues that reduces dry weight of
tissues by about 10% is considered toxic.
Excess of one element may inhibit the uptake of another element.
Excess of Manganese shows Brown spots in the leaf surrounded by
chlorotic veins. Excess of Manganese competes with other micro
elements (Fe, Ca and Mg) and inhibits their uptake. So symptoms of Mn
toxicity
also results in the deficiency symptoms of Fe, Ca and Mg.
Q: Explain the steps involved in the formation of root nodule?
A: The following steps are observed in the formation of root nodule:
1) Bacteria are attracted by sugars, amino acids and flavonoids
secreted by the roots of leguminous plants.
2) Legumes select compatible strains of Rhizobium by specific proteins
called Lectins.
3) Bacteria multiply and secrete curling factor. Root hair shows
curling which is called Shepherd's crook.
4) Bacteria invade the root hair and produce cellulase, pectinase to
digest the cell wall.
5) Bacteria breach the cell wall. Plasma membrane invagination forms
infection thread and carries bacteria into cortical cell.
6) Cortical cells produce Auxins, multiply and root nodule is formed
into which bacteria are released.
7) Bacteria also stimulate pericycle cells to divide.
8) The nodule develops vascular connection.
9) It consists Leghaemoglobin (Pink coloured), Nitrogenase. The
bacterial cells in the nodule become spherical & called bacteroid.
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