1Center of Agriculture Resource, Institute of Genetics and Developmental Biology, China Academy of Science, Huaizhong Road 286, Shjiazhuang, Hepei Province, 050021, R P China; Email email@example.com
2 Institute of Crop Germplasm, Chinese Academy of Agriculture Science, Beijing, 100081; R P China;
Crop high water use efficiency (WUE) breeding will play an important role in the future. Much genetic diversity has been found in crop WUE especial in wheat, reviewed in this paper. Different WUE genes have been located by wheat aneuploids and detected by molecular markers in other crops; one WUE related gene has been cloned, and some genes associated with drought resistance (HVA1) and high photosynthesis (Asr1) transferred in wheat and maize. We believe that more genes controlling WUE could be precisely mapped and cloned, and more genes associated with drought resistance and photosynthesis and yield could be transferred into crops for improving WUE and yield in favourable and unfavourable environments.
New technology is allowing scientists to find the genes that make some varieties of crops use water much more efficiently.
Crop, water use efficiency, genetics, improving.
In the Northwest of China, the annual precipitation ranges from 500-600 mm, but wheat yield has increased gradually from hundreds kg/ha before 1950 to >5000 kg/ha. Wheat WUE has increased slowly with yield . In our opinion, a crop water saving breeding program could combine high yield, high WUE and good drought resistance traits together in one variety. So water saving crop breeding is important in both irrigated land and dryland (Zhang et al. 1998b).
Briggs and Shantz first studied the water requirement of different species (including corn, sorghum, millet, wheat, oat, barley, potato, alfalfa and soybean) for a whole growth period. (Briggs et al., 1914; Shantz et al., 1927). From these early data, we can find that C4 plants had higher WUE than C3 plants. Briggs and Shanntz (1913) already said: “differences in the water requirement also exist between different varieties of the same crop, and this suggests the possibility of developing … strains which are still more efficient in the use of water”. Under well-irrigated conditions, significant variation for WUEg from 34- 46.9μmol/mmol among fifteen winter wheat genotypes was due to genotypic differences in both photosynthetic capacity and gs (Morgan et al., 1991).
Richards (1987) found they cultivated wheats had a higher WUE than their diploid and tetraploid ancestors in glasshouse experiments but did not find consistent differences between new and old varieties. Siddique et al. (1990) found WUE (grain yield/ amount of water used) of modern cultivars was higher than old varieties among 9 Australian varieties. Improved WUE for grain in modern wheat cultivars was associated with higher harvest index, mainly by reducing plant height with use of dwarfing genes.
Under irrigated and dryland condition, the flag leaf WUE of 44 wild species and cultivars [5 diploid (AA, DD, RR), 7 tertaploid (AABB, AAGG), 13 hexaploid (AABBDD, AAAAGG, AABBRR), 1 AABBDDRR, 7 irrigated land varieties and 11 dryland varieties] was measured by LCA-4 model photosynthesizes apparatus, the results showed that flag leaf WUE of diploid and tetraploid increased with domestication. Flag leaf WUE increased as ploidy increased ( 2x→4x→6x ) in wheat evolution . Among modern cultivars, flag leaf WUE of irrigated varieties is higher than dryland varieties (Zhang et al., 1998a). In the evolution of wheat (2x→4x→6x), the flag leaf area and stomata size change from small to large, but the frequency of stomata changes from high to low. That is the main reason for the trend of decreasing stomatal conductance (G), photosynthetic rate (P) and transpiration rate (T) and the concentration of carbon dioxide in cell (Ci). Photosynthetic rate decreased less than transpiration rate, therefore the leaf WUE (=P/T) increases in the evolution of wheat (Zhang et al., 2003).
Zhang et al. (2003) reported that because the wheat wild species have small leaves and slow early growth, from seedling in October to ear emergence in the following April at Shijiazhuang in China. In diploid species (AA, BB, DD) stem elongation stage is too late at beginning of May; and they grow more biomass and little grain yield in late stage, from anthesis in the end of May to maturity at June 20 or even later, so field evaporation is greater in early growth stage but grain yield is less than that of modern hexaploid cultivars. Modern hexaploid dwarf cultivars have large leaves and grow more biomass quickly in early stage from seedling to anthesis, and grow less biomass and large grain yield in late stage from grain-filling to maturity. Modern cultivars have early vigor and higher yield and harvest index than that of ancient species, therefore WUE of different ploidy wheat has increased in wheat evolution from 2x→4x→6x →modern cultivars.
Before the 1990s, because of the complexity and difficulty of measuring different level WUE of a large number of breeding lines under field conditions, there was a need to find an alternative to the conventional approach for the improvement of WUE of field crops. Farquhar et al. (1982) and Farquhar and Richards (1984) proposed that δ13C in leaf tissue is negatively correlated with WUE in many crop species. Handly et al. (1994) found that chromosome 4 controls potential water use efficiency (measured by δ13C) in barley. Gorny et al. (1999) found that genes located on almost all D-genome chromosomes improved the efficiency of water use (WUE) in the vegetative plant tissues, with the strongest effects associated with chromosome 7D. Zhang et al. (2000) reported that the order of flag leaf WUE of different genomes is AA>BB>DD>RR. Among twenty Chinese spring ditelosomic lines, the flag leaf WUE of A ditelosomic group is the highest, the high WUE genes located on 1AL, 2AS and 7AS chromosome arm. Among seven wheat–rye addition lines, the high WUE genes were located on 4R chromosome. The flag leaf WUE of 5R addition lines is the lowest.
Martin et al. (1989) were first to report combining molecular markers and δ13C technologies to identify potential genes associated with WUE in tomato. Main et al. (1996) combined leaf ash and RFLP technologies to identify a QTL associated with WUE in Soybean. Main et al. (1998) had identified two previously unreported QTLs (one on LG C1, one on LG L) in another population. Zhang et al. (2002; 2003) reported that two QTL controlling leaf WUE, ten QTL significantly affecting per plant WUE (Total dry matter/ amount of water used by per plant); six QTL significantly controlling leaves and stems WUE (Dry weight of stem and leaves/amount of water used by per plant), two pairs interaction QTL influencing leaves and stems WUE; five QTL significantly controlling roots WUE (Dry weight of roots /amount of water used by per plant), three pairs interaction QTL influencing root WUE. There were gene clusters constituted by two or four QTL linking closely together found on 1A, 3B, 4A and 6D chromosomes. More and more scientists are giving more attention to study genetic background and genetic manipulation for improving crop WUE. The water use efficiency genomic project was set up by UAS national science fund in 2001, the aim is to use stable isotope technique to screen genotypes of modern crops, as represented by tomatoes, rice and their wild relatives, for differences in WUE.
To date, no gene with direct or major control over WUE has been cloned. But some scientists want to identify, clone, and characterize genes associated with water-use efficiency (WUE) by using the differential display and other molecular genetics methods. If major loci that directly control plant WUE can be defined in further work, WUE gene(s) could cloned by chromosome walking, chromosome microdissection and microcloning, orthologous gene cloning and other gene cloning methods.
WUE is a trait under multigenic control. By transferring some drought resistance genes, photosynthesis-increasing and yield-increasing genes in crops, WUE could be improved directly or indirectly. For example, the ABA-responsive barley gene HVA1 was introduced into spring wheat. The transgenic HVA1 lines had significantly higher water use efficiency (Sivamani et al., 2000). A candidate maize gene Asr1 (a putative transcription factor for genetically linked drought tolerance QTLs), was used to modify CO2 fixation rates in leaves through changes of C4 phosphoenolpyruvate carboxylase (C4-PEPC) activity. The highest C4-PEPC overexpressing line exhibited an increase (+30%) in intrinsic WUE accompanied by a dry weight increase (+20%) under moderate drought conditions (Jeanneau et al., 2002).
In the future, we believe that more and more genes directly controlling WUE and associated with drought resistance, yield and photosynthesis could be transfered into crops for improving yield and WUE in favorable and unfavorable environments.
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