INTRODUCTION
Rice (O. sativa L.) is at the heart of the culture and the staple food of 557 million people in Southeast Asia (Manzanilla et al., Reference Manzanilla, Paris, Vergara, Ismail, Pandey, Labios, Tatlonghari, Acda, Chi, Duoangsila, Siliphouthone, Manikmas and Mackill2011). Amid this demand, rice productivity in Southeast Asia is seen to suffer serious losses mainly because of erratic rainfall patterns and increasing risks from typhoon and rainfall-induced flood. Plants require water for growth but excess water that occurs during submergence is harmful or even lethal (Nishiuchi et al., Reference Nishiuchi, Yamauchi, Takahashi, Kotula and Nakazono2012). Prolonged submergence is a major production constraint and affects 22 million ha of rainfed lowland rice in South and Southeast Asia (Khush, Reference Khush1984; Mackill et al., Reference Mackill, Ismail, Singh, Labios and Paris2012), of which over 6 million ha in India (Khush, Reference Khush1984; Sarkar et al., Reference Sarkar, Reddy, Sharma and Ismail2006). Flooding is the third most important abiotic stress affecting rice productivity in Asia, surpassed only by drought and weeds (Widawsky and O’Toole, Reference Widawsky and O’Toole1990). The adverse effects of flooding constitute a complex phenomenon that varies with genotype and pretreatments, stage of the plant when flooding occurs, duration and severity of flooding (Setter et al., Reference Setter, Waters, Atwell, Kupkanchanakul, Greenway and Crawford1987), carbohydrate status during submergence in seeds (Ella and Setter, Reference Ella and Setter1999) and shoots (Palada and Vergara, Reference Palada and Vergara1972; Sakagami and Kawano, Reference Sakagami and Kawano2011), Pn rate of submerged rice (Mazaredo and Vergara, Reference Mazaredo and Vergara1982; Panda et al., Reference Panda, Sharma and Sarkar2008; Setter et al., Reference Setter, Greenway and Kupkanchanakul1989) and degree of turbidity of floodwater (Das et al., Reference Das, Panda, Sarkar, Reddy and Ismail2009; Setter et al., Reference Setter, Ramakrishnayya, Ram and Singh1995). Underwater photosynthesis can improve survival by enhancing internal aeration, increasing carbohydrate reserves or both and supported by the observation, that blocking early ethylene-induced chlorophyll degradation improves survival (Ella et al., Reference Ella, Kawano, Yamauchi, Tanaka and Ismail2003). Ethylene production and accumulation occur during stress such as submergence and that ethylene triggers LS, which consequently reduces Pn rate during and after submergence (Bradford and Yang, Reference Bradford and Yang1980; Ella et al., Reference Ella, Kawano, Yamauchi, Tanaka and Ismail2003; Jackson et al., Reference Jackson, Waters, Setter and Greenway1987; Voesenek et al., Reference Voesenek, Banga, Their, Muddle, Harren, Barendse and Blom1993). Submergence subjects plants to the stresses of low light, limited gas diffusion, effusion of soil nutrients, mechanical damage and increased susceptibility to pests and diseases (Greenway and Setter, Reference Greenway and Setter1996; Ram et al., Reference Ram, Singh, Singh, Singh, Singh, Setter, Singh and Singh1999; Setter et al., Reference Setter, Ellis, Laureles, Ella, Senadhira, Mishra, Sarkarung and Datta1997).
Progress has been made in developing more tolerant germplasm for flood-prone ecosystems, but fewer efforts have been devoted to identifying proper nutrient management options (Ella and Ismail, Reference Ella and Ismail2006), but are gradually gaining ground with increasing attention of the research community to unfavourable environments. Rice grain yield may decrease about 21% when water depth increased from 30 to 90 cm (Kupkanchanakul, Reference Kupkanchanakul1979). The reduction in yield has been attributed to a decrease in the proportion of ripened grains due to fertilization failure (Matsushima, Reference Matsushima1962; Reddy and Mittra, Reference Reddy and Mittra1985a; Singh et al., Reference Singh, Mackill and Ismail2009). In rice, the reproductive stage is the most sensitive to complete submergence, followed by the seedling and the maximum tillering stages (Gautam et al., Reference Gautam, Nayak, Lal, Bhattacharyya, Tripathi, Shahid, Mohanty, Raja and Panda2014a; Matsushima, Reference Matsushima1962; Pearson and Jacobs, Reference Pearson and Jacobs1986). Rice genotypes tolerant of complete submergence at the vegetative stage, such as the Indian landrace FR13A, were identified that can survive submergence for over 2 weeks, and a single gene responsible for tolerance (SUB1A) was cloned and its role in conferring tolerance established (Bailey-Serres et al., Reference Bailey-Serres, Fukao, Ronald, Ismail, Heuer and Mackill2010; Mazaredo and Vergara, Reference Mazaredo and Vergara1982; Setter et al., Reference Setter, Ellis, Laureles, Ella, Senadhira, Mishra, Sarkarung and Datta1997; Xu et al., Reference Xu, Xu, Fukao, Canlas, Maghirang-Rodriguez, Heuer, Ismail, Bailey-Serres, Ronald and Mackill2006). Low yield of FR13A hindered its use by farmers (Mackill et al., Reference Mackill, Coffman and Garrity1996, Reference Mackill, Ismail, Singh, Labios and Paris2012; Mohanty et al., Reference Mohanty, Mallik and Grover2000). However, developing cultivars with submergence tolerance matching that of FR13A, coupled with high yield and good grain quality, could have enormous impact through increasing and stabilizing the productivity of rice in rainfed areas, since 25–30% of the world's rice-growing areas are prone to submergence (Singh et al., Reference Singh, Mackill and Ismail2009). The varieties developed through marker assisted back crossing system and by transferring Sub1 gene into popular rice varieties showed a yield advantage of 1 to >3 t/ha over the original varieties following submergence for a few days to 18 days (Mackill et al., Reference Mackill, Ismail, Singh, Labios and Paris2012; Neeraja et al., Reference Neeraja, Maghirang-Rodriguez, Pamplona, Heuer, Collard, Septiningsih, Vergara, Sanchez, Xu, Ismail and Mackill2007).
Optimal nutrition before flooding is necessary to equip plants with cellular and metabolic requirements essential for survival of flooding, and also for fast recovery after floodwater recedes (Ella and Ismail, Reference Ella and Ismail2006; Gautam et al., Reference Gautam, Nayak, Lal, Bhattacharyya, Tripathi, Shahid, Mohanty, Raja and Panda2014a; Lal et al., Reference Lal, Gautam, Rath, Haldar, Panda, Raja, Shahid, Tripathi, Bhattacharyya, Mohanty and Nayak2014). The farmers’ practice of applying only nitrogenous fertilizers in the nursery to get taller and greener seedlings may not have positive consequences in flood-prone areas because high N content in seedlings before flooding adversely affects survival after flooding and results in poorer recovery (Ella and Ismail, Reference Ella and Ismail2006). However, when high N is accompanied by high P supply on P-deficient soils, seedling survival seems to improve, with effectively better recovery after submergence (Gautam et al., Reference Gautam, Lal, Raja, Baig, Haldar, Rath, Shahid, Tripathi, Mohanty, Bhattacharyya and Nayak2014b; Jackson and Ram, Reference Jackson and Ram2003). Here, we quantified the impact of IR64-Sub1 with IR64 under non-submerged conditions as well as under complete submergence (14 days) at AT, PI and H stage with different nutrient combination applied before submergence and after desubmergence on growth, Pn rate and yield. This information is essential in providing insights into the usefulness of application time of fertilizers in typical farmers’ fields.
MATERIALS AND METHODS
Plant material and growth conditions
The experiment was conducted under natural conditions during 2012–13 with two Indica rice (O. sativa L.) cultivars i.e. IR64-Sub1 having submergence tolerant Sub-1 gene and IR64 without Sub-1 gene; six schedules of nutrient application viz., control, P (phosphorus), NBS (nitrogen before submergence), NAS (after submergence), NPBS and NPAS (details of fertilizer application schedule given in Table 1) and three different stages of submergence viz., AT (21 days after transplanting (DAT)), PI (45 DAT) and H stage (65 DAT). The experiment was arranged in a factorial randomized block design with four replications. One 15 days old seedling of each cultivar was transplanted in the pots containing 8 kg of farm soils (Sandy clay loam, pH 6.4, EC-0.079 dSm−1, available N, P and K-5.89, 0.45 and 6.47 mg kg−1 of soil, respectively). 1.75 g (NH4)2SO4, 0.95 g KH2PO4 and 0.52g KCl were applied to each pot as per the treatments (dose of fertilizer was kept as 100:50:50 kg ha−1 for NPK, respectively). The amount of K applied through KH2PO4 was adjusted while applying KCl as source of K. Plants were irrigated with fresh water to maintain 2 cm standing water except during the period when rice plants were subjected to submergence. Potted plants were completely submerged in a concrete tank containing fresh water and the water depth was maintained at 30 cm above the top of the plant canopy for 14 days at three different times of crop duration i.e. 21, 45 and 65 DAT, which coincides with AT, PI and H stages of crop, respectively.
Table 1. Fertilizer application schedule followed in the experiment.
AT: active tillering, PI: panicle initiation, H: heading, BS: before submergence, AS: after submergence, N-nitrogen, P-phosphorus.
*Nitrogen was applied in equal amounts (25% of total dose) in each split, full dose of P and K and first split of N was applied as basal at the time of
transplanting.
After desubmergence, plants were allowed to recover for 7 days, and plant survival was recorded. Plant samples were collected just before submergence and soon after desubmergence for various measurements of plant responses during submergence. Light transmission (photosynthetically active radiation) through the floodwater, water temperature, dissolved oxygen (DO), pH, electrical conductivity (EC), redox potential (ORP), total dissolved salts (TDS) were determined. Light intensity was measured at 12:00 h using LICOR light meter (LI-COR, USA), whereas temperature and other water quality parameters were determined by using U-50 multiparameter water quality meter (HORIBA, Japan).
Assessment of phenology, yield attributes and yield
Plant height and dry matter was determined at maturity, total number of tillers was counted during different crop growth stages, whereas number of tillers producing panicles out of maximum number of tillers was represented as percentage productive tillers. Total leaf area was recorded by putting every fresh leaf of one plant from each treatment in digital leaf area meter just before submergence and 7 days after desubmergence. The area thus obtained was divided by the area of ground to get leaf area index (LAI). Percent change in LAI was recorded after desubmergence over control/non-submerged plants. Number of panicles was counted from each treatment, five panicles per plant were randomly selected and panicle length and panicle weight was determined. The crop was harvested and sun dried, then total produce was weighed and recorded as total biomass. The produce was then threshed and grains were separated, dried (up to 14% moisture content with grain moisture meter (Model- MB 45, OHAUS grain moisture meter, Switzerland)) and weighed.
Measurement of photosynthetic (Pn) rate
Net photosynthetic rate rate and stomatal conductance (Gs) of rice plants 7 days after submergence and non-submerged plants were measured with an infrared gas analyser (LI-6400, LI-COR, Lincoln, NE) around 11:00 AM. The conditions in the assimilation chamber were kept as follows: air humidity, 70%; leaf temperature, 35°C; light intensity (PAR), 1200 μmol m−2 s−1, CO2 concentration of 380 μmol CO2 mol−1. Middle portion (3 cm long) of the fully expanded and not senescent leaf blade was selected for Pn rate measurement rate.
Measurement of Leaf senescence (LS)
LS senescence is characterized by dramatic yellowing resulting from chloroplast degeneration. Submergence tolerant cultivars were able to retain green leaves for longer time than intolerant cultivars. LS was assessed immediately after submergence on per plant basis using a visual scale of 1 to 5. This visual score was based on the yellow proportion of leaves: 1 = all leaves green; 5 = all leaves completely yellow or degenerated (Toojinda et al., Reference Toojinda, Siangliw, Tragoonrung and Vanavichit2003). A chlorophyll meter (Minolta SPAD-502) was used to confirm the LS based on the measurement of amount of chlorophyll or greenness of leaves.
Measurements of ethylene concentration
Ethylene was measured according to procedure described by Kende and Hanson (Reference Kende and Hanson1976). The internodes (2 cm long) of plant (2 from each treatment) were placed in 30 ml test tubes with 2 ml of water or test solution. The tubes were stoppered with serum vial caps and kept horizontally. Ethylene was sampled by first injecting 1 ml of air into each tube with a tuberculin syringe, pumping the syringe several times, and then withdrawing 1 ml for analysis. Ethylene was determined by gas chromatography (GC Model Chemito CERES 800 Plus, Thermo Scientific) equipped with Porapak-Q column (6 feet long, 1/8 inch outer diameter, 80/100 mesh size, stainless steel column) and flame ionization detector. The oven, injector and detector temperatures were set at 100, 300 and 150°C respectively and the flow of carrier N2 gas, air and H2 were maintained at 15, 285, 30 ml per minute, respectively. The amount of ethylene produced from samples was expressed by comparing with the standard curve of pure ethylene standard gas (9.12 ppm in N2, Matheson Tri Gas) and under afore mentioned GC conditions, ethylene was detected at retention time of 2.247 minutes.
Statistical analysis
The statistical analysis was carried out for each parameter studied based on factorial randomized block design model using SAS software, 2012 (version 9.2). Associations between parameters were studied using correlation and linear regression analysis. Means were compared by least significant difference (LSD) test if the f value is significant.
RESULTS
Floodwater characteristics
The temperature of floodwater during the crop growth period ranged from 22.6 to 27.1 °C in the morning (06:30 AM) and from 24.5 to 29.5 °C in the afternoon (02:00 PM). The temperature was slightly lower during submergence at AT stage (21 DAT), as compared to 45 and 65 DAT, due to prevailing weather conditions. The pH, DO and EC of floodwater ranged from 8.3–8.5, 7.3–8.2 mg l−1 and 0.24–0.27 dSm−1, respectively. Water depth did not have significant effects on the temperature, pH, DO and EC, but substantially affect light penetration. When calculated as the percentage of total incidence irradiance above the water surface, under-water light intensity decreased by 35.9% at 5 cm below the water surface, and by an additional 53.7% decrease at 50 cm and 77% at 75 cm.
Growth and metabolic activities affected due to submergence
Leaf area index
LAI before submergence was almost similar in both the cultivars but decreased significantly after submergence with more devastating effects in IR64, and percentage change in LAI was higher when submergence induced at H stage, as compared to other stages and non-submerged plants (Figure 1). The decrease in LAI after submergence was 54.2% in IR64 and 14.7% in IR64-Sub1 as compared to non-submerged conditions. N application before submergence (48 h before submergence) resulted in highest percentage change in LAI (96%) which indicates that before submergence N application contributed to more chlorophyll degeneration, damage was more severe when basal P was not applied. Minimum percentage change in LAI (23.1%) was observed when N applied after desubmergence (48 h after submergence) along with basal P (Figure 1).
Figure 1. Leaf area index (LAI) and percentage change in LAI of IR64 and IR64-Sub 1 under non-submerged conditions and submergence at 21, 45 and 65 days after transplanting (DAT) (vertical bars in each line and column represents standard error). (a) pre-submergence N application, (b) post-submergence N application, (c) basal P and pre-submergence N application and (d) basal P and post-submergence N application. Vertical bars on primary axis represent LAI and lines on secondary axis represent percentage change in LAI.
Photosynthetic rate and stomatal conductance
Pn rate and Gs of both cultivars was almost similar under non-submerged conditions. The leaf Pn rate decreased in both the cultivars with the progression of time of submergence but decrease was more in IR64 (44.1%) than in IR64-Sub1 (17.8%) as compared to non-submerged conditions (Figure 2). Higher value of leaf Gs in IR64-Sub1 after submergence resulted in 27.7% higher Pn rate over IR64 (Figure 2). (Pn) rate and the factor controlling Pn rate varied significantly with time of submergence and stage of crop. Pn rate remained highest when plants were submerged at AT stage whereas, minimum Pn rate and Gs was observed during H stage submergence. Nitrogen application after desubmergence considerably improved Pn rate and Gs, this improvement was more prominent when basal P was applied along with N. Application of N after desubmergence along with basal P resulted in 78.7% higher Pn rate as compared to control. Pn rate was lowest when N was applied before submergence, and much less when no basal P was applied with N (Figure 2).
Figure 2. Photosynthetic (Pn) rate (μmol CO2m−2s−1) and stomatal conductance (Gs) (mol H2O m−2 s−1) of IR64 and IR64-Sub 1 under non-submerged conditions and submergence at 21, 45 and 65 days after transplanting (DAT) (vertical bars in each line and column represents standard error). Vertical bars on primary axis represent Pn rate and lines on secondary axis represent Gs. Control - no N and P application, P-basal P application only, N Bs-pre-submergence N application, N As - post-submergence N application, NP Bs - Basal P and pre-submergence N application and NP As - Basal P and post-submergence N application.
Ethylene accumulation and leaf senescence
Submergence enhanced the ethylene accumulation in both the cultivars, and accumulation was 22.5% higher in IR64 over IR64-Sub1 (Figure 3). Stage of submergence also significantly influenced the ethylene accumulation irrespective of the cultivars; the order of ethylene accumulation was H > PI > AT stage. Plants enriched with nutrients reduced the accumulation of ethylene; P has the major role in low ethylene production and accumulation. Basal P application resulted in 22.1% lower accumulation of ethylene as compared to no P application.
Figure 3. Ethylene concentration (ppm) and leaf senescence score of IR64 and IR64-Sub1 after submergence at 21, 45 and 65 days after transplanting (DAT) (vertical bars in each line and column represents standard error). Vertical bars on primary axis represent ethylene concentration and lines on secondary axis represent leaf senescence. Control - no N and P application, P-basal P application only, N Bs - pre-submergence N application, N As - post-submergence N application, NP Bs - Basal P and pre-submergence N application and NP As - Basal P and post-submergence N application.
LS occurs when plants were exposed to complete submergence for 14 days submergence at AT, PI and H stage (Figure 3). LS increased significantly after submergence and effect was higher in IR64. IR64-Sub1 was superior in terms of more number of green leaves after desubmergence. Submergence at H stage resulted in 24.1 and 18.9% higher LS IR64 and IR64-Sub1 respectively, over submergence at AT stage. LS was accelerated by N application before submergence; with more damaging effect when basal P was not applied.
Phenology, yield attributes and yield affected due to submergence
Biomass and phenology
Under non-submerged conditions, biomass accumulation remained at par in both the cultivars. After introduction of submergence, significant reduction in biomass over non-submerged plants was observed in both the cultivars but more in IR64. Biomass accumulation in IR64 decreased to extent of 13.2%, and in IR64-Sub1 decreased only by 1.8%, over non-submerged conditions (Figure 4). Biomass decreased variably with time of submergence compared to non-submerged conditions with progression of time of submergence, biomass reduced consistently from AT, PI to H stages. Application of N and P and application time of N significantly contributed to the biomass production at the time of harvest. Basal P and N application after desubmergence resulted in 36.5 and 45.1% higher biomass production in IR64 and IR64-Sub1, respectively over control. Whereas, N before submergence and no P application reduced biomass production up to 59.7 and 43.2% in IR64 and IR64-Sub1, respectively over basal P and N application after desubmergence (Figure 4).
Figure 4. Biomass production (g plant−1) in IR64 and IR64-Sub 1 at harvest, under non-submerged conditions and submergence at 21, 45 and 65 days after transplanting (DAT) (vertical bars in each line represents standard error). Control - no N and P application, P - basal P application only, N Bs - pre-submergence N application, N As - post-submergence N application, NP Bs - Basal P and pre-submergence N application and NP As - Basal P and post-submergence N application.
Under non-submerged condition, IR64 and IR64-Sub1 flowered at the same time. Delay in flowering was observed after submergence in both the cultivars and more in IR64. Flowering was substantially delayed for both the genotypes, by about 30 days in IR64 and 18 days in IR64-Sub1 (Table 2). The apparent delay in maturity was mostly because of the delay in flowering, which was more affected when submergence induced at reproductive stage. Submergence for 14 days delayed flowering and maturity by 29, 20 and 10 days when submergence induced at H, PI and AT stage, respectively. Application of N prior to submergence substantially delayed flowering (27 days) and delay was more (33 days) when basal P was not applied with N. Flowering and maturity in both the cultivars were took less time (18 days) when N was applied after desubmergence with basal P as compared to control (Table 2).
Table 2. Phenology as affected by cultivar, time of submergence and nutrient application.
In a column, values followed by a common letter are not significantly different at p < 0.05 using least significance difference test.
BS-before submergence, AS-after submergence, N-nitrogen, P-phosphorus.
Yield attributes and yield
Cultivar, stage of submergence and nutrient application had significant effect on yield attributes (Table 2). IR64-Sub1 produced maximum number of productive tillers (85.5%), panicles per plant (20.9), spikelets per panicle (81.2) and fertility percentage (75.6%) over IR64 (Tables 2 and 3). H stage submergence resulted in maximum damage of all the yield attributes, whereas, submergence at AT stage reflected the least damage. Nitrogen application before submergence resulted in less number of productive tillers (75.4%), panicles (13.8), spikelets (65.7) and grain filling (65.8%), these attributes affected more when basal P was not applied. Maximum number of tillers (28.2), panicles (25.8), spikletes (86.7) and highest grain filling (83.3%) was achieved with post-submergence N and basal P application (Table 3). Yield of the Sub 1 introgression line were similar to that of its recurrent parent under non-submerged condition. The exposure to submergence had left detrimental effects on yield of both the cultivars, grain yield decreased up to 35.5 and 13.8% in IR64 and IR64-Sub1, respectively as compared to non-submerged condition (Table 4). The Sub 1 genotype did not have any apparent negative effect on grain yield under controlled conditions, but considerably a yield advantage of 21.8% over IR64 under submerged condition. Submergence at different stages of crop growth caused a significant reduction in grain yield in both the genotypes. Fourteen days complete submergence at H, PI and AT stage resulted in 58.7, 29.8 and 7.6% yield loss, respectively, as compared to non-submerged conditions (Figure 5). Grain yield was influenced by application of both N and P, positive influence was reflected in case of basal P and post-submergence N application whereas, negative effect was seen when N applied before submergence. When N applied after desubmergence without basal P resulted in 16.7% yield reduction but when it combined with basal P then yield loss was only 4.2% as compared to non-submerged condition. Whereas, N application before submergence along with basal P resulted in yield reduction of 35.3% but when no basal P was supplied, yield subdued up to 44.7% indicating maximum damage (Table 4).
Table 3. Yield attributes as affected by cultivar, time of submergence and nutrient application.
In a column, values followed by a common letter are not significantly different at p < 0.05 using least significance difference test.
BS-before submergence, AS-after submergence, N-nitrogen, P-phosphorus.
Table 4. Grain yield (g plant−1) influenced by interaction effect of cultivar, time of submergence and nutrient application.
In a column, values followed by a common letter are not significantly different at p < 0.05 using least significance difference test.
BS- before submergence, AS-after submergence, N-nitrogen, P- phosphorus, V-variety, S-stage of submergence, N-nutrient.
Figure 5. Percentage reduction in yield of IR64 and IR64-Sub 1 when subjected to submergence at active tillering, panicle initiation and heading stage (which coincides with 21, 45 and 65 days after transplanting (DAT)) over non-submerged conditions (vertical bars in each column represents standard error).
DISCUSSION
Submergence substantially decreased LAI, Pn rate, biomass, yield because chlorophyll contents decreased during submergence, and failed to recover following a return to non-submerged conditions. One of the reasons for the decreased Pn rate may be ethylene accumulation in shoots. Complete submergence enhances the accumulation of ethylene due to increased synthesis which triggers chlorophyll degradation and LS of the submerged plants through suppression of abscisic acid synthesis but enhanced synthesis and sensitivity to gibberellins (Fukao and Bailey-Serres, Reference Fukao and Bailey-Serres2008) resulting in lower Pn rate and yield. Another reason may be low light reaching at canopy level is injurious resulting in slower photosynthesis, reduction in production of respirable assimilates (Jackson and Ram, Reference Jackson and Ram2003) and low LAI, biomass and yield. The depletion of Pn rate under submergence has been documented previously and primarily it was based on the loss of chlorophyll fluorescence (Panda et al., Reference Panda, Sharma and Sarkar2008), lowering of Gs, intercellular CO2 concentration (Ismail et al., Reference Ismail, Johnson, Ella, Vergara and Baltazar2012) as well as denaturing of the Pn machineries (Mackill et al., Reference Mackill, Ismail, Singh, Labios and Paris2012). Survival of the Sub 1 lines was substantially higher after submergence than that of the non-Sub 1 varieties, and this is consistently reflected in a yield advantage of 1 to over 3.5 t ha−1 based on the stage at which submergence occurs, duration of submergence and the conditions of floodwater (Das et al., Reference Das, Panda, Sarkar, Reddy and Ismail2009; Gautam et al., Reference Gautam, Lal, Raja, Baig, Mohanty, Tripathi, Shahid, Bhattacharyya and Nayak2014c; Ismail et al., Reference Ismail, Singh, Singh, Dar and Mackill2013).
LAI, Pn rate, biomass and yield was higher but LS and ethylene accumulation was lower in IR64-Sub1 compared to IR64 after submergence. This might be because Sub 1 genotype increases the chances of survival and after growth in two ways: first, less energy is wasted on elongation (Das et al., Reference Das, Panda, Sarkar, Reddy and Ismail2009); second, the intact chlorophyll helps rice plants to generate more energy during submergence (Singh et al., Reference Singh, Mackill and Ismail2009) and to perform better photosynthesis after desubmergence. Under submergence, plants receive less light and oxygen compared to control conditions, Pn apparatus suffered greater damage when plants exposed to air. The tolerant cultivar, however, adjusted to the new environment quickly, and the susceptible cultivar failed to adjust with the upcoming conditions (Panda et al., Reference Panda, Sharma and Sarkar2008) and Pn rate decreased further.
Introgression of Sub 1 gene narrowed the delay in flowering and suppressed ethylene induced LS caused by submergence, possibly by maintaining healthier plants that can resume faster, as Sub1 is known to enhance chlorophyll retention and conserve carbohydrate reserves through reducing leaf and stem elongation (Das et al., Reference Das, Sarkar and Ismail2005; Fukao et al., Reference Fukao, Xu, Ronald and Bailey-Serres2006) and by halting ethylene accumulation. In all cases, submergence induced reductions in yield attributes were more severe in IR64 because the ability for faster recovery and early tiller formation following submergence is probably one of the most important traits of Sub 1 genotype for higher grain yield because only early tillers will become effective in contributing to grain yield.
Submergence at H stage proved most detrimental and resulted in lowest LAI, Pn rate, biomass and yield but maximum LF and ethylene accumulation, reverse was the case for AT stage submergence. Plants at active tillering stage are tender and able to recover fast, as plant age increases hardiness, decreases the ability of recovery or regeneration, which reflected in grain yield. Reddy and Mittra (Reference Reddy and Mittra1985a) reported that decrease in grain yield on submergence at reproductive stage was due to impaired anthesis causing high sterility as evidenced by higher number of unfilled spikelets. Moreover, the growth of the endosperm is suppressed after fertilization and this finally leads to abortive kernels with under developed endosperm (Matsushima and Wada, Reference Matsushima, Wada and Matsubayshi1966). Panicle weight and filled spikelets are less due to improper grain filling after submergence. Improper grain filling following submergence might attribute to a reduction in source capacity to provide sufficient carbohydrate, as well as a reduced translocation of assimilates to the sink (Palada and Vergara, Reference Palada and Vergara1972). Delay in flowering might also have contributed to loss in grain yield as compared to non-submerged conditions, strong positive correlation between delay in flowering and percentage reduction in yield supported the statement (Figure 6). Singh et al. (Reference Singh, Mackill and Ismail2009) also reported that delay in flowering and maturity could result in severe damage during grain formation and filling caused by stresses such as terminal drought and cold weather at reproductive stage.
Figure 6. Association of delay in flowering time with corresponding percentage reductions in grain yield caused by submergence at 21, 45 and 65 DAT (coinciding with active tillering, panicle initiation and heading stage, respectively) in IR64 (closed circles) and IR64-Sub 1 (open circles).
Nitrogen application before submergence resulted in higher LS and ethylene accumulation but lower Pn rate because of chlorosis and degeneration of leaves and this damage was fatal without basal P application. Application of basal P alone delayed LS as evident from higher SPAD values, more green leaves or less senescence in Sub1 genotype was also confirmed by the SPAD values, which was higher in Sub1 cultivars (Figure 7). The strong negative correlation between leaf senescence and SPAD values but positive correlation between ethylene accumulation and LS (Figure 7); and negative correlation between ethylene accumulation and Pn rate (Figure 8) confirmed the above findings.
Figure 7. Relationship between leaf senescence and ethylene accumulation (closed circles); leaf senescence and SPAD values (open circles).
Figure 8. Relationship between ethylene accumulation and photosynthetic rate.
Nitrogen application after desubmergence along with basal P resulted in more number of green leaves, higher Pn rate, biomass and yield. It might be due to leaf sheath N could have a major accumulative and supportive role during submergence, perhaps by protein acting as respirable reserves. Addition of P to soil prior to seed sowing can stimulate overall growth in height, dry mass and it also helps in reducing under water elongation and more strong recovery after desubmergence (Jackson and Ram, Reference Jackson and Ram2003). Nitrogen application after desubmergence along with basal P contributed to fast recovery of surviving plants leading to early flowering and maturity and this is of particular relevance to actual field condition in some areas (Gautam et al., Reference Gautam, Lal, Raja, Baig, Haldar, Rath, Shahid, Tripathi, Mohanty, Bhattacharyya and Nayak2014b; Lal et al., Reference Lal, Gautam, Rath, Haldar, Panda, Raja, Shahid, Tripathi, Bhattacharyya, Mohanty and Nayak2014). Reddy and Mittra (Reference Reddy and Mittra1985b) reported that the N requirement being more during vegetative stage, the N and P fertilization resulted in higher number of tillers and ultimately the number of panicle bearing tillers. This indicates the crop fertilized with these two nutrients could withstand better the onslaught of complete plant submergence than that of unfertilized crop and resulted in the grain yield 9–14% over N alone under deepwater conditions. Grain yield is the ultimate result of various interacting growth factor, photosynthesis, interdependence on growth, development and yield attributing characters and is reflected in the strong positive correlation among yield attributing characters (r = 0.89), LAI (r = 0.81) and photosynthetic rate rate (r = 0.74) with grain yield (Table 5).
Table 5. Correlation coefficients for the association among yield attributes, Pn rate, elongation and survival as affected by nutrient application with grain yield.
Pn rate: photosynthetic rate, LAI: leaf area index.
CONCLUSIONS
The results of this study showed that the IR64-Sub1 could maintain better phenology and yield attributes after submergence, which subsequently reflected higher yields after submergence. The tolerant genotypes followed quiescence strategy during submergence period had higher Pn rate, and lower ethylene concentration during recovery period than genotypes followed escape strategy. Submergence at H stage resulted in poor phenology and lowest grain yield. Application of P helped in improving the phenology, Pn rate and yield attributes, post-submergence N application along with basal P proved its superiority in terms of better leaf area, photosynthesis and led to higher grain yield. This study suggests that productivity could be enhanced in areas where untimely flooding is anticipated by applying basal P and adjusting the time of N application, if combined with tolerant germplasm, this approach could contribute to enhanced productivity and production of rice in flood-prone lowlands.
