Any plant breeding has a limit to the number of person-hours that can be devoted to it. Consequently, any labour-saving device will permit an increase in the number of crosses, and the number of plants in the screening population. When doing recurrent mass selection , there is no need to label crosses, or individual plants, or to keep detailed records of parasitism, etc. The only thing that matters is that the final selections must be the best plants of that generation and, the larger the screening population, the greater the genetic advance. Labour-saving is not laziness. It is increased productivity. Lactuca sativa

Lettuce. This crop is a member of the Compositae family, and it is the main component of salads. There are four basic types known as 'crisphead', 'butterhead', 'romaine', and 'leaf. All types show great variation and there is considerable scope for increased horizontal resistance. The chief disease is downy mildew caused by the fungus Bremia lactucae. Past resistance breeding has involved vertical resistance, and there is scope for horizontal resistance breeding by amateurs. Ladybirds

Beetles of the family Coccinellidae. These beetles are distinctively oval, almost spherical, with a flat under-surface, and they are coloured red or orange, with conspicuous black spots. Both the adults and the larvae of many species of ladybird feed on other insects, particularly aphids, which are crop parasites, and the ladybirds are valuable agents of biological control. Lagenaria siceraria

The bottle gourd. A monoecious member of the Cucurbitaceae, this is a very ancient crop that pre-dates pottery in many tropical areas. It is apparently the only crop that was common to both the Old World and New World before the development of transoceanic travel. Gourds are believed to have originated in Africa, and to have floated across the Atlantic Ocean to Brazil at a very early date. They also spread to India and China, and all parts of S.E. Asia. The dried hard shells of the fruit have a wide range of uses including bottles, bowls, spoons, ladles, tobacco pipes, musical instruments, and floats for fishing nets. The crop has a limited scope for local amateur breeders . Landrace

A cultivated plant population which is genetically diverse and genetically flexible. A landrace can respond to selection pressures during cultivation. The maize crops of tropical Africa, which were so vulnerable to tropical rust, were landraces, and they responded to the selection pressure for resistance. Prior to the discovery of Johansen's pure lines in 1905, most crop varieties in the industrial world were landraces, and most subsistence crops in the non-industrial world are still landraces. See also: cultivar, Ecotype, Micro-evolution. Larch

See: Larix spp. Larix spp.

Larch, which is used as a plantation forest species of softwood. Not recommended for amateur breeders . Larvae

The early instars of an insect are generally called larvae (singular, larva), particularly in insects that exhibit metamorphosis. Thus caterpillars are the larvae of butterflies and moths. It is often these juvenile stages that are voracious feeders, and that constitute some of the most serious insect parasites of crops. This term should not be confused with the molten rock that comes out of volcanoes, and is spelled lava. See also: Grub. Late selection

Traditionally, selection is conducted on highly heterozygous individuals which then become the parents of the next screening generation. This is now called early selection. Late selection involves self-pollinating the variable progeny of a cross for 3-4 generations, using either the bulk breeding method or single seed descent, and producing a mixed population of relatively homozygous individuals. The late selection is made among these homozygous individuals. Late selection is efficient because it produces plants with a reduced hybrid vigour, which can be misleading during the screening process, and it also produces a greater expression of recessive alleles, which are exhibited only in the homozygous state. The features of late selected plants thus have a higher heritability than those of early selected plants. However, this advantage must be equated with the longer breeding cycle required by late selection.

Was this article helpful?

0 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook

Post a comment