(a) Describe the botanical features, ecology and propagation of rubber tree. How is rubber obtained and processed from rubber…

**(a) Rubber tree (Hevea brasiliensis): botany, ecology, propagation and rubber**

Botany: Hevea brasiliensis (Euphorbiaceae) is a tall, semi-deciduous tree from the Amazon basin, which sheds leaves in the dry season. Leaves are long-petioled and trifoliate, with extrafloral nectaries at the petiole apex. Flowers are small, unisexual, in axillary panicles, with male and female flowers on the same plant (monoecious). The fruit is a three-lobed explosive capsule with large mottled seeds. Latex, an aqueous suspension of rubber (cis-1,4-polyisoprene) particles, is held in articulated non-anastomosing laticifers in the bark phloem.

Ecology: warm humid tropics, about 25–28°C, well-distributed rainfall of about 2000 mm or more, and deep well-drained loamy or laterite soils of pH 4.5–6.5. In India it is grown mainly in Kerala, the chief producer, and also in Tamil Nadu, Karnataka and the north-east.

Propagation: mainly vegetative, by budding high-yielding clones (e.g. RRII 105) on seedling rootstocks, as seedlings vary greatly in yield.

Obtaining and processing: the bark is tapped early in the morning with a thin, downward half-spiral cut, cutting the latex vessels, on alternate days. Latex flows into cups and is collected. It is (1) coagulated with dilute formic or acetic acid, rolled into sheets and smoke-dried as ribbed smoked sheet; (2) processed into crepe rubber by washing and milling; or (3) concentrated by centrifugation and ammonia-preserved for dipped goods. Rubber is vulcanised with sulphur.

**(b) Classification of maize (Zea mays) varieties**

On endosperm:

• Flint (var. indurata): hard, rounded kernels with a hard starchy endosperm.
• Dent (var. indentata): soft starch at the crown shrinks on drying, leaving a dent; most widely grown.
• Flour (var. amylacea): soft starchy endosperm, used for flour.
• Sweet (var. saccharata): recessive su gene gives sugary, wrinkled kernels.
• Waxy (var. ceratina): endosperm almost wholly amylopectin.
• Popcorn (var. everta): small, hard kernels that burst on heating.
• Opaque-2 (quality protein maize): high in lysine and tryptophan.

On glume and floral features:

• Pod corn (var. tunicata): each kernel is enclosed by long glumes, due to the dominant Tu gene; the ear is a botanical curiosity and not commercially grown.

(c) Somaclonal variation

Causes: Variation that arises in plants regenerated from culture. Causes are pre-existing variation among somatic cells of the explant, and culture-induced changes such as chromosomal rearrangements and ploidy change, point mutations, transposon activation, gene amplification and epigenetic changes (DNA methylation), especially after a long callus phase, high 2,4-D and stress.

Identification:

• Morphological: changes in height, leaf shape, pigmentation and yield.
• Cytological: chromosome count, karyotype, flow cytometry.
• Biochemical: isozyme and protein patterns.
• Molecular: RFLP, RAPD, SSR and AFLP markers.

Isolation: Somaclones are isolated by in vitro selection, growing cells or calli on a medium with a selective agent such as salt, a toxin (e.g. of Helminthosporium), herbicide or an amino-acid analogue, and regenerating plants from the surviving cells. They are then tested over several generations (progeny testing) for heritable stability before use.