Deficiency diseases are among the non-parasitic physiological disorders, which are due mainly to nutritional deficiencies or toxicities. Each nutritional element produces its own deficiency symptoms. Within one plant, mobile elements can be taken from old tissues to feed the young tissues, and the symptoms then appear mainly in the older tissues. Conversely, immobile elements cannot be re-allocated in this way, and the main deficiency symptoms then appear in the youngest tissues. Deficiency symptoms are easily confused with herbicide injury, and a specialist should usually be consulted. For the symptoms of each mineral deficiency, see Nitrogen, Phosphorus, Potassium, Magnesium, Calcium, Boron, Sulphur, Iron, Zinc, Copper, and Manganese. Defoliation
Loss of leaf. Defoliation can be (1) natural, as with leaf-fall in a deciduous tree or shrub; (2) pathologically induced by the activities of parasites; (3) induced by the misuse of herbicides; or (4) from abnormal environmental conditions, such as severe drought. Dehiscent
This term means that a seed capsule, anther, etc, opens spontaneously when mature. Democratic plant breeding
The converse of autocratic plant breeding. With democratic plant breeding, as many breeders as possible are producing as many cultivar as possible so that the farmer has a wide choice of cultivars. This approach is possible with the use of horizontal resistance because breeding for this kind of resistance is so easy. In many instances, the farmers themselves will do their own plant breeding. Once there are enough amateur plant breeders, the whole system of crop improvement will become self-organising. Density-dependent selection
The limiting of the size of a population (e.g., a vertical pathotype) by mechanisms that are themselves controlled by the size of that population. This is a probable genetic mechanism for controlling the system of locking of the n/2 model, ensuring that all the n/2 biochemical locks and keys occur with an equal frequency. The rarity of a vertical pathotype or pathodeme is a reproductive advantage that leads to commonness. And commonness is a reproductive disadvantage that leads to rarity. Derris elliptica
The powdered dried root of this Leguminous plant contains rotenone and other toxic compounds that are used as an insecticide and a fish poison in S.E. Asia. The insecticide is used mainly as a hair wash to control lice. This derris insecticide is stable, as no derris-resistant strains of insects have appeared during centuries of use. Derris dust can also be used as an insecticide on crops. The pounded roots are soaked in water to produce a fish poison, and the poisoned fish can be eaten without risk. Selected clones of the crop are propagated vegetatively by cuttings of mature stems. But seed set is common and improvement by amateurs is feasible. However, there are no serious parasite of derris, and breeding for horizontal resistance appears to be unnecessary. Derris is a short-day plant that needs a tropical forest ecology, and there appears to be no possibility of producing temperate cultivar.
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