Published in Crop Sci. 43:1983-1988 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
CROP BREEDING, GENETICS & CYTOLOGY
Root Growth Parameters of Converted Race Stocks of Upland Cotton and Two BC2F2 Populations
H. Basala,
P. Bebelib,
C. W. Smith*,c and
P. Thaxtonc
a Dep. of Crop Sciences, Faculty of Agriculture, Adnan Menderes Univ., Aydin 09100 Turkey
b Dep. of Plant Breeding and Biometry, Agriculture Univ. of Athens, Iera Odos 75, Athens 11855 Greece
c Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474
* Corresponding author (cwsmith{at}tamu.edu).
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ABSTRACT
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Genetic modification of the rooting system may lead to more drought tolerant upland cotton, Gossypium hirsutum L. This experiment was designed to evaluate rooting traits at the seedling stage for 68 Converted Race Stocks (CRS) compared with TAM94L-25 and ‘Lankart 142’, and to investigate the recoverability of robust rooting traits in two BC2F2 populations derived by crossing a robust (M-9044-0031) and a nonrobust (M-9044-0057) rooting CRS donor parent with TAM94L-25. Initial screen of the CRS showed that genetic variation occurred among the 68 accessions for root length (RL), lateral root number (LRN), root fresh weight (RFW), lateral root dry weight (LRDW), and total root dry weight (TRDW). There was a 2.3-, 2.6-, 4.6-, 4.8-, and 4.4-fold difference among the 11 most robust and 11 least robust rooting CRS accessions for RL, LRN, RFW, LRDW, and TRDW, respectively. However, no CRS were identified that were superior to the elite germplasm and BC2F2 recurrent parent, TAM94L-25. The robust rooting population of M-9044-0031/3*TAM94L-25 produced longer RL and more LRN per plant than the nonrobust rooting population of M-9044-0057/3*TAM94L-25. However, no differences were observed between the two BC2F2 populations for root weight parameters. Results suggest that the day-neutral CRS accessions have useful genetic variability for root growth parameters, that robustness of seedling rooting parameters can be recovered easily, and that seedling rooting robustness can be improved by crossing robust rooting parents.
Abbreviations: LRDW, lateral root dry weight • LRN, lateral root number • RFW, root fresh weight • RL, root length • and TRDW, total root dry weight
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INTRODUCTION
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WATER AVAILABILITY is a determining factor in plant growth and yield of all agricultural commodities. While demands on water resources for agricultural purposes is increasing, declining water availability, changing climate conditions, and increasing human demands are limiting its availability for agriculture (Reddy et al., 1996).
Water deficit (drought) is one of the common stress conditions that adversely affects plant growth and yield. Cotton is grown in semiarid regions of the USA where water often limits yield. Many areas of the U.S. Cotton Belt receive inadequate amounts or inadequate distribution of rainfall. Decreasing ground water supplies and high energy cost also affect production of irrigated cotton. Therefore, selection for drought tolerance is a major interest of plant breeders in cotton, as well as other agricultural crop commodities. A number of studies have focused on either modifying root systems to increase water use efficiency or determining the effects of plant growth regulators on cotton roots for increasing drought resistance (Bland and Dugas, 1989; Xu and Taylor, 1992; Ball et al., 1994; McMichael et al., 1999; Nepomuceno et al., 1998; Kasperbauer, 1999; Pace et al., 1999; Howard et al., 2001).
Root characteristics can be important in determining the response of plants to drought. Water deficit not only decreases shoot growth rate, plant height, and yield; it also affects root growth. However, root growth is less sensitive to drought than shoot growth according to Malik et al. (1979), Saab and Sharp (1989), Creelman et al. (1990), McMichael and Quisenberry (1991), and Ball et al. (1994). Pace et al. (1999) reported that drought-stressed cotton seedlings showed some increase in root length but reduced root diameter. Ball et al. (1994) and Prior et al. (1995) showed that inadequate soil moisture reduced cotton root elongation while Plaut et al. (1996) found reduced root length density at 42 and 70 d after emergence. Malik et al. (1979) reported an effect of drought stress on root distribution.
A number of different root and physiological traits have been suggested as important mechanisms of drought tolerance of cotton. These include distance from transition zone to the first main lateral root, taproot weight, number of lateral roots, seedling vigor, rapidity of root system development, and root-to-shoot ratio (Cook, 1985); longer taproot length (Pace et al., 1999); reduced transpiration (Quisenberry et al., 1982); and stomatal conductance and photosynthetic rate (Nepomuceno et al., 1998).
As the number of vascular bundles increased, high branching intensities of lateral roots also increased in 7-d-old seedlings of exotic cotton (McMichael et al., 1987). Quisenberry et al. (1981) reported significant variability for taproot length and number of lateral roots among exotic cotton germplasm in greenhouse-grown, 35-d-old plants. McMichael and Quisenberry (1991) evaluated exotic cotton genotypes as well as modern cultivars grown in containers for 60–70 d and found significant differences in root growth and branching.
In sorghum [Sorghum bicolor (L.) Moench] (Nour and Weibel, 1978) and wheat (Triticum aestivum L.) (Sandhu and Laude, 1958), higher root/shoot ratios have been shown to be associated with greater drought resistance. Studies with rice (Oryza sativa L.) showed that root length, number of roots, and root density in the 20 to 40 cm soil layer were positively correlated with drought tolerance (Ekanayake et al., 1985; Ingram et al., 1995). Superior drought resistance was associated with enhanced root growth, rapid root water uptake at deeper soil layers, maintenance of root viability at the soil surface, and rapid root regeneration after rewatering in seven warm season turf grasses (Huang et al., 1997).
Plant breeders primarily use current and obsolete cultivars along with public germplasm in developing new cultivars. Thus, it is important to add new alleles from exotic genotypes to expand genetic diversity. Roark and Quisenberry (1977) suggested that the genetic variability in current cotton cultivars potentially is low for many drought tolerant traits, since most of the current cultivars have been selected under humid and high rainfall conditions. The primitive race stocks of upland cotton have been identified as potential sources of traits associated with drought tolerance (Roark and Quisenberry, 1977; Quisenberry et al., 1981). These accessions are photoperiodic and thus will not flower in temperate regions such as the southern USA until late in the production season or until early autumn. McCarty and Jenkins (1993) reported converting 79 of the approximately 600 primitive race stock accessions to day neutrality, referred to as Converted Race Stocks (CRS). Additional primitive accessions have been converted since 1993 (McCarty and Jenkins, 2002). The day neutral or CRS is given a designation that includes the letter M, followed by four digits that indicate the year of release and the BCnFn generation at the time of release, followed by the four digits of the Texas (T-) Cotton Germplasm Collection accession number of the unconverted primitive stock. Liu et al. (2000) surveyed the molecular variation in 97 CRS to determine the genetic distance of each race stock from a typical G. hirsutum cultigen by using simple sequence repeat (SSR) DNA markers. They found strong evidence that the accessions were heterozygous or heterogeneous and the majority of the accessions had genetic distances <0.25 from the G. hirsutum genetic standard TM1. Thus, many of the CRS may be similar phenotypically to commercial upland cotton.
In the study reported herein, 68 CRS accessions were evaluated for root and root growth traits. Two of these 68, one identified as having a robust seedling root system and one as having a nonrobust seedling root system, their BC2F2 populations with TAM94L-25 as the recurrent parent (Smith, 2003), TAM94L-25, and Lankart 142 were evaluated for root characteristics. The specific objectives of this research were (i) to determine rooting traits for 68 CRS compared with a breeding line developed at Texas A&M University, TAM94L-25, and an obsolete commercial cultivar, Lankart 142, adapted to the limited moisture conditions of central Texas; (ii) to identify CRS having robust seedling root traits and nonrobust root traits; and (iii) to determine the recoverability of robust root traits in a BC2F2 population developed by crossing a robust and a nonrobust CRS parent as the donor parents to TAM94L-25.
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MATERIALS AND METHODS
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Sixty-eight CRS accessions were planted in pots (20-cm height x 11-cm diam.) filled with fritted clay (Absorb-N-Dry, Balcones Co., Flatonia, TX) on 22 May 2002. Ten pots of each CRS, TAM94L-25, and Lankart 142 were established by seeding four seeds and thinning the resulting seedlings to one plant per pot 2 wk post emergence. The experiment was conducted in a greenhouse at the Borlaug Biotechnology Center on the campus of Texas A&M University with 32/27°C day/night temperatures and 57/67% day/night relative humidity. The pots were watered each day and fertilized with 20-20-20 NPK fertilizer (Peterson Professional All Purpose Plant Food, Spectrum Group, Division of United Industries Corp., St. Louis, MO) and micronutrients (Peterson Professional M-77 Soluble Trace Element Mic.) added three times to the irrigation water on the 8th, 12th and 16th day of the 20-d experiment. Half of the plants from each genotype were harvested 19 d after planting (DAP) and half were harvested 20 DAP on 10-11 June 2002, respectively. Plants were taken from pots and washed free of clay, then spread on paper for determination of RL and LRN. Plants were cut into root and shoot, and fresh weight measured. Shoot, taproot, and lateral roots were dried for 48 h at 38°C and dry weight recorded. Genotypes were evaluated for RL, LRN, RFW, LRDW, and TRDW. Among the 68 CRS lines, 11 robust and 11 nonrobust rooting potential CRS were selected for retesting.
The experiment was repeated with the 11 putative robust and 11 putative nonrobust rooting CRS identified in the first experiment along with TAM94L-25 and Lankart 142. All genotypes were planted in tubes (70-cm height x 11-cm diam.) filled with fritted clay on 19 June 2002. Ten tubes of each genotype were established by planting four seeds and thinning the resulting seedlings to one plant per tube. Plants in the tubes were grown under the same conditions as described above for pots. Twenty days after planting, 9 July 2002, plants were taken from the tubes and root parameters measured as described above. Converted race stocks M-9044-0031 and M-9044-0057 were selected as representative of robust and nonrobust rooting lines, respectively.
Two BC2F2 populations, M-9044-0031/3*TAM94L-25 and M-9044-0057/3*TAM94L-25, the recurrent parent, TAM94L-25, which has robust roots, donor parents M-9044-0031 and M-9044-0057, and Lankart 142, which exhibited nonrobust root growth parameters, were compared using similar protocol as described above. M-9044-0031, M-9044-0057, TAM94L-25, and Lankart 142 were planted in ten tubes and 100 tubes of each of the selected BC2F2 populations were established by the same methods as described above on 19 July and repeated on 13 August 2002, respectively. Plants were removed from the 19 July planting on 8 August and on 2 September for the 13 August planting. Plants and roots were evaluated according to the protocols described above for the screening.
The two backcross populations had been developed previously by Smith et al. (Smith, unpublished data, 2002) by crossing each of the CRS to TAM94L-25 and backcrossing without selection the F1 and BC1F1 to TAM94L-25. The resulting BC2F1 was allowed to open pollinate under field conditions at College Station, TX, to produce the BC2F2 generation.
The distribution of RL and LRN within each of the two BC2F2 populations were compared with that of TAM 94L-25 by means of Z = (y M µ)/
, where Z = normal curve area, y = value of random variable, µ = population mean, and = population standard deviations (Snedecor and Cochran, 1967). Both populations had normal distribution of variances according to the Bartlett distribution test.
Data from all experiments were analyzed as a one-way analysis of variance with sampling by the SAS System (SAS Institute, Cary, NC). Repeated experiments were combined for analysis purposes.
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RESULTS AND DISCUSSION
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Initial Screen of CRS
Significant genetic variation was found among the 68 CRS genotypes for RL, LRN, RFW, LRDW, and TRDW (Table 1). No CRS had better root parameters than TAM94L-25. However, 32, 35, 7, 57, and 58 of the 68 CRS accessions had significantly higher RL, LRN, RFW, LRDW, and TRDW, respectively, than Lankart 142, which is an obsolete cotton cultivar adapted to the limited water conditions of central Texas. Tap RL of the 68 CRS ranged from 22.7 cm (M-9044-0045) to 34.7 cm (M-9044-0156). Thirty-one of the CRS accessions were not different from M-9044-0156 in RL. There was a 2.3-fold difference between the lowest CRS (M-9044-0045) for LRN and the highest CRS (M-8744-0106). M-8744-0106, averaging 65 lateral roots, had more lateral roots than 30 of the CRS, but was not different than the other 37. Root fresh weight, LRDW, and TRDW were at least five times larger in M-9044-0062 than for M-9044-0045. Even though M-9044-0062 had the highest RFW, LRDW, and TRDW, it was not different than 55, 19, and 23 CRS, respectively, for these traits.
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Table 1. Root characteristics of 68 CRS of upland cotton, an advanced breeding line, and a commercial cotton cultivar grown under greenhouse cultural at College Station, TX, in 2002.
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Second Screen of Robust and Nonrobust Rooting CRS
Genotypic differences were found for several root parameters, thus defining the putative 11 robust and 11 nonrobust rooting CRS accessions (Table 2). M-9044-0061 and M-8744-0175 exhibited taproot lengths over 30 cm at 20 DAP, which was 2.3 times longer than M-9044-0045. These RLs were not different than eight of the 11 putative robust rooting CRS and three of the putative nonrobust rooting CRS. There was a 2.6- and 4.6-fold difference between the smallest, M-9044-0057, and the largest, M-9044-0031, CRS for LRN and RFW, respectively. M-9044-0031, averaged 70.2 lateral roots, which was not different than six of the 11 robust rooting CRS and three of the nonrobust rooting CRS. M-8744-0175 averaged 4.8 and 4.4 times heavier LRDW and TRDW, respectively, than M-9044-0045. M-8744-0175 produced a more massive LRDW and TRDW than four of the putative robust rooting CRS and eight of the 11 putative nonrobust rooting CRS. Even though M-9044-0072, M-8744-0106, and M-9044-0182 were chosen as robust rooting lines in the first screening experiment, the root parameters of these lines were found to be less than expected during the second screening. On the other hand, M-9044-0061, M-9044-0068, and M-9044-0074, identified as nonrobust rooting CRS lines in the first screening, showed more robust rooting performance in the second evaluation.
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Table 2. Root characteristics of 22 putative robust and nonrobust rooting CRS of upland cotton, an advanced breeding line, and a commercial cotton cultivar grown under greenhouse culture at College Station, TX, in 2002.
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Two CRS accessions, M-8744-0175 and M-9044-0031, from the robust rooting group and one CRS accession, M-9044-0061, from the nonrobust rooting group had longer RL than TAM94L-25 and Lankart 142, which were not different in the second screen. Seven nonrobust rooting CRS accessions had lower LRN than TAM94L-25 and 12 of the 22 had greater LRN than Lankart 142. The LRDW of M-8744-0175 and M-9044-0031, and TRDW of M-8744-0175, M-9044-0031, and M-9044-0062 of the robust rooting CRS were higher than TAM94L-25. All CRS accessions, selected as being putative robust rooting, and seven of the 11 nonrobust rooting group established greater root weight, defined as LRDW and TRDW, than Lankart 142. Also, a significant difference was observed between TAM94L-25 and Lankart 142 for LRN, LRDW, and TRDW. Converted race stocks M-9044-0031 and M-9044-0057 were selected for further study on the basis of their overall root parameters as robust and nonrobust rooting accessions, respectively. These two accessions were identified as drought tolerant and drought sensitive, respectively, on the basis of unpublished seedling drought tolerance evaluation tests (Smith, unpublished data, 2002).
The results of screening Exp. I and II showed phenotypic variation for root traits among the CRS lines that may be associated with drought tolerance. These results indicated that the day neutral CRS accessions might have useful genetic variability for cotton drought tolerance improvement programs, which was pointed out by Roark and Quisenberry (1977), Quisenberry et al. (1981), McMichael and Quisenberry (1991), and Liu et al. (2000).
Smith et al. (Smith, unpublished data, 2002) established BC2F2 populations with a number of the CRS as donor parents and TAM94L-25 as the recurrent parent as part of their ongoing cotton breeding activities. This provided the opportunity to determine recovery of root robustness without selection following the cross and two backcrosses with a seedling root robust and nonrobust donor.
Variation among the two BC2F2 populations, recurrent (TAM94L-25) and donor (M-9044-0031 and M-9044-0057) parents, and Lankart 142 was significant for all root characters (Table 3). Interaction between genotype and experimental run was nonsignificant (P > 0.05) for all root measurements, so data were averaged over experimental runs.
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Table 3. Mean square values for root parameters of two BC2F2 populations, their parents, and Lankart 142 when grown under greenhouse culture at College Station, TX, in 2002.
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Root length of the BC2F2 population utilizing the robust rooting CRS M-9044-0031 ranged from 64 to 120.5 cm with a mean length of 92.1 cm (Table 4). This BC2F2 population produced significantly longer RL than the recurrent parent (TAM94L-25) and the BC2F2 population of M-9044-0057/3*TAM94L-25, the BC2F2 developed with the nonrobust rooting donor parent, M-9044-0057. On the other hand, no differences were detected between the robust rooted BC2F2 population and its donor parent (M-9044-0031) for RL. The RL of the nonrobust rooted BC2F2 population ranged from 17 to 121 cm with a mean length of 83.9 cm. This population had more variation for RL than did the robust rooted population but did not differ from the recurrent parent for RL while averaging longer RL than M-9044-0057, the nonrobust rooting donor parent. Both BC2F2 populations had longer tap roots than Lankart 142.
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Table 4. Root characteristics of two BC2F2 populations, their parents, and Lankart 142 when grown under greenhouse culture at College Station, TX, in 2002.
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The robust rooting BC2F2 population produced more LRN per plant, averaging 95.3, than the nonrobust rooting BC2F2 population which averaged 83.2 lateral roots and Lankart 142 which averaged only 31.9 (Table 4). However, no significant differences were observed with the recurrent parent, TAM94L-25. Differences also were detected between the nonrobust rooting BC2F2 population mean and donor parent M-9044-0057 and Lankart 142. While about 64 and 34% of the BC2F2 robust rooting population had longer RL and more LRN, respectively, than the recurrent parent, TAM94L-25, these ratios decreased to 40 and 16% in the nonrobust rooting BC2F2 population (data not shown). The percentage of plants, having higher LRDW, RFW, and TRDW than the recurrent parent in the robust rooting population were 23, 20, and 25%, respectively, and 17, 17, and 16% respectively, in the nonrobust rooting population. These results support the idea of using robust rooting CRS accessions as sources of genetic variation for root traits that may be associated with drought tolerance. However, they also suggest that root robustness, as defined by these traits, can be recovered easily when crossing a nonrobust rooting CRS with a robust rooting recurrent parent. However, the BC2F2 derived by crossing two robust rooting parents averaged longer RL and more LRN than the BC2F2 population derived by crossing a nonrobust rooting parent followed by two backcrosses to a robust rooting recurrent parent. The longer seedling RL and more LRN suggest that robust rooting populations may be able to establish deeper and more diffuse root systems which is important obviously in exploring soil volume for water and nutrients (Quisenberry and McMichael, 1996; McMichael et al., 1999). Rapid establishment of an extensive seedling root system is advantageous when plants are subjected to early season water deficit (McMichael, 1986).
While the robust-rooting BC2F2 population had a significantly longer average RL and more LRN than the nonrobust rooting BC2F2 population, no differences were observed among the robust-rooting BC2F2 population, the nonrobust rooting BC2F2 population, TAM94L-25, and donor parent M-9044-0031 for root weight parameters, with the exception of the nonrobust rooting BC2F2 for LRDW (Table 4). However, this may not be as important as RL and LRN according to Bohm (1979) who reported that the absorption area of root systems is not related to root dry weight.
The data reported herein suggest that seedling root parameters vary among CRS and two genotypes adapted to central Texas. Converted Race Stocks were identified that equaled the better of the two adapted genotypes. No CRS were identified that were superior to the most robust control, TAM94L-25.
Seedling root growth parameters in BC2F2 populations developed with a robust rooting CRS donor parent and a nonrobust rooting CRS donor parent suggest that robustness of seedling rooting parameters can be recovered easily. However, the data also suggest that seedling root robustness can be improved by crossing robust-rooting parents. Such a strategy for improving root growth parameters should be investigated further.
Received for publication January 27, 2003.
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TOP
ABSTRACT
INTRODUCTION
