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Published online 20 June 2006
Published in Crop Sci 46:1673-1681 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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CROP PHYSIOLOGY & METABOLISM

Seedling Nutrient Status before Submergence Affects Survival after Submergence in Rice

Evangelina S. Ella and Abdelbagi M. Ismail*

Crop, Soil, and Water Sciences Division, International Rice Research Institute DAPO, Box 7777, Metro Manila, Philippines

* Corresponding author (abdelbagi.ismail{at}cgiar.org)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Vast rainfed lowland areas in Asia periodically experience flash floods that adversely affect survival and productivity of rice (Oryza sativa L.). Progress has been made in developing more tolerant germplasm, but fewer efforts have been devoted to identifying proper management options. We evaluated the effects of nitrogen (N) and phosphorus (P) added before submergence on seedling survival after submergence. Two experiments were conducted using two cultivars, submergence-tolerant FR13A and intolerant IR42. In the first experiment, N was applied at two different times and, in the second, N and P were used in a fertile and a P-deficient soil. Seedlings (21-d-old) were submerged in concrete tanks for 12 d. Addition of P seems to enhance tolerance of plants grown on P-deficient soils. Survival of both cultivars decreased substantially in seedlings with high N concentration, with higher survival in FR13A. Leaf N and chlorophyll concentration before submergence were higher in N-treated seedlings, whereas chlorophyll a/b ratio was lower. In both cultivars, photosynthesis and root–shoot ratio decreased and chlorophyllase activity increased after submergence with increasing N and with higher activity of chlorophyllase in IR42. Survival was negatively correlated with leaf N concentration, but positively correlated with root–shoot ratio and stem starch concentration before submergence and with chlorophyll concentration and chlorophyll a/b ratio after submergence. Crop establishment could therefore be enhanced in areas where untimely flooding is anticipated by avoiding excessive N application.

Abbreviations: QTLs, quantitative trait loci • PAR, photosynthetically active radiation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
EARLY flooding that persists for several days usually reduces plant survival and productivity of rice in rainfed lowlands and more than 20 million hectare are annually affected in South and Southeast Asia. This damage may have several causes linked to floodwater interference in normal gas exchange and illumination. The adverse effects of flooding constitute a complex phenomenon that varies with genotype and pretreatments, carbohydrate status of the plant before and after submergence, developmental stage of the plant when flooding occurs, duration and severity of flooding, and degree of turbidity of floodwater (Setter et al., 1995; Ramakrishnayya et al., 1999; Jackson and Ram, 2003; Das et al., 2005). Setter et al. (1987a) reported that the environmental factors associated with flooding are variable in different locations, even over short distances.

Since gas diffusion in water is 10 000 times slower than in air (Armstrong, 1979), limited O2 and CO2 diffusion is considered the most limiting environmental factor under flooded conditions because O2 is required for the production of energy necessary for growth and maintenance processes and CO2 is required for photosynthesis. The plant hormone ethylene also accumulates in plants during submergence because diffusive escape is strongly inhibited by floodwater. Ethylene produced during submergence accumulated up to 0.49 µM in the gas phase in shoots of 12-d-old seedlings of cultivar IR42 when submerged for up to 55 h (Jackson et al., 1987). Being trapped in submerged tissues, ethylene becomes more available to promote both (i) underwater elongation during submergence, as observed in rice (Ku et al., 1970; Suge, 1985; Lee and Lin, 1996), the floating aquatic plant Callitriche platycarpa Keutz. (Musgrave et al., 1972), and the wetland dicotyledonous plant Rumex palustris L. (Voesenek et al., 1993), and (ii) leaf senescence (Jackson et al., 1987; Ella et al., 2003) that may reduce photosynthetic carbon fixation during and after submergence. Both elongation growth and reduction in concurrent carbon fixation during submergence can result in depletion of carbohydrate reserves with a consequent reduction in seedling survival.

Elongation during submergence is a disadvantage because it competes with maintenance processes for stored energy. The adverse effect of elongation growth on tolerance of complete submergence has been demonstrated in a few studies (Jackson et al., 1987; Setter and Laureles, 1996; Singh et al., 2001; Das et al., 2005). Submergence tolerance of rice cultivars could therefore be enhanced by selecting lines with slower elongation and by using management practices that suppress accelerated growth in environments where submergence occurs for a shorter duration. The negative effect of ethylene during submergence can be effectively overcome by treating seedlings with ethylene action inhibitors before submergence (Ella et al., 2003). Blocking the action of ethylene decreased chlorophyll degradation, lowered the activity and gene expression of chlorophyllase, and improved the survival of intolerant cultivar IR42. Another environmental factor to consider is poor light transmission under water, resulting in lower irradiance, particularly in the presence of thick algal growth, and consequently limiting photosynthesis (Whitton et al., 1988; Setter et al., 1995).

Numerous reports on attempts to improve rice productivity in flood-prone areas through germplasm enhancement are available. Incorporating tolerance for complete submergence is typically achieved through conventional breeding or through marker-assisted breeding using DNA markers associated with QTLs (quantitative trait loci) involved in submergence tolerance (HilleRisLambers and Vergara, 1982; Mackill et al., 1993; Mackill and Xu, 1996; Singh and Dwivedi, 1996; Siangliw et al., 2003; Pamplona et al., 2004). Other attempts also involved genetic transformation (Quimio et al., 2000; Dennis et al., 2000; Rahman et al., 2001). However, there is little published information regarding the use of crop and nutrient management strategies that can help enhance seedling survival during submergence and recovery afterward. Ensuring proper nutrient management early in the season before submergence may help improve seedling survival after submergence.

A specific nutrient management strategy has not been developed for flood-prone areas and farmers normally adopt management practices developed for intensive irrigated systems when resources are available. This study aims to evaluate the effect of N and P management before submergence on the survival of rice seedlings after a few days of complete submergence, typical of the conditions experienced in flash-flood-prone areas. Attempts were also made to partially unravel the physiological bases of the effects of high N before submergence on survival after submergence. An association during senescence between inactivation of photosynthesis and chlorophyll loss has been established in many crops, e.g., wheat (Triticum aestivum L., Wittenbach, 1979; Camp et al., 1982) and rice (Uchida et al., 1982). Here, we also examined changes in chlorophyll concentration during submergence and their relationship with seedling survival after submergence.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plant Material
Seeds of two rice cultivars, FR13A (submergence-tolerant) and IR42 (-intolerant), were sown in 4-L pots with fertilized soil and with 20 seedlings of each cultivar per pot. The seeds were first incubated in a small amount of water for 3 d at 30°C to germinate in the dark before direct sowing in the fertilized soil. All experiments were conducted under greenhouse conditions.

Experiment I
This experiment was conducted with N treatments applied once but at different times before submergence: no N (control), early N at sowing of pregerminated seeds, and late N at 11 d after sowing with the amounts of fertilizer per liter of soil as follows: 1 g (NH4)2SO4 as N source, 0.5 g P2O5 as P source, and 0.5 g K2O as potassium (K) source. P and K were applied as basal and N was applied based on the treatment. Pregerminated seeds were sown into pots filled with Maahas clay soil (430 g kg–1 clay, 440 g kg–1 silt, and 130 g kg–1 sand; pH 5.9; Tirol-Padre and Ladha, 2004). At 18 d after sowing, seedlings were either not submerged (control) or completely submerged in concrete tanks for 12 d inside a greenhouse, at which time the intolerant cultivar, IR42, showed visual symptoms of injury. The seedlings were allowed to recover for an additional 21 d after submergence when final survival was recorded. The experiment was conducted twice, during May through June 2001 (Experiment IA) and October through November 2001 (Experiment IB).

Experiment II
This experiment was conducted to evaluate the effects of N and P applied before submergence on seedling survival. The treatments were no N and no P (control), and P, N, and NP, incorporated into the soil once at sowing using the same NP rates as in Experiment I, whereas K was applied to all pots. Two soil types were used: Maahas clay and a P-deficient Pangil soil (590 g kg–1 clay, 380 g kg–1 silt, and 30 g kg–1 sand; pH 5.7; Tirol-Padre and Ladha, 2004). Control and submergence treatments were conducted as in experiment I, with submergence started 18 d after sowing and continued for 12 d in concrete tanks. The seedlings were allowed to recover for 21 d after submergence, when final survival was recorded. This experiment was also conducted twice, during April–May 2002 (Experiment IIA) and July–August 2002 (Experiment IIB). In all experiments, algal growth was minimized by partially removing algae from the water surface daily using a small fish-net filter. Extra care was taken when scooping the algae to minimize disturbance of the floodwater.

Measurements
During both experiments, the following were monitored two times daily (0800 and 1300 h): incident light (photosynthetically active radiation, PAR) using a light meter (LI-COR 250, Lincoln, NE), relative humidity, and air temperature inside the glasshouse before and after submergence; and light, O2 level, pH, and temperature of floodwater at 5-, 50-, and 75-cm water depths in the concrete tanks during submergence. Dissolved O2 and floodwater temperature were measured using a dual temperature and oxygen meter (Syland Scientific GMBH Simplair F5 model 4000, Heppenheim, Germany) and floodwater pH was measured using a pH meter (ORION Model 230A, Beverly, MA).

Leaf, stem, and root samples were harvested before and after submergence in both experiments. All samples were freeze-dried and dry weights were recorded before laboratory analysis. Shoot length, chlorophyll concentration, and nonstructural carbohydrate concentration (ethanol-soluble sugars and starch) were measured before and after submergence. Seedlings were counted before submergence and 21 d after desubmergence. Leaf N concentration, chlorophyllase activity, and photosynthetic CO2 fixation rate of the youngest fully expanded leaf were measured in Experiment IA.

Dried leaf samples collected before submergence were digested with concentrated H2SO4 and 30% (v/v) H2O2 following the method of Thomas et al. (1967). Nitrogen concentration of the leaf digest was determined following the salicylate method of Kempers and Zweers (1986). Ethanol-soluble sugar and starch concentration in leaf, root, and stem were determined before and after submergence. Samples were harvested and frozen in liquid N2, freeze-dried, and weighed to obtain their dry weights. The dried sample was extracted in 80% ethanol (v/v) and used for soluble carbohydrate analysis with anthrone reagent (Fales, 1951). The residue was washed several times and used for starch analysis following the method of Setter et al. (1989). Starch was solubilized in boiling water for 3 h with further hydrolysis using amyloglucosidase (Sigma Chemicals, St. Louis, MO) and subsequently analyzed for free sugars using glucose oxidase (Sigma Chemicals, St. Louis, MO) as described by Kunst et al. (1988).

Chlorophyll concentration was determined on a subsample of seedling leaves harvested before submergence and at Day 0 of recovery following the method of Mackinney (1941) in acetone extracts and then chlorophyll a/b molar ratio was determined. Levels of chlorophyllase activity in leaves were assayed after submergence at Day 0 of recovery following the method described by Tsuchiya et al. (1997). Photosynthetic gas exchange of the youngest fully expanded leaf was measured at Day 3 of recovery using an LI-6400 Photosynthesis Measurement System (LICOR Inc., Lincoln, NE). The LI-6400 meter was programmed to maintain CO2 concentration of 380 µmol m–2 s–1, artificial red light of 1000 µmol m–2 s–1 (supplied with an LED light source, model 6400–02 Red LED), and relative humidity of 40 to 60%. Leaves were allowed to equilibrate to the chamber conditions (CV < 0.4%) before measurements were taken. The average irradiance during measurements in the greenhouse was about 1300 µmol m–2 s–1.

Statistical Analysis
The experiments were conducted following a split-plot design with five replications. The main plot constitutes submergence treatments. The subplots constitute 3 N treatments x 2 cultivars or 4 fertilizer treatments x 2 soil types x 2 cultivars in the first and second experiments, respectively. Data were subjected to analysis of variance (ANOVA) using IRRISTAT for Windows version 4.4 (IRRI, 2004). Simple linear correlations were performed using Microsoft Excel software. Interaction means are presented in the tables when statistically different.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Environmental Characterization
Climatic conditions in the greenhouse were similar during Experiments IA, IB, and IIA, where average maximum and minimum air temperatures were 38.2°C (range of 37.8–38.6°C) and 27.4°C (range of 26.7–27.7°C), respectively, and average irradiance and relative humidity were 1363 µmol m–2 s–1 and 43%, respectively, when measured at 1300 h. However, during experiment IIB, average maximum (35.3°C) and minimum (24.6°C) air temperatures were noticeably lower and air relative humidity higher (55%). This is probably because of the pervasiveness of cloudiness during this time resulting in substantially lower average incident irradiance of only 562 µmol m–2 s–1. The pH of the floodwater (range of 6.8–7.0) did not vary much with time of day, water depth, or experiment. Similar trends were also observed for dissolved O2, except that it was slightly higher when measured at 1300 h (5.5 mg L–1) than at 0800 h (5.2 mg L–1), but both values are less than that of air-saturated water at ambient temperature (8.2 mg L–1). Floodwater was cooler in experiment IIB (average of 28.2°C) but warmer (31.8–33.1°C) in the other experiments, with similar values obtained at the three depths, and was about 1°C warmer in all experiments when measured at 1300 h rather than at 0800 h. Photosynthetically active radiation (PAR) decreased dramatically with water depth, to about 40.3, 28.6, and 19.8% of total incident PAR when measured at 5, 50, and 75 cm from the water surface.

Plant Survival after Submergence
In Experiment I, submergence reduced seedling survival of both cultivars with significantly greater effects on submergence-intolerant cultivar IR42 than on submergence-tolerant FR13A. Nitrogen treatment before submergence resulted in lower seedling survival, with significant interaction between N and the cultivar because of the greater decrease in IR42, whose survival approached zero (Fig. 1a ). Similar trends were observed in Experiment II, in which addition of N at sowing substantially reduced survival of both cultivars but with a more dramatic effect on intolerant cultivar IR42, whose survival approached zero, regardless of P level (Fig. 1b, c). Addition of P seems to have enhanced survival of plants grown in the P-deficient Pangil soil (Fig. 1b). However, this enhancement was not apparent when this experiment was repeated (Fig. 1c).


Figure 1
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  		Fig. 1. Effect of fertilizer treatment before submergence on survival of FR13A ({square}) and IR42 ({blacksquare}) after submergence in (a) Experiment I (IA and IB combined), (b) Experiment IIA, and (c) Experiment IIB. Data are averages of five replications in each experiment and vertical bars are LSD0.05 values.

 
Leaf N Concentration before Submergence and Chlorophyll Concentration before and after Submergence
Before submergence, higher leaf N concentration was observed in N-treated seedlings (3.85%) than in seedlings receiving no N treatment (3.32%). Leaf chlorophyll concentration also increased significantly with the addition of N (Fig. 2 ) and this was also apparent visually as greener leaves. However, the two cultivars had similar levels of leaf N and chlorophyll concentrations. The addition of P together with N resulted in significantly higher leaf chlorophyll concentration in the P-deficient soil (Fig. 2). When expressed relative to the control, chlorophyll concentration of both cultivars decreased significantly after submergence and with greater effects when N was applied (Fig. 3 ). IR42 showed a higher reduction in chlorophyll concentration than FR13A (Fig. 3b). The relative concentration of both chlorophyll a and b, measured 3 d after submergence, decreased significantly, with a greater decrease in chlorophyll a than in chlorophyll b in both cultivars (Fig. 4a ). The molar ratio of chlorophyll a/b was similar in both cultivars and was not affected by N treatment when seedlings were not submerged (Fig. 4b). However, after submergence, this ratio decreased with N treatment, indicating that chlorophyll degradation during submergence was more in chlorophyll a than in chlorophyll b.


Figure 2
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  		Fig. 2. Effect of N and P applied before submergence on chlorophyll concentration. Data are combined means of FR13A and IR42 with five replications each in Experiment IIB. Vertical bar is LSD0.05 value.

 

Figure 3
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  		Fig. 3. Effect of fertilizer treatment before submergence on relative chlorophyll concentration after submergence of (a) FR13A ({square}) and IR42 ({blacksquare}) at Day 3 of recovery after submergence in Experiment IB and (b) across two soil types at Day 0 of recovery after submergence in Experiment IIB. Values are means of five replications in each experiment and vertical bars are LSD0.05 values.

 

Figure 4
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  		Fig. 4. Effect of time of N treatment before submergence on (a) chlorophyll a and chlorophyll b content of FR13A ({square}) and IR42 ({blacksquare}) and (b) chlorophyll a/b molar ratio of the not-submerged and submerged plants at 3 d after submergence. Data are means of five replications in Experiment IB and vertical bars are LSD0.05 values.

 
Chlorophyllase Activity and Photosynthetic Gas Exchange after Submergence
Activity of chlorophyllase, the enzyme involved in the initial steps of chlorophyll breakdown, was measured to assess the effects of N treatment before submergence on chlorophyll stability during submergence (Fig. 5 ). In both cultivars, chlorophyllase activity decreased slightly with N treatment in seedlings that were not submerged, but increased significantly with submergence and progressively with the delay in N treatment before submergence. This increase in chlorophyllase activity after submergence was substantially higher in IR42, in which it increased by 48, 139, and 259% compared with 20, 97, and 157% in FR13A under no N-, early N-, and late N-treatments before submergence, respectively.


Figure 5
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  		Fig. 5. Effect of time of N treatment before submergence on chlorophyllase activity in control ({square}) and submerged ({blacksquare}) seedlings of FR13A and IR42 at Day 0 of recovery after submergence. Data are means of five replications in Experiment IA and the vertical bar represents the LSD0.05 value.

 
Photosynthetic gas exchange of the youngest, fully expanded leaf decreased significantly with submergence and with N treatment before submergence by up to 25% (data not shown). Tolerant cultivar FR13A showed a higher rate of photosynthetic gas exchange (43.8 µmol CO2 m–2 s–1) than IR42 (36.3 µmol CO2 m–2 s–1). A strong negative correlation was observed between photosynthetic CO2 fixation rate at Day 3 of recovery and leaf N concentration before submergence (Fig. 6 ).


Figure 6
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  		Fig. 6. Relationship between leaf N content before submergence and photosynthetic gas exchange rate measured at Day 3 of recovery after submergence in FR13A ({square}) and IR42 ({blacksquare}) in experiment IA. Data points represent three measurements in each of 3 N treatments. ** P < 0.01.

 
Shoot Elongation during Submergence and Root–Shoot and Leaf–Stem Ratios before and after Submergence
Extent of shoot elongation during submergence was expressed relative to initial shoot length before the start of the submergence treatment. Nitrogen treatment before submergence significantly enhanced shoot elongation during submergence (Table 1). Intolerant cultivar IR42 showed greater shoot elongation than FR13A when submerged, whereas the latter showed greater elongation than IR42 when not submerged.


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  		Table 1. Effect of genotype and N treatment given before submergence on shoot elongation during submergence (Experiment IA). Shoot elongation is expressed as percentage increase in shoot length relative to length at the start of the submergence.

 
Root–shoot ratio before submergence decreased in both cultivars with N treatment. However, the ratio was greater in IR42 than in FR13A (Tables 2, 3), and greater in the P-deficient Pangil soil than in the fertile Maahas soil (Table 3). However, P supplementation did not influence seedling root–shoot ratio. Submergence resulted in a substantial reduction in root–shoot ratio by about 60 and 38% in Experiments IA and IIB, respectively (Tables 2, 3). Moreover, N treatment also significantly decreased root–shoot ratio in submerged seedlings in both experiments.


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  		Table 2. Effect of time of N treatment on the partitioning of dry matter as measured by root–shoot and leaf–stem dry weight ratios before submergence and at Day 6 of recovery after submergence in Experiment IB.

 

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  		Table 3. Effect of fertilizer treatment before submergence on the partitioning of dry matter as measured by root–shoot and leaf–stem dry weight ratios before submergence and at Day 0 of recovery after submergence in Experiment IIB.

 
The dry weight ratio of leaf blades relative to the weight of the rest of the shoot (stem and/or overlapping leaf sheaths) was measured in both experiments. For simplicity, this ratio is referred to as leaf–stem ratio henceforth. When measured before submergence, leaf–stem ratio increased with N treatment (Tables 2, 3), and IR42 had a greater ratio than FR13A. Submergence increased leaf–stem ratio by about 38 and 40% in the first and second experiments, respectively (Table 2, 3). The significant interaction between submergence and cultivar was due to a greater increase in leaf–stem ratio in FR13A than in IR42 (Table 2).

Association of Seedling Survival with Selected Physiological Parameters Measured before and after Submergence
Significant negative correlations were observed between seedling survival after submergence and leaf N concentration before submergence, and also with chlorophyllase enzyme activity after submergence (Table 4). Correlations with leaf–stem ratio before submergence were negative but not significant (R = –0.49 and –0.46 for FR13A and IR42, respectively). However, correlations of plant survival with root–shoot ratio and stem starch concentration before submergence were positive. Associations of seedling survival with relative chlorophyll concentration (R = 0.76) and chlorophyll a/b molar ratio (R = 0.92) measured after submergence were strongly positive.


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  		Table 4. Correlation coefficients for the association between seedling survival after submergence and different parameters measured before and after submergence. Data are from Experiment IB (n = 30).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Flash flooding that results in complete inundation for a few days is a serious problem in rainfed lowlands in Asia and tropical Africa because it normally results in partial to complete crop failure. Reasonable progress through both conventional and marker-assisted breeding has been made in developing rice cultivars that can tolerate complete submergence for a few days (Mackill and Xu, 1996; Singh and Dwivedi, 1996; Siangliw et al., 2003; Pamplona et al., 2004). However, management practices that can further enhance survival during submergence and recovery afterward have not been sufficiently explored. In this study, we investigated the effect of N and P enrichment before submergence on seedling survival after submergence using one tolerant (FR13A) and one intolerant (IR42) rice cultivar.

Survival of both cultivars decreased considerably with submergence and this decrease became greater in N-enriched seedlings, and was substantially more obvious in IR42 than in FR13A (Fig. 1). The increase in mortality in N-treated seedlings suggested that an optimum N level in seedlings before submergence is probably needed to ensure higher seedling survival after submergence.

We investigated the effects of high N before submergence on some of the physiological traits that are known to be associated with tolerance, particularly chlorophyll stability, nonstructural carbohydrate levels, shoot elongation, and partitioning of dry matter (Jackson and Ram, 2003; Ella et al., 2003). High N seems to enhance shoot growth over root growth with the subsequent depletion of carbohydrates stored in shoots, both of which are known to increase mortality in submerged seedlings. The addition of P in P-deficient soils seems to be beneficial when the stress is not too severe, as in Experiment IIA (Fig. 1b). In this experiment, light transmission under water was higher (36.4%) and plant survival was higher than in the repeat experiment where light transmission was lower (20.5%, Fig. 1c). Ramakrishnayya et al. (1999) reported that the addition of P into floodwater reduced rice survival during submergence by up to 35%. This was mainly because of competition between algae and the submerged rice plant for CO2 and, perhaps, for light due to thick algal growth on the water surface limiting underwater photosynthesis. However, application of P to the soil at planting, rather than to floodwater, could be beneficial in P-deficient soils.

Nitrogen-treated seedlings had higher chlorophyll concentration before submergence (Fig. 2), but maintained much lower levels after submergence, with a greater decrease in IR42 than in FR13A (Fig. 3a, b). This suggests greater chlorophyll degradation during submergence in N-enriched seedlings and in IR42 than in FR13A. Figure 4 suggests that chlorophyll a was degraded more than chlorophyll b, resulting in a significant decrease in chlorophyll a/b molar ratio after submergence in N-treated seedlings. A reduction in chlorophyll a/b molar ratio was also observed before in senescing leaves of rice (Youn and Ota, 1973; Kura-Hotta et al., 1987; Hidema et al., 1992) and wheat (Patterson and Moss, 1979).

To further assess the effects of N treatment before submergence on chlorophyll stability during submergence, chlorophyllase activity was measured. Chlorophyllase activity has been shown to increase in the presence of ethylene (Trebitsh et al., 1993; Jacob-Wilk et al., 1999; Ella et al., 2003). In this study, chlorophyllase activity following submergence was greater in N-treated seedlings (Fig. 5), suggesting that higher leaf N before submergence could enhance senescence during submergence. The greater chlorosis observed in leaves of submerged rice plants was previously attributed to accumulated ethylene in plant tissues during submergence (Ella et al., 2003). Chlorophyll degradation in plants subjected to high ethylene concentrations was mitigated when CO2 concentrations were high (Jackson et al., 1987; Setter et al., 1988), where CO2 had possibly acted as an inhibitor of ethylene on chlorosis (Burg and Burg, 1965; Abeles, 1973), and also when carbohydrate level during submergence was high (Setter et al., 1988).

Photosynthetic gas exchange was significantly lower after submergence, and correlated negatively with N concentration before submergence (R = –0.78, Fig. 6). Furthermore, there were strong positive correlations (Table 4) between plant survival and chlorophyll content (R = 0.76), as well as with chlorophyll a/b molar ratio after submergence (R = 0.92). However, from these data, it is not clear whether the reduced chlorophyll content and molar ratio are a cause or effect of senescence and ultimate seedling death.

Greater shoot elongation during submergence was observed in IR42 than in FR13A (Table 1) and this is consistent with earlier reports (Setter and Laureles, 1996; Ella et al., 2003; Das et al., 2005). Nitrogen enrichment seems to have further enhanced underwater elongation of intolerant cultivar IR42. A high level of ethylene was reported to accumulate in this cultivar during submergence (Jackson et al., 1987) and ethylene is known to promote leaf elongation. The slower shoot elongation in FR13A when submerged is beneficial in conserving carbohydrates that can be used for growth and/or maintenance processes (Penning de Vries et al., 1983). Besides, the energy reserves could also be used for the synthesis of proteins necessary for survival. The deleterious effects of high N applied before submergence could therefore be partially attributed to enhanced shoot elongation with the subsequent depletion of carbohydrate reserves.

Nitrogen treatment seems to affect assimilate partitioning as seen in the reduced root–shoot ratio of both cultivars before and after submergence (Tables 2, 3). Rice is known for its shallow root system and this further reduction in root growth relative to shoot growth under high N applied before submergence may have contributed to a greater reduction in survival of seedlings after submergence. Reduced root growth might have slowed nutrient uptake during and after submergence.

High N before submergence increased leaf–stem ratio in both cultivars. This ratio also tends to increase during submergence (Tables 2, 3). The relatively higher increase in leaf blade biomass is probably an adaptive response to underwater low-light intensity, but this could be at the expense of carbohydrate reserves. High N also seems to have an adverse effect on the shoot nonstructural carbohydrates remaining after submergence because of enhanced shoot growth and leaf area expansion. This is supported by the positive correlation between leaf N concentration before submergence and the change in soluble sugar concentration during submergence (R = 0.78). A high level of nonstructural carbohydrate concentration before submergence was previously found to be positively associated with survival after submergence (Palada and Vergara, 1972; Setter et al., 1987b; Chaturvedi et al., 1995), which is consistent with the high correlation observed between stem starch concentration before submergence and survival after submergence (Table 4). However, a recent study showed that carbohydrates remaining after submergence (which is a function of content before submergence, photo-assimilation and consumption during submergence) are more important for survival than the initial level alone (Das et al., 2005).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Seedling enrichment with N before submergence adversely affected survival after submergence. It also resulted in an increase in leaf N and chlorophyll concentration before submergence and a decrease in root–shoot dry weight ratio. Additionally, chlorophyll concentration and chlorophyll a/b molar ratio decreased and chlorophyllase activity increased after submergence. This study suggests that rice seedling survival could be enhanced in areas where untimely flooding is anticipated by avoiding excessive N application early in the season. If combined with tolerant germplasm, this approach could contribute to enhanced and stable productivity of rice in flood-prone rainfed lowlands.


   ACKNOWLEDGMENTS
 
We thank Lamberto Licardo for his technical assistance.

Received for publication August 25, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome