Plant Physiology and Biochemistry Questions and Answers

Answer: D

Root nodules are characteristic of Fabaceae.

Legumes such as pea, gram, soybean and groundnut form root nodules in which bacteria of the Rhizobium group fix atmospheric nitrogen. The bacteria enter through root hairs by an infection thread, and the plant provides carbohydrates and leghaemoglobin while receiving ammonia. This is why legumes enrich the soil in crop rotation.

Lamiaceae (the mint family), Asteraceae (the sunflower family) and Acanthaceae do not form such nitrogen-fixing nodules. (A few non-legumes such as Casuarina and Alnus do form nodules, with Frankia, but they belong to none of these families.)

Answer: D

The key gives potassium nitrate, because nitrate is the form of nitrogen the plant takes up directly.

Nitrate needs no conversion in the soil and moves quickly to the roots. Urea has first to be hydrolysed by soil enzymes to ammonium, which takes from about a day to a week depending on temperature, and ammonium itself is mostly nitrified by soil microbes over one to several weeks. So on the reading of 'availability' as nitrogen that is available to the plant at once, potassium nitrate comes first.

Ammonium sulphate and ammonium chloride supply ammonium, which plants can absorb directly but at lower rates. Urea is slow acting in cold or dry soil. Note the limit of the question: if 'availability' were read as nitrogen content per unit weight, urea (about 46 per cent N) would lead, so the question rests on the form of nitrogen, not its percentage. We confirmed the nitrate reading in fertiliser extension notes but could not find the University's own source for it.

Answer: C

In the polymer-trapping model, raffinose and stachyose are made in the intermediary cell.

In plants such as cucurbits that load phloem symplastically, sucrose made in the mesophyll diffuses through plasmodesmata into specialised companion cells called intermediary cells. There it is combined with galactose to form the larger sugars raffinose and stachyose. These are too large to diffuse back through the narrow plasmodesmata towards the bundle sheath but can pass on into the sieve elements, so they are trapped in the phloem and concentrated there.

The mesophyll and bundle sheath supply sucrose, not the polymers, and the sieve element transports them but does not synthesise them.

Answer: D

The CO₂ of photorespiration is released from glycine.

Rubisco's oxygenase reaction splits ribulose 1,5-bisphosphate into 3-phosphoglycerate and 2-phosphoglycolate in the chloroplast. Phosphoglycolate becomes glycolate, which is oxidised to glyoxylate in the peroxisome and converted to glycine. In the mitochondrion the glycine decarboxylase complex then converts two molecules of glycine into one serine, releasing CO₂ and NH₃.

Ribulose 1,5-bisphosphate, 2-phosphoglycolate and glyoxylate are earlier intermediates of the pathway; none of their conversions gives off CO₂.

Answer: D

The answer is leghaemoglobin.

Nitrogenase, the enzyme that reduces N₂ to ammonia, is irreversibly damaged by oxygen, yet the bacteroids in a nodule need oxygen for respiration to supply the ATP the reaction consumes. Leghaemoglobin, a red oxygen-binding protein in the nodule cells, solves this by binding oxygen and delivering it at a low, steady concentration. Effective nodules are pink inside because of it.

Flavonoids (spelt Flavanoids in the paper) released by legume roots act as signals that start nodulation, but they play no part in the fixation reaction. Anthocyanins are pigments, and fatty acids have no special role in nitrogen fixation.

Answer: B

The compound is kinetin.

Kinetin (6-furfurylaminopurine) was discovered by Miller and Skoog in the 1950s in autoclaved herring sperm DNA, as a breakdown product formed during heating. It is a synthetic cytokinin used widely in tissue culture, but it is not one of the regulators plants make, and it is not a normal base of DNA, which is the statement the question quotes.

The other three are natural plant hormones. Indole-3-acetic acid is the main natural auxin, zeatin is the main natural cytokinin, first isolated from maize, and gibberellins are a family of natural growth regulators first found in the fungus Gibberella and in plants.

Answer: A

The one not caused by mineral deficiency is etiolation.

Etiolation is the pale, spindly growth of a plant kept in darkness or very low light: long internodes, small unexpanded leaves and no chlorophyll. Its cause is lack of light, not of any mineral nutrient, and it reverses when the plant is put in light.

The other three are classic deficiency symptoms. Chlorosis (yellowing) follows a lack of nitrogen, magnesium, iron or sulphur; necrosis (death of tissue) follows a lack of calcium, potassium or magnesium; and shortened internodes, as in the rosetting of zinc deficiency, follow a lack of zinc or boron.

Answer: C

The phenomenon is positive heliotropism.

Heliotropism is the movement of a plant organ that follows the sun across the sky. Young sunflower heads and leaves face east in the morning and turn west during the day, then reset at night; growing towards the sun makes the response positive.

Phototropism is growth towards a light source in general rather than daily solar tracking, and photoperiodism is the response of flowering to day length. Negative heliotropism would mean turning away from the sun.

Answer: B

The hormone is abscisic acid.

Under water stress, the roots and leaves make abscisic acid (ABA), which acts on guard cells: it opens ion channels so that potassium and anions leave the guard cells, water follows by osmosis, and the cells lose turgor and close the stomatal pore. This cuts water loss by transpiration, which is why ABA is called the stress hormone.

Ethylene regulates ripening and senescence, and ferulic acid and coumarin are phenolic compounds, not the stomatal signal of water stress.

Answer: A

The incorrect pair is \(\mathrm{C_{3}}\) plants - Maize.

Maize is a \(\mathrm{C_{4}}\) plant: it first fixes carbon dioxide in the mesophyll into the four-carbon oxaloacetate, then releases it to the Calvin cycle in the bundle sheath. Wheat and rice are typical \(\mathrm{C_{3}}\) plants.

The other pairs are correct. \(\mathrm{C_{4}}\) plants have Kranz anatomy, a wreath of bundle-sheath cells round each vein; the first stable product of the Calvin cycle is 3-phosphoglycerate (PGA); and the first product of the Hatch and Slack pathway is oxaloacetic acid.