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Published online 23 February 2005
Published in Crop Sci 45:766-771 (2005)
© 2005 Crop Science Society of America
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Seedling Drought Tolerance in Upland Cotton

H. Basala, C. W. Smithb,*, P. S. Thaxtonb and J. K. Hemphillc

a Dep. of Crop Sciences, Faculty of Agriculture, Adnan Menderes Univ., Aydin 09100 Turkey
b Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843
c Dep. of Forest Science, Texas A&M Univ., College Station, TX 77843

* Corresponding author (cwsmith{at}tamu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Combining root morphological and plant physiological traits associated with drought tolerance should result in improved drought resistance in cotton (Gossypium hirsutum L.). This study was designed to evaluate the response of root growth of selected Converted Race Stocks (CRS), TAM94L-25, and ‘Lankart 142’ (LK 142) under water stress and nonstressed conditions, and to compare their excised leaf water loss (ELWL). Two putative seedling drought tolerant and two putative nonseedling drought tolerant CRS lines, plus TAM94L-25 and LK 142, were grown with and without moisture stress in a greenhouse at College Station, TX, in 2002 and 2003. The robust rooting CRS, M-9044-0031-R and M-8744-0175-R, had longer tap root length (RL), higher lateral root number (LRN), greater total root dry weight (TRDW), greater root weight per unit length of tap root (W/L), and greater shoot dry weight (SDW) than the nonrobust CRS M-9044-0045-NR and M-9044-0057-NR, and LK 142 regardless of water regime. Two cycles of seedling drought resulted in an increase in LRN in M-9044-0057-NR, TAM94L-25, and LK 142. SDW and TRDW were highly correlated and SDW was associated with W/L. The excised leaf water loss (ELWL) tended to be greater in the NR CRS lines when measured at 30-m intervals for 8 h. Results suggested (i) that root parameters, initial water content (IWC), and ELWL, could be used as a reliable selection criteria for drought tolerance and (ii) that day-neutral CRS accessions could be used as a source of useful genetic variability for a cotton drought tolerance improvement program.

Abbreviations: ELWL, excised leaf water loss • IWC, initial water content • LRN, lateral root number • RL, root length • SDW, shoot dry weight • SDWR, shoot dry weight reduction • TRDW, total root dry weight • W/L, root weight per unit length of tap root


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
DROUGHT TOLERANCE is a complex agronomic trait with multigenic components, which interact in a holistic manner in plant systems (Ingram and Bartels, 1996; Cushman and Bohnert, 2000). Production in many regions of the U.S. Cotton Belt is limited by inadequate amounts or inadequate distribution of rainfall. Decreasing ground water supplies and high energy costs affect production of irrigated cotton in other regions. Therefore, selection for drought tolerance is a major interest of plant breeders in cotton, as well as other agricultural commodities. Previous drought tolerant studies have focused on either root morphology or plant physiological characters (Quisenberry et al., 1982; Chaves and Rodrigues, 1987; Winter et al., 1988; Bland and Dugas, 1989; Ball et al., 1994; Xu and et al., 1995; Kumar and Singh, 1998; Kasperbauer, 1999; Pace et al., 1999).

Root characteristics logically play an important role in determining the response of plants to drought. Water deficit decreases shoot growth rate, plant height, and yield, but 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 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 that suboptimal soil moisture reduced root length and density at 42 and 70 d after emergence. A number of different morpho-physiological traits have been suggested as important relative to drought tolerance in 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); stomatal conductance and photosynthetic rate (Nepomuceno et al., 1998); and leaf water content and carbon isotope discrimination (Leidi et al., 1999).

Another important aspect of drought tolerance may be the plant's ability to reduce water loss by early stomata closure or leaf morphological structures (Levitt, 1980, p. 395–434.; Fernandez and McCree, 1991; Fambrini et al., 1995; Franca et al., 2000). Therefore, many studies have emphasized the association of drought resistance with the rate of excised leaf water loss, stomatal closure, and abscisic acid (ABA) accumulation in the plants under water deficient conditions. Reports have noted that stomatal closure under drought stress is controlled essentially by the concentration of ABA transported in the xylem from the root to shoot and perceived at the guard cell apoplast (Ackerson, 1980; Hartung et al., 1998; Anderson et al., 1994; Borel and Simonneau, 2002). In addition to the stomatal behavior, the rate of excised-leaf water loss has been proposed as a possible indicator of drought resistance in winter wheat (Triticum aestivum L.) (Winter et al., 1988; McCaig and Romagosa, 1989), durum wheat (Triticum turgidum L. var. durum) (Yang et al., 1991), Brassica species (Kumar and Singh, 1998), and cotton (Roark et al., 1975; Quisenberry et al., 1982). Clarke et al. (1989) reported that wheat accessions with low rates of water loss from excised leaves had higher yield than accessions with higher rates of water loss from excised leaves when produced under water-limited conditions.

Currently, plant breeders develop new cultivars by recycling elite cultivars as parents of new populations while sometimes utilizing newer public germplasm releases in their breeding programs. Thus, it is essential to either identify specific alleles for drought resistance in adapted elite germplasm or add novel alleles from exotic sources to expand genetic diversity for drought avoidance or resistance.

The primary objective of this research was to investigate the differential response in root growth of selected putative drought tolerant and susceptible CRS lines, an adapted germplasm line bred for long fiber length, and an obsolete commercial cultivar that was popular for a number of years under dryland conditions in Texas when grown under water stressed and nonstressed conditions. A second objective was to compare the excised leaf water loss of these genotypes when grown under well-watered conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
CRS lines varying in robustness of seedling root systems were identified by Basal et al. (2003) and used in the present study. Two CRS lines with robust (R) seedling rooting traits, M-9044-0031-R and M-8744-0175-R, and two CRS lines with nonrobust (NR) seedling rooting traits, M-9044-0045-NR and M-9044-0057-NR, germplasm line TAM94L-25, (Smith, 2003), and cultivar LK 142 were grown in greenhouse culture at College Station, TX, under water stressed and nonstressed conditions. Seeds of these cotton genotypes were planted (4 seeds per container) in 9-L pots and 70- by 11-cm tubes filled with 5.6 kg of fritted clay (Absorb-N-Dry, Balcomes Co., Flatonia, TX). Twenty tubes and 10 pots of each genotype were established and maintained in a greenhouse at approximately 32/27°C and 57/67% relative humidity (day/night) conditions. Each tube and pot was subsequently thinned to a single plant and were watered and fertilized (Peters 20-20-20 NPK and Peters M-77 micronutrients) for normal production. Fertilization was performed by aspiration during watering.

All plants in tubes were watered and fertilized in accordance with the procedure described above until the plants reached the first true-leaf stage. Subsequently, the tubes were distributed randomly into two groups, each containing 10 tubes of each genotype. One group of seedlings was watered at regular intervals (control) and another group was subjected to two consecutive drought cycles. This drought stress regime was initiated by first withholding water when the plants reached the first true-leaf stage. Plants were watered to field capacity 12 h after visual signs of wilting. At the end of second drought cycle, plants from each group were harvested. Roots were washed free of clay, and then spread on paper for determination of root length and lateral root number. Plants were cut into two parts, root and shoot, and fresh weight determined. Shoot and total root mass of each plant was dried for 48 h at 90°C and dry weight determined. RL was determined by direct measurement of fresh tap roots; LRN was determined by direct count of roots before drying; TRDW was determined by direct measurement after drying for 48 h at 90°C; W/L was determined by dividing the TRDW (mg) by the tap root length (cm); SDW was determined by direct measurement after drying; and shoot dry weight reduction (SDWR) was determined by calculation of the difference in fresh shoot weight (data not shown) and SDW.

The plants in pots were grown under well-watered conditions described above until the 3rd main stem leaf was fully expanded, approximately 44 d after emerging. Fully expanded 3rd main stem leaves were excised from each plant at noon. Each leaf was immediately weighted for initial leaf weight and then placed in a growth room under a light bank (photosynthetically active radiation at 340 µmol m–2 s–1) at 30°C and 45 ± 5% RH. Rate of water loss was determined for each leaf blade by weighing at 30-m intervals for 8 h.

Shoot dry weight reduction values (%) were transformed before ANOVA, and other data had normal distributions of variance according to the Bartlett distribution test. The experiment was repeated using a completely randomized design and data analyzed by using the SAS System (SAS Institute, Cary, NC). Means of genotypes within water regimes were separated by the Waller–Duncan LSD at k = 100 which approximates p = 0.05. The response of each genotype to water regime was also evaluated as a one-way ANOVA (data not shown).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Tube Experiment
Significant genetic variation was found in both watered and water stressed treatments (Table 1). Tap root length of genotypes ranged from 47.9 cm (LK 142) to 86.6 cm (TAM94L-25) under watered conditions, and from 48.3 cm (LK 142) to 82.5 cm [M-9044-0031-R] after two drought cycles (Table 2). The rank of genotypes for RL was similar in both treatments in which TAM94L-25, M-9044-0031-R, and M-8744-0175-R had the longest taproots. Water stress reduced (P = 0.05) RL only in two genotypes, TAM94L-25 and M-8744-0175-R.


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  		Table 1. Mean square of four CRS lines, TAM 94L-25 and Lankart 142 when grown in greenhouse culture at College Station, TX, in 2002–2003.

 

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  		Table 2. Root characteristics of four CRS, TAM94L-25, and Lankart 142 when grown in tubes under greenhouse culture at College Station, TX, in 2002–2003.

 
The robust rooting CRS lines had higher LRN than the nonrobust lines and LK 142 under either water treatment (Table 2). While two cycles of seedling drought resulted in either no change or a decrease in RL [M-8744-0175-R and TAM94L-25], drought stress resulted in greater LRN for M-9044-0057-NR, TAM94L-25, and LK 142. The increase in LRN under water stress may indicate differential response to drought due to the positive association between LRN and drought tolerance reported in previous studies (McMichael et al., 1999). This result provides evidence that the expression of genotypic root growth potential could be influenced by the availability of water. Under watered condition, only M-8744-0175-R produced more LRN than TAM94L-25. Under drought, TAM94L-25 had more LRN than all other genotypes except M-9044-0031-R. The nonrobust rooting CRS lines had higher LRN than LK 142 but fewer LRN than TAM94L-25 within both moisture treatments.

The robust rooting CRS had greater (P = 0.05) TRDW than the nonrobust CRS and LK 142 regardless of moisture treatment (Table 2). TRDW decreased in TAM94L-25 and M-8744-0175-R under water stress. This reduction may be attributed to shorter RL under water stressed conditions because LRN of M-8744-0175-R did not change and the LRN of TAM94L-25 actually increased. M-8744-0175-R and M-9044-0031-R had higher TRDW than TAM94L-25 under watered and water stressed treatments, respectively. Nonrobust rooting CRS lines, M-9044-0045-NR and M-9044-0057-NR, produced higher TRDW than LK 142, which shows seedling root potential of CRS lines as a source of novel alleles to expand genetic diversity for drought tolerance.

Genotypes differed in W/L under both water and water stressed treatments. While M-8744-0175-R had the highest W/L under watered conditions, M-9044-0031-R had higher W/L after two drought cycles. Since W/L was calculated as root dry weight/root length, higher W/L represents a greater mass per unit root length regardless of RL. M-8744-0175-R and M-9044-0031-R had greater W/L than TAM94L-25 under watered and water stressed treatments, respectively. However, W/L of the nonrobust seedling rooting CRS lines, M-9044-0045-NR and M-9044-0057-NR, were not different from LK 142 under watered condition, but W/L was greater (P = 0.05) under water stressed conditions.

The robust rooting CRS exhibited at least twice (P = 0.05) as much shoot weight as the nonrobust CRS by the termination of the experiment at 49 d post planting. M-8744-0175-R had the highest SDW under watered conditions, and M-9044-0031-R and M-8744-0175-R produced more SDW than the remaining genotypes under water stressed conditions. All genotypes produced less (P = 0.05) SDW under water stressed conditions than watered conditions. In addition, the smaller SDW decrease in M-9044-0031-R may result from greater root size (the highest RL, LRN, TRDW, and W/L) allowing the plants to store more carbohydrate to better tolerate stress (De Souze and Vieria da Silva, 1987). The W/L of genotypes was not significantly different between water stressed and watered plants except for M-8744-0175-R. However, SDW was lower for the stressed treatment than the watered treatment in all genotypes, indicating that root growth was less sensitive to water deficit than shoot growth as found by (Ball et al., 1994), suggesting that SDW or SDWR could be used as selection criteria for drought tolerance because of their ease of their measurement and reliability.

The RL, LRN, TRDW, W/L, and SDW all were positively and significantly correlated regardless of water regime. The highest correlation was observed between W/L and TRDW (r = 0.94 for nonstressed and stressed regimes) (Tables 3 and 4). SDW was highly associated with TRDW (r = 0.92, for watered; r = 0.87, for water stressed) (Tables 3 and 4). Vigorous shoot growth corresponds to vigorous root growth under both watered and water stressed treatments.


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  		Table 3. Correlation coefficients among investigated characters for four CRS, TAM94L-25, and LK 142 when grown in tubes under nonstressed condition at greenhouse culture at College Station, TX, in 2002–2003.

 

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  		Table 4. Correlation coefficients among investigated characters for four CRS, TAM94L-25, and LK 142 when grown in tubes under water stressed condition at greenhouse culture at College Station, TX, in 2002–2003.

 
Pot Experiment
Differences in initial excised leaf water content (IWC) were detected among genotypes (Table 5). IWC of nonrobust CRS lines, M-9044-0045-NR and M-9044-0057-NR, were lower than TAM94L-25 and M-9044-0031-R, while M-9044-0031-R, M-8744-0175-R, TAM94L-25, and LK 142 exhibited similar IWC (Table 6). Figure 1 displays the ELWL data in Table 6 that occurred as stomatal and cuticular transpiration. Following excision, all genotypes lost water rapidly through the open stomata for 30 m, which was as expected since mean stomatal closure time in cotton ranges from 21 to 26 m according to Quisenberry et al. (1982). After stomatal closure, transpiration water loss is through cuticular conductance. ELWL data in Table 6 and Fig. 1 support the data of Quisenberry et al. (1982) as the transpiration rate from excised leaves decreased between the 30- and 60-m measurements. The ELWL at 30 m was the highest in M-9044-0045-NR (1.444 g H2O g–1 dry wt.) followed by M-9044-0057-NR, M-8744-0175-R, LK 142, M-9044-0031-R, and TAM94L-25. While, TAM94L-25 lost the least water, M-9044-0045-NR lost the most water from excised leaves after 30 m (Table 6). These results suggest that the stomata of TAM94L-25, M-9044-0031-R, and LK 142 closed earlier than other genotypes. Rapid stomatal closure could be a drought mitigation response allowing plants to maintain relatively more water in their tissues (Ludlow, 1980). The higher IWC and lower ELWL, and lower IWC and higher ELWL for TAM94L-25 and M-9044-0045-NR, respectively, agree with the results of Clarke et al. (1989) and McCaig and Romagosa (1989) that IWC and ELWL behave independently. The ranking of genotypes for ELWL were similar from 60 m through 480 m.


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  		Table 5. Mean square values for initial leaf water content (IWC) and excised leaf water loss (ELWL) of four CRS lines, TAM 94L-25 and LK 142 when grown in greenhouse culture at College Station, TX, in 2002–2003.

 

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  		Table 6. Initial excised leaf water content (IWC) and excised leaf water loss (ELWL) at 30 m intervals for four CRS, TAM94L-25, and LK 142 genotypes when grown under greenhouse culture at College Station, TX, in 2002–2003.

 

Figure 1
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  		Fig. 1. Excised leaf water loss (%) of four CRS, TAM94L-25, and LK 142 during 8 h.

 
Low water loss from excised leaves has been proposed as a simple but relatively reliable indicator of drought resistance in wheat (Yang et al., 1991), in cotton (Quisenberry et al., 1982), and in sorghum [Sorghum bicolor L (Moench.)] (Jordan et al., 1984). TAM94L-25 and M-9044-0031-R exhibited higher IWC and lower ELWL at 30 m, indicating that both characteristics may contribute to the maintenance of leaf water status during water deficit.

Information from these experiments indicates that genotypic differences for IWC and ELWL exist. If an extensive root system could be combined with the ability to maintain high leaf water content, the result could be superior adaptation to dry land environments (Hurd and Spratt, 1975). The robust rooted CRS line M-9044-0031-R displayed superior root and shoot mass, especially following two cycles of drought stress and it displayed higher IWC and lower ELWL at 30 m, both of which have been proposed as reliable drought selection criteria for different plant species, including cotton (Roark and Quisenberry, 1977; Quisenberry et al., 1981; McCaig and Romagosa, 1989; McMichael and Quisenberry, 1991, and Yang et al., 1991). These results indicate that IWC and ELWL with seedling robust rooting parameters might be used as selection criteria in drought tolerance breeding program. Validation of these findings under field conditions and with mature cotton is the necessary next step. The data reported herein also suggest that the CRS accessions, such as M-9044-0031-R, might be useful in developing cultivars with more robust rooting systems and lower ELWL in a drought tolerance improvement program.


   ACKNOWLEDGMENTS
 
This research was supported by a grant from Fulbright Council for International Exchange of Scholars program and TAMU. We thank the Cotton Improvement Lab and the Borlaug Biotechnology Center personnel for their excellent technical assistance throughout this research.

Received for publication July 16, 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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