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a CSIRO Tropical Agriculture, 306 Carmody Rd., St. Lucia, QLD 4067, Australia
b Pioneer Hi-Bred International, Inc., 7431 Kaumualii Highway, P.O. Box 596 Kekaha, HI 96752 USA
edmeadgreg{at}phibred.com
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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. Grain yield, EPP, and GPE were strongly correlated with ASI across entries under drought
, though not when water was plentiful. The use of managed stress environments that consistently reveal genetic variation for these traits at specific times during crop development is endorsed for selection purposes.
Abbreviations: AD, days from sowing to 50% anthesis • ASI, anthesis-silking interval • EPP, fertile ear number per plant • GPE, grain number per fertile ear • GNA, grain number per square meter • GW1000, 1000 grain weight • GY, grain yield • TBN, tassel primary branch number • WW, well watered
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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These populations were selected with an index of traits that included the primary trait, GY. Secondary traits chosen for the index were thought to improve performance in water-limited environments. An ideal secondary trait should be genetically associated with GY under drought, carry no yield penalty under favorable conditions, be heritable, cheap and rapid to measure, stable over the measurement period, and be able to be observed at or before flowering so that undesirable parents are not crossed (Edmeades et al., 1998). Unfortunately, the measurement of many putative drought-adaptive traits is too slow and/or costly for routine use in a conventional breeding program, though some indirect indicators of their function, such as canopy temperature, can be measured rapidly. The use of secondary traits with GY, rather than selection for GY alone, has been shown to increase selection efficiency by about 20% in maize grown under stress induced by low nitrogen status (Bänziger and Lafitte, 1997). Ludlow and Muchow (1990) have reviewed secondary traits associated with tolerance of intermittent water deficits, and recommended osmotic adjustment, improved rooting depth and density, delayed foliar senescence, and an increased capacity to remobilize stem reserves as potentially useful traits in a selection program.
In the case of osmotic adjustment of shoots, Bolaños and Edmeades (1991), after screening a large number of tropical maize genotypes, found limited genetic variation for the trait and no clear association between osmotic content and productivity under water stress. In the lowland maize population, `Tuxpeño Sequía', Bolaños et al. (1993) reported no changes in osmotic adjustment or in the amount of water extracted from the soil under water-limited conditions after eight cycles of selection.
Labile stem reserves can contribute to grain filling under water stress in maize (McPherson and Boyer, 1977). Blum (1998) has described screening techniques for this trait in wheat involving chemical defoliation during grain filling, and reported that tall plants had a greater reserve level than short plants. However, it is variation in grain number rather than grain weight that has the greatest effect on yield in maize under drought (Bolaños and Edmeades, 1996). The deleterious effects of water stress at flowering on grain number can be largely offset by stem infusions of sucrose, an intervention that resembles an increase in current assimilation rate (Boyle et al., 1991). Short maize plants have been reported as more tolerant of drought at flowering than taller plants (Fischer et al., 1983), and selection for reduced tassel size has been shown to increase ear size near flowering (Fischer et al., 1987). These studies suggest that competition for assimilates between competing organs at flowering affects ear growth and grain number in maize. Other reports have also suggested that successful grain set is more closely related to current photosynthesis than to reserves formed in the pre-anthesis period (Schussler and Westgate, 1995; Zinselmeier et al., 1995). One indicator of the flux of current assimilate to the developing ear in maize is the anthesis-silking interval (ASI), which increases markedly under drought (DuPlessis and Dijkhuis, 1967; Edmeades et al., 1993) and has been closely associated with GY and barrenness (Hall et al., 1981; Bolaños and Edmeades, 1996).
Drought stress during grain filling accelerates lower leaf senescence (Aparicio-Tejo and Boyer, 1983; Wolfe et al., 1988), thus eliminating future assimilation by those leaves and reducing GY. In low N situations, however, grain filling may be enhanced by foliar senescence, which releases leaf N to the grain (Uhart and Andrade, 1995). Smart et al. (1995), however, suggest that enhanced staygreen does not always ensure that the age-related decline in photosynthesis is delayed. Leaf rolling also reduces leaf area exposed to radiation. It has been associated with cultivars having low leaf water status (Sobrado, 1987) and is usually selected against in breeding programs.
Genetic correlations (rg) between GY under severe drought stress and secondary traits, obtained from trials of inbred progenies from maize populations used in the present study, have been reported by Bolaños and Edmeades (1996). They showed a strong dependence of GY on ears per plant (EPP)
and grains per ear (GPE)
, while the correlation between GY and weight per grain was weak
. A moderately strong correlation
was reported between GY and ASI, while genetic correlations between GY and leaf rolling, staygreen, leaf angle, canopy temperature, tassel branch number, leaf chlorophyll concentration, and plant height were generally less than |0.20|. Similar results were reported for two of these populations by Guei and Wassom (1992). It should be noted that EPP is a measure of barrenness rather than of prolificacy in these and the present studies.
An earlier evaluation of changes due to selection for drought tolerance in Tuxpeño Sequía reported reductions in ASI, tassel size, and plant height, and increases in GY, EPP, and ear biomass at anthesis (Bolaños and Edmeades 1993a,b), but no measurable effect on plant water status or leaf senescence (Bolaños et al., 1993). Encouraged by yield gains in this population, recurrent selection was started in two others with S1 rather than full-sib progenies and a greater selection intensity. At the same time, two populations that could serve as sources for a diversity of drought-adaptive traits (`DTP1' and `DTP2') were developed by combining drought tolerant sources from Mexico, USA, Africa, and Thailand. Changes in biomass, GY, and harvest index with selection in these populations have been reported (Edmeades et al., 1999).
The objectives of the present study were to ascertain direct and correlated changes in secondary traits in three tropical populations resulting from recurrent selection for drought tolerance and to compare these with changes in conventionally selected check entries and in populations comprised of drought tolerant source germplasm.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Checks provided comparisons of progress for drought tolerance derived from different selection strategies such as conventional or single trait selection vs. selection specifically for drought tolerance alone. `Pop. 43 C6' and La Posta Sequía C0 are identical populations, and `Pool 26 C20' and Pool 26 Sequia C0 are virtually identical (Edmeades et al., 1999). Check entries `Pop. 43 C9' and `Pool 26 C23' differed from La Posta Sequía C0 and Pool 26 Sequía C0 in that they had been subjected to three additional cycles of conventional selection in CIMMYT's maize breeding program, where progenies are tested internationally (Pop. 43) or at a single site in Mexico (Pool 26), largely under WW conditions. Progress under the different schemes can be gauged by comparing Pop. 43 C9 with La Posta Sequía C3, or Pool 26 C23 with Pool 26 Sequía C3. The third check, `TLWD-ELV', was a late-maturing Tuxpeño-based synthetic selected for upright leaves in the absence of drought. The effects of erect leaves on GY under drought can be gauged to some degree by comparing the performance of TLWD-ELV with either La Posta Sequía C0 or Tuxpeno Sequía C0. Two other entries, `TL89DTP1 C5' and `DTP2 C2', exhibit a good level of drought tolerance, obtained not from recurrent selection but by assembling these populations from sources with putative or proven levels of drought tolerance. The efficacy of this strategy, relative to recurrent selection, can be estimated by comparing these two with La Posta Sequía C3, Pool 26 Sequía C3, and Tuxpeno Sequia C8.
Progress due to selection was evaluated in 10 environments (Table 1) that differed mainly in available water, and ranged in yield from 1.01 to 10.40 Mg ha-1 (Edmeades et al., 1999). Sixteen entries, comprised of cycles of selection and checks, were included in each environment. In several trials, another entry was substituted for Pool 26 Sequía C0 and data from this entry have been discarded from the present analysis. Trial design at each site was an alpha (0,1) lattice in three replications, and plot size was four rows 5 m in length, with an established plant density of 53000 plants ha-1 in nine sites and 40 400 plants ha-1 at one site. In five well-watered (WW) trials, irrigation was applied every 10 d if rain was insufficient. The five water-deficit trials were managed by withdrawing or delaying irrigation during flowering and grain filling. Further details on cultural practices, and grain and biomass harvests are described by Edmeades et al. (1999).
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When stress was evident in the drought treatments in four trials, leaf rolling was scored (1 = no rolling; 5 = complete rolling) on two or three occasions. At the time of scoring in the 1993A severely stressed trial, four observations of the leaf-air temperature differential were made per plot between 1100 and 1400 h with an infrared thermometer (Model AG42, Telatemp Corp, Fullerton, CA) focused on sunlit upper leaves. After anthesis, in four irrigated trials only, leaf erectness was scored (1 = erect, 5 = lax), and the number of primary tassel branches (TBN) on 10 plants per plot was counted. From two to five scores of leaf senescence were made at about 5-d intervals in seven trials in the last half of grain filling. The scores were visual estimates of the proportion of total leaf area that was senesced, determined by a scale from 0 to 10, where each unit represented 10% of the total leaf area.
At final harvest, numbers of plants, lodged plants, and ears with at least one filled grain, were recorded from a bordered area of each plot. Final harvest density, EPP, and percentage lodged plants were calculated from these data. Grain was dried to constant weight in a forced air oven at 80°C and shelled. A representative sample of 200 grains from each plot was immediately weighed and used to determine 1000 grain weight (GW1000), number of grains per fertile ear (GPE), and grain number per square meter (GNA). All yield data are expressed on an oven dry weight basis.
Statistical Analysis
Statistical analyses are described by Edmeades et al. (1999). In brief, following analyses of variance for each variable at each site, lattices were analyzed across sites, considering entries as fixed effects, environments as random effects, and incomplete blocks as random within replicates. For entries derived from recurrent selection within a population, entry sums of squares were partitioned into linear, quadratic, and residual components. Progress from selection was estimated by regression of entry means on cycle number. Percentage change per cycle was computed as the ratio of the regression slope to the mean of C0 as estimated by that same regression. Where repeated scores for a trait were taken at a site, these were averaged within a site prior to analysis across sites. Phenotypic correlations were computed with entry means within or across environments.
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Results and discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Water deficits increased the average ASI for all entries to 4.5 d from an average of 1.0 d under WW conditions (Table 2). In general, ASI under drought increased with AD by 0.43 d d-1 (P < 0.01; 13 df). When only the initial cycles of selection and the check entries were included, this fell to 0.16 d d-1 (P < 0.05; 5 df), indicating that selection influenced this relationship. Among check entries ASI was not different from the initial selection cycles of each population. On the basis of previous results obtained in Tuxpeno Sequia (Fischer et al., 1989; Bolaños and Edmeades, 1993a), reductions in ASI were expected in all three populations. Across all cycles in La Posta Sequía and Pool 26 Sequía, ASI decreased by about 60% under drought (Table 2). Selection changed ASI under drought by an average of -1.34 d cycle-1 under S1 recurrent selection and by -0.34 d cycle-1 under full-sib selection. Changes in WW environments were -0.38 d cycle-1 and -0.11 d cycle-1, or about 30% of those observed under water deficits.
Differences among entries for EPP generally paralleled those observed in ASI (Table 2). Barrenness, considered here to be when mean EPP < 1.00, occurred in water-stressed environments where EPP averaged 0.88. Barrenness under drought was least for the short duration entries, especially TL89DTP1 C5, DTP2 C2, and Pool 26 Sequía C3. Under water deficits EPP changed with AD at -0.022 d-1 (P < 0.01) across all entries, but was not related to AD under WW conditions. Barrenness under drought was considerably reduced by selection—EPP increased on average by 0.070 cycle-1 with S1 selection, by 0.025 with full-sib selection, and at about 60% of these rates under WW conditions. Pool 26 Sequía and La Posta Sequía, when evaluated under drought, showed an increase in EPP from initial to final cycle of 27% (Table 2). This result is similar to those reported in previous studies of Tuxpeño Sequía (Fischer et al., 1989; Bolaños and Edmeades, 1993b), and confirms that the major change due to selection was a reduction in frequency of alleles that confer barrenness under a drought stress that coincides with flowering.
Leaf senescence was strongly affected by AD, and was more rapid in the earliest materials, TL89DTP1 C5 and DTP2 C2. The score changed with AD by -0.16 d-1 (P < 0.001) under drought, and by -0.29 d-1 (P < 0.001) under WW conditions. Thus a delay of 7 d in AD would result in 11 and 20% more green leaf area under the two regimes near the end of grain filling, when senescence was occurring rapidly. Under drought, senescence was accelerated by selection in Pool 26 Sequía, though not in La Posta Sequía, and it was only under full-sib selection in Tuxpeño Sequía that senescence was delayed (Table 2). Under WW conditions, however, all advanced selection cycles had a smaller proportion of green leaf area on any given date than initial cycles or checks, though the rate of change with selection was only significant for Tuxpeño Sequía. The largest difference within germplasm groups was the significantly smaller senescence score of Pop. 43 C9, in contrast to all cycles of La Posta Sequía. When leaf senescence averaged over all entries was about 50% complete, Pop. 43 C9 had an estimated 8% more of its total leaf area as green than La Posta Sequía C3 under drought and 29% more under WW conditions. However, when the difference in AD between these two entries (3.5 d) was taken into account by the regression of senescence score on AD for all entries, these differences disappeared. In general, about 55% of the differences among cycles of selection of La Posta Sequía and Pool 26 Sequía under drought, and all of the differences in foliar senescence under well-watered conditions, can be explained by differences in AD.
Selection for delayed senescence under drought has been recommended (Aparicio-Tejo and Boyer, 1983) and practiced in this study, yet foliar senescence was slightly accelerated by selection using the index in these populations. Estimates published by Bolaños and Edmeades (1996) indicate that staygreen is a moderately heritable trait, so improvements under drought were expected. The apparent lack of progress in selecting for this trait suggests that increased demands for N by the larger ear resulting from selection are met by mining N from the leaves. This induces senescence (Wolfe et al., 1988; Muchow, 1994) because additional N uptake is restricted in dry soil. If average N content of grain under drought is 15 mg g-1 (1998, unpublished data), the observed increase in GY of 0.78 Mg ha-1 over three cycles in these two populations (Edmeades et al., 1999) would increase N requirements of the ear by about 12 kg ha-1. Assuming leaves during late grain filling contain 2% N (A. Elings, 1998, personal communication), then this demand, if met entirely from leaves, would require the complete mobilization of N from approximately 0.6 Mg ha-1 of leaves, or about 30% of the entire leaf biomass present under drought (Bolaños and Edmeades, 1993a). Given limited N uptake from dry soil during grain filling, the demand for N from the rest of the plant almost certainly increased with selection. Sinclair (1998) has noted that as harvest index increases the demand for increased N uptake also rises because grain has a considerably higher N content than stover. In this context then, an absence of accelerated leaf senescence must be regarded as progress from selection. Confirmation of actual progress for delayed foliar senescence was reported by Bänziger et al. (1999), who showed that when these same contrasting cycles of selection were grown under low soil N in the absence of drought, significant improvements in staygreen with selection were observed.
Primary tassel branch number decreased significantly by 18%, or 5.9% per cycle, during S1 recurrent selection. Under full-sib selection, where TBN was not included in the selection index, it declined by 24%, or 3.1% per cycle (Table 2). In both La Posta and Pool 26, TBN was significantly less in the checks than in the initial cycles. The decline in TBN in La Posta Sequía and Pool 26 Sequía was expected, since it was included in the selection index of these populations. This result in Tuxpeño Sequía, however, strongly suggests that selection for increased ear growth rates at flowering through reduced ASI (Edmeades et al., 1993) produces a correlated reduction in tassel size. The growth rate of the tassel in the period 1 to 5 d after 50% anthesis in temperate germplasm, measured with 14C, has been estimated to be 45% of ear growth rate at that developmental stage (Edmeades and Daynard, 1979). Furthermore, direct selections for reduced tassel size have resulted in correlated reductions in ASI in other studies in tropical germplasm (Fischer et al., 1987). Taken together, these results suggest that a further reduction in tassel size could lead to increased ear growth and improved ear fertility under stress.
No significant differences due to selection were observed in leaf rolling scores across water-stressed environments. Canopy temperatures, measured as the canopy-air temperature differential under drought in one site, were lower for TS6 C1 (-1.6 °C) than for Tuxpeño Sequía C0 (2.1 °C), but no other comparisons among entries were significant. Scores of leaf erectness in three irrigated environments indicated that leaves of TLWD-ELV were always more upright (score = 1.4) than those of other entries (average score 3.1). Leaf erectness did not change because of selection in Pool 26 Sequía, but in La Posta Sequía and Tuxpeño Sequía it increased by 0.5 to 0.8 units (P < 0.05) between initial and final cycles. Lack of major changes in canopy temperature, leaf erectness, and leaf rolling with selection reflects their lack of adaptive value under drought and the small weighting they received during selection, since it is known that the latter two have quite high heritabilities (Bolaños and Edmeades, 1996).
Responses in Unselected Traits
Mean GW1000 ranged from 122 g in the severely stressed Tlaltizapán 1993A winter environment to 321g in the well-watered Tlaltizapán 1992A winter environment, and entry means for this trait were unaffected by AD. Drought stress reduced average weight per grain by 46% (Table 3)
. Check entries had similar grain size to their drought tolerant counterparts. Changes in GW1000 with selection were non-significant in La Posta Sequía, but there was a slight but significant decline in Pool 26 Sequía under drought and a 1% cycle-1 increase in Tuxpeño Sequía in both water regimes. The small magnitude of these changes in GW1000 with selection is surprising, given the significant increase in grain number per plant and the concomitant increase in intraplant competition for assimilates. The sharp increase in GY when irrigation was applied to WW sites (Fig. 1)
illustrates clearly that GW1000 has the capacity to respond to an increased supply of assimilate per grain during grain filling.
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In Tuxpeño Sequía, selection did not change GPE under drought. It seems probable that the absence of change in GPE in Tuxpeño Sequía under drought may be due to an 18% reduction in spikelet number per ear from C0 to C8 (Edmeades et al., 1993) that has limited the capacity of fertile ears to set more grains. Selection has also reduced spikelet numbers per ear in the other two populations but not to the same degree (M. Bänziger, 1994, unpublished data). A similar but smaller (6%) reduction in ear spikelet number was also observed in an unrelated population selected for improved performance under low N (Lafitte and Edmeades, 1995). The trend is also consistent with a 6% reduction in GPE under WW conditions following selection in La Posta Sequía and Pool 26 Sequía, and a 10% reduction in Tuxpeño Sequía. Whatever the cause of the decline in spikelets initiated on the upper ear, an increase in prolificacy in these populations may be the best insurance against loss of yield in conditions where high yields are favored and GPE is limiting. A significant increase in EPP with selection (Table 2) suggests that prolificacy is already increasing in the two populations subjected to S1 recurrent selection.
Grain number per square meter (GNA) varied in a similar manner to its major components, EPP and GPE, and their combined impact was an increase of 36% in GNA over all populations under drought. This marked response in grain numbers to water stress at flowering confirms previous reports (Hall et al., 1981; Grant et al., 1989; Bolaños and Edmeades, 1993b). The coefficient of regression of GNA on AD under drought was -61 m-2 d-1 (P < 0.001), but was not significant under WW conditions. Under drought, GNA increased with selection at an average rate of 167 and 44 m-2 cycle-1 under S1 and full-sib selection, or, as expected, at about the same relative rates (12% and 3–4% cycle-1) as GY (Edmeades et al., 1999). Under WW conditions, GNA increased with selection in Pool 26 Sequía, but was unaffected in the other two populations.
Although there were marked increases under drought in GNA because of selection, some of this change, and many of the differences among populations, can be accounted for by differences in AD. Despite considerable care taken during selection, AD did not remain constant, but became earlier by 0.3 to 1.0 d cycle-1 (Table 2). Given the general relationships previously noted for all entries between AD and ASI, EPP, GNA, and GY across sites, this change in maturity is thought to account for 23% of the gains in EPP, 23% in ASI, 27% in GPE, 27% in GNA, and 29% for GY. As noted by Edmeades et al. (1999), selection has increased this rate of change in these traits. Nonetheless, it illustrates the need to prevent changes in AD when selecting under drought stress that increases with intensity over time, so that tolerance rather than escape is obtained. Escape through early maturity is a very important means by which stable but often relatively low GY is obtained, and it has proven relatively easy to attain through selection. Tolerance, on the other hand, is more elusive, but can stabilize GY at a higher level in later maturing cultivars because their yield potential is usually greater than their early-flowering counterparts.
Total leaf number was greater in later flowering genotypes. It was least (20.3 leaves) for the DTP germplasm and advanced cycles of Tuxpeño Sequía, and greatest (22.2 leaves) in the Pop. 43 C9 and in the La Posta Sequía selections. Across all entries, an increase in final leaf number by 1.0 leaf delayed AD by 3.5 d (P < 0.01), and changes in total leaf number closely paralleled changes in AD. Plant height and ear height increased with AD across entries by 3 to 4 cm d-1 under WW conditions (Table 3), though these increases were not significant under drought. Averaged across both water regimes, advanced cycles of selection were shorter (by 10 to 30 cm) than original cycles and their respective checks, with changes in height ranging from around -4 cm cycle-1 under S1 selection to -1.5 cm cycle-1 under full-sibs (-2 to -1% cycle-1) (Table 3). Similar changes were observed in ear height. These results suggest that selection for increased ear growth rates at flowering through reduced ASI (Edmeades et al., 1993) produces a correlated reduction in plant height. Intraplant competition for assimilates during reproductive development is further confirmed by a 14 to 24% decline in stem biomass at maturity in these populations with selection (Edmeades et al., 1999). This considerably exceeds the 6 to 7% decline in plant height observed over the same period (see Table 3). In the period 1 to 5 d after 50% anthesis, growth rates of the stem vs. the ear in temperate germplasm have been reported to be in the ratio of 4:1 (Edmeades and Daynard, 1979). Direct selections for reduced plant height and tassel size have resulted in correlated reductions in ASI in other studies in related germplasm (Johnson et al., 1986; Fischer et al., 1987), indicating that competition among ears, stems, and tassels for assimilate, especially at flowering, is a significant factor determining ear fertility under stress.
Relationships among Traits
Across Entries and Environments
The matrix of correlation coefficients among major traits (Table 4) across all entries within droughted or WW sites indicated a strong positive association between GY under drought and traits associated with grain number (EPP and GPE), harvest index, and to a lesser degree with biomass yield and senescence score. Variation in GNA accounted for 83% of the variation in GY in drought environments and for 64% in WW environments (Table 4). The consistent and close dependence of GY on GNA within and among environments is illustrated in Fig. 1. Grain number per square meter under drought was more strongly associated with
than with
. Strong negative correlations between ASI and grain numbers, and ASI and harvest index, were also evident in water stressed environments, the correlation between ASI and GNA being -0.97 (P < 0.001). Under WW conditions GY was positively correlated with GNA but with no other trait (Table 4), and negative correlations existed between GPE and GW1000 and between harvest index and ASI. Correlations between GY and EPP became non-significant at yield levels above about 6 Mg ha-1, while those between GY and GPE ceased being significant at yields greater than approximately 4 Mg ha-1.
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While the correlations among EPP, GNA, GY, and ASI under drought have been demonstrated previously (DuPlessis and Dijkhuis, 1967; Fischer et al., 1989; Bolaños and Edmeades, 1996), it is of interest to examine these effects within a population. La Posta Sequía C0 and C3 and their check entry, Pop. 43 C9, were chosen to typify the relationships observed in all three populations (Fig. 2)
. A single linear regression common to all three entries described well the relationship between EPP and ASI (Fig. 2a). The relationships between GPE, GNA and GY, and the independent variable, ASI, were better described (R2 > 0.8) by functions of the form:
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Comparisons with Check Genotypes and Source Populations
During conventional selection the appropriate checks for La Posta Sequía and Pool 26 Sequía (i.e., Pop. 43 C9 and Pool 26 C23) became significantly later to flower and somewhat taller (Table 2; Table 3). They did not, however, produce more biomass or GY than the advanced drought cycles under WW conditions (Edmeades et al., 1999). Under drought, EPP, GNA, and GY of both checks had apparently not been improved through conventional selection.
Populations formed from diverse sources of drought tolerance, TL89DTP1 C5 and DTP2 C2, demonstrated values of ASI, EPP, GNA, and drought tolerance (Chapman et al., 1997) that were similar to those of Pool 26 Sequía C3, but with improved yield potential in WW environments (Edmeades et al., 1999). Their general performance indicates that they have reached a level where they are competitive with elite drought-tolerant germplasm as alternative sources of alleles conferring drought tolerance. The diversity among the components used in their formation suggests they could serve as future sources for a range of drought-adaptive responses.
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Conclusions |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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Future research should focus on the N economy of crops grown in very dry soil, its implications for sink-source relationships, and the appropriate optimization of C and N during pollination and grain filling. The relationships between ASI, spikelet number, ovule fertility, and prolificacy, and their interaction with pollination method and stress level also merit further study.
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ACKNOWLEDGMENTS |
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Received for publication August 27, 1998.
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionConclusionsREFERENCES |
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in all cases. The coefficients of a and b for La Posta Sequía C0 (solid line), La Posta Sequía C3 (dotted line), and their related check (Pop. 43 C9, dot-dash line) are: for (b): 6.33, -0.106; 6.24, -0.132; 6.27, -0.116; for (c): 8.19, -0.197; 8.07, -0.193; 8.10, -0.171; and for (d): 2.21, -0.247; 2.08, -0.302; 2.21, -0.238