沙漠研究 22-1, 175 -178 (2012 )"ICAL 1 / DT X"

Journal of Arid Land Studies－Refereed Paper－Transpiration Ratio in Sorghum [Sorghum bicolor (L.) Moench]for Increased Water-use Efficiency and Drought Tolerance
Mohankumar H. KAPANIGOWDA1), William A. PAYNE*1,2), William L. ROONEY1) and John E. MULLET3)
Abstract: There is an increasing need to improve crop water-use efficiency (WUE) (i.e., the ratio of whole-plant biomass tocumulative transpiration) due to decreased water availability and increased food and energy demands throughout the world. Inprevious studies, the pre-flowering transpiration ratio (A:E) [CO2 assimilation rate (A) divided by transpiration rate (E)] of sorghum
leaves was correlated with WUE. The present greenhouse study was conducted to examine pre-flower A:E in 70 inbred sorghum(Sorghum bicolor L. Moench) lines and two parents (Tx430 and Tx7078) in terms of phenotypic and genetic variation. Parents wereselected with contrasting A:E based on previous studies. The experimental design was a Randomized Complete Block, withgenotype (70 inbred lines and the parents) and water regime (40 and 80% of “field capacity” (FC)) as experimental factors, and four
replications. Genotype had a highly significant effect on A, E and A:E. Frequency distribution for A:E revealed the normal
distribution indicating the polygenic segregation among the progenies. Average A:E was 3.07 mmol CO2 mol-1 H2O for Tx430 and2.80 for Tx7078. These results provide further evidence that there is genetic variability among genotypes for gas exchange rates at
pre-flowering in sorghum suggesting scope for improved WUE and productivity.
Key Words: Drought, Sorghum, Transpiration ratio, WUE

1. Introduction
Sorghum (Sorghum bicolor (L.) Moench) is the fifth mostimportant cereal crop after wheat, maize, rice and barley (FAO,2009) and it is a staple food crop for 500 to 700 million people
in arid and semiarid regions primarily of Africa and some ofAsia. In terms of utilization, worldwide, almost half of thegrain sorghum produced is used as animal feed. Morerecently sorghum has been proposed as a dedicated cellulosic
bioenergy feedstock (Rooney et al., 2007).
With the rapid increase in population and scarcity of thefresh water resource, water shortage (or drought) has become
the key factor that constrains crop production worldwide.
Therefore, strategies to increase the food productivity whileconserving the water become increasingly important (Ungerand Howell, 1999). These include selection of droughttolerant crops and improvement of WUE. Sorghum is known
for its extensive phenotypic and genotypic variation in
response to drought (Blum, 1979; Doggett, 1988). The
superior drought tolerance in the crop is likely due to its
evolution in Sub-Saharan Africa, a region characterized bypredictably low and erratic rainfall patterns. Thus, it is one of
the most drought tolerant crops and serves as a model forstudying the genetic and physiological mechanisms of droughttolerance in cereal crop species. Even though considerable

work has been done on plant response to moisture stress(Tuinstra et al., 1996; Xu et al., 2000; Kebede et al., 2001;
Haussmann et al., 2002), there has been little emphasis on theuse of specific physiological traits to enhance the drought stress
tolerance. Specific physiological trait like transpiration ratio(A:E), which is defined as change in CO2 assimilation rate (A)per unit change in transpiration rate (E), is one potential way toincrease water productivity under drought stress condition.
1.1. Gas exchange rates and WUE
Several field and greenhouse studies have found significant
genetic variation for A:E in grain sorghum under water limited
conditions (Krieg and Hutmacher, 1986; Kidambi et al., 1990a,
Peng et al., 1991; Peng and Krieg, 1992; Krieg et al., 1992;
Balota et al., 2008). Significant variation in the ratio of A tostomatal conductance (g) has been observed (Kidambi et al.
(1990a). In their study, g was relatively conservative,
suggesting that it may be possible to select for increased A
without a concurrent increase in g in sorghum. Genotypicvariation in A was associated with increased leaf area and shootbiomass production; this occurred without significant increasein water use or leaf transpiration (Peng and Krieg, 1992).
Further, they concluded that measurements of A and leaf area
could be used as selection criteria for higher WUE in grainsorghum under field conditions. Peng et al. (1991) reported astrong correlation between A and total biomass production in* Corresponding Author: w-payne@tamu.edu
Research Director,Teague Building suite 123, College Station, TX 77843-2477
1) Department of Soil and Crop Science, Texas A&M University, USA.
2) Norman Borlaug Institute for International Agriculture, Texas A&M University, USA

3) Dept. Bio-physics and Bio-chemistry, Texas A&M University, USA22 sorghum genotypes.
They also suggested that single leaf measurement of A
could be used to select for higher productivity among sorghum
genotypes. Balota et al. (2008) examined four sorghum
parental inbred lines and 12 of their hybrids for transpiration
ratio under water limited and well watered conditions. They
found that average A:E over both water conditions was 3.10mmol CO2 mol-1 H2O for Tx430 and 2.91 for Tx7078. These
two genotypes also had the highest A. They concluded that
there is genetic variation for pre-flower A, E, and A:E rating aswell as WUE in sorghum genotypes.
Even though
pre-flowering drought-stress commonly occurs in sorghum
production environments (Rosenow et al., 1996), very few
genetic analyses have been completed for pre-floweringdrought-stress. Therefore, the objective of this research wasto determine the genetic variation for A:E related to pre-flower
drought tolerance in sorghum recombinant inbred lines
population under controlled conditions.

2. Material and Methods
A total of 70 F6 generation recombinant inbred lines(RILs) and two parents (Tx430 and Tx7078) were developedin the Texas AgriLife Breeding Program directed by Dr.
William L. Rooney. The parents were selected based oncontrasting values of transpiration ratio (A:E). The RILs andtheir parents were grown in a greenhouse at the Texas AgriLifeResearch and Extension Station in Bushland, TX, (35°11’ Nlat; 102°06’ W long; 1170 m elevation) to measure gas
exchange rates. The experimental design was a randomized
complete block with four replications, with genotype and water
regime (40 and 80 % FC) as experimental factors.
Each pot contained 2.5 kg finely screened Pullman clay
loam soil (fine, mixed, superactive, thermic Torrertic
Paleustolls. The two water treatments, i.e. 80 % and 40% FC,
were based on soil water retention curves determined by apressure plate apparatus (Klute, 1986) and correspond to
gravimetric soil water contents of 0.17 kg kg-1 at a pressure
potential of -0.1 MPa and 0.09 kg kg-1 at -1 MPa. Each pot
was lined with a plastic bag, which was wrapped around thestems to minimize water evaporation from the soil surface.
For the first 15 days after planting (DAP) all the pots weremaintained under well watered conditions. Water treatments
were imposed at 16 d after planting and maintained by daily
weighing and watering, using an electronic balance withincrements of 5 g. Gas-exchange measurements were takenfrom 32 - 34 DAP using LI-6400 Infrared Gas Analyzer(IRGA) portable photosynthesis system (LI-COR, Lincoln,NE) on 70 inbred lines and two parents. Genotype effect on
A, E, and A:E was analyzed with ANOVA from the GLM
procedure of SYSTAT 10.2 (2002, SYSTAT Software Inc.,
Richmond, CA) using genotype and replication as independent
variables and each pot as an experimental unit (Balota et al.,2008).

3. Results and Discussion
The RILs for mean values of A:E, A and E were normal
distributed, as expected for a quantitative trait both under 80%and 40% FC except in A:E under 40% FC (Fig. 1). The
normal distribution of these traits indicated polygenic
segregation. However, skewed distribution of A:E under
40% FC suggested the involvement of a single gene with large
effects controlling this physiological trait.
Genotype had a highly significant effect on A (P < 0.0001
and P<0.0001), E (P < 0.002 and P<0.007) and A:E (P <0.0001 and P<0.0001) under 80% and 40% FC, respectively(Table 1). Genetic variation in A and E among sorghum lines
and their hybrids have been reported previously by several
authors (Kidambi et al., 1990b: Peng and Krieg, 1992; Balotaet al., 2008). Kidambi et al. (1990a) reported substantial
genetic variation in the A: g (stomatal conductance)
relationship that caused by significant genetic variation in A.
However, g was relatively more conservative to increasingwater stress. They proposed that selection for high A might
directly contribute to greater WUE and higher droughttolerance. However, Balota et al. (2008) suggested thatconcomitant selection for high A:E and A may be necessary
when high biomass and WUE are both desired. In our study,
we found genotypes had highly significant effect on A, E and
A:E, which provides further evidence that when we desired
to have potentially greater drought tolerance, it might benecessary to select for both higher A:E and A.
Between the two levels of water regimes in experiment 1,values for A, E, and A:E were generally larger under 80% FC
than those in the 40% FC among the RILs (Table 2).
However, RILs produced ~10% more biomass per unit oftranspiration under 40% FC than 80% FC in experiment 1. In
the study by Balota et al. (2008), which included both low
water (LW) and high water treatment, LW treatment resulted inlower biomass and cumulative transpiration in sorghum similar
to our study. They predicted that lower biomass under LW
might be due to the acclimatization of plants to LW byregulating the transpiration without partial stomatal closure
which is more sensitive to moisture stress.

Table 1.

80% Water regime

Mean squares and probability levels (p) by environment(water regime) from ANOVAs for gas exchange traits
related to pre-flower drought tolerance in sorghum RIL
population derived from Tx430 x Tx7078, Bushland, Texas
during 2008.

Tx707825Tx430

Number of RIL population

2015A

DfM S¶pMSp

Genotype
Replication

72
370823<0.001<0.0010.88
132<0.001<0.001

Error
Genotype
Replication
Error

681
72
3601

38114
4146
32

<0.001<0.001

0.19
1.02
116
0.09

<0.001<0.001

10

50
1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

Transpiration efficiency (A:E) (mmol CO2 mol-1 H2O)

40% Water regime

Units: A- μmol CO2 m−2 s−1; A:E - mmol CO2 mol-1 H2O kPa-1; ¶ Mean square.

25Tx7078

Tx430

Nember of RIL population

20

Table 2.

15

Mean values for CO2 assimilation rate (A), transpiration (E) andtranspiration ratio (A:E) for recombinant inbred lines (RILs),
Bushland, TX.
RILs mean (SD)†10Trait
540% FC

80%FC

30.12

37.21

(µmol CO2 m s )Transpiration (E)(4.14)†

-2.68

6.65

8.37

(mmol H2O m-2 s-1)

-0.73

-0.72

CO2 assimilation rate (A)
-20
1.0

1.2

1.4

1.6

1.8

2.02.22.4

2.6

Transpiration efficiency (A:E) (mmol CO2 mol-1 H2O)

80% Water regime
30

A:ESource

Tx430

-1

Transpiration ratio (A:E)

1.93

2.29

(mmol CO2 mol-1 H2O)

-0.44

-0.31

†Standard deviation
20

Tx7078

Number of RIL population

25

15

4. Conclusion

10

5026

28

30

32

3436

38

40

42

44

CO2 assimilation rate (A) (µmol CO2 m-2 s-1)

40% Water regime
30

Tx7078Tx430

Number of RIL population

25

These results provide further evidence that there is genetic
variability among genotypes for gas exchange rates (A, E andA:E) at pre-flowering in sorghum suggesting scope for
improved WUE and productivity. We believe that our results
are potentially useful to develop the genetic map and identify
the genes involved in pre-flower drought tolerance, particularly
at the GS2 stage, and increased sorghum production under U.S.
Great Plains environments, Africa and parts of India.

20References
15105015

202530354045

CO2 assimilation rate (A) (µmol CO2 m-2 s-1)

Fig. 1. Frequency distribution of transpiration ratio (A:E) and CO2
assimilation rate (A) at 80% and 40% FC in 70 recombinant
inbred lines and two parents. The mean A:E values forTx7078 and Tx430 are indicated by arrows.

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