AOBPreview originally published online on October 21, 2008
Annals of Botany 2009 103(2):171-180; doi:10.1093/aob/mcn201
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Contrasting physiological responses by cultivars of Oryza sativa and O. glaberrima to prolonged submergence
1 Japan International Research Center for Agricultural Sciences, 1-1 Ohwashi, Tsukuba, Ibaraki 305-8686, Japan
2 University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
* For correspondence. E-mail sakajun{at}affrc.go.jp
Received: 2 August 2008 Returned for revision: 11 August 2008 Accepted: 28 August 2008 Published electronically: 21 October 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Background and Aims Oryza glaberrima: is widely grown in flood-prone areas of African river basins and is subject to prolonged periods of annual submergence. The effects of submergence on shoot elongation, shoot biomass, leaf area and CO2 uptake were studied and compared with those of O. sativa.
Methods: A wide selection of lines of O. sativa and O. glaberrima, including some classified as submergence tolerant, were compared in field and pot experiments. Plants were submerged completely for 31 d in a field experiment, and partially or completely for 37 d in a pot experiment in a growth chamber.
Key Results: Leaf elongation and growth in shoot biomass during complete submergence in the field were significantly greater in O. glaberrima than in O. sativa. So-called submergence-tolerant cultivars of O. sativa were unable to survive prolonged complete submergence for 31–37 d. This indicates that the mechanism of suppressed leaf elongation that confers increased survival of short-term submergence is inadequate for surviving long periods underwater. The O. sativa deepwater cultivar ‘Nylon’ and the ‘Yélé1A’ cultivar of O. glaberrima succeeded in emerging above the floodwater. This resulted in greatly increased shoot length, shoot biomass and leaf area, in association with an increased net assimilation rate compared with the lowland-adapted O. sativa ‘Banjoulou’.
Conclusions: The superior tolerance of deepwater O. sativa and O. glaberrima genotypes to prolonged complete submergence appears to be due to their greater photosynthetic capacity developed by leaves newly emerged above the floodwater. Vigorous upward leaf elongation during prolonged submergence is therefore critical for ensuring shoot emergence from water, leaf area extension above the water surface and a subsequent strong increase in shoot biomass.
Key words: Flooding, leaf area, net assimilation rate, Oryza glaberrima, O. sativa, photosynthesis, rice, stress adaptation, submergence escape
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Oryza glaberrima is a monocarpic annual derived from O. barthii (Sakagami et al., 1999), and grown in traditional rice production in wetlands of West Africa. It is highly adapted to deepwater inundation in countries such as Gambia, Guinea, Mali, Niger, Senegal and Sierra Leone (Inouye et al., 1989). Work with O. sativa has shown that cultivars of rice that exhibit superior abilities to survive submergence and yield a crop predominantly adopt one of two contrasting strategies (Jackson and Ram, 2003). The ‘quiescence strategy’ features a slowing of ethylene-promoted leaf elongation by seedlings to conserve energy and carbohydrates during flash floods of up to about 2 weeks complete submergence. It is a feature of lines derived from a traditional farmer variety of indica rice that contain a mutated form of the Sub1A gene (Xu et al., 2006). The second strategy inherent in most cultivars of O. sativa and especially in deepwater and floating rice lines is the ‘escape strategy’. This involves the promotion of elongation of leaves and/or stems by entrapped ethylene (Jackson, 2008). This enables plants to resume aerobic metabolism and photosynthetic fixation of CO2 by raising their shoots above water. The escape strategy based on elongation by the stem is a prominent characteristic of deepwater and floating rice lines that are grown where submergence continues for >1 month in water deeper than 50 cm (Kawano et al., 2008). Rapid elongation by leaves of young plants in response to short-term submergence (e.g. flash flooding for up to 2 weeks), however, adversely affects flash flood tolerance by depleting carbohydrates that would otherwise support survival during submergence and after desubmergence (Chaturvedi et al., 1995; Setter and Laureles, 1996; Kawano et al., 2002; Ram et al., 2002; Jackson and Ram, 2003; Joho et al., 2008). In Guinea, costal or lowland areas are heavily affected by submergence during the rainy season and rice plants are often partially or completely submerged for more than a month. Such prolonged submergence thus often triggers crop failures.
The results of a previous study showed that O. glaberrima can lodge readily after desubmergence due to weakening of the shoot base, resulting in leaf elongation during 10 d complete submergence. This, in turn, can increase plant mortality (Joho et al., 2008). Oryza glaberrima generally is susceptible to short-term complete submergence, but genotypes may adapt to prolonged flooding by shoot elongation that restores aerial photosynthesis, thereby ensuring survival. Two important points are to be noted in plant responses to prolonged submergence in O. glaberrima. First, the plants are able to cope with prolonged submergence by rapid shoot elongation before exhausting their stored carbohydrates and other energy resources. Secondly, these rice plants increase photosynthetic capacity in the canopy that develops above the floodwater to maintain their carbohydrate supply as a major physiological contribution to their survival and grain-yielding capacity.
The overall objective of this study was to identify differences in response to prolonged submergence between the already much studied deepwater rice genotypes of O. sativa and the relatively neglected cultivars of O. glaberrima from Guinea and other parts of West Africa. Such findings are expected to help breed and select superior lines for use in West Africa.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Experiment 1: field experiment
This study compared 35 Oryza sativa and 27 O. glaberrima genotypes. They represented a spread of abilities to adapt to lowland, deepwater and partial submergence, and to upland environments (Table 1). Submergence-tolerant genotypes of O. sativa that express a quiescence response to complete submergence were donated by the International Rice Research Institute. Oryza glaberrima genotypes were cultivars from Guinea, Mali, Niger and Senegal. Submergence tolerance is defined here as the ability of rice plants to survive without marked shoot elongation for 10–14 d under complete submergence and resume growth when the floodwater subsides (Setter et al., 1995).
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Rice seedlings were transplanted into an experimental lowland field at the Foulaya Agricultural Research Center in the Republic of Guinea (10°0'N, 12°9'W, elevation 330 m) 15 d after seeding in 30 hills (3 rows x 10 hills) at a density of 16·7 hills m–2 (30 x 20 cm). Paddy soil was a ferralsol, a sandy type. Soil pH was 4·2 before transplanting. Chemical fertilizer was applied at N 30 kg ha–1, P2O5 30 kg ha–1 and K2O 30 kg ha–1 before transplanting. Fifteen days after transplanting, the plants were submerged completely for 31 d in 50 cm of water (complete submergence plots) maintained by means of an overflow system. The floodwater was usually clear during submergence; however, it became turbid after a heavy rain due to a high clay content. Complete submergence is defined as floodwater covering the plants completely on the first day of submergence. The leaf tips were allowed to reach the water surface if underwater elongation was sufficiently vigorous. The distance from the tip of the shoot to the water surface was an average of 20·5 cm at the start of complete submergence for all cultivars. Seedlings of the same genotypes were also transplanted into field plots without submergence treatment (non-submergence). Here, water was maintained about 2 cm deep throughout. Water temperature and dissolved oxygen were measured by a dissolved oxygen analyser (OM-51-2, Horiba Co., Ltd, Tokyo, Japan) at the four corners of a plot at depths of 5, 10 and 20 cm between 08:00 and 18:00 h on the 14th and 30th day of submergence (14 and 30 DAS). Average daytime water temperature was 25·6 °C at 14 DAS and 33·1 °C at 30 DAS. Average dissolved oxygen in daytime floodwater was 10·8 mg L–1 at 14 DAS and 9·8 mg L–1 at 30 DAS.
The experiment was a randomized complete block design with two replications of 30 plants per cultivar. Three plants in the centre of each replication were selected for shoot length and shoot dry weight determined 1 d before submergence (DBS), and also for shoot dry weight determined 1 d after draining (DAD). The timing of measurement was described as the number of days from the start of submergence. The whole shoot (i.e. leaf blade and leaf sheath) was dried in an oven for 48 h at 80 °C for dry weight measurement.
Experiment 2: pot experiment
The intention behind the pot experiment was to confirm the results of the field experiment and analyse growth responses to submergence in more detail. Four of the O. sativa cultivars and one of the O. glaberrima cultivars used in the field experiment were selected on the basis of shoot elongation responses to submergence in the field. For O. sativa, the lowland cultivar ‘Banjoulou’ was selected as a typical slowly elongating line when submerged, ‘Nylon’ and ‘IR71700-247-1-1-2’ (IR71700) were selected for intermediate elongation, and ‘IR73020-19-2-B-3-2B’ (IR73020) was selected as a control cultivar (Table 1). From O. glaberrima, ‘Yélé1A’ was selected for intermediate shoot elongation. ‘IR71700’ and ‘IR73020’ were donated by the International Rice Research Institute (Woonho et al., 2007). ‘Banjoulou’, ‘Nylon’ and ‘Yélé1A’ are cultivars developed in Guinea.
The experiment was conducted in a laboratory growth chamber kept at 28 °C from 06:00 to 18:00 h and at 25 °C from 18:00 to 06:00 h. Artificial light was provided for 12 h during the daytime. Mean irradiation 0·7 m above the tank water surface was 905 µmol m–2 s–1 PAR. Seedlings were grown in a 0·5 x 0·3 x 0·2 m nursery box filled with nursery soil (pH, 5·0; N, 0·75 %; P2O5, 13·5 %; K2O, 11 %) for 20 d and transplanted into 1·5 L pot with 1 g of N, 1 g of P2O5 and 1 g of K2O per pot filled with dried clay soil from a paddy field. Plants were placed in a 600 L water tank 15 d after transplanting in water maintained at a depth of 79·6–85·3 cm for 37 d as the complete submergence treatment. The water inside the tank was not replaced during the experiment. The tips of the leaves were allowed to reach the surface where underwater elongation was sufficiently vigorous. The distance from the tip of the shoot to the water surface was an average of 23·3 cm at the start of complete submergence for all cultivars. The same rice genotypes were also set in a tank with water only 35 cm deep as a partial submergence treatment for the same period. Partial submergence is defined as plants being submerged for about half of the plant height in water at the start of submergence. Non-submergence treatments were set up adjacent to submerged plants. The experiment was a randomized complete block with three replications. Three plants per replication were selected from each genotype to measure shoot length, shoot dry weight and leaf area at 1 DBS and 1 DAD. Photosynthetic rate was measured at the youngest fully expanded leaf of the main shoot with a portable photosynthesis analyser (LI-6400, Li-Cor, Lincoln, NE, USA) at 1 DBS and 37 DAS. Shoots were dried as in Experiment 1. Water temperature and dissolved oxygen were measured at a depth of 20 cm as in Experiment 1. Dissolved O2 averaged between 8·5 and 11·2 mg L–1. Water temperatures averaged between 25·5 and 29·0 °C.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Shoot elongation and shoot biomass during submergence in field and pot experiments
All cultivars of O. sativa with submergence tolerance based on the quiescence strategy, i.e. ‘FR13A’, ‘IR67520’, ‘IR70027’, ‘IR73018’ and ‘IR73020’, failed to regain contact with the aerial environment and died during 31 d submergence in the field experiment (Table 2). In contrast, all O. glaberrima genotypes resurfaced and survived submergence. Shoot length and biomass results in Table 2 exclude those of the five non-surviving cultivars. Shoot lengths of O. sativa cultivars averaged 28·2 cm 1 DBS and 78·8 cm 1 DAD, and O. glaberrima shoot length averaged 31·0 cm 1 DBS and 91·9 cm 1 DAD in submerged plots. The shoot elongation rate was calculated as elongation during submergence divided by the submerged period in days. The shoot elongation rate of non-submerged plants averaged 1·20 cm d–1. Completely submerged plants of O. sativa elongated 1·63 cm d–1 and those of O. glaberrima elongated 1·97 cm d–1. The shoot elongation rate of all species was significantly (P < 0·01) greater in submerged plants than in non-submerged plants in the field. Shoot elongation by completely submerged O. glaberrima was significantly (P < 0·01) faster than that of O. sativa but no statistically significant difference was seen between species in non-submerged plots.
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Growth in shoot biomass by both species measured 1 DAD was slower in completely submerged plants than in non-submerged plants. Completely submerged O. sativa plants averaged 0·11 g at 1 DBS and 1·32 g at 1 DAD. For O. glaberrima the equivalent values were 0·12 and 3·38 g. The average rate of increase in shoot biomass by completely submerged O. glaberrima (0·1 g per plant d–1) was faster (P < 0·01) than that of O. sativa (0·04 g per plant d–1). No statistically significant difference was seen between non-submerged plants of either species.
All plants of ‘IR71700’ with reputed lowland adaptability and ‘IR73020’ with known submergence tolerance through quiescence died during complete submergence in the pot experiment (Table 3). Shoot length 1 DBS across the genotypes was 54·7 cm in non-submerged plots, 55·4 cm in partially submerged plots and 58·0 cm in completely submerged plots. By 1 DAD, these lengths had increased to 79·4, 107·1 and 102·5 cm, respectively. The shoot elongation rate during submergence for all genotypes was significantly greater (P < 0·01) for partially and completely submerged plants than for non-submerged plants. Shoot elongation rates were not significantly different between genotypes when partially submerged. However, under complete submergence, elongation rates for O. glaberrima Yélé1A' (1·49 cm d–1) and O. sativa ‘Nylon’ (1·27 cm d–1) were significantly greater than that of O. sativa ‘Banjoulou’ (0·80 cm d–1). Shoot elongation was much faster under partial than under completely submerged conditions for ‘Banjoulou’ and ‘Nylon’, but ‘Yélé1A’ elongated faster when submerged completely than when partially submerged. In pot-grown plants, shoot biomass 1 DAD (Table 4) ranged from 2·89 to 10·6 g per plant for partial submergence plots, indicating statistically significant differences between cultivars. For completely submerged plants, shoot biomass was notably greater for ‘Nylon’ and ‘Yélé1A’ than for ‘Banjoulou’. Complete submergence depressed shoot biomass more than partial submergence. In ‘Banjoulou’, the dry mass of completely submerged plants was only 0·74 g compared with 8·83 g for partially submerged plants. Equivalent values for ‘Nylon’ were 2·92 and 7·25 g, and for ‘Yélé1A ‘were 3·75 and 6·12 g.
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Effect of submergence on leaf area and photosynthetic rate in pot experiment
Leaf area per plant 1 DAD in the pot experiment (Table 5) varied from 3·71 to 8·19 m2 for partially submerged plants and from 0·69 to 3·02 m2 for completely submerged plants. A statistically significant difference (P < 0·05) was found between genotypes in partial and complete submergence plots 1 DAD. Leaf areas were greater in partially submerged plants than in non-submerged plants for ‘Banjoulou’ (x1·25), ‘Nylon’ (x1·88) and ‘IR73020’ (x2·19). The leaf area of ‘Banjoulou’ decreased more in response to complete submergence than that of ‘Nylon’ or ‘Yélé1A’. Leaf areas of all completely or partially submerged plants were smaller than those of non-submerged plants.
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The photosynthetic rates of the youngest fully expanded leaf of the main shoot of pot-grown plants were measured (Table 6). The non-submerged rate for ‘IR73020’ (30·9 µmol cm–2 s–1) 1 DBS was significantly above those for other genotypes. The rate for ‘Yélé1A’ (6·74 µmol cm–2 s–1) was the lowest. It was, however, significantly greater for ‘Yélé1A’ than for other genotypes in non-submergence plots (29·2 µmol cm–2 s–1), in partial submergence plots (30·3 µmol cm–2 s–1) and in complete submergence plots (34·8 µmol cm–2 s–1) 37 DAS. No statistically significant difference was seen between genotypes except for partially submerged ‘Yélé1A’. The ratio of the photosynthetic rate of completely submerged plants to non-submerged plant was 0·67 for ‘Banjoulou’, 1·19 for ‘Nylon’ and 1·19 for ‘Yélé1A’. The net assimilation rate (NAR) during submergence for 37 d correlated positively with the photosynthetic rate at 37 DAS (r = 0·58, P < 0·05) in the pot experiment (Fig. 1). NAR varied from 0·89 to 2·15 g m–2 d–1 in non-submerged plants, from 1·19 to 2·24 g m–2 d–1 in partially submerged plants, and from –0·21 to 4·33 g m–2 d–1 in completely submerged plants. NAR for ‘Yele1A’ (4·24 g m–2 d–1) and ‘Nylon’ (4·33 g m–2 d–1) was much greater when completely submerged than when partially submerged (‘Yélé1A’ 2·24 g m–2 d–1, ‘Nylon’ 1·32 g m–2 d–1).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Effect of submergence on rice growth
The objective of the present study was to determine the response to prolonged complete submergence by a wide range of O. sativa and O. glaberrima cultivars in field and pot experiments. Rice plants are thought to overcome submergence stress using either an escape or a quiescence strategy. The former involves stress avoidance through the development of anatomical and morphological traits, such as fast upward shoot extension, and the latter comprises submergence tolerance achieved through a repression of cell elongation and carbohydrate metabolism (Bailey-Serres and Voesenek, 2008). Oryza sativa ‘FR13A’, ‘IR67520’, ‘IR70027’, ‘IR73018’ and ‘IR73020’ died during 31 d complete submergence in the field (Table 2). ‘IR73020’ also died when completely submerged in the pot experiment. Although each of these genotypes has been identified as submergence tolerant (IRRI 2004), they were unable to survive prolonged (>31 d) complete submergence using the tolerance mechanism of suppressing shoot elongation under water. None of these cultivars regained contact with the air and remained totally submerged throughout the experiment. Some O. sativa genotypes with submergence tolerance, however, survived partial submergence well in the pot experiment and did so while elongating their shoots. It is therefore concluded that responses other than suppression of underwater shoot extension are required to survive prolonged and initially complete submergence. What appears to be required is an ability to elongate with sufficient vigour to allow contact to be made with the air. The average time taken for the upper leaf tip to reach the water surface after the start of complete submergence by species of O. sativa (n = 27) was 7·4 d (minimum, 2; maximum, 15; median, 7·0) and that of O. glaberrima (n = 27) was 2·8 d (minimum, 2; maximum, 7; meduan, 2·5) in the field experiment. The time taken by cultivars of O. sativa was 4·7 d for ‘Banjoulou’ and 2·3 d for ‘Nylon’ in the pot experiment. The time taken for the O. glaberrima ‘Yélé1A’ was only 2 d. It is likely that this greater shoot elongation in O. glaberrima is caused by the ethylene-producing capacity trapped within the plants under water (Khan et al., 1987). ‘Yélé1A’ also showed greater shoot elongation during complete submergence than under partial submergence. These responses constitute the submergence escape. Mochizuki et al. (1996) described internode elongation ability under rising water conditions using a floating Asian cultivar that responds to submergence by rapid internode elongation (Vergara et al., 1976). It was observed that both leaf blade and sheath development contributed to shoot elongation in the present experiments. Internode elongation was not observed during 31–37 d submergence where rapid submergence mimicked flash flooding in the field. Leaf elongation fully accounted for the rapid shoot elongation during submergence of the initially 30- and 35-d-old plants. Although the cultivars tested may lack the capacity for rapid internodal stem elongation, they clearly have a capacity for fast leaf elongation underwater (Singh et al., 1989). The average shoot elongation rate of 27 O. glaberrima genotypes was significantly greater than that of 30 O. sativa genotypes. The results thus agree well with those of Futakuchi et al. (2001), who pointed out that O. glaberrima shows greater shoot elongation than O. sativa when flooded.
The increase in shoot biomass and leaf area during submergence was strongly affected by depth of submergence (Tables 4 and 5). Mazaredo and Vergara (1982) showed that submergence-tolerant genotypes tended to have greater leaf area with higher levels of carbohydrate during submergence compared with submergence-susceptible genotypes. In a previous study, submergence-tolerant genotypes had a larger leaf area and dry weight in the leaf blade and sheath when submerged for 10 d in water 45 cm deep (Joho et al., 2008). Under prolonged partial submergence treatment in this study, ‘IR73020’, a submergence-tolerant cultivar of O. sativa, produced the greatest shoot biomass and largest leaf area apart for ‘Banjolou’ (Table 4). However under complete submergence for a similar period (37 d), all died, indicating the very different requirements for tolerance of partial and complete submergence.
The relationships between (a) shoot elongation rate and increase in shoot biomass during submergence; (b) shoot elongation rate and increase in leaf area during submergence; and (c) increase in shoot biomass and increase in leaf area during submergence of non-, partial and completely submerged plants in the pot experiment (Fig. 2) were examined. Shoot biomass clearly affected leaf area in both partial and complete submergence plots. Shoot elongation further affected leaf area and shoot biomass in complete submergence plots. Rice plants clearly use photosynthesis to expand leaf area above the water surface, made possible by vigorous leaf elongation during the initial complete submergence phase. Complete submergence adversely affected leaf area, indicating that contact with the air and the associated photosynthesis and a fully aerobic state is required. Emerged leaves of partially and completely submerged plants displayed an increase in photosynthetic rate. The photosynthetic rate at 37 DAS in partial and complete submergence was closely related to the NAR during submergence in the pot experiment (Fig. 1). The submergence response in deepwater rice has been associated with a 26-fold increase in the translocation of photosynthetic assimilates, synthesized during 3 d submergence, from the leaves to the rapidly growing internodes and younger regions of the culms (Raskin and Kende., 1984). The photosynthetic rate was higher in deeply submerged plants than in non-submerged plants for ‘Nylon’ and ‘Yélé1A’. This resulted in a rapidly increasing leaf area above the water surface (Yamaguchi et al., 1989). The first point requiring clarification is that parameters related to photosynthesis and growth traits increased in complete submergence plots. The most critical differences between ‘Banjoulou’, ‘Nylon’ (O. sativa) and ‘Yélé1A’ (O. glaberrima) under submergence appears to be the photosynthetic rate. That of ‘Yélé1A’ is significantly above that of the other genotypes in both partial and complete submergence plots. The difference between deepwater cultivars of O. sativa and O. glaberrima thus seems to lie in their response to prolonged complete submergence. Based on the present analysis, an increase of photosynthesis at 37 DAS was controlled by shoot elongation and NAR during submergence. Important traits for submergence escape are thus fast shoot elongation and dry matter production per unit leaf area to improve photosynthesis capacity above the water surface.
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O. glaberrima escape from complete submergence
The escape from complete submergence of ‘Yélé1A’, a typical deepwater genotype of O. glaberrima grown in flood-prone areas in Guinea, was examined. It was found that ‘Yélé1A’ had an especially large capacity for shoot elongation when submerged. Watarai and Inouye (1998) noted that high internodal elongation contributes to shoot elongation using O. glaberrima under different flooding regimes. It has been shown that the faster shoot elongation of O. glaberrima genotypes underwater is mainly caused by leaf elongation. Thus, internode and leaf elongation underwater share certain similarities in O. glaberrima, both presumably being stimulated by ethylene. The capacity for elongation among submergence-tolerant genotypes was limited in complete submergence for 31 d, resulting in high mortality. The response of ‘Yélé1A’ to complete submergence showed no significant difference in field and pot experiments (Tables 2 and 3). ‘Yélé1A’ thus seems to be especially well adapted to complete submergence. Shoot elongation during complete submergence competes with the demands of maintenance of energy and for respirable carbohydrates (Setter and Laureles, 1996). If leaves can contact air through shoot elongation under water, the plants can then recoup losses involved in supporting a fast elongation rate and go on to accelerate rates of leaf expansion and growth in shoot dry weight. It is concluded that escape from complete submergence over a prolonged period such as 31–37 d requires the ability for strong leaf elongation underwater that leads to later benefits of enhanced expansion of leaf area and photosynthesis. ‘Yélé1A’, a West African O. glaberrima cultivar, possesses these attributes in large measure. It therefore holds promise for selecting and breeding rice genotypes for use in different flood-prone environments in Africa.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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We thank Dr Sekou Beavogui of the Guinean Agricultural Research Institute for supporting the work in Guinea.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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