Identification of physiological indicators for establishing screening techniques related to tolerance of excess water in soybean [Glycine max(L.) Merrill].
1The National Institute of Crop Science, Korea, www.nces.go.kr Email : sbplje@rda.go.kr
2 The Chungbuk National Univ., Korea, www.chungbuk.ac.kr Email : hongsigk@chungbuk.ac.kr
A series of experiments were carried out under greenhouse conditions to identify physiological indicators for increasing the efficiency in breeding soybean cultivars that tolerate excess water. Waterlogging tolerance was assessed in five cultivars at early vegetative stage (V5) and early reproductive stage (R2) by maintaining the water level at the soil surface for 10 days. Results of the study indicated that Pungsannamulkong was the most tolerant and Jangyupkong was the most susceptible to excess water at the two growth stages. 5 physiological characters, including leaf water use efficiency, were promising indicators for screening soybean which is tolerant of excess water.
Nodule activity, photosynthetic rate, stomatal conductance, water use efficiency, ureide content in petiole and the root distribution are promising indicators for screening soybeans tolerant of excess water.
Key Words
Soybeans, Excess water, Waterlogging tolerance, Screening indicator
Stress from excess water is the most harmful limiting factor in soybean yield during the wet season under the climate conditions in Korea. Soybean is very sensitive to excess water compared to other crops. Griffin et al. (1988) and Scott et al. (1989) reported that flood inundation of more than 48 hours resulted in yield decrease and the degree of decrease was more severe at reproductive growth stage than at the vegetative stage. The response to excess water in soybeans is associated with a number of biochemical, morphological and anatomical changes in both the root and the shoot (Richard et al. 1994; Bacanamwo and Harper 1997). Bacanamwo and Purcell (1999) reported that morphological mechanisms of acclimation to flooding stress in soybean appear to involve an avoidance of water loss by transpiration and a facilitated transport of atmospheric O2 to the submerged roots through the flood-induced formation of adventitious roots and aerenchyma. In general, the petiole ureide concentration is increased by drought stress in soybean. By contrast, Puiatti and Sodek (1999) reported that ureide concentration in xylem decreased under flooded conditions. These physiological and morphological changes by waterlogging stress can be promising indicators for screening soybeans for tolerance of excess water. Our objective was to identify physiological parameters for establishing a screening system for waterlogging tolerance in soybean.
The experiments were conducted in 2002 and 2003 at Suwon, Korea with 5 soybean cultivars (Jangyupkong, Pungsannamulkong, Myungjunamulkong, Muhankong, and Peking). These 5 varieties were planted on May 29 in 2002 and 2003 and 2 plants per pot were grown by thinning at the the emergence stage (VE). The experimental design was randomized block arrangement by three replications. Waterlogging treatment was conducted by maintaining the water level at the soil surface for 10 days at the early vegetative growth stage (V5) and the flowering stage (R2). The growth stage was identified as in Fehr & Caviness (1977). Six soybean plants were sampled at the treatment completion day of V5 stage, at the treatment completion day of R2 stage, at 31days after R2 stage treatment and R8 stage. Nodule activity, photosynthetic rate, stomatal conductance, leaf water use efficiency, ureide content in the petiole and root distribution were measured. The collected data were analyzed using the SAS statistical package.
In waterlogging treatment of the 5 soybean test varieties, Jangyupkong, Myungjunamulkong were susceptible and Pungsannamulkong, Muhankong were tolerant in nodule activity, photosynthetic rate, stomatal conductance, water use efficiency, ureide content in petiole and root distribution. The specific nodule weight was reduced in all test varities in response to waterlogging (Table 1), but the degree of decrease was much less in Pungsannamulkong and Muhankong than Jangyupkong and Myungjunamulkong. The similar tendencies were showed in photosynthetic rate, stomatal conductance and water use efficiency (Table 2 and Table 3). In general, the leaf water potential decreases under excess water in all plants. So stomatal conductance decreases to minimize the water content loss in plant tissue, as found here too. But the water use efficiency also decreased here, indicating damage to the photosynthetic machinery from waterlogging. Photosynthetic rate decreased in all cultivars. The decrease was smallest in Pungsannamulkong and Muhankong and these varieties showed smallest reduction in water use efficiency and smallest reduction in stomatal conductance (Table 2 and Table 3).
Table 1. Changes of nodule activity according to waterlogging at V5 stage for 10 days in different soybean varieties.
Variety |
Dry wt. of nodules(g/plant) |
SNW* (mg/nodules) | ||||
Control(A) |
Treatment(B) |
B/A(%) |
Control(A) |
Treatment(B) |
B/A(%) | |
Jangyupkong |
2.77a |
2.16b |
78 |
10.09a |
3.30b |
33 |
Pungsannamulkog |
1.70a |
1.56b |
92 |
7.12a |
6.05b |
85 |
Myungjunamulkong |
1.23a |
0.86b |
70 |
9.46a |
4.66b |
49 |
Muhankong |
2.20a |
1.98b |
90 |
11.48a |
8.09b |
70 |
Peking |
1.23a |
1.02b |
83 |
6.93a |
5.85b |
85 |
* Specific Nodule Weight : Dry wt. of nodule per plant/No. of nodules per plant
Table 2. Changes of photosynthetic rate, stomatal conductance and WUE* according to waterlogging at V5 stage for 10 days in different soybean varieties.
Jangyupkong |
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Pungsannamulkog |
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Myungjunamulkong |
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Muhankong |
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Peking |
* Water use efficiency : CO2 assimilation rate/Transpiration rate
Table 3. Changes of photosynthetic rate, stomatal conductance and WUE* according to waterlogging at the flowering stage for 10 days in different soybean varieties.
Jangyupkong |
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Pungsannamulkog |
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Myungjunamulkong |
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Muhankong |
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Peking |
* Water use efficiency : CO2 assimilation rate/Transpiration rate
Ureide contents in petiole can be a promising physiological indicator of N2 fixation and transportation ability under excessive water stress. The ureide contents of petiole in response to watelogging was reduced in all test varieties (Figure 1 and Figure 2). But the degree of decrease was less in Pungsannamulkong and Muhankong than Jangyupkong and Myungjunamulkong. When soybean is under hypoxia conditions, adventitious roots are formed on the upper part of root close to soil surface where oxygen partial pressure is high. The number of adventitious roots also can be promising physiological indicator of morphological adaptation to the hypoxia conditions (Photo 1).
Fig 1. Changes of ureide content in petiole according to waterlogging at V5 stage for 10 days in different soybean varities.
Fig 2. Changes of ureide content in petiole according to waterlogging at the flowering stage for 10 days in different soybean varities.
Pungsannamulkong (tolerant) |
Jangryupkong (susceptible) |
Photo 1. Comparison of root distribution affected by waterlogging treatment at the flowering stage for 10 days in different soybean varieties.
The specific nodule weight, photosynthetic rate, stomatal conductance, water use efficiency, ureide content in petiole and the number of adventitious root were identified as promising physiological and morphological indicator for screening soybeans which are tolerant of excess water.
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