IMPROVEMENT
(A) CROP RESISTANCE TO WATER DEFICIT CAN BE IMPROVED:
Improving drought resistance is an important aim of plant breeders.
Four basic approaches to the drought resistance are being used :
1. breed for high yields under optimal condition ; i.e breed for yield potential – assuming this will provide yield advantage under suboptimal condition.
2. breed for maximum yield in the target environment.
3. select and incorporate morphological and physiological mechanisms of drought resistance into traditional breeding programmes.
4. do not use multiple physiological selection criteria , but established without doubt that a single drought-resistance character will benefit yield under water limited conditions , and then incorporate the character into an existing yield breeding programme.
Using molecular techniques several classes of genes have been identified that confer resistance to water deficit . Some of the genes could be used to engineer plant for drought resistance and better crop yield under drought condition. First the enzymes that synthesize osmoprotectants , small molecules that accumulate in the cytoplasm of drought stress plants , have been identified.
Plants genetically engineered with the genes encoding these enzymes are more drought tolerant. Second the genes that encode transcription factors that regulate entire metabolic pathways leading to drought adaptation were identified. By incorporating such genes , one can hope to ensure that plants respond rapidly and efficiently to any water deficit and continue all their developmental processes.
( B ) Better performance on saline soil.
Salt tolerance is a complex , quantitative , genetic trait controlled by many genes. Recently a few genes have been identified that provide information useful in screening and selection programmes for salt tolerance.
Four major stratergies that to develop salt tolerant crops are :
1. gradually improve the salt tolerance to conventional breeding and selection.
Example : development of salt tolerance in rice ( Pokkali Rice) of kerala,
2. Introduce traits for salt tolerance from wild relatives into the crops by the process of back crossing.
Example : tomato (Lycopersicon esculentum),
Barley (Hordeum vulgare ) and Wheat ( triticum aestivum).
3. Domesticate wild species that currently inhabit saline environment ( halophytes ) by breeding and selecting for improved agronomic characteristic.
4. use molecular techniques to identify genes associated with salt tolerance , and enhance their expression in the crop species or transfer the genes from the non crop to a crop species. Example : On the molecular front , the genes involved in sensing salt in the environment ( signal- transduction ) , transcription factor genes that turn on batteries of other genes that prepare cell to withstand a higher rate of salt influx , and genes that are a part of plant’s adaptation to the presence of salt are being identified. An example of the later category is the gene that encodes that vacuolar sodium pump. Plants that can turn this gene on rapidly when the cells are exposed to salt , will be able to transport the salt from the cytoplasm into the vacuole , there by detoxifying the cytoplasm. Example : Lycopersicon esculentum ( tomato)
CONCULSION :
Conventional and GM breeding are complementary approaches and can be expected to enhance the draught resistance and yield of crops. People have entered in new era in which enhance knowledge of both the physiology of yield accumulation and the physiological basis of genetic variation in both salt and draught resistance traits offer the potential for improving breeding efficiency for major food crops in different target environments. Using physiological knowledge and powerful tools
