Plant Physiology and Biochemistry Questions and Answers

Answer: A

The red light receptors are phytochromes.

Phytochromes exist in two interconvertible forms: Pr absorbs red light (about 660 nm) and converts to the active Pfr form, which absorbs far-red light (about 730 nm) and reverts. They control seed germination, de-etiolation, shade avoidance and flowering.

Cryptochromes and phototropins are blue-light and UV-A receptors; cryptochromes regulate de-etiolation and the circadian clock, phototropins phototropism and chloroplast movement. Cytochromes are haem proteins of electron transport, not photoreceptors.

Answer: B

Nitrogen metabolism is impossible without molybdenum.

Molybdenum is a component of two key enzymes. Nitrogenase, which fixes atmospheric nitrogen, has an iron-molybdenum cofactor, and nitrate reductase, which reduces nitrate in plants, contains a molybdenum cofactor. Molybdenum deficiency therefore shows as nitrogen deficiency.

Zinc is needed for auxin synthesis and many enzymes, boron for cell walls and pollen tube growth, and manganese for water splitting in photosynthesis. None is central to nitrogen fixation and nitrate reduction in the way molybdenum is.

Answer: A

Anthocyanins are coloured flavonoids.

Anthocyanins are water-soluble flavonoid pigments stored in the vacuole. They give red, purple and blue colours to flowers, fruits and leaves, the colour shifting with vacuolar pH.

Alkaloids are nitrogen-containing compounds such as nicotine and morphine. Terpenoids are built from isoprene units; the carotenoid pigments are terpenoids, but anthocyanins are not. Tannins are polyphenols known for astringency rather than as the red-blue pigments.