Morphological and Physiological Responses of Sorghum
(Sorghum bicolor L. Moench) to Waterlogging
The aim of this study was to investigate the effect of waterlogging on morphological and physiological traits of sorghum (Sorghum bicolor L. Moench) cultivars. Four sorghum cultivars, cv. Wray, Keller, Bailey (sweet cultivar) and cv. SP1 (forage cultivar) at five expanded leaf stage were subjected to 20 days of waterlogging and drained pots were kept as the control. Twenty days of waterlogging did not cause a significant difference in shoot and root biomass among cultivars. Flooding reduced leaf area (69%), plant height (30%) and youngest leaf expansion rate of all cultivars but severely reduced in SP1 (35-80%). Flooding promoted leaf senescence of all cultivars and biomass allocation to shoot (increase in shoot/root) in Wray, Keller and Bailey, but increased biomass partitioning to root in SP1. The initiation of new nodal root was noted in SP1, whereas the ability to maintain root surface area by increase in longest root length and nodal root development near soil surface was found in Wray. Photosynthetic rate, stomatal conductance and transpiration rate were severely reduced under waterlogging conditions of sweet cultivars (65-78%), but enhanced over the control in forage cultivar (56%). The ability to conserve root surface area, allocate more biomass to shoot during waterlogging and develop root near soil surface may support new growth in Wray, whereas the ability to maintain leaf gas exchange parameters in SP1 was due to the active nodal root growth. Nevertheless, there was no relationship between photosynthetic rate and shoot growth of sorghum under anaerobic conditions.
March 12, 2010; Accepted: May 28, 2010;
Published: June 19, 2010
One of the considered influencing factors for the recent world food crisis
is renewable energy from agriculture products. It has been criticized as a humiliation
by diverting crops from food to bio-fuel feedstock and resources competition,
water or land, leaving 800 million people in hunger or undernourishment worldwide.
Second-generation bio-fuel technologies, based on lignocellulosic feedstock,
have been proposed to reduce this competition. Native or external plant species
with fast growing rate, high net energy yield and low input requirements, particularly
capable of being grown on marginal lands, are being intensively researched such
as sugarcane (Saccharum officinarum L.) (Rabelo et
al., 2008) or switchgrass (Panicum virgatum L.) (Schmer
et al., 2008). One potential energy crop is Sorghum bicolor
(L.) Moench. (Reddy et al., 2005; Corredor
et al., 2008). In addition, land use intensification such as sequential
cropping, intercropping or crop rotation of energy crop with food crop is one
of the solutions (Malezieux et al., 2009). However,
in the tropical regions, most of the crops grown during the summer-rainy season
frequently suffer intermittent or long-term waterlogging or flooding due to
excess irrigation, storms, poor soil drainage or overflowing of the rivers.
If this energy crop is incorporated into the paddy fields, a massive occupied
in tropical and sub-tropical areas, waterlogging has a particularly heavy effect
on the crop, since repeated puddling breaks capillary pores, reduces void ratio,
destroys soil aggregates and disperses fine clay particles as well as when a
rising water table and rainfall intensity combine with low evapotranspiration
These lead energy crops to be more commonly subjected to waterlogging or flooding. Therefore, information about sorghum responses to waterlogging is essential.
Growth and physiological processes are detrimentally affected by waterlogging.
Waterlogging decreases the leaf elongation rate and leaf area of plants (Orchard
and Jessop 1984; Dias-Filho and de Carvalho, 2000;
Malik et al., 2001, 2002;
Henshaw et al., 2007) and consequently reduces
plant height and ultimately suppresses root and shoot production (Haung et
Stomata closure, reduction of transpiration and inhibition of photosynthesis
are common responses that can occur in hours or days, depending on the tolerance
to waterlogging of each species and cultivar. When waterlogging is prolonged,
waterlogging-intolerant plants such as wheat (Triticum aestivum) (Malik
et al., 2001) or maize (Zea mays L.) (Zaidi
et al., 2003) drastically reduce their physiological activities and
are often killed in a short time, whereas, in waterlogging-tolerant plants the
same parameters could even be enhanced or have less effect due to the ability
of roots to acclimate to waterlogging, such as by the ability to produce adventitious
roots and aerenchyma formation (Dias-Filho and de Carvalho,
2000; Pang et al., 2004; Striker
et al., 2005; Irving et al., 2007;
Li et al., 2007; Mollard
et al., 2008).
Growth related to physiological response under waterlogging stress in sorghum has not yet been elucidated. Understanding how plants respond to waterlogging is important in determining their potential for use in habitat prone to this stress.
Therefore, the objectives of this study were to: (1) investigate morpho- and physiological changes in sorghum (Sorghum bicolor L. Moench) in response to waterlogging; and (2) compare the waterlogging tolerance of four sorghum cultivars, Wray, Keller, Bailey and Supanburi 1(SP1) during early vegetative stage.
MATERIALS AND METHODS
The experiment was conducted at the Moorbank Botanical Garden at the University
of Newcastle (UK) School of Biology, from February to April 2008 (54.987°N
and 1.635°W ). Seeds of sweet sorghum (Sorghum bicolor (L.) Moench)
cv. Wray, Keller, Bailey and forage sorghum (Sorghum bicolor (L.) Moench)
cv. Suphanburi 1 (SP1) (Supanburi Field Crop Research Centre, Thailand) were
sown in 15 cm diameter pots with 450 g of 6:2:1 (peat: unsterile sand: potting
base) culture. The potting base contained 7.5% nitrogen, 3.6% phosphorus and
5.2% potassium. Prior to planting, pots were watered to Field Capacity (FC)
and then three to five sweet sorghum seeds were drilled into each pot at 2 cm
depth. At the two-leaf stage, plants were thinned to one plant per pot. Water
was applied daily to FC by weighing random selected pots on a balance. Waterlogging
was applied at the vegetative stage (five fully expanded leaves) of each cultivar
(Vanderllip and Reeves, 1972). Pots containing growing
plants were placed in a larger pot and water was applied daily to the soil surface
for 20 days. Control pots were free-draining and were watered daily. To create
similar growing conditions between waterlogged and control pots, control pots
were also placed in the empty larger pots (Irving et
al., 2007). Pots were arranged in environmentally controlled conditions,
where the temperature was 13/26°C (min/max) and with a 14 h photoperiod
with the additional light of 400-watt high-pressure sodium bulbs (Pang
et al., 2004). The pots were arranged in a 2x4 factorial randomized
complete block design with four replications of waterlogging treatment and two
replications of control treatment (Irving et al.,
Plant growth measurements: During the waterlogging period, the length
of the youngest expanding leaf was measured daily using a ruler (Dias-Filho
and da Carvalho, 2000). Plant height (cm plant-1), senescent
leaf number per plant and leaf area (cm2 plant-1) were
recorded at four-day intervals. A non-destructive leaf area measurement method
was adopted, according to Wright (1981). At the end of
the waterlogging treatment, 20 Days after Waterlogging (DAW), plants were destructively
harvested. The length of the longest nodal root and nodal root number were recorded
(Mcfarlane et al., 2004). Root, leaf and culm
parts were dried at 80°C for 48 h and then the plants dry weight was
recorded. Shoot: root biomass ratio was calculated (Ye et
Photosynthetic measurements: Photosynthetic rate (A, μmol m-2
sec-1), stomatal conductance (gs, mol m-2 sec-1)
and transpiration rate (E, mmol m-2 sec-1) were collected
with a LCi portable photosynthesis system (ADC Bioscientific Ltd., Hoddesdon,
2004, UK). Measurements were taken from the youngest fully expanded leaf of
each plant (Pang et al., 2004). These measurements were made from 11.00
to 13.00 hrs, with the following specifications/adjustments: leaf surface area,
6.25 cm2; ambient CO2 concentration, 385 μmol mol-1;
and temperature of leaf chamber varying from 32.37°C to 34.33°C. Photosynthetically
Active Radiation (PAR), provided by 400-watt high-pressure sodium bulbs, was
set at the leaf surface to 700 μmol m-2 sec-1 (Ashraf
and Ur-Rehman, 1999). Measurements were conducted at 0, 4, 8, 12, 16 and
20 Days after Waterlogging (DAW).
Statistical analysis: The data for all physiological and growth parameters
were subjected to Analysis of Variance (ANOVA) using Statistix 8 software (Anonymous,
2003). The mean values were compared using the least significant difference
Shoot growth response to waterlogging: Waterlogging significantly reduced
plant height and the adverse effects were more pronounced when duration of waterlogging
increased. At 4 Days after Waterlogging (DAW), cv. Wray and Bailey could maintain
plant height to control plants but cv. Keller and SP1 showed a 10% reduction
from their respective controls. However after 8 DAW onwards, a higher height
reduction with longer waterlogging duration was found. At 20 DAW, height of
Keller was reduced to 30% (Fig. 1c), Bailey was reduced to
33% (Fig. 1e) and SP1 was reduced to 35% (Fig.
1g) as compared to the control. In contrast, cv. Wray after 12 DAW onwards
had height reduction maintained at 18-20% lower than the control (Fig.
1a). Reduction in plant height was consistent with the decrease in Youngest
Leaf Expansion Rate (YLER). Waterlogging significantly reduced YLER of all cultivars,
being lowest in cv.
||Effect of waterlogging on plant height (a, c, e, g) and Youngest
Leaf Expansion Rate (YLER) (b, d, f, h) of 4 sorghum cultivars. Solid lines
and open symbols represent control plants, broken lines and closed symbols
represent waterlogged plants. Error bars represent standard errors
Wray (Fig. 1b), while progressively decreasing in cv. Keller
(Fig. 1d), Bailey (Fig. 1f) and being highest
in SP1 (Fig. 1h). LA was significantly affected by waterlogging,
with advancing reduced when waterlogging duration was extended. Different responses
were noted among studied cultivars. Cv. Wray could maintain LA similar to control
plants at 4 DAW and began to reduce at 8 DAW while the other three cultivars
started to decline at 4 DAW onwards. After 12 DAW to 20 DAW LA reductions in
cv. Wray were between 56-65% (Fig. 2a). However, in cv. Keller
it increased from 43% at 12 DAW to 62% at 20 DAW (Fig. 2c),
or 34 to 72% in cv. Bailey (Fig. 2e) and the highest persistence
was in cv. SP1 (74 to 80%) (Fig. 2g). The reduction in LA
was concurrent with the acceleration in leaf senescence. Waterlogging significantly
increased the senescent leaf number of all studied cultivars in terms of the
onset and change during the waterlogging period. Senescent leaf no. gradually
increased at 8 DAW in cv. Wray and Bailey and reached a maximum at 16 DAW and
20 DAW, respectively (Fig. 2b and f). However,
a sharp rise to maximum was noted in cv. Keller at 8 DAW onwards (Fig.
2d). Nevertheless, the development of senescent leaf no. was delayed in
cv. SP1, started at 12 DAW. Shoot biomass was severely affected by 20 days of
waterlogging with 71% compared to the control.
||Effect of waterlogging on leaf area, LA, (a, c, e, g) and
leaf senescent number (b, d, f, h) of 4 sorghum cultivars. Solid lines and
open symbols represent control plants, broken lines and closed symbols represent
waterlogged plants. Error bars represent standard errors
This was due to the decrease in culm dry weight (DW) (68%) and leaf DW (72%).
All cultivars had a similar response and no interaction between water regimes
and cultivars was found (Table 1). However, under waterlogging
conditions, cv. Wray showed relatively higher shoot DW, followed by cv. Keller
and Bailey and lowest in cv. SP1.
Root growth: Waterlogging significantly reduced Nodal Root (NR) number
plant-1, longest root length (LRL) and consequent root DW, compared
to the control plants. NR number plant-1, LRL and root DW were reduced
to 24, 28 and 71% compared to the control plants, respectively (Table
1). The differences in cultivar responses were found in all root growth
parameters. Interestingly, compensation between NR number and LRL was observed.
The lowest NR number was found in cv. Wray (16 plant-1), but it had
the longest LRL (399 mm plant-1) (Table 1). Interactions
between water regimes and cultivars on NR number and LRL were also noted. Cv.
Bailey had the highest NR number (30 plant-1), whereas the lowest
was found in waterlogged Wray (13 plant-1) (Fig. 3a).
LRL was highest in the control treatment of cv. Wray (502.75 mm plant-1)
and lowest in waterlogged cv.
||Effects of 20 days of waterlogging on above-ground and below-ground
growths of 4 sweet sorghum cultivars
|*,**Significant at p<0.05 or 0.01, respectively and ns;
not significantly different at 0.05 of probability. Values followed by the
same letter in the same column are not significantly different at 0.05 and
0.01 probability. DW: dry weight
||(a) Effects of 20 days of waterlogging on nodal root number
per plant, (b) longest root length and (c) shoot/root ratio of 4 sorghum
cultivars. Values followed by the same letter in each growth stage are not
significant different at 0.05 and 0.01 probability. Error bars represent
Bailey (272 mm plant-1) (Fig. 3b). However, no
interaction was noted in terms of root DW.
Shoot/root ratio: Shoot /root ratio (S/R) was significantly higher (3.56) in plants under waterlogged conditions than in freely drained pots (3.17) (Table 1). A statistically different interaction was found. Further analysis showed that waterlogged SP1 had the lowest S/R, while the highest was found in waterlogged Wray (Fig. 3c).
Physiological response to waterlogging: Leaf gas-exchange characteristics
of all sorghum cultivars were significantly affected by waterlogging varying
in cultivars studied. The intensification was more pronounced with longer duration
of waterlogging. Photosynthetic rate (A) of sweet sorghum, cv. Wray, Keller
and Bailey, started to decline at 4 DAW, with 29, 58 and 31%, respectively,
compared to their controls but unchanged in cv. SP1. The degree of the reduction
was maintained to 8 DAW and accelerated to 57% at 12 DAW and was lowest at 20
DAW 78% in cv. Wray (Fig. 4a). In cv. Keller, the adverse
effect was alleviated at 8 DAW (35%) and 12 DAW (40%). However, prolonged waterlogging
duration reduced A of Keller to 65-70% (Fig. 4c). In cv. Bailey,
the response was found similar to cv. Wray (Fig. 4e). In contrast
to sweet sorghum, waterlogging reduced A of forage sorghum, cv.
||Effect of waterlogging on photosynthetic rate (A) (a, c, e
and g) and stomatal conductance (gs) (b, d, f and h) of 4 sorghum cultivars.
Solid lines and open symbols represent control plants, broken lines and
closed symbols represent waterlogged plants. Error bars represent standard
SP1 only at 8 DAW (11%). And starting from 12 DAW onwards, A was increased
over the control, giving 56% higher than controls at 20 DAW (Fig.
Stomatal conductance (Fig. 4b, d, f
and h) and transpiration rate (data not shown) followed a
similar pattern to that observed for photosynthesis.
Four sorghum genotypes in this experiment responded to waterlogging similarly in terms of shoot and root biomass accumulations but study of root growth, dry matter partition and leaf gas exchange parameter showed drastic differences in their responses.
The significant shoot growth reduction (71%) was due to the restricted development
of plant height (30%), LA (69%) and thus decreased dry matter accumulation in
leaf (72%) and culm (68%). This finding is in agreement with the findings in
wheat (Triticum aestivum) (Malik et al., 2001),
mungbean (Vigna radiate L. Wilczek) (Ahmed et
al., 2002), ryegrass (Lolium perenne L.) (Mcfarlane
et al., 2004), maize (Zea mays L.) (Zaidi
et al., 2003, 2004), barley (Hordeum vulgare)
(Pang et al., 2004), buckwheat (Fagopyrum
sp.) (Matsuura et al., 2005) and soybean (Glycine
max (L.) Merrill ) (Henshaw et al., 2007).
The pronounced reduction in shoot growth in this study is described by its
high susceptibility to waterlogging at vegetative stage of S. bicolor
due to its lack of nodal root development as compared to other growth stages
(Our experiment, unpublished data). In this study, even though plant height
and leaf area were reduced, waterlogging did not cause a significant reduction
to biomass of shoot and root among cultivars. This indicates that these cultivars
are tolerant to waterlogging conditions. Our previous experiment demonstrated
that continuous flooding sweet sorghum from 30 DAE (8-10 leaf stage) until harvest
only reduced shoot biomass and stalk yield to 20 and 22%, respectively. However,
to investigate up to what extent they can withstand continuous flooding at the
early vegetative stage further experiment is required. No significant difference
in shoot biomass but significant reduction in shoot length and LA in response
to waterlogging is also reported in Hibicus esculentus (Ashraf
and Arfan, 2005).
The present study showed that even with a lack of differences in total biomass
accumulation between aerobic and anaerobic treatment, significant differences
in biomass allocation patterns was found. This is consistent with the response
to waterlogging of Sporobolus virginicus (L.) Kunth (Naidoo
and Mundree, 1993).
Results of this experiment indicate that the biomass partitioned to root in
cv. SP1 is utilized for the initiation of new nodal roots. However, there was
a competition for assimilates between shoot and root growth, resulting in slow
shoot growth. This is in contrast to cv. Wray, where more assimilates was allocated
to support shoot growth. This supports the result of Ye
et al. (2003), who indicated that a shift of biomass from root to
shoot is an adaptation to prolonged waterlogging in the higher waterlogging
tolerance mangrove species (Kandelia candel). The significant reduction
in photosynthate allocation to belowground parts, while maintaining aboveground
biomass accumulation is reported as an effective metabolic strategy to reduce
belowground oxygen demand and to increase the potential of shoots to transport
oxygen to root of Sporobolus virginicus (L.) Kunth (Niadoo and Mundree,
1993). Therefore, it could possibly be concluded that in sorghum biomass portioning
to shoot is an acclimation response to long-term waterlogging at early growth
Leaf elongation rate has been proposed as an early detection mechanism for
relative flood tolerance in grass species such as Brachiaria spp. (Dias-Filho
and de Carvalho, 2000; Dias-filho, 2002) or maize
(Zea mays L.) (Lizaso and Ritchie, 1997). Our
results show that cv. Wray has the ability to extend the youngest leaf and produce
new leaves, indicating that this cultivar is relatively tolerant to waterlogging.
However in cv. SP-1, YLER was sharply decreased at day 2 after applying waterlogging
and continued decreasing over time, indicating that cv. SP-1 is relatively intolerant
to waterlogging. Cv. Keller (as well as cv. Bailey) has the ability to maintain
YLER for at least five days after waterlogging and then gradually decrease.
This may imply that these two cultivars are quite tolerant to short-term waterlogging.
Coincident increasing in senescent leaf number of cv. Wray, Keller and Bailey
during waterlogging implies that these new growths may be supported by the remobilization
of nutrients from older parts. This finding is consistent with the response
of lucerne (Medicago sativa) to waterlogging (Irving
et al., 2007).
Adventitious root or nodal root development has been reported as the key root
acclimation to waterlogging or flooding (Pardales et
al., 1991; McDonald et al., 2002; Pang
et al., 2004; Polthenee et al., 2008;
Changdee et al., 2009). However, Van
Noordwijk and Brouwer (1993) suggested that more developed roots may be
less able to adapt morphologically (such as development of aerenchyma)
under stress conditions. A relatively extensive aerenchyma spaces noted
in cv. Wray than cv. SP1 in previous experiment may support the previous concept.
Zaidi et al. (2003) indicated that an early
adventitious rooting is one of the crucial morphological traits of maize to
tolerate excess soil moisture stress. This is in agreement with our results,
indicating that sweet sorghum possesses this root acclimation trait, while fibre
sorghum does not. In addition, our results showed that in cv. Wray even with
lowest nodal root number, it had significant highest individual root length.
This may presumably be a partial compensation to conserve root area for water
and nutrient uptake during waterlogging. It was also observed that during waterlogging,
the nodal root of cv. Wray was located near the soil surface indicating that
this cultivar possesses root acclimation to reach atmospheric oxygen. This supports
the results of Niadoo and Mundree (1993), who indicated that waterlogging tolerant
Sporobolus virginicus (L.) possesses morphological responses to waterlogging
such as fewer, but taller and more mature culm with greater aerenchyma
spaces and production of aboveground adventitious roots close to the aerobic
Thus, the present experiment indicates that the early nodal root development, the ability to maintain root surface area for water and nutrient uptake and to develop aerenchyma spaces in existing roots and nodal root development near the soil surface during the susceptible stage are root morphological acclimations to survive and concurrently sustain plant growth under prolonged waterlogging conditions.
The decreased photosynthetic rate, as well as transpiration rate, in our experiment
may be partially regulated by stomatal closure, due to a high positive correlation
between stomatal conductance and photosynthesis under both control and waterlogged
conditions (Fig. 5). Other findings (Huang
et al., 1994; Malik et al., 2001;
Ashraf, 2003; Striker et al.,
2005) have also shown that stomatal conductance is the major factor effecting
photosynthesis under waterlogging conditions in plants.
The lower leaf photosynthesis of cv. Wray, Keller and Bailey may also possibly
be regulated by sink strength in terms of root growth, since these cultivars
have relatively low nodal root numbers compared to significantly higher ones
in cv. SP1. Strong regulation of leaf photosynthesis by sink strength in wheat
(Triticum aestivum) (Malik et al., 2002)
and sugarcane (Saccharum officinarum L.) (McCormick
et al., 2006) has also been reported. In this study, a positive significant
correlation coefficient between photosynthetic rate and nodal root number of
cv. SP1 may support this finding (Fig. 6). In addition, factors
regulating photosynthesis in plants grown in waterlogged soil may be reduced
CO2 transfer conductance from sub-stomatal cavities to the site of
carboxylation, or activity of photosynthetic enzymes at the carboxylation point.
||Relationships between photosynthetic rate (A) and stomatal
conductance (gs) of sorghum. Open symbols represent control plants and filled
symbols represent waterlogged plants. Asterisks represent responses in waterlogged
||Relationships between NR number per plant and photosynthetic
rate (A) under waterlogging conditions
A marked increase in sub-stomatal CO2 (Ci) noted in waterlogged
cv. Wray, Keller and Bailey in relation to control plants, compared with comparative
levels between waterlogged and freely drained plant in cv. SP1 (data not shown),
may confirm that point. This is consistent with the finding of Malik
et al. (2001), who reported that this may be responsible for decreased
photosynthesis in waterlogged wheat. However, to draw such relationship in sorghum
in response to waterlogging, a further experiment is needed. Nevertheless, the
observed response of photosynthesis to CO2 indicates that a relevant
limitation to photosynthesis of S. bicolor under waterlogging conditions
may be caused by a reduced capacity of RuBisCo for CO2 fixation,
not translocation of CO2 such as occurs in drought stressed sweet
sorghum (Massacci et al., 1996). Reduction of
RuBisCO is closely related to total soluble protein content (Irving
et al., 2007) and is logically related to total leaf nitrogen content.
The concurrent higher SPAD Chlorophyll Meter Reading (SCMR) and photosynthetic
rate (A) in cv. SP1, as well as a significant decrease in SCMR and A of cv.
Wray, Keller and Bailey, throughout the experiment (data not shown), possibly
implies that the leaf photosynthetic rate in waterlogged sorghum is partly regulated
However, in sweet sorghum, particular cv. Wray, it is likely that the growth
did not relate to chlorophyll. This is consistent with the finding in Blue panicgrass
response to waterlogging (Ashraf, 2003). This may be
due to the remobilization of nitrogen from older plant parts to support shoot
growth during waterlogging causing a reduction in photosynthetic rate. Under
waterlogging or flooding conditions, a positive relation between photosynthetic
capacity and growth has been reported in signal grass (Brachiaria brizantha
(A. Rich.) Stapf) (Dias-filho, 2002), Blue panicgrass (Panicum antidotale
Retz.) (Ashraf, 2003), barley (Hordeum vulgare)
(Pang et al., 2004), bird's-foot trefoils (Lotus
sp.) (Striker et al., 2005) or dallisgrass
(Paspalum dilatatum) (Mollard et al., 2008),
due to the development of nodal roots or adventitious roots, which form extensive
aerenchyma spaces which create a relatively low-resistance internal pathway
and by enhancing mass flow transport of oxygen, carbon dioxide and ethylene
between plant parts above water and submerged tissues (Jackson
and Colmer, 2005). This is in contrast to our findings. The present study
showed that under waterlogging conditions cv. SP1 produces nodal roots similar
to control plants and increased photosynthetic rate higher than the control
at the end of the experiment. However, its height, LA, leaf DW and shoot DW
had highest decrease. In contrast to cv. Wray, waterlogging significantly reduced
nodal root number and photosynthetic rate. However, its height, Youngest Leaf
Expansion Rate (YLER), LA and shoot biomass was less affected than cv. SP1.
This indicates that there is no relationship between growth and photosynthetic
capacity in sorghum under waterlogging conditions. This supports the results
of Ashraf and Arfan (2005), who found that under waterlogging
Okra (Hibiscus esculentus) decreases photosynthetic rate but not its
shoot biomass or wheat (Triticum aestivum) under salt stress (Hawkins
and Lewis, 1993). The concomitant reduction in shoot biomass and unaffected
leaf photosynthetic rate is found in Sporobolus virginicus (L.) Kunth
in response to the combination effects of waterlogging and salt stress (Naidoo
and Mundree, 1993), which is consistent to the response of forage sorghum,
cv. SP1, in this experiment.
Nevertheless, in comparison to wheat response to severe waterlogging (Malik
et al., 2001), photosynthesis is reduced to 82% compared with the
control at 5 d after waterlogging. Our results indicate that sorghum has higher
waterlogging tolerance than wheat since it took 20 days to suppress photosynthesis
to a similar value (average 73% as compared to the control plants). Significantly
higher plant height, or the ability to extend the youngest leaf expansion rate
per day, results in relatively higher shoot biomass during waterlogging in cv.
Wray and it can thus be concluded that cv. Wray may be the most waterlogging
tolerant from an agronomic point of view, the maintenance of relatively high
yield (Setter and Waters, 2003). Cv. Keller and Bailey
are intermediate and cv. SP1 is sensitive to waterlogging. This indicates that
sweet sorghum; especially cv. Wray has the potential to grow on waterlogging
prone areas, whereas forage sorghum is more preferable on upland areas. In cv.
SP1, leaf photosynthetic rate was less affected by short-term waterlogging and
during the long-term waterlogging duration leaf photosynthetic rate as well
as transpiration rate and stomatal conductance increased over the control. It
can thus be concluded that cv. SP1 is the most waterlogging tolerant from a
physiological point of view, survival or maintenance of high growth rate under
waterlogging, relative to non-waterlogged conditions (Setter
and Waters, 2003). Cv. SP1 may be the most valuable cultivar for use in
further breeding programs. At the same time, from an agronomic point of view,
cv. Wray may be the most suitable for immediate use by agronomists. Therefore,
it can be seen that physiological measurements can help in assessing highly
physiology tolerant cultivars, which only a high-yield basis assessment may
In conclusion, waterlogging reduced plant growth of four sorghum genotypes similarly but study of root growth, dry matter partition and leaf gas exchange parameter showed drastic differences in their responses. Cultivar, maintaining the higher shoot growth, was not associated with the ability to develop higher nodal root and retain or least affect leaf photosynthetic rate. But it is related to biomass partitioning to shoot during long- term flooding, early nodal root development, accompanied with the ability to conserve root surface area for water and nutrient uptake as well as the ability to remobilize nutrients from older parts to support shoot growth such as cv. Wray. In contrast, keeping high photosynthetic rate by partitioning biomass to develop new roots was at the expense of shoot such as cv. SP1. Nevertheless, to comprehend the physiological response of this cultivar, further study is needed.
This research was financially supported by the Royal Golden Jubilee program of the Thailand Research Fund. The authors acknowledge the kind assistance of Brian Brown, technical manager from the School of Agriculture, Food and Rural Development and Moorbank Botanical Garden, School of Biology, University of Newcastle, UK, for providing equipment and facilities during the conduction of the experiment.
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