18
Plants with zinc deficiency show reduced biosynthesis of
Explanation
Plant with zinc deficiency shows, reduced biosynthesis of auxin. Zinc is the microelement that is absorbed by almost all parts of the plant in the form of $\mathrm{Zn}^{2+}$ ion.
It functions as a constituent of carbonic anhydrase and auxin. It also activates various enzymes especially carboxylases and dehydrogenases.
19
Yellowish edges appear in leaves deficient in.
Explanation
Yellowish edges or chlorosis appears in the leaves, deficient in nitrogen. Nitrogen deficiency also causes delaying of flowering, interference in protein synthesis and dormacy of lateral buds.
20
Name the macronutrient which is a component of all organic compounds but it not obtained from soil.
Explanation
Carbon is an essential element. Plant take it from atmosphere in the form of $\mathrm{CO}_2$. It's concentration in atmosphere is about $0.03 \%$. Plants use $\mathrm{CO}_2$ for photosynthesis (as a source of carbon) to synthesises glucose.
21
Name one non-symbiotic nitrogen fixing prokaryote.
Explanation
Azotobacter is a non-symbotic nitrogen fixing prokaryote. It flourishs in the rice fields.
22
Rice fields produce an important green house gas. Name it.
Explanation
Rice fields remain logged with excess water which harbour the great microbial activity. Many anaerobic bacteria also grow in these areas and release methane which is a green house gas.