(a) Enumerate the functions of Endoplasmic reticulum and Golgi apparatus. 10 (b) Explain inbreeding depression and its effect in…

(a) Functions of the endoplasmic reticulum and Golgi apparatus

Endoplasmic reticulum. The rough ER, studded with ribosomes, makes secretory, membrane and lysosomal proteins, folds them with chaperones, adds core N-linked sugars and checks quality before sending them out in transport vesicles. The smooth ER synthesises lipids, membrane phospholipids and, in animals, steroids; detoxifies drugs and poisons; stores calcium (as sarcoplasmic reticulum in muscle); and forms a network between cell parts, with plasmodesmata links in plants.

Golgi apparatus. A stack of cisternae with a cis face receiving ER vesicles and a trans face dispatching them. It modifies glycoproteins and glycolipids (trimming and adding sugars, sulphation, proteolytic processing), sorts products at the trans-Golgi network for secretion, membrane or lysosomes, forms lysosomes and secretory vesicles, and in plants makes cell-wall matrix polysaccharides (hemicellulose, pectin) and supplies vesicles for the cell plate at cytokinesis.

(b) Inbreeding depression: effects and degrees

Inbreeding depression is the loss of vigour and fitness when normally cross-pollinated crops are self-pollinated or mated between relatives. It arises because inbreeding raises homozygosity and exposes deleterious recessive alleles hidden in heterozygotes, and, in part, because overdominant heterozygote advantage is lost.

Effects in crops: reduced plant height, yield, seed set and vigour, more weak and sterile plants, lethal and chlorophyll-deficient types, greater susceptibility to disease and lodging, and loss of lines in early generations, as in maize, where the decline is steepest in the first generations and then stabilises.

Degrees: (1) High in strictly cross-pollinated crops such as maize, alfalfa, rye and carrot. (2) Moderate in often cross-pollinated crops such as sorghum, pearl millet and cotton. (3) Low in crops such as cucurbits and sunflower. (4) None in self-pollinated crops such as rice, wheat and barley, which are already homozygous.

Inbreeding depression is exploited in developing inbred lines for hybrids and explains why cross-pollinated crops are improved by hybrids and synthetics.

(c) Requirements for certified seed production

Certified seed is the progeny of foundation seed and is produced under the Seeds Act 1966 and Seed Rules 1968 and the Minimum Seed Certification Standards, with certification by the State Seed Certification Agency.

The requirements are: source seed of the prescribed class (foundation or certified, with tags); a field free of volunteer plants and with a suitable previous crop history; isolation distance (for maize, 400 m for foundation and 200 m for certified seed); field inspections at critical stages (vegetative, flowering, pre-harvest); roguing of off-types and diseased plants and of objectionable weeds; harvest, threshing and processing with care to avoid mixtures; laboratory tests meeting standards for physical purity (about 98%), germination (for example 90% maize, 80% rice, 85% wheat), moisture and seed health; and labelling with a blue certification tag, which remains valid for a stated period (nine months, extendable after retest).

(d) Classification of soil water and moisture availability by soil type

Physical classification: gravitational water, which drains by gravity from macropores and is available only briefly; capillary water, held in micropores at field capacity down to the permanent wilting point and mainly available to plants; and hygroscopic water, bound to particles and unavailable. Biological classification: available, unavailable and superfluous water. Moisture constants are saturation, field capacity (about 1/3 bar) and permanent wilting point (15 bar).

Availability by soil type. Available water is the difference between field capacity and wilting point. Sandy soils hold little (about 6–8% by volume) and dry quickly; clays hold the most water at field capacity but keep much of it at the wilting point, so their available water is moderate (about 12–16%); loams and silt loams give the highest available water (about 15–20%) with good aeration. Hence sandy soils need frequent light irrigation and clays less frequent but well-drained irrigation.

(e) Origin and domestication of rice

Cultivated Asian rice, Oryza sativa, arose from the wild perennial or annual Oryza rufipogon, with O. nivara as the annual form. Vavilov placed its centre of origin in the Indo-Burma region, and archaeological finds at Koldihwa in Uttar Pradesh and in the middle Ganga plain suggest domestication there about 7,000–9,000 years ago, while remains at Hemudu and the Yangtze and Pearl River valleys in China show domestication there around 9,000 years ago.

Genome studies are debated: one view holds a single origin of japonica in the Pearl River valley with indica formed by later crossing with local wild rice in India, another holds independent domestication. Indica is generally linked to India and japonica to China. Domestication selected non-shattering grain (sh4 gene), reduced dormancy, larger grains and erect plants. African rice, O. glaberrima, was domesticated from O. barthii in the Niger delta.