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Effects of Aerated Irrigation on Leaf Senescence at Late Growth Stage and Grain Yield of Rice
Rice Science, 2012, 19(1): 4448 Copyright © 2012, China National Rice Research Institute Published by Elsevier BV. All rights reserved
Effects of Aerated Irrigation on Leaf Senescence at Late Growth Stage and Grain Yield of Rice ZHU Lian-feng, YU Sheng-miao, JIN Qian-yu (State Key Laboratory of Rice Biology, China National Rice Research Institute, Hangzhou 310006, China)
Abstract: With the japonica inbred cultivar Xiushui 09, indica hybrid combinations Guodao 6 and Liangyoupeijiu as materials, field experiments were conducted in 2007 and 2008 to study the effects of aerated irrigation on leaf senescence at late growth stage and grain yield of rice. The dissolved oxygen concentration of aerated water evidently increased and decreased at a slow rate. The soil oxidation-reduction potential under aerated irrigation treatment was significantly higher than that of the CK, contributing to significant increases in effective panicles, seed setting rate and grain yield. In addition, the aerated irrigation improved root function, increased superoxide dismutase activity and decreased malondialdehyde content in flag leaves at post-flowering, which delayed leaf senescence process, prolonged leaf functional activity and led to enhanced grain filling. Key words: rice; aerated irrigation; physiological characteristics; leaf senescence; yield
Plant roots consume oxygen during growth and development, and sufficient oxygen should be available for root consumption. Rice is one of the wetland plants which has well-developed aerenchyma, a continuous system of air spaces in the plant that allows diffusion of O2 from the shoot or the air to the roots. Although the aerenchyma system avoids inhibition of respiration due to lack of O2, which is inevitable for the plants non-adapted to wet soils, soil aeration is still a key factor that affects the growth and development of rice (Zhang et al, 2000; Stefano et al, 2002; Fu et al, 2007; Ding et al, 2008; Dai et al, 2008). Our predecessors understood the relation between oxygen and rice growth mostly through the study of rice irrigation patterns. Aerobic irrigation measurements, such as intermittent irrigation, wetting irrigation and dry-wet alternate irrigation, increased soil oxygen content, improved soil environment, and enhanced rice growth and grain yield by appropriate adjustments of the time, duration and quantity of irrigation (Cannell et al, 1985; Zhang et al, 2002; Pan et al, 2009; Liu et al, 2009). However, these irrigation methods seldom yielded satisfactory results when applied in large-scale rice production, because of the difficulty in practical operation, varied weather conditions and other factors. Air pump aeration and some other measurements were widely used in drip irrigation system and horticulture to increase the oxygen content of the water. These aerated irrigations increased crop rhizosphere oxygen content and promoted root growth, leading to Received: 25 July 2011; Accepted: 12 October 2011 Corresponding author: JIN Qian-yu ([email protected])
improved water and fertilizer use efficiency and enhanced rice production (Zhang et al, 2002; Song et al, 2005). Super-micro bubble (SMB) technology was a new invention that produced numerous air bubbles with an average diameter of less than 3 m in treated water, and the dissolved oxygen concentration exceeded the oxygen saturation value in non-treated water. At present, the studies of super-micro bubble technology mainly referred to contaminated water purification or wastewater treatment, while its application on rice irrigation had not been reported (Masayuki et al, 2003; Chen et al, 2008). This study treated irrigation water applying super-micro bubble technology and assessed the effects of the aerated irrigation on leaf senescence and grain yield of rice. We also explored the relationship of oxygen with rice growth and development for the purpose of enriching the cultivation theory and providing reference for super high yield production of rice.
MATERIALS AND METHODS Design of experiment The treatments consisted of two kinds of irrigation, i.e. conventional irrigation with river water (CK) and aerated irrigation with river water treated by supermicro bubble generating system (Aeration). Aerating steps: Dig a pit of 3 m × 2 m × 2 m at the irrigation water inlet to the experiment field, and place in the super-micro bubble generating system (model MBO75ZS); treat the water over 30 minutes before irrigation, and then let the river water flow into the top layer of
ZHU Lian-feng, et al. Effects of Aerated Irrigation on Yield and Leaf Senescence at Late Growth Stage of Rice
Measurement and analysis The effects of super-micro bubble generating system on the dissolved oxygen concentration were investigated. The dissolved oxygen concentrations (mg/L) of the aerated water and non-aerated water were measured every 12 h using a portable dissolved oxygen meter (YSI 550A, YSI Environmental, USA). At the heading stage, select three representative plants of each treatment. Cut the stems at 10 cm from the ground with scissors, cover the cutting edge with pre-weighted cotton, and then wrap with plastic film. Collect the exudates from18:00 pm to 6:00 am, weigh the cotton with root exudates, then separate the exudates into centrifuge tube for analysis. The contents of total soluble sugar and free amino acids in the root exudates were determined according to the methods of Kong et al (2008). Fresh flag leaf samples were collected every 7 d
after flowering. Superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were determined according to the methods of Zhao et al (2002). Plants in each plot were harvested at maturity for the determination of grain yield. Yield components including number of panicles per m2, number of spikelets per panicle, spikelet filling percentage and grain weight were determined from the plants of 5 hills. Spikelet filling rate was calculated as the number of grains that sank to the bottom of a beaker filled with salt solution with specific gravity of 1.06 as percentage of total spikelets. The results were analyzed for variance by ANOVA, and statistical differences among means of the three replicates were tested by least significant difference at the 0.05 probability level with the SPSS13.0 for Windows. Only the 2008 data were analyzed because the two separate experiments in 2007 and 2008 gave very similar results.
RESULTS Oxygenation effects of super-micro bubble technology Fig. 1 showed the dynamics of dissolved oxygen concentration in 2000 L non-treated water (CK) and aerated water (water aerated for 30 min). After aeration, the concentration of dissolved oxygen in irrigated water was 7.93 mg/L, while that of CK was 3.16 mg/L. In natural conditions, the oxygen in the air will dissolve into water, which finally comes to an equilibrium. Because the concentration of dissolved oxygen for CK was lower initially, with the dissolving of oxygen from air, the oxygen content increased in CK. The dissolved oxygen concentration reached super-saturation by aeration, and the super-micro bubbles collapsed as time passed. The concentration
Dissolved oxygen concentration (mg/L)
the pit water for aeration treatment while the aerated water outflow from the bottom of the pit through plastic pipe into the experimental plots. Besides the two treatments, a standing water layer was maintained during plant recovery stage, and intermittent irrigation was applied during the other stages. Field trials were conducted at the China National Rice Research Institute, Hangzhou, Zhejiang Province, China, during the rice growing season (May to November) of two successive years 2007 and 2008. The materials used in 2007 were indica hybrid rice Guodao 6 and japonica inbred rice Xiushui 09, and indica hybrid rice Liangyoupeijiu and japonica inbred rice Xiushui 09 were used in 2008. The three rice materials were chosen because they were planted to a large extent in Zhejiang Province. Seedlings were raised in the field with sowing date on 24 May (hybrid rice) or 30 May (inbred rice) and transplanted on 24 June at a hill spacing of 0.3 m × 0.2 m with one seedling per hill (hybrid rice) or at a hill spacing of 0.2 m × 0.2 m with four seedlings per hill (inbred rice). The plot dimensions were 8.0 m × 5.2 m. A completely randomized block design with three replications was used in the study. The soil of the field was loam clay with 36.8 g/kg organic matter and available N-P-K at 142.0, 17.0 and 14.1 mg/kg, respectively. A sum of 63.0 kg N, 72.0 kg P2O5 and 67.5 kg K2O as compound fertilizer with N:P:K = 14:16:15, and 27.0 kg of nitrogen per hectare were applied in the form of urea and incorporated before transplanting. Nitrogen as urea was applied at mid-tillering (45.0 kg/hm2) and panicle initiation stage (15.0 kg/hm2), and potassium as potassium chloride (60% K2O) was also applied at panicle initiation stage (52.5 kg/hm2).
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Fig. 1. Dissolved oxygen concentrations in the aerated water and non-treated water.
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of dissolved oxygen declined slowly and decreased to 6.87 mg/L after 104 h, which equals to that of CK. Using the aeration technology, the dissolved oxygen concentration is enhanced, exceeding that of CK within 100 h after the treatment, and its decline is delayed. Soil oxidation-reduction potential increased from panicle initiation stage of rice plants (Fig. 2), which may be attributed to the water regime adopted in this experiment. Comparison between the two treatments showed that the soil oxidation-reduction potential of aeration treatment was higher than that of CK during the entire measurement periods, especially for heading stage. The effects of aeration on the two conventional cultivars were identical, but more obvious on the hybrid Liangyoupeijiu. Under the same conditions, the aeration can increase the oxygen content in soil significantly, and improve the oxidation-reduction potential in the root zone. Effects of aerated irrigation on rice grain yield and its components In the two years’ experiment, the grain yields of the three materials by aerated irrigation were significantly higher than those of CK (Table 1), and the increasing
rates were 8.45% (2007) and 6.23% (2008) for Xiushui 09, 9.13% for Guodao 6 (2008), and 7.49% for Liangyoupeijiu (2008), respectively. For yield components, the panicle numbers per hectare by aeration were all higher than those of CK. The tiller dynamics showed that aerated irrigation mainly increased the tiller emergence at early tillering stage, but slightly influenced the maximum tiller number, indicating that the early tillers at the lower nodes contributed to the increase of the panicle number. Compared with CK, the panicle number of aerated irrigation treatment was increased at 4.8%–7.4%. In addition, the number of spikelets per hectare and seedsetting rate of aerated irrigation were significantly higher than those of CK, and the 1000-grain weight of aerated irrigation also increased but at non-significant level. The number of spikelets per panicle of aeration treatment was equal to or a little below the number of CK, with the lowest for Xiushui 09. Furthermore, aerated irrigation increased the seed-setting rate, completely-filled grain number and 1000-grain weight, which led to significantly increased grain yield. Effects of aerated irrigation on contents of soluble sugar and free amino acids in the root exudates Rice has maximum exudate quantity at heading
Fig. 2. Effects of aerated irrigation on soil oxidation-reduction potential. T, Tillering stage; PI, Panicle initiation stage; H, Heading stage; M, Milky ripening stage. * and ** indicate significant differences at the 0.05 and 0.01 levels, respectively, compared with conventional irrigation (CK).
Table 1. Effects of aerated irrigation on rice grain yield and its components. Year
Rice material Xiushui 09
Treatment
No. of panicles per hectare (×104)
Aerated irrigation 334.9* CK 302.8 2007 Guodao 6 Aerated irrigation 188.7* CK 178.1 Xiushui 09 Aerated irrigation 307.1* CK 270.3 2008 Liangyoupeijiu Aerated irrigation 253.8* CK 237.9 * indicates significant difference at the 0.05 level.
No. of spikelets per panicle 131.0 136.7 194.7 196.5 115.5 122.0 155.9 154.7
No. of spikelets Seed-setting per hectare (×104) rate (%) 49343.9* 48036.7 48804.1* 47713.5 39262.8* 37289.9 52907.8* 51068.3
91.13* 86.15 75.30* 71.26 90.36* 85.28 74.79* 70.38
1000-grain weight (g)
Grain yield (t/hm2)
25.54 24.90 29.24 28.74 25.28 24.67 25.42 24.94
7.83* 7.22 6.93* 6.35 7.50* 7.06 8.32* 7.74
ZHU Lian-feng, et al. Effects of Aerated Irrigation on Yield and Leaf Senescence at Late Growth Stage of Rice Table 2. Influence of aerated irrigation on exudate quantity and its soluble sugar and free amino acid contents. Total exudates Soluble sugar Free amino Variety Treatment per tiller content acids content (mg/h) (μg/mL) (μg/mL) Aeration 51.46** 25.54** 106.18** Xiushui 09 CK 38.59 15.96 75.97 Aeration 60.90** 14.46** 59.66** Liangyoupeijiu CK 49.55 12.75 50.80 ** indicates significant difference at the 0.01 level.
stage. Shown as the Table 2, the exudate quantity, contents of total soluble sugar and free amino acid in the root exudates of Xiushui 09 and Liangyoupeijiu by aerated irrigation were significantly increased in comparison with those of CK (P < 0.01). Effects of aerated irrigation on SOD activity and MDA content in flag leaf at the late growth stage of rice From Fig. 3, we can see that the flag leaf SOD activity of Xiushui 09 and Liangyoupeijiu by aerated irrigation was significantly higher than that of CK from flowering to 21 d after flowering. The SOD activity of the two varieties under both treatments decreased from flowering, and the decreasing speed was slow during the first 14 d after flowering. However the SOD actitiviy decreased faster after 14 days from flowering. Compared with CK, the decreasing speed of flag leaf SOD activity of aerated irrigation was smaller. The dynamics of leaf MDA content showed that the MDA content of aerated irrigation was lower than that of CK from flowering to 21 days after flowering and that the content difference was small at the first 7 days, then became large from 7 to 21 d after flowering. Compared with CK, the MDA content of aerated irrigation was lower from flowering, and the difference became greater from 7 d after flowering.
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DISCUSSION Root activity is a key requirement for the improvement of fertilizer and water use efficiency, prevention of leaf premature senescence and maintenance of high photosynthetic capacity at the late growth stage of rice. Oxygen-rich environment is very conducive to the emergence and growth of roots and to the maintenance of high root activity. Lin et al (2005) reported that aerobic irrigation improved the growth environment of rice root, leading to significantly increased root exudates, root metabolism and nutrient use efficiency. Root activity has immediate and direct effects on leaf photosynthesis (Zhao et al, 2007). Our experiment showed the exudate quantity of rice roots under aerated irrigation was obviously higher than that of CK, namely higher root activity at heading stage. Moreover, the contents of free amino acids and soluble sugar in the root exudates of rice by aerated irrigation were also higher than those of CK, and this was indicative of increased nitrogen absorption and assimilation, which would satisfy the nitrogen requirement of the plants and in return, translocate adequate photosynthates to root system. Higher free amino acids content was favorable to the synthesis of chlorophyll and proteins in leaves, and conducive to the prevention of protein decomposition, which may result from the lack of free amino acid. Stronger root activity ensured adequate water supply for shoots, which enhanced the stomatal conductance and transpiration rate. Therefore, the photosynthetic capacity of leaves increased with the improvement of root function, and this is in agreement with the result of Zhang et al (2008). Leaf senescence at the late growth stage of rice will lead to loss of photosynthetic capacity and serious
Fig. 3. Influence of aerated irrigation on SOD activity and MDA content in rice flag leaf.
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impact on grain filling. Wang et al (2008) reported that the increase of relative content of reactive oxygen species was the main reason for impaired photosystem of plants. The integrity of the membrane system is essential for normal photosynthetic growth of green plants, while the SOD plays a major role in the protection of the integrity of the membrane system. This experiment showed that aerated irrigation obviously increased the SOD activity of flag leaves at post-flowering, and compared with CK, the flag leaf SOD activity of aerated irrigation decreased slowly. The MDA content of aerated irrigation was lower than that of CK, and increased slowly as the growth progressed. This indicates that aerated irrigation enhanced the activity of antioxidant enzymes of rice leaves. As a result, the production and scavenging of reactive oxygen species comes to equilibrium, and the damage by reactive oxygen species decreases. Consequently, leaf senescence process is delayed, leaf functional activity prolonged and grain filling promoted, which contribute to the increase of grain yield.
REFERENCES Cannell R Q, Belford R K, Blackwell P S, Govi G, Thomson R J. 1985. Effects of waterlogging on soil aeration and on root and shoot growth and yield of winter oats (Avena sativa L.). Plant & Soil, 85: 361–373. Chen Z Q, Li S G, Han G Y, Xie Y C. 2008. Micro-bubble pure oxygen aeration that used in renew oxygen of rivers and lakes. Water Sci Eng Technol, (1): 17–19. (in Chinese with English abstract) Dai G J, Lwayishi S, Miki T, Hua Z T, Wang Y R. 2008. Effects of root sistribution, growth and vigor of japonica rice with different cultivations and fertilizations. J Shenyang Agric Univ, 39: 274–278. (in Chinese with English abstract) Ding C P. 2008. Oxidation-reduction regimes and characteristics of natural soil, upland soil and paddy soil in China. Acta pedolog sin, 45: 66–75. (in Chinese with English abstract) Fu Z Q, Huang H, He B L, Xie W, Liao X L. 2007. Correlation between rice plant aerenchyma system and methane emission from paddy field. Acta Agron Sin, 33: 1458–1467. (in Chinese with English abstract) Fukagawa M, Imai T, Tanaka T, Fujisato T. 2003. Improvement of water quality by removal of suspended solids and high-speed biological filtration in closed water body. J Water Environ Technol, 1: 233–238. Lin X Q, Zhu D F, Li C S, Yuan G H, Zhang Y P, Chen H Z. 2005.
Physiological characteristics of high-yielding rice under different irrigation methods. Chin J Rice Sci, 19: 328–332. (in Chinese with English abstract) Liu L J, Xue Y G, Sun X L, Wang Z Q, Yang J C. 2009. Effects of water management methods on grain yield and fertilizer-nitrogen use efficiency in rice. Chin J Rice Sci, 23: 282–288. (in Chinese with English abstract) Mancuso S, Boselli M. 2002. Characteristion of the oxygen fluxes in the division, elongation and mature zones of Vitis roots: Influence of oxygen availability. Planta, 214: 767–774. Maricle B R, Lee R W. 2007. Root respiration and oxygen flux in salt marsh grasses from different elevational zones. Mar Biol, 151: 413–423. Kong X S, Yi X F. 2008. Experimental Technology of Plant Physiology. Beijing: China Agriculture Press. (in Chinese) Pan S G, Cao C G, Cai M L, Wang J P, Wang R H, Yuan B Z, Zhai J. 2009. Effects of nitrogen application on nitrogen use efficiency, grain yields and qualities of rice under different water regimes. Plant Nutr & Fertil Sci, 15: 283–289. (in Chinese with English abstract) Song W T, Gao L H, Zhang S G, Teng G H, Huang Z D. 2005. Experimental study on root-zone oxygen environment in DFT for tomato cultured. Chin Agric Sci Bull, 21: 219–223. (in Chinese with English abstract) Wang G M, Chen G X, Zhang M P, Hao Z B. 2008. Characteristics of flag leaves of high yield rice hybrids after the leave fully spread. Acta Agric Nucl Sin, 22: 697–700. (in Chinese with English abstract) Zhang R P, Ma J, Wang H Z, Li Y, Li X Y, Wang R Q. 2008. Effects of different irrigation regimes on some physiology characteristics and grain yield in paddy rice during grain filling. Acta Agron Sin, 34: 486–495. (in Chinese with English abstract) Zhang W F, Wang Z L, Yu S L, Li S K, Cao L P, Ren L Y. 2002. Effect of under-mulch-drip irrigation on canopy apparent photosynthesis, canopy structure and yield formation in high-yield cotton of Xinjiang. Sci Agric Sin, 35: 632–637. (in Chinese with English abstract) Zhang Y S, Lin X Y, Luo A C. 2000. Chemical behavior of phosphorus in paddy soil as affected by O2 secretion from rice root. Chin J Rice Sci, 14: 208–212. (in Chinese with English abstract) Zhang Y S. Scherer H W. 2002. Mechanisms of fixation and release of ammonium in paddy soils after flooding. IV. Significance of oxygen secretion from rice roots on the availability of non-exchangeable ammonium–a model experiment. Biol Fert Soils, 35: 184–188. Zhao S J, Shi A G, Dong X C. 2002. Experiment Directions of Plant Physiology. Beijing: China Agricutural Science and Technology Press. (in Chinese) Zhao Q Z, Qiao J F, Liu H, Tian Z Q. 2007. Relationship between root and leaf photosynthetic characteristic in rice. Sci Agric Sin, 40: 1064–1068. (in Chinese with English abstract)
Effects of Aerated Irrigation on Leaf Senescence at Late Growth Stage and Grain Yield of Rice ZHU Lian-feng, YU Sheng-miao, JIN Qian-yu (State Key Laboratory of Rice Biology, China National Rice Research Institute, Hangzhou 310006, China)
Abstract: With the japonica inbred cultivar Xiushui 09, indica hybrid combinations Guodao 6 and Liangyoupeijiu as materials, field experiments were conducted in 2007 and 2008 to study the effects of aerated irrigation on leaf senescence at late growth stage and grain yield of rice. The dissolved oxygen concentration of aerated water evidently increased and decreased at a slow rate. The soil oxidation-reduction potential under aerated irrigation treatment was significantly higher than that of the CK, contributing to significant increases in effective panicles, seed setting rate and grain yield. In addition, the aerated irrigation improved root function, increased superoxide dismutase activity and decreased malondialdehyde content in flag leaves at post-flowering, which delayed leaf senescence process, prolonged leaf functional activity and led to enhanced grain filling. Key words: rice; aerated irrigation; physiological characteristics; leaf senescence; yield
Plant roots consume oxygen during growth and development, and sufficient oxygen should be available for root consumption. Rice is one of the wetland plants which has well-developed aerenchyma, a continuous system of air spaces in the plant that allows diffusion of O2 from the shoot or the air to the roots. Although the aerenchyma system avoids inhibition of respiration due to lack of O2, which is inevitable for the plants non-adapted to wet soils, soil aeration is still a key factor that affects the growth and development of rice (Zhang et al, 2000; Stefano et al, 2002; Fu et al, 2007; Ding et al, 2008; Dai et al, 2008). Our predecessors understood the relation between oxygen and rice growth mostly through the study of rice irrigation patterns. Aerobic irrigation measurements, such as intermittent irrigation, wetting irrigation and dry-wet alternate irrigation, increased soil oxygen content, improved soil environment, and enhanced rice growth and grain yield by appropriate adjustments of the time, duration and quantity of irrigation (Cannell et al, 1985; Zhang et al, 2002; Pan et al, 2009; Liu et al, 2009). However, these irrigation methods seldom yielded satisfactory results when applied in large-scale rice production, because of the difficulty in practical operation, varied weather conditions and other factors. Air pump aeration and some other measurements were widely used in drip irrigation system and horticulture to increase the oxygen content of the water. These aerated irrigations increased crop rhizosphere oxygen content and promoted root growth, leading to Received: 25 July 2011; Accepted: 12 October 2011 Corresponding author: JIN Qian-yu ([email protected])
improved water and fertilizer use efficiency and enhanced rice production (Zhang et al, 2002; Song et al, 2005). Super-micro bubble (SMB) technology was a new invention that produced numerous air bubbles with an average diameter of less than 3 m in treated water, and the dissolved oxygen concentration exceeded the oxygen saturation value in non-treated water. At present, the studies of super-micro bubble technology mainly referred to contaminated water purification or wastewater treatment, while its application on rice irrigation had not been reported (Masayuki et al, 2003; Chen et al, 2008). This study treated irrigation water applying super-micro bubble technology and assessed the effects of the aerated irrigation on leaf senescence and grain yield of rice. We also explored the relationship of oxygen with rice growth and development for the purpose of enriching the cultivation theory and providing reference for super high yield production of rice.
MATERIALS AND METHODS Design of experiment The treatments consisted of two kinds of irrigation, i.e. conventional irrigation with river water (CK) and aerated irrigation with river water treated by supermicro bubble generating system (Aeration). Aerating steps: Dig a pit of 3 m × 2 m × 2 m at the irrigation water inlet to the experiment field, and place in the super-micro bubble generating system (model MBO75ZS); treat the water over 30 minutes before irrigation, and then let the river water flow into the top layer of
ZHU Lian-feng, et al. Effects of Aerated Irrigation on Yield and Leaf Senescence at Late Growth Stage of Rice
Measurement and analysis The effects of super-micro bubble generating system on the dissolved oxygen concentration were investigated. The dissolved oxygen concentrations (mg/L) of the aerated water and non-aerated water were measured every 12 h using a portable dissolved oxygen meter (YSI 550A, YSI Environmental, USA). At the heading stage, select three representative plants of each treatment. Cut the stems at 10 cm from the ground with scissors, cover the cutting edge with pre-weighted cotton, and then wrap with plastic film. Collect the exudates from18:00 pm to 6:00 am, weigh the cotton with root exudates, then separate the exudates into centrifuge tube for analysis. The contents of total soluble sugar and free amino acids in the root exudates were determined according to the methods of Kong et al (2008). Fresh flag leaf samples were collected every 7 d
after flowering. Superoxide dismutase (SOD) activity and malondialdehyde (MDA) content were determined according to the methods of Zhao et al (2002). Plants in each plot were harvested at maturity for the determination of grain yield. Yield components including number of panicles per m2, number of spikelets per panicle, spikelet filling percentage and grain weight were determined from the plants of 5 hills. Spikelet filling rate was calculated as the number of grains that sank to the bottom of a beaker filled with salt solution with specific gravity of 1.06 as percentage of total spikelets. The results were analyzed for variance by ANOVA, and statistical differences among means of the three replicates were tested by least significant difference at the 0.05 probability level with the SPSS13.0 for Windows. Only the 2008 data were analyzed because the two separate experiments in 2007 and 2008 gave very similar results.
RESULTS Oxygenation effects of super-micro bubble technology Fig. 1 showed the dynamics of dissolved oxygen concentration in 2000 L non-treated water (CK) and aerated water (water aerated for 30 min). After aeration, the concentration of dissolved oxygen in irrigated water was 7.93 mg/L, while that of CK was 3.16 mg/L. In natural conditions, the oxygen in the air will dissolve into water, which finally comes to an equilibrium. Because the concentration of dissolved oxygen for CK was lower initially, with the dissolving of oxygen from air, the oxygen content increased in CK. The dissolved oxygen concentration reached super-saturation by aeration, and the super-micro bubbles collapsed as time passed. The concentration
Dissolved oxygen concentration (mg/L)
the pit water for aeration treatment while the aerated water outflow from the bottom of the pit through plastic pipe into the experimental plots. Besides the two treatments, a standing water layer was maintained during plant recovery stage, and intermittent irrigation was applied during the other stages. Field trials were conducted at the China National Rice Research Institute, Hangzhou, Zhejiang Province, China, during the rice growing season (May to November) of two successive years 2007 and 2008. The materials used in 2007 were indica hybrid rice Guodao 6 and japonica inbred rice Xiushui 09, and indica hybrid rice Liangyoupeijiu and japonica inbred rice Xiushui 09 were used in 2008. The three rice materials were chosen because they were planted to a large extent in Zhejiang Province. Seedlings were raised in the field with sowing date on 24 May (hybrid rice) or 30 May (inbred rice) and transplanted on 24 June at a hill spacing of 0.3 m × 0.2 m with one seedling per hill (hybrid rice) or at a hill spacing of 0.2 m × 0.2 m with four seedlings per hill (inbred rice). The plot dimensions were 8.0 m × 5.2 m. A completely randomized block design with three replications was used in the study. The soil of the field was loam clay with 36.8 g/kg organic matter and available N-P-K at 142.0, 17.0 and 14.1 mg/kg, respectively. A sum of 63.0 kg N, 72.0 kg P2O5 and 67.5 kg K2O as compound fertilizer with N:P:K = 14:16:15, and 27.0 kg of nitrogen per hectare were applied in the form of urea and incorporated before transplanting. Nitrogen as urea was applied at mid-tillering (45.0 kg/hm2) and panicle initiation stage (15.0 kg/hm2), and potassium as potassium chloride (60% K2O) was also applied at panicle initiation stage (52.5 kg/hm2).
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Fig. 1. Dissolved oxygen concentrations in the aerated water and non-treated water.
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of dissolved oxygen declined slowly and decreased to 6.87 mg/L after 104 h, which equals to that of CK. Using the aeration technology, the dissolved oxygen concentration is enhanced, exceeding that of CK within 100 h after the treatment, and its decline is delayed. Soil oxidation-reduction potential increased from panicle initiation stage of rice plants (Fig. 2), which may be attributed to the water regime adopted in this experiment. Comparison between the two treatments showed that the soil oxidation-reduction potential of aeration treatment was higher than that of CK during the entire measurement periods, especially for heading stage. The effects of aeration on the two conventional cultivars were identical, but more obvious on the hybrid Liangyoupeijiu. Under the same conditions, the aeration can increase the oxygen content in soil significantly, and improve the oxidation-reduction potential in the root zone. Effects of aerated irrigation on rice grain yield and its components In the two years’ experiment, the grain yields of the three materials by aerated irrigation were significantly higher than those of CK (Table 1), and the increasing
rates were 8.45% (2007) and 6.23% (2008) for Xiushui 09, 9.13% for Guodao 6 (2008), and 7.49% for Liangyoupeijiu (2008), respectively. For yield components, the panicle numbers per hectare by aeration were all higher than those of CK. The tiller dynamics showed that aerated irrigation mainly increased the tiller emergence at early tillering stage, but slightly influenced the maximum tiller number, indicating that the early tillers at the lower nodes contributed to the increase of the panicle number. Compared with CK, the panicle number of aerated irrigation treatment was increased at 4.8%–7.4%. In addition, the number of spikelets per hectare and seedsetting rate of aerated irrigation were significantly higher than those of CK, and the 1000-grain weight of aerated irrigation also increased but at non-significant level. The number of spikelets per panicle of aeration treatment was equal to or a little below the number of CK, with the lowest for Xiushui 09. Furthermore, aerated irrigation increased the seed-setting rate, completely-filled grain number and 1000-grain weight, which led to significantly increased grain yield. Effects of aerated irrigation on contents of soluble sugar and free amino acids in the root exudates Rice has maximum exudate quantity at heading
Fig. 2. Effects of aerated irrigation on soil oxidation-reduction potential. T, Tillering stage; PI, Panicle initiation stage; H, Heading stage; M, Milky ripening stage. * and ** indicate significant differences at the 0.05 and 0.01 levels, respectively, compared with conventional irrigation (CK).
Table 1. Effects of aerated irrigation on rice grain yield and its components. Year
Rice material Xiushui 09
Treatment
No. of panicles per hectare (×104)
Aerated irrigation 334.9* CK 302.8 2007 Guodao 6 Aerated irrigation 188.7* CK 178.1 Xiushui 09 Aerated irrigation 307.1* CK 270.3 2008 Liangyoupeijiu Aerated irrigation 253.8* CK 237.9 * indicates significant difference at the 0.05 level.
No. of spikelets per panicle 131.0 136.7 194.7 196.5 115.5 122.0 155.9 154.7
No. of spikelets Seed-setting per hectare (×104) rate (%) 49343.9* 48036.7 48804.1* 47713.5 39262.8* 37289.9 52907.8* 51068.3
91.13* 86.15 75.30* 71.26 90.36* 85.28 74.79* 70.38
1000-grain weight (g)
Grain yield (t/hm2)
25.54 24.90 29.24 28.74 25.28 24.67 25.42 24.94
7.83* 7.22 6.93* 6.35 7.50* 7.06 8.32* 7.74
ZHU Lian-feng, et al. Effects of Aerated Irrigation on Yield and Leaf Senescence at Late Growth Stage of Rice Table 2. Influence of aerated irrigation on exudate quantity and its soluble sugar and free amino acid contents. Total exudates Soluble sugar Free amino Variety Treatment per tiller content acids content (mg/h) (μg/mL) (μg/mL) Aeration 51.46** 25.54** 106.18** Xiushui 09 CK 38.59 15.96 75.97 Aeration 60.90** 14.46** 59.66** Liangyoupeijiu CK 49.55 12.75 50.80 ** indicates significant difference at the 0.01 level.
stage. Shown as the Table 2, the exudate quantity, contents of total soluble sugar and free amino acid in the root exudates of Xiushui 09 and Liangyoupeijiu by aerated irrigation were significantly increased in comparison with those of CK (P < 0.01). Effects of aerated irrigation on SOD activity and MDA content in flag leaf at the late growth stage of rice From Fig. 3, we can see that the flag leaf SOD activity of Xiushui 09 and Liangyoupeijiu by aerated irrigation was significantly higher than that of CK from flowering to 21 d after flowering. The SOD activity of the two varieties under both treatments decreased from flowering, and the decreasing speed was slow during the first 14 d after flowering. However the SOD actitiviy decreased faster after 14 days from flowering. Compared with CK, the decreasing speed of flag leaf SOD activity of aerated irrigation was smaller. The dynamics of leaf MDA content showed that the MDA content of aerated irrigation was lower than that of CK from flowering to 21 days after flowering and that the content difference was small at the first 7 days, then became large from 7 to 21 d after flowering. Compared with CK, the MDA content of aerated irrigation was lower from flowering, and the difference became greater from 7 d after flowering.
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DISCUSSION Root activity is a key requirement for the improvement of fertilizer and water use efficiency, prevention of leaf premature senescence and maintenance of high photosynthetic capacity at the late growth stage of rice. Oxygen-rich environment is very conducive to the emergence and growth of roots and to the maintenance of high root activity. Lin et al (2005) reported that aerobic irrigation improved the growth environment of rice root, leading to significantly increased root exudates, root metabolism and nutrient use efficiency. Root activity has immediate and direct effects on leaf photosynthesis (Zhao et al, 2007). Our experiment showed the exudate quantity of rice roots under aerated irrigation was obviously higher than that of CK, namely higher root activity at heading stage. Moreover, the contents of free amino acids and soluble sugar in the root exudates of rice by aerated irrigation were also higher than those of CK, and this was indicative of increased nitrogen absorption and assimilation, which would satisfy the nitrogen requirement of the plants and in return, translocate adequate photosynthates to root system. Higher free amino acids content was favorable to the synthesis of chlorophyll and proteins in leaves, and conducive to the prevention of protein decomposition, which may result from the lack of free amino acid. Stronger root activity ensured adequate water supply for shoots, which enhanced the stomatal conductance and transpiration rate. Therefore, the photosynthetic capacity of leaves increased with the improvement of root function, and this is in agreement with the result of Zhang et al (2008). Leaf senescence at the late growth stage of rice will lead to loss of photosynthetic capacity and serious
Fig. 3. Influence of aerated irrigation on SOD activity and MDA content in rice flag leaf.
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impact on grain filling. Wang et al (2008) reported that the increase of relative content of reactive oxygen species was the main reason for impaired photosystem of plants. The integrity of the membrane system is essential for normal photosynthetic growth of green plants, while the SOD plays a major role in the protection of the integrity of the membrane system. This experiment showed that aerated irrigation obviously increased the SOD activity of flag leaves at post-flowering, and compared with CK, the flag leaf SOD activity of aerated irrigation decreased slowly. The MDA content of aerated irrigation was lower than that of CK, and increased slowly as the growth progressed. This indicates that aerated irrigation enhanced the activity of antioxidant enzymes of rice leaves. As a result, the production and scavenging of reactive oxygen species comes to equilibrium, and the damage by reactive oxygen species decreases. Consequently, leaf senescence process is delayed, leaf functional activity prolonged and grain filling promoted, which contribute to the increase of grain yield.
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