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III. Enhancement Strategy for Multiple-Stress Resistance
R.R.DUNCAN AND R. N. CARROW
dicates that different mechanisms or degrees of expression in each mechanism are
Specific resistance mechanisms must be associated with adaptive genes. Once
the gene-mechanism relationship is established within a species, this can be used
to identify ecotypes with this resistance mechanism. Gene expression of the mechanism often requires that the abiotidedaphic stresses be present. Also, once a specific stress mechanism is clarified, a search of the scientific literature may reveal
that the associated genes have been identified in other plant species-one of the
objectives of this review article is to assess the current status of gene identification for various abiotic and edaphic stresses. Thus, this component of an overall
strategic framework aids in focusing biotechnology methods and manipulaton on
high-priority genes that are responsible for the specific mechanism@)that causes
a high level of stress resistance.
A third component is to integrate laboratory-generated biotechnology stressenhanced germplasm back into the traditional breeding and genetics programs for
turfgrass improvement. This includes evaluation of germplasm performance under severely stressed and nonstressed field conditions. The following are reasons
for using this approach: (i) Unless the “enhanced germplasm” is placed under the
stress in question, success cannot be determined; (ii) germplasm that has been enhanced for a specific stress mechanism must still perform under multiple stresses;
and (iii) the germplasm can be incorporated into other germplasm by conventional breeding to enhance a broader germplasm pool within a species. Therefore, this
third component integrates biotechnology methods with traditional breeding and
genetic protocols for the purpose of maximizing the efficiency and magnitude of
progress toward greater turfgrass stress performance.
The common abiotic and edaphic stresses that affect turfgrass stress response
plasticity and therefore persistence include
Moisture deficiency/excess problems
Extreme soil/air temperatures
Salinities/poor water quality
High soil strength/traffic/compaction
Low soil oxygen
Reduced light intensity/shade
Low nutrient availability
Many of these constraints interact to create significantgenotype-environment interactions and directly influence turf quality and performance. Turfgrass stress re-
TURFGRASS MOLECULAR GENETIC IMPROVEMENT
sistance can involve several mechanismsthat may be exhibited at the whole plant to
subcellular level. Each mechanism is genetic based. Most turfgrasses are grown in
multiple-stress (drought-high temperature and soil acidity-drought) environments.
The common plant feature that links all these constraints together is the root system.
The first line of defense in adaptation to abiotic and edaphic stresses is the root
system, which provides essential nutrients and water (i.e., drought resistance) for
critical turfgrass functions. Unfortunately, most turf breeding programs do not address root plasticity (functional root volume and viability under cyclic stresses) either directly or indirectly. Root system improvement should be the first step in
a comprehensive abiotic/edaphic stress-resistancebreeding program because
this strategy addresses primary components of stress response that directly in fluence the turf plant capability to acquire essential nutrients and water and to ultimately persist. Many diverse field situations limit turfgrass rooting, but only six
primary soil chemical and physical constraints account for restricted rooting in
turfgrasses in these field stress situations (Table XIII). These primary stresses can
be incorporated into breeding programs as either single- or multiple-stress screening protocols (Carrow and Duncan, 1996; Duncan and Carrow, 1997; Maranville,
1993). Gene technology can be integrated with this traditional breeding strategy
to enhance genetic-based root plasticity; discern multiple stress tolerance mechanisms; locate, sequence,clone, and map stress-responsivegenes; and utilize marker-assisted selection techniques.
1. Genetic Potential for Rooting
Turf species vary in their genetic potential for rooting depth. Table XIV compares several cool- and warm-season grasses for general root depth potential.
Genotypes within a species can also exhibit inherent differences in rooting depth
under nonlimiting soil conditions (Lehman and Engelke, 1991), but rooting depth
potential is only one component of the overall rhizosphere stress adaptation response mechanism because multiple abiotic and edaphic stresses often limit maximum rooting depth.
While root morphology is governed by genetics (Aeschbacher et al., 1994), developmental plasticity in response to environmental stimuli (light, nutrients, temperature, aeration, water, physical barriers, microorganisms, gravity, competition
from adjacent roots, and chemical barriers) (Schiefelbein and Benfey, 1991) will
ultimately determine the final configuration of the root system (Fitter and Stickland, 1992;Lynch, 1995; Schiefelbein et al., 1997).Developmental alterations occur in the form of changes in the direction of growth after perception of an external signal, transduction of the signal, alteration in gene regulation and protein activity, and modification of cell division-expansion-differentiation (Aeschbacher
et al., 1994). Turf species differ in their capacity for enhanced root growth and
rapid root water uptake at deeper soil layers, maintenance of root viability at the
R. R.DUNCAN AND R. N. CARROW
Six Primary Soil Physical and Chemical Constraintsof Rooting, Associated Field Situations in
Which the StressesAre Expressed, and Relative Importance of Stress on 'hrfgrasses
Relative importance of stress
Associated field problems
Layers with few macropores
Low soil 0,
High water table
Poor surface drainage
Layer impeding percolation
Acid soil root toxicities (T)/ Acid soil complex (T, D)
Nutrient deficiencies (Ca, Mg, K)
Usually low organic matter
Usually high soil strength
Moderately acid soil (D)
Acid sulfate soil (T.D)
Acid mine spoils (T,D)
Sodic soil (T, D)
Salt root toxicities/
Na, CI, B, OH toxicities
K, Mg, Ca deficiencies
High soil strength
Low soil 0,
Saline soil (T, D)
Saline-sodic soil (T, D)
Direct high-temperature root injury
High soil temperature
Indirect high-temperatureb stress
limits root development,
High soil strength
"High organic matter alleviates the Al toxicity factor.
"Indirect high-temperature stress is the major factor limiting cool-season grass adaptation into
warmer temperature climatic zones because it determines carbohydrate status for maintaining root viability. It becomes a site-specific problem when site conditions inhibit canopy cooling. High root temperatures enhance indirect high-temperature stress just as high aerial temperatures will do so.
"The more Xs, the greater the importance.
TURFGRASS MOLECULAR GENETIC IMPROVEMENT
Genetic Potential for Rooting Depth among lbrfgrasses"
Q p e of grass
Argentine Bahia grass
Seashore paspalurn (Adalayd)
Common St. Augustine
Texturf 10 Bermuda
"Reproduced with permission from C m o w (1989).
surface drying layer, and rapid root regeneration after rewatering under drought
conditions (Huang et af., 1997). Roots vary morphologically and physiologically
in response to variable soil nutrient distributions (Robinson, 1996) and to mechanical impedants such as high soil bulk densities or compacted layers (Bengough and Young, 1993; Carrow and Petrovic, 1992; Materechera et af., 1992;
Wiecko et al., 1993). Heritability estimates are quite variable, depending on growing conditions during evaluation and species differences (Browning er af., 1994;
Lehman and Engelke, 1991).
Many of the edaphic and abiotic stress-responsive genes involve the root system. QTLs linked to root morphological characters (Champoux et af., 1995) and
root penetration ability into compacted soils (Ray et af., 1996) have been identified, with potential application in turfgrass transformation studies (Table XV). Increased root density and depth provide an avoidance mechanism in response to
abiotic stress, particularly drought (O'Toole and De Datta, 1986).Compacted soil
layers can impede depth of rooting and negatively enhance the overall stress response in turf. Root penetration ability varies both interspecifically (Assaeed ef al.,
1990; Materechera et af., 1992) and intraspecifically (Kasperbauer and Busscher,
199I ; Masle, 1992) in plants. Screening systems are available to effectively measure root penetration variability among genotypes (Huang et al., 1997; Yu ef aZ.,
1995) and select in the field for root plasticity under stress (Duncan and Carrow,
1997; Erb, 1993; Montpetit and Coulman, 1991).Transgenes governing root gen-
R. R. DUNCAN AND R. N. CARROW
QTLs L i e d to Root System Enhancement in Stressed Environments
Root thickness, rootshoot ratio, root dry weight per
tiller, deep root dry weight per tiller, maximum root
Root penetration ability into compacted soil layers
Other root-inducing genes (plasmid root-inducing)
rol (root loci: A, B, C, D)
am (auxin synthetic: 1,2)
Champoux ef al. (1995)
Ray et al. ( 1996)
Chriqui er al. (1996)
eration and growth have been expressed in plants (Chriqui et al., 1996). Several
RFLP probes are available to screen for root elongation growth and drought tolerance (Price and Tomos, 1994).Alteration of leaf cytosolic pyruvate kinase can affect source-sink relationships as well as root biomass (Knowles et al., 1998).
C. ROLEOF TURFGRASS
While various abiotic and edaphic stresses and their related mechanisms are
genetically controlled, long-term management strategies and variable climatic/
growth conditions will govern turfgrass quality, performance, and persistence.
Most management practices are conducted to alleviate or prevent specific stresses or constraints. Because of the three-way interactions between turf species and
cultivar, specific multiple stresses, and the environment, management strategies
must be adjusted to site-specific situations. Managing the turfgrass root system for
maximum development (depth, volume, and plasticity) and viability/functionality is the key to maintaining high-quality turf in stress environments.
1. Root Management
Root systems in perennial turfgrasses are dynamic or ever-changing (Fig. 3).
Seasonal weather patterns govern growth cycles and affect root topology (branching capacity), root distribution (total biomass, which includes root length and
depth of penetration into the soil), and functionality of roots (root dieback). Most
turf roots survive from 6 months to 2 years, depending on species, management
conditions, and environmental constraints (Carrow, 1989). Duration of exposure
and severity of a stress or multiple stresses have profound influences on turfgrass
persistence mainly because carbohydrates produced in green shoot tissues by pho-
TURFGRASS MOLECULAR GENETIC IMPROVEMENT
Figure 3 Seasonal root growth rates of turfgrasses.
tosynthesis are usually utilized first for shoot growth and maintenance and secondarily for root growth and maintenance. Severe environmental stress will create
unbalanced carbohydrate demands in turf plants that can enhance root mortality or
decrease root functionality and ultimately diminish turf quality.
Proper root management to minimize stress is essential to turf quality longevity (Carrow, 1989, 1995a):
1. Select species and cultivars within species with the best root plasticity capability.
2. Promote maximum net carbohydrate production by
a. Optimizing leaf area, which ensures maximum photosynthesis, by increasing mowing height, decreasing wear damage, and controlling biotic
b. Optimizing leaf chlorophyll content by avoiding (i) Fe, Mn, Mg, S, and
M deficiencies, (ii) low soil oxygen or waterlogged conditions, and
(iii) prolonged water-deficit conditions.
c. Promoting good light capture conditions by (i) pruning trees and removing excess grass clippings and (ii) selecting appropriate cultivars.
3. Avoid depletion of carbohydrate reserves in the crown region by minimizing excessive and frequent N applications (especially fast-release N sources),
overwatering, and close mowing. Modify high soil temperatures that contribute to
the depletion of carbohydrates with imgation, drainage, cultivation, or by increasing mowing height.
R. R. DUNCAN AND R. N. CARROW
4. Correct soil physical problems as follows: Correct high soil strength (i.e.,
high bulk density and heavy clay soils) and low soil oxygen with cultivation (aeration) and additions of peat or gypsum; excessively dry soils with irrigation and
additions of organic matter to increase water-holding capacity; low soil oxygen
with cultivation and surface/subsurface drainage; soil layering with cultivation;
and cold soils in the spring with cultivation and proper drainage.
5. Correct poor soil chemical conditions as follows: Correct acid/high Al soils
with lime; very alkaline soils with S, H,SO,, or acidic N carriers; infertile soils
with fertilizers or microbial amendments; and salt-affected soils with cultivation,
gypsum, or sulfur amendments, drainage, and use of alternative water sources.
Avoid toxins by limiting excessive use of herbicides or other chemicals, limiting
heavy metal-containing soil amendments, and judicious application of macro- and
6. Correct soil biotic problems as follows: Correct root-feeding insects, diseases, and nematodes with preventive, cultural, or chemical control treatments;
thatch by mechanical removal, cultivation, and promotion of microbial degradation.
Perennial grasses will always be subjected to fluctuating multiple stresses. Traditional breeding programs can address specific environmental constraints and, as
mechanisms governing stress response become better understood, these programs
can focus on specific components of these mechanisms. Gene technology provides
an enhancement strategy for these traditional breeding approaches. An increasing
number of genes are being identified, sequenced, and cloned. Transformation and
regeneration technology is available for implementation into turfgrass stressresistance programs. With the release of new “biotech” turf cultivars in the twenty-first century, management strategies will have to be adjusted to maximize performance and persistence.
Enhanced abiotidedaphic stress tolerance in turf will provide
1. Improvements in performance under environmental extremes
2. Functional root systems that perform equally well in stressed and nonstressed environments
3. Improved water use efficiency
4. Improved nutrient uptake/utilization efficiency
5. Better adapted cultivars for niche environments
6. More high-quality and environmentally compatible turfgrasses under abiotic/edaphic stressed conditions
TURFGRASS MOLECULAR GENETIC IMPROVEMENT
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