Effects of elevated CO2 on plant growth and nutrient partitioning of rice (Oryza sativa L.) at rapid tillering and physiological maturity

Abstract

This work investigates the relationship between plant growth, grain yield, nutrient acquisition and partitioning in rice (Oryza sativa L.) under elevated CO₂. Plants were grown hydroponically in growth chambers with a 12-h photoperiod at either 370 or 700 µmol CO₂ mol^?1 concentration. Plant dry mass (DM), grain yield and macro- and micronutrient concentrations of vegetative organs and grains were determined. Elevated CO₂ increased biomass at tillering, and this was largely due to an increase in root mass by 160%. Elevated CO₂ had no effect on total nutrient uptake (N, P, K, Mg and Ca). However, nutrient partitioning among organs was significantly altered. N partitioning to leaf blades was significantly decreased, whereas the N partitioning into the leaf sheaths and roots was increased. Nutrient use efficiency of N, P, K, and Mg in all organs was significantly increased at elevated CO₂. At harvest maturity, grain yield was increased by 27% at elevated CO₂ while grain (protein) concentration was decreased by a similar magnitude (28%), suggesting that critical nutrient requirements for rice might need to be reassessed with global climate change.     

Diminution of photosynthesis in rice (Oryza sativa L.) seedlings under elevated CO2 concentration and increased temperature

The photosynthetic responses to elevated CO₂ concentration (EC) at ambient and ambient +4°C temperature were aßsessed in the second leaf of rice (Oryza sativa L.) seedlings. The duration of different leaf developmental phases, as characterised by changes in photosynthetic pigment contents and photochemical potential, was protracted in the seedlings grown under EC. On the other hand, a temporal shift in the phases of development with an early onset of senescence was observed in the seedlings grown under EC at ambient +4°C temperature. The contents of carotenoids, ß-carotene, and xanthophyll cycle pigments revealed that EC downregulated the protective mechanism of photosynthetic apparatus against oxidative damages, whereas this mechanism assumed higher significance under EC at ambient +4°C temperature. We observed an enhancement in electron transport activity, photochemical potential, and net photosynthesis in spite of a loss in photostasis of photosynthesis under EC. On the other hand, the loss in photostasis of photosynthesis was exacerbated under EC at ambient +4°C temperature due to the decline in electron transport activity, photochemical potential, and net photosynthesis.     

Arsenic uptake and accumulation in rice (Oryza sativa L.) irrigated with contaminated water

Long-term use of arsenic contaminated groundwater to irrigate crops, especially paddy rice (Oryza sativaL.) has resulted in elevated soil arsenic levels in Bangladesh. There is, therefore, concern regarding accumulation of arsenic in rice grown on these soils. A greenhouse pot experiment was conducted to evaluate the impact of arsenic-contaminated irrigation water on the growth and uptake of arsenic into rice grain, husk, straw and root. There were altogether 10 treatments which were a combination of five arsenate irrigation water concentrations (0–8 mg As l^–1) and two soil phosphate amendments. Use of arsenate containing irrigation water reduced plant height, decreased rice yield and affected development of root growth. Arsenic concentrations in all plant parts increased with increasing arsenate concentration in irrigation water. However, arsenic concentration in rice grain did not exceed the maximum permissible limit of 1.0 mg As kg^–1. Arsenic accumulation in rice straw at very high levels indicates that feeding cattle with such contaminated straw could be a direct threat for their health and also, indirectly, to human health via presumably contaminated bovine meat and milk. Phosphate application neither showed any significant difference in plant growth and development, nor in As concentrations in plant parts.     

Heterosis breeding in rice (Oryza sativa L.)

Summary Studies conducted at the International Rice Research Institute (IRRI) during 1980 and 1981 have shown up to 73% heterosis, 59% heterobeltiosis and 34% standard heterosis for yield in rice. The latter was estimated in comparison to commercial varieties: IR36 and IR42 (yield 4–5 t/ha in wet season trials and 7–8 t/ha in dry season trials). Generally speaking, absolute yield was lower and extent of standard heterosis was higher in wet season than in dry season with some exception. Yields up to 5.9 t/ha (22% standard heterosis) in the wet season and 10.4 t/ha (34% standard heterosis) in the dry season were obtained. Most of the hybrids performed better in some season while some performed better in both seasons. Hybrids showed better lodging resistance although they were 5–10 cm taller. F₁ hybrids had significant positive correlations with the parental traits viz., yield (r = 0.446), tillering (r = 0.746), height (r = 0.810) and flowering (r = 0.843). Selection of parents among elite breeding lines on the basis of their per se yield performance, diverse origin and resistance to insects and diseases should give heterotic combination. Yield advantage of hybrids was due primarily to increase in number of spikelets per unit area even though tiller number was reduced. Grain weight was either the same or slightly higher. High yielding hybrids also showed significant heterosis and heterobeltiosis for total dry matter and harvest index. For commercial utilization of heterosis in rice, effective male sterility and fertility restoration systems are available and up to 45% natural outcrossing on male sterile lines has been observed. Consequently, F₁ rice hybrid have been successfully developed and used in China. Prospects of developing hybrid rice varieties elsewhere appear bright especially in countries that have organized seed production, certification and distribution programs and where hybrid seed can be produced at a reasonable cost.     

Arsenic toxicity to rice (Oryza sativa L.) in Bangladesh

Natural contamination of groundwater with arsenic (As) occurs around the world but is most widespread in the river basin deltas of South and Southeast Asia. Shallow groundwater is extensively used in the Bengal basin for irrigation of rice in the dry winter season, leading to the possibility of As accumulation in soils, toxicity to rice and increased levels of As in rice grain and straw. The impact of As contaminated irrigation water on soil-As content and rice productivity was studied over two winter-season rice crops in the command area of a single tubewell in Faridpur district, Bangladesh. After 16–17 years of use of the tubewell, a spatially variable build up of As and other chemical constituents of the water (Fe, Mn and P) was observed over the command area, with soil-As levels ranging from about 10 to 70 mg kg^?1. A simple mass balance calculation using the current water As level of 0.13 mg As L^?1 suggested that 96% of the added arsenic was retained in the soil. When BRRI dhan 29 rice was grown in two successive years across this soil-As gradient, yield declined progressively from 7–9 to 2–3 t ha^?1 with increasing soil-As concentration. The average yield loss over the 8 ha command area was estimated to be 16%. Rice-straw As content increased with increasing soil-As concentration; however, the toxicity of As to rice resulted in reduced grain-As concentrations in one of the 2 years. The likelihood of As-induced yield reductions and As accumulation in straw and grain has implications to agricultural sustainability, food quality and food security in As-affected regions throughout South and Southeast Asia.     

Mechanism associated with nonstructural carbohydrate accumulation in submergence tolerant rice (Oryza sativa L.) cultivars

Nonstructural carbohydrate (NSC) accumulation in submergence tolerant rice cultivars (cv) was studied in six Indica rice [Oryza sativa (L.)] cv under control and simulated submerged conditions. Tolerant cultivars accumulated greater contents of NSC compared to the susceptible cultivars. Starch and total NSC content showed significant positive association with survival percentage. On the other hand, elongation due to submergence was significantly a negative association with survival. The CO₂ photosynthetic rate, chlorophyll content, maximum photochemical efficiency of PS II (Fv/Fm), and activities of Rubisco were not significantly different between tolerant and susceptible cv under control condition. The ADP glucose pyrophosphorylase (AGPPase) activity was significantly higher in the tolerant cv and was a positive association with starch/NSC, whereas Fructose 1,6-diphosphatase (FDPase) activity was significantly higher in susceptible cv compared to tolerant cv and was a negative association with starch/NSC. Greater activities of AGPPase along with lower activities of FDPase might facilitate greater accumulation of NSC in tolerant rice cultivars.     

Assimilation of ammonium ions and reutilization of nitrogen in rice (Oryza sativa L.)

A major source of inorganic nitrogen for rice plants grown in paddy soil is ammonium ions. The ammonium ions are actively taken up by the roots via ammonium transporters and subsequently assimilated into the amide residue of glutamine (Gln) by the reaction of glutamine synthetase (GS) in the roots. The Gln is converted into glutamate (Glu), which is a central amino acid for the synthesis of a number of amino acids, by the reaction of glutamate synthase (GOGAT). Although a small gene family for both GS and GOGAT is present in rice, ammonium-dependent and cell type-specific expression suggest that cytosolic GS1;2 and plastidic NADH-GOGAT1 are responsible for the primary assimilation of ammonium ions in the roots. In the plant top, approximately 80% of the total nitrogen in the panicle is remobilized through the phloem from senescing organs. Since the major form of nitrogen in the phloem sap is Gln, GS in the senescing organs and GOGAT in developing organs are important for nitrogen remobilization and reutilization, respectively. Recent work with a knock-out mutant of rice clearly showed that GS1;1 is responsible for this process. Overexpression studies together with age- and cell type-specific expression strongly suggest that NADH-GOGAT1 is important for the reutilization of transported Gln in developing organs. The overall process of nitrogen utilization within the plant is discussed.     

Epigenetic responses to drought stress in rice (Oryza sativa L.)

Cytosine methylation polymorphism plays a key role in gene regulation, mainly in expression of genes in crop plants. The differential expression of cytosine methylation over drought stress response was analyzed in rice using drought susceptible but agronomically superior lines IR 20 and CO 43, and drought tolerant genotypes PL and PMK 3 and their F₁ hybrids. The parents and hybrids were subjected to two moisture regimes viz., one under drought condition and another under control condition. The cytosine methylation polymorphism in genomic DNA was quantified under both the conditions at the reproductive stage of the plant using the Methylation Sensitive Amplified Polymorphism (MSAP) technique devised by Xiong et al. (261:439–446, 1999). The results depicted that under drought condition, hyper-methylation was predominant in the drought susceptible genotypes while drought tolerant genotypes presented hypo-methylation behavior. While imposing drought, spikelet sterility per cent was positively correlated to percentage of methylation whereas, panicle length, number of seed per panicle, panicle weight, 100 seed weight, and yield/plant were negatively correlated indicating the role of epigenetic regulation in yield attributing traits in response to drought. Thus, methylation can be considered as an important epigenetic regulatory mechanism in rice plants to adapt drought situation. From this study, we speculate that the hyper- methylation may be an indicator of drought susceptibility and the hypo-methylation for drought tolerance and this methylation polymorphism can be effectively used in drought screening program.     

开放式空气CO2浓度增高对水稻N素吸收利用的影响

在大田栽培条件下 ,研究空气中CO2 浓度增高 (FACE) 2 0 0 μmol·mol-1对水稻N素吸收及其利用效率的影响 .结果表明 ,FACE处理使水稻不同生育时期的植株含N率显著下降 ;由于干物质生产量显著增大 ,FACE处理使水稻不同生育时期的N素累积量有所提高 ,但无显著影响 ;FACE处理能够显著提高移栽后 2 8d、抽穗期以及成熟期单位N素的干物质生产效率、单位N素的籽粒生产效率和显著提高水稻的N素收获指数 .高N处理的植株含N率、N素累积量均有所增加 ,但使N素生产效率呈现下降趋势 .     

Growth,CO2 exchange rate and dry matter partitioning in mungbean (Vigna radiata L.) grown under elevated CO2

This study was conducted to determine the effects of anticipated future level of CO2 on growth and dry matter partitioning of mungbean (Vigna radiata). Plants were grown from seedlings to maturity inside the open top chamber under amhient CO2 (350 +/- 25 microL L(-1)) and elevated CO2 (600 +/- 50 microL L(-1)) at Indian Agricultural Research Institute, New Delhi (India). Plants were harvested at 20, 35 and 50 days after germination. Mungbean plants grown under elevated CO2 concentration resulted in greater photosynthetic rate on a leaflet area basis and no acclimation in photosynthesis was recorded due to high CO2. Plants grown under CO, enrichmcnt were taller and attained greater leaf area along with more dry matter than ambient CO2 grown plants at all growth stages. Response to high CO, depends upon the growth stage of the plant and it was more at early growth stages compared to maturity stages. The high CO2 grown mungbean plants also exhibited increased root growth along with stem and leaves. There was a substantial increase in pod number and seed number/plant under elevated CO2 conditions. The increase in dry matter and growth of root, stem and leaves proved that CO2 enrichment of the atmosphere can stimulate photosynthetic rate which can ultimately lead to an increase in dry matter and growth.     

Seasonal changes in canopy photosynthesis and respiration, and partitioning of photosynthate, in rice (Oryza sativa L.) grown under free-air CO2 enrichment

An increase in atmospheric CO(2) concentration ( [CO(2)]) is generally expected to enhance photosynthesis and biomass. Rice plants (Oryza sativa L.) were grown in ambient CO(2) (AMB) or free-air CO(2)-enrichment (FACE), in which the target [CO(2)] was 200 micromol mol(-1) above AMB. (13)CO(2) was fed to the plants at different stages so we could examine the partitioning of photosynthates. Furthermore, canopy photosynthesis and respiration were measured at those stages. The ratio of (13)C content in the whole plant to the amount of fixed (13)C under FACE was similar to that under AMB at the vegetative stage. However, the ratio under FACE was greater than the ratio under AMB at the grain-filling stage. At the vegetative stage, plants grown under FACE had a larger biomass than those grown under AMB owing to enhancement of canopy photosynthesis by the increased [CO(2)]. On the other hand, at the grain-filling stage, CO(2) enrichment promoted the partitioning of photosynthate to ears, and plants grown under FACE had a greater weight of ears. However, enhancement of ear weight by CO(2) enrichment was not as great as that of biomass at the vegetative stage. Plants grown under FACE did not necessarily show higher canopy photosynthetic rates at the grain-filling stage. Therefore, we concluded that the ear weight did not increase as much as biomass at the vegetative stage owing to a loss of the advantage in CO(2) gain during the grain-filling period.     

Low irradiance stress tolerance in rice (Oryza sativa L.)

The effect of low irradiance on three rice cultivars (shade tolerant cvs. Swarnaprabha and CO 43 and shade susceptible cv. IR 20) was studied. The large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase with molecular mass of 55 kDa was reduced in cv. IR 20 grown under low irradiance (LI). Native protein profile studied showed, under LI, reduction in the contents of proteins with RF values 0.03, 0.11 and 0.37. Analysis of chloroplast polypeptides revealed an induction of light-harvesting chlorphyll-protein 2 (LHCP2) under shade. The induction was more expressed in cv. CO 43 than in cv. IR 20. Under LI, in vivo labelled protein bands in the molecular range of 26 - 27 kDa were induced. These proteins were highly turned over in the LI-grown plants of cv. CO 43 than in cv. IR 20. A signal for rbcL gene sequences in EcoRI digested lanes was also found. Isozyme analysis of peroxidase showed an induction of a new band with RF 0.43 in cv. IR 20 subjected to LI. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetics of gel consistency in rice (Oryza sativa L.)

Inheritance of gel consistency in rice was studied in crossés involving highamylose, low-gelatinizalion temperature parents with hard, medium, and soft gel consistency. The results of single-grain analysis of parents, F₁, F₂, B₁F₁, B₂F₂, and their reciprocal crosses from a single-season harvest showed that the differences between hard and soft, hard and medium, and medium and soft gel consistency are under monogenic control and that modifiers affect the expression of the trait. Multiple alleles at the same locus, hereby designated asgec ^a for medium gel consistency andgec ^b for soft gel consistency, were recessive to the wild type allele for hard gel consistency andgec ^a was dominant overgec ^b. The results indicate that selection for desired gel consistency can effectively be done in early segregating generations.     

The roles of jasmonate signalling in nitrogen uptake and allocation in rice (Oryza sativa L.)

Herbivore damage by chewing insects activates jasmonate (JA) signalling that can elicit systemic defense responses in rice. Few details are known, however, concerning the mechanism, whereby JA signalling modulates nutrient status in rice in response to herbivory. (¹⁵NH₄)₂SO₄ labelling experiments, proteomic surveys, and RT‐qPCR analyses were used to identify the roles of JA signalling in nitrogen (N) uptake and allocation in rice plants. Exogenous applications of methyl jasmonate (MeJA) to rice seedlings led to significantly reduced N uptake in roots and reduced translocation of recently‐absorbed ¹⁵N from roots to leaves, likely occurring as a result of down‐regulation of glutamine synthetase cytosolic isozyme 1–2 and ferredoxin–nitrite reductase. Shoot MeJA treatment resulted in a remobilization of endogenous unlabelled ¹⁴N from leaves to roots, and root MeJA treatment also increased ¹⁴N accumulation in roots but did not affect ¹⁴N accumulation in leaves of rice. Additionally, proteomic and RT‐qPCR experiments showed that JA‐mediated plastid disassembly and dehydrogenases GDH2 up‐regulation contribute to N release in leaves to support production of defensive proteins/compounds under N‐limited condition. Collectively, our results indicate that JA signalling mediates large‐scale systemic changes in N uptake and allocation in rice plants.