Phosphorus cycling in rainfed lowland rice ecosystems on sandy soils

Phosphorus cycling in rainfed lowland rice ecosystems is poorly understood. Soil drying and grazing of rice straw during the long dry season, the growth of volunteer pastures during the early wet season, and intermittent loss of soil-water saturation while the rice crop is growing are important distinguishing characteristics of the rainfed lowlands in relation to P cycling. We studied P cycling in an acid sandy rainfed lowland soil that covers about 30% of the rice growing area of Cambodia. Soils with similar properties in comparable rainfed sub- ecosystems occur in Laos and northeast Thailand. We developed a general schema of P pools and fluxes in the crop and soil for rice-based cropping systems in the rainfed lowlands of Cambodia. The schema was derived from a number of field experiments carried out over five consecutive cropping seasons to quantify the residual value of P fertiliser, P mass balances, soil P fractions, the effect on subsequent rice crops of crop residues and volunteer pastures incorporated into the soils, and the dynamics of P turnover in the soil. With a single rice crop yielding 2.5–3 t ha⁻¹, application of 8–10 kg P ha⁻¹ maintained yields and a small positive P balance in the soil. However, the soil P balance was sensitive to the proportion of rice straw returned to the soil. Volunteer pastures growing during the early wet season accumulated significant amounts of P, and increased their P uptake when soils were previously fertilised with P. These pastures recycled 3–10 kg P ha⁻¹ for the succeeding rice crops. While inorganic soil P pools extractable with ion exchange resins and 0.1 M NaOH appeared to be the main source of P absorbed by rice, microbial and organically-bound P pools responded dynamically to variation in soil water regimes of the main wet, dry and early wet seasons. The schema needs to be developed further to incorporate site-specific conditions and management factors that directly or indirectly affect P cycling, especially loss of soil-water saturation during the rice cropping cycle. The paper discusses the application of results for acid sandy soils to other significant rice soils in the rainfed lowlands of southeast Asia.     

Leaf gas exchange of upland and lowland rice cultivars

Leaf gas exchange of upland and lowland rice cultivars were measured during late vegetative and during grain filling stages in the field under upland and lowland growth conditions. The rate of photosynthesis and water use efficiency (the rate of photosynthesis/the rate of transpiration) under upland conditions decreased with ageing, but generally varied little among four cultivars. At mid-grain filling under lowland conditions, upland cultivars showed lower rates of photosynthesis and transpiration than the lowland cultivars with concomitant reduction in whole plant conductance. At this stage, water use efficiency was higher under upland conditions than under lowland conditions, particularly in the upland cultivars. Water stress reduced the rate of photosynthesis without altering water use efficiency.     

Performance of different fish species for controlling weeds in rainfed lowland rice field

A field experiment was conducted during wet season to evaluate the performance of different fish species for biocontrol of weeds in rainfed waterlogged rice fields with fingerlings of three exotic carps – grass carp, Ctenopharyngodon idella (Val.) (Cyprinidae, Cypriniformes), silver barb, Puntius gonionotus (Bleeker) (Cyprinidae, Cypriniformes) and common carp, Cyprinus carpio (L.) (Cyprinidae, Cypriniformes) – and three Indian major carps – rohu, Labeo rohita (Ham.) (Cyprinidae, Cypriniformes), catla, Catla catla (Ham.) (Cyprinidae, Cypriniformes), mrigal, Cirrhinus mrigala (Ham.) (Cyprinidae, Cypriniformes). A total of 13 major weeds under the categories of grassy, sedges, broadleaf and aquatic weeds were observed in the rice fields. Grass carp reduced maximum weed biomass (weed control efficiency [WCE] 63% at 60 days after transplanting [DAT] and 62% at 100 DAT) followed by silver barb and common carp. Among the Indian carps, only rohu was effective in control of weeds (WCE, 23% at 60 DAT). The grain yield of rice (variety Varshadhan) slightly increased (4.2–4.5 t/ha), but straw yield was significantly higher (10.2–10.7 t/ha) under rice-fish farming. Fish yield was significantly higher in exotic carps (270–288 kg/ha/90days) due to higher specific growth rates (1.8–2% body weight/day). The study indicated that exotic carps (grass carp, silver barb and common carp in order) were more effective than Indian carps for control of weed in rainfed lowland rice fields and among the Indian carps, rohu showed potential for weed control.     

Root growth response of rainfed lowland rice to aerobic conditions in northeastern Thailand

Background and aims

Rice plants alternately experience anaerobic and aerobic conditions during their life cycle in rainfed lowlands. Each condition affects root growth differently. Our objective was to clarify the specific rice root response to aerobic conditions in rainfed lowlands.

Methods

At the Ubon Ratchathani Rice Research Center in northeastern Thailand, we obtained root samples from 17 ‘Surin1’ (Thai variety) BC₃-derived lines and 7 CT9993-5-10-1-M × IR62266-42-6-2 doubled-haploid lines from flooded and non-flooded paddy fields at the reproductive stage in 2010 and 2011.

Results

In the non-flooded trial, rice was grown aerobically by draining the perched water; soil moisture at a depth of 20 cm fluctuated between ?10 and ?30 kPa. Deep rooting was likely promoted under aerobic conditions, but slightly drier soils under longer dry spells seemed to restrict root penetration, as the topsoil rapidly hardened during dry spells of only a few days. Fine-root development in the topsoil was inhibited under aerobic conditions.

Conclusions

Even without drought stress, rice roots respond significantly to the disappearance of standing water in rainfed lowlands via deep rooting and root branching. We identified one promising ‘Surin1’ BC₃-derived line showing an adaptive response of deep rooting under aerobic conditions, which can be used as a breeding material for rainfed lowland rice in Thailand.     

Mapping QTLs for root morphology of a rice population adapted to rainfed lowland conditions

In the rainfed lowlands, rice ( Oryza sativa L.) develops roots under anaerobic soil conditions with ponded water, prior to exposure to water stress and aerobic soil conditions that arise later in the season. Constitutive root system development in anaerobic soil conditions has been reported to have a positive effect on subsequent expression of adaptive root traits and water extraction during progressive water stress in aerobic soil conditions. We examined quantitative trait loci (QTLs) for constitutive root morphology traits using a mapping population derived from a cross between two rice lines which were well-adapted to rainfed lowland conditions. The effects of phenotyping environment and genetic background on QTLs identification were examined by comparing the experimental data with published results from four other populations. One hundred and eighty-four recombinant inbred lines (RILs) from a lowland indica cross (IR58821/IR52561) were grown under anaerobic conditions in two experiments. Seven traits, categorized into three groups (shoot biomass, deep root morphology, root thickness) were measured during the tillering stage. Though parental lines showed consistent differences in shoot biomass and root morphology traits across the two seasons, genotype-by-environment interaction (GxE) and QTL-by-environment interaction were significant among the progeny. Two, twelve, and eight QTLs for shoot biomass, deep root morphology, and root thickness, respectively, were identified, with LOD scores ranging from 2.0 to 12.8. Phenotypic variation explained by a single QTL ranged from 6% to 30%. Only two QTLs for deep root morphology, in RG256-RG151 in chromosome 2 and in PC75M3-PC11M4 in chromosome 4, were identified in both experiments. Comparison of positions of QTLs across five mapping populations (the current population plus populations from four other studies) revealed that these two QTLs for deep root morphology were only identified in populations that were phenotyped under anaerobic conditions. Fourteen and nine chromosome regions overlapped across different populations as putative QTLs for deep root morphology and root thickness, respectively. PC41M2-PC173M5 in chromosome 2 was identified as an interval that had QTLs for deep root morphology in four mapping populations. The PC75M3-PC11M4 interval in chromosome 4 was identified as a QTL for root thickness in three mapping populations with phenotypic variation explained by a single QTL consistently as large as 20-30%. Three QTLs for deep root morphology were found only in japonica/indica populations but not in IR58821/IR52561. The results identifying chromosome regions that had putative QTLs for deep root morphology and root thickness over different mapping populations indicate potential for marker-assisted selection for these traits.     

Internal efficiency, nutrient uptake, and the relation to field water resources in rainfed lowland rice of northeast Thailand

Rice-based (Oryza sativa L.) rainfed lowlands are the major cropping system in northeast Thailand. Average yields are low, which is generally explained by frequent drought events, low soil fertility, and poor fertilizer response. However, neither the relative importance of these factors nor their interaction is well understood. Therefore, we analyzed an existing database on fertilizer trials conducted between 1995 and 1997 at eight different sites in northeast Thailand with the objective to determine indigenous nutrient supplies, internal efficiencies, and recovery efficiencies of applied nutrients in rainfed lowland rice. Of particular interest was the effect of variety type (traditional) and water supply on these components. Comparison of N, P, and K concentrations in grain and straw (average N–P–K grain concentration of 11.0–2.7–3.4 g kg⁻¹; average N–P–K straw concentration of 5.2–0.9–16.4 g kg⁻¹) in the traditional-type varieties used at all trial sites with literature values showed no differences for these parameters between traditional and modern-type varieties or between irrigated and rainfed environments. In contrast, internal efficiencies of N, P, and K (average IEN: 46 kg grain per kg N uptake; IEP: 218 kg grain per kg P uptake; IEK: 25 kg grain per kg K uptake) were much lower than reported for irrigated systems, and the difference was greatest for K, which is mainly accumulated in the straw. Indigenous nutrient supply (average INS: 38 kg ha⁻¹; IPS: 10 kg ha⁻¹; IKS: 89 kg ha⁻¹) and recovery efficiency (average REN: 0.28 kg kg⁻¹; REP: 0.13 kg kg⁻¹; REK: 0.49 kg kg⁻¹) were low but comparable to the lower values reported from irrigated systems. Average seasonal field water resources seemed to reduce the indigenous nutrient supply but had no or little effect on internal efficiency and recovery efficiency. We concluded that the main reason for the low system productivity without and with fertilizer in northeast Thailand is the dominant use of traditional-type varieties with low harvest indices, which was the dominant cause for the observed low internal nutrient efficiency. Therefore, intensification of rainfed systems through substantially increased nutrient inputs can be recommended only where varieties with an average harvest index of close to 0.4 or higher are available.     

Seed vigour contributes to yield improvement in dry direct‐seeded rainfed lowland rice

Dry direct‐seeded rice (DSR) cultivation is widely spreading in tropical Asia, but drought and nutrient deficiency stresses often cause crop failure in rainfed lowlands. The objective of this study was to dissect the physio‐morphological characteristics associated with crop establishment and early vigour of DSR under drought and P deficiency conditions in the Philippines. It was found that new drought‐resistant cultivars bred for DSR (Rc348 and Rc192) had faster germination and sprout growth than popular irrigated rice cultivars (Rc222 and Rc10) under soil water deficit due to rapid moisture acquisition by the germinating seeds from drying soils. There was a significant correlation between seed moisture content and the reduction in seed dry weight, and between reduction in seed dry weight and shoot elongation under both control and drought stress treatments at the germination stage. At the seedling stage, the root growth of Rc348 under drought tended to be more vigorous with its higher root‐to‐shoot ratio compared to Rc222 and Rc10. The seedling vigour of Rc348 under P deficiency was also greater than that of Rc222 due to its greater root growth and P uptake. The yields of Rc348 and Rc192 grown under rainfed condition at the target drought‐prone site where a dry spell of 13 days occurred during crop establishment were higher (4.0–4.1 t ha^?1) than the yield of Rc10 (3.0 t ha^?1). These results suggest that quick germination and seedling vigour with quick root anchorage and great nutrient uptake capacity, even with limitations of soil moisture and nutrients, would be important traits for DSR in rainfed lowlands.     

UV-B induced stress responses in three rice cultivars

UV-B responses of three rice (Oryza sativa L.) cultivars (Sasanishiki, Norin 1 and Surjamkhi) with different photolyase activity were investigated. Carbon dioxide assimilation data support that Sasanishiki was less sensitive to UV-B than Norin 1 and Surjamkhi. UV-B radiation sharply decreased the content of Rubisco protein in Surjamkhi and has no effect in Sasanishiki. The photochemical activities of photosystem (PS) 1 and PS 2 was slightly affected by UV-B treatment. The content of H₂O₂ and the activities of antioxidant enzymes, catalase (CAT), peroxides (POX) and superoxide dismutase (SOD) were enhanced after UV-B treatment. The activities of CAT and POX isoenzymes in Sasanishiki were more enhanced by UV-B radiation than those in Norin 1 and Surjamkhi.     

不同水稻品种对虫害胁迫的生理响应

以褐飞虱-水稻为模式,研究虫害胁迫下植物的化学防御生理生态特征,测定了可溶性糖含量、叶绿素含量、光合速率、蒸腾速率、气孔导度以及细胞间隙二氧化碳浓度、防御酶的时间变化特征.结果表明,经虫害诱导,不同品种水稻可溶性糖含量降低;同时,光合速率以及叶绿素含量也相应下降;蒸腾速率、气孔导度和细胞间隙二氧化碳浓度的变化不显著。虫害对水稻叶片多酚氧化酶、过氧化物酶和脂氧合酶均具有诱导作用.这种诱导作用具有时间效应,且不同品种诱导作用不一致.     

The chemistry of the lowland rice rhizosphere

Models and experimental studies of the rhizosphere of rice plants growing in anaerobic soil show that two major processes lead to considerable acidification (1–2 pH units) of the rhizosphere over a wide range of root and soil conditions. One is generation of H⁺ in the oxidation of ferrous iron by O₂ released from the roots. The other is release of H⁺ from roots to balance excess intake of cations over anions, N being taken up chiefly as NH₄ ⁺. CO₂ exchange between the roots and soil has a much smaller effect. The zone of root-influence extends a few mm from the root surface. There are substantial differences along the root length and with time. The acidification and oxidation cause increased sorption of NH₄ ⁺ ions on soil solids, thereby impeding the movement of N to absorbing root surfaces. But they also cause solubilization and enhanced uptake of soil phosphate.     

Comparison on physiological adaptation and phosphorus use efficiency of upland rice and lowland rice under alternate wetting and drying irrigation

As one of the most widely promoted water-saving irrigation strategies for rice, alternate wetting and drying irrigation (AWD) can not only save water but also increase mineral nutrient use efficiency. In this study, we compared the growth conditions of four rice varieties (two lowland and two upland varieties) under three irrigation regimes: continuously flooded (CF), alternate wetting and moderate soil drying (AWD15) and alternate wetting and severe soil drying (AWD30). AWD15 and AWD30 enabled the plants to receive fewer irrigation events and less irrigation water than CF, thereby saving both water resources and labor. AWD15 reduced redundant vegetative growth, promoted root growth, and increased the root-shoot ratio and harvest index. AWD15 increased the grain yield, water use efficiency (WUE) and phosphorus use efficiency (PUE) of upland rice and maintained the grain yield while increasing the WUE and PUE of lowland rice. More developed root systems under AWD helped upland rice to maintain a higher water status than lowland rice when plants were subjected to soil drying, which resulted in superior performance in grain yield in upland rice. AWD30 could not reconcile the demands of higher yield and the desire to reduce irrigation water use because it decreased grain yield. The results indicate that AWD15 irrigation of rice can not only increase rice yield and WUE but also enhance PUE, which can potentially reduce the use of phosphorus fertilizers. The results provide theoretical and technical support for improving rice cultivation.     

Differential antioxidative responses of indica rice cultivars to drought stress

The present study investigated the linkages between drought stress, oxidative damages and variations in antioxidants in the three rice varieties IR-29 (salt-sensitive), Pokkali (salt-tolerant) and aromatic Pusa Basmati (PB), to elucidate the antioxidative protective mechanism governing differential drought tolerance. Water deficit, induced by 20% (w/v) polyethylene glycol (PEG-6000), provoked severe damages in IR-29 and PB in the form of huge chlorophyll degradation and elevated H₂O₂, malondialdehyde and lipoxygenase (LOX, EC 1.13.11.12) levels as compared to Pokkali. The protein oxidation was more conspicuous in IR-29. Increment in antioxidants, particularly flavonoids and phenolics was several folds higher over control in Pokkali, while much lesser in IR-29 and PB. The activity of catalase (CAT, EC 1.11.1.6) and superoxide dismutase (SOD, EC 1.15.1.1) were decreased in IR-29 and PB, but unaltered in Pokkali. However, marked drought-induced increase in guaiacol peroxidase (GPX, EC 1.11.1.7) activity was noted in both IR-29 and PB. Induction in radical scavenging activity, being the maximum in IR-29, and increased reducing power ability in all the cultivars, accompanied with drought stress, were observed as a defense mechanism. The novelty of our work is that it showed the aromatic rice PB behaving more closely to IR-29 in greater susceptibility to dehydration stress, while the salt-tolerant Pokkali also showed effective drought tolerance properties.     

不同水层和密度条件下化感作用水稻对无芒稗的干扰控制作用

不同水层下水稻对无芒稗的干扰控制作用研究表明,水稻化感作用品种吓一跳、谷梅2号和中156对无芒稗的株高抑制作用比无化感作用品种秀水63和春江11呈显著性差异．随盆栽水稻密度增加,水稻对无芒稗的干扰控制作用加大．在每盆4株栽植密度下,水稻化感作用品种鸡早籼、谷梅2号显著抑制无芒稗植株株高;在每盆32株密度时,水稻化感作用品种吓一跳、PI312777、TN1等对受控无芒稗的干扰控制与无化感作用品种春江11相比,呈显著性水平．     

Upland rice and lowland rice exhibited different PIP expression under water deficit and ABA treatment

Aquaporins play a significant role in plant water relations. To further understand the aquaporin function in plants under water stress, the expression of a subgroup of aquaporins, plasma membrane intrinsic proteins （PIPs）, was studied at both the protein and mRNA level in upland rice （Oryza sativa L. cv. Zhonghan 3） and lowland rice （Oryza sativa L. cv. Xiushui 63） when they were water stressed by treatment with 20% polyethylene glycol （PEG）. Plants responded differently to 20% PEG treatment. Leaf water content of upland rice leaves was reduced rapidly. PIP protein level increased markedly in roots of both types, but only in leaves of upland rice after 10 h of PEG treatment. At the mRNA level, OsPIP1,2, OsPIP1,3, OsPIP2;1 and OsPIP2;5 in roots as well as OsPIP1,2 and OsPIP1;3 in leaves were significantly up-regulated in upland rice, whereas the corresponding genes remained unchanged or down-regulated in lowland rice. Meanwhile, we observed a significant increase in the endogenous abscisic acid （ABA） level in upland rice but not in lowland rice under water deficit. Treatment with 60 μM ABA enhanced the expression of OsPIP1;2, OsPIP2;5 and OsPIP2;6 in roots and OsPIP1;2, OsPIP2;4 and OsPIP2;6 in leaves of upland rice. The responsiveness of PIP genes to water stress and ABA were different, implying that the regulation of PIP genes involves both ABA-dependent and ABA-independent signaling oathways during water deficit.     

The influence of water depth and flow regime on phytoplankton biomass and community structure in a shallow,lowland river

Hydrobiologia - The taxonomic composition and biomass of phytoplankton in the San Joaquin River, California, were examined in relation to water depth, flow regime, and water chemistry. Without...     

The effects of irrigation regimes on the water use,dry matter production and physiological responses of paddy rice

In most cases, rice production is associated with flooding irrigation and the efficiency of irrigated water use (WUEi) is generally lower for production of rice than for other crops. We have examined the effects of various irrigation regimes on water consumption in a well-puddled paddy field, as well as on dry matter production, grain yield and physiological responses of the plants. Four sets of conditions were studied, with two replications, in the well-puddled paddy field: Continuous flooding irrigation treatment (CSF); three intermittent irrigation treatments, designated II-0, II-1 and II-2, in which plants were re-irrigated when the water potential of the soil fell below 0, –10, and –20 kPa at a depth of 5 cm, respectively. Water consumption was lower during II-0 than during CSF because the percolation rate was reduced by the reduction in the hydraulic head of the ponded water. Intermittent irrigation led to the repeated shrinking and swelling of soil during II-1 and II-2 and, therefore, soil cracks developed rapidly. Since they became the major routes of water percolation, these soil cracks increased water consumption during II-1 and II-2 above that during CSF and II-0. There were no significant differences in dry matter production and grain yield between CSF and II-0, but both were significantly greater than in the case of II-1 and II-2. Therefore, WUEi increased in the following order: II-0, CSF, II-2, II-1, although the difference was very small between II-1 and II-2. A lower crop growth rate (CGR) resulted from a decrease in the net assimilation rate (NAR) during II-1 and II-2, and there was also a reduction in the leaf area index (LAI) during II-2. Early senescence with ripening and water stress around midday decreased the rate of photosynthesis in leaves, causing the lower NAR. These physiological responses of the plants were responsible for the reduction on the dry matter production and grain yield in the intermittent irrigation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of phosphorus application on transformation of zinc fraction in soil and on the zinc nutrition of lowland rice

We have studied in the laboratory the effect of different levels of P application on the transformation on native as well as of applied zinc in a rice-growing soil under two moisture regimes viz., flooded and nonflooded. Application of P caused a decrease in the water soluble plus exchangeable and organic complexed with a concomitant increase in the amorphous and crystalline sesquioxide bound forms of native soil zinc. Application of P also caused a decrease in the transformation of applied Zn into the water soluble plus exchangeable and organically complexed and an increase in the amorphous and crystalline sesquioxide bound forms of zinc. The above effects of P were more pronounced in soil under flooded than under nonflooded moisture regimes. The water soluble plus exchangeable and the organically complexed forms of Zn are considered to play an important role in Zn nutrition of lowland rice, while the role of the amorphous and crystalline sesquioxide bound forms are less important in this regard.The results of greenhouse experiments showed that P application caused a progressive decrease in the Zn concentration in shoot and root. This was attributed at least partly to the decrease in the water soluble plus exchangeable and organically complexed forms of Zn and an increase in the amorphous and crystalline sesquioxide bound forms in soil due to P application.