Kehui Cui

Improving nitrogen fertilization in rice by site-specific N management. A review

Excessive nitrogen (N) application to rice (Oryza sativa L.) crop in China causes environmental pollution, increases the cost of rice farming, reduces grain yield and contributes to global warming. Scientists from the International Rice Research Institute have collaborated with partners in China to improve rice N fertilization through site-specific N management (SSNM) in China since 1997. Field experiments and demonstration trials were conducted initially in Zhejiang province and gradually expanded to Guangdong, Hunan, Jiangsu, Hubei and Heilongjiang provinces. On average, SSNM reduced N fertilizer by 32% and increased grain yield by 5% compared with farmers’ N practices. The yield increase was associated with the reduction in insect and disease damage and improved lodging resistance of rice crop under the optimal N inputs. The main reason for poor fertilizer N use efficiency of rice crop in China is that most rice farmers apply too much N fertilizer, especially at the early vegetative stage. We observed about 50% higher indigenous N supply capacity in irrigated rice fields in China than in other major rice-growing countries. Furthermore, yield response of rice crop to N fertilizer application is low in China, around 1.5 t ha− on average. However, these factors were not considered by rice researchers and extension technicians in determining the N fertilizer rate for recommendation to rice farmers in China. After a decade of research on SSNM in China and other Asian rice-growing countries, we believe SSNM is a matured technology for improving both fertilizer N use efficiency and grain yield of rice crop. Our challenges are to further simplify the procedure of SSNM and to convince policy-makers of the effectiveness of this technology in order to facilitate a wider adoption of SSNM among rice farmers in China.

Impact of high-temperature stress on rice plant and its traits related to tolerance

The predicted 2–4°C increment in temperature by the end of the 21st Century poses a threat to rice production. The impact of high temperatures at night is more devastating than day-time or mean daily temperatures. Booting and flowering are the stages most sensitive to high temperature, which may sometimes lead to complete sterility. Humidity also plays a vital role in increasing the spikelet sterility at increased temperature. Significant variation exists among rice germplasms in response to temperature stress. Flowering at cooler times of day, more pollen viability, larger anthers, longer basal dehiscence and presence of long basal pores are some of the phenotypic markers for high-temperature tolerance. Protection of structural proteins, enzymes and membranes and expression of heat shock proteins (HSPs) are some of the biochemical processes that can impart thermo-tolerance. All these traits should be actively exploited in future breeding programmes for developing heat-resistant cultivars. Replacement of heat-sensitive cultivars with heat-tolerant ones, adjustment of sowing time, choice of varieties with a growth duration allowing avoidance of peak stress periods, and exogenous application of plant hormones are some of the adaptive measures that will help in the mitigation of forecast yield reduction due to global warming.

Rice management interventions to mitigate greenhouse gas emissions: a review

Global warming is one of the gravest threats to crop production and environmental sustainability. Rice, the staple food of more than half of the world’s population, is the most prominent cause of greenhouse gas (GHG) emissions in agriculture and gives way to global warming. The increasing demand for rice in the future has deployed tremendous concerns to reduce GHG emissions for minimizing the negative environmental impacts of rice cultivation. In this review, we presented a contemporary synthesis of existing data on how crop management practices influence emissions of GHGs in rice fields. We realized that modifications in traditional crop management regimes possess a huge potential to overcome GHG emissions. We examined and evaluated the different possible options and found that modifying tillage permutations and irrigation patterns, managing organic and fertilizer inputs, selecting suitable cultivar, and cropping regime can mitigate GHG emissions. Previously, many authors have discussed the feasibility principle and the influence of these practices on a single gas or, in particular, in the whole agricultural sector. Nonetheless, changes in management practices may influence more than one gas at the same time by different mechanisms or sometimes their effects may be antagonistic. Therefore, in the present attempt, we estimated the overall global warming potential of each approach to consider the magnitude of its effects on all gases and provided a comprehensive assessment of suitable crop management practices for reducing GHG emissions in rice culture.

Crop Plant Hormones and Environmental Stress

Plant hormones play vital roles in the ability of plants to acclimatize to varying environments by mediating growth, development and nutrient allocation. Hormones move through specific pathways to regulatory sites where they respond to stress at awfully low concentration. All biological activities are directly or indirectly affected by both phytohormones. Here we review the role of hormones against abiotic tolerance in crop plants. The main findings are: (1) abscisic acid act as a mediator in plant responses to many stresses, including salt stress. (2) Stress modifies the level of indole acetic acid (IAA) thus reducing growth. (3) Functional analysis of cytokinin receptor mutants show that cytokinin receptors of Arabidopsis act as negative regulators in abscisic acid (ABA) signaling and in osmotic stress response. (4) The mechanisms by which gibberellic acid (GA) priming could induce salt tolerance in plants are not yet clear. Salinity perturbs the hormonal balance in plants. Under salt stress hormonal homeostasis might be the possible mechanism of GA3-induced plant salt tolerance. (5) A low level of salicylic acid and jasmonate is effective against abiotic stress by enhancing physiological processes and improving tolerance. (6) Role of brassinosteroids and triazole during environmental stress is emerging. (7) Ethylene is considered as a stress hormone; however, the role of ethylene in salt stress is equivocal. The present review focus on abscisic acid, indole acetic acid, cytokinins, gibberellic acid, salicylic acid, brassinosteroids, jasmonates, ethylene and triazole.

Mapping QTLs for seedling characteristics under different water supply conditions in rice (Oryza sativa).

A greater understanding of the genetics of responses to water deficit/drought may be helpful in improving water-deficit resistance in the early stages of growth in rice. A recombinant inbred population derived from a cross between Zhenshan 97 and Minghui 63 was grown in hydroponic culture to characterize the responses of seedlings to water deficit imposed by PEG 6000 and to identify quantitative trait loci (QTLs) for seedling characteristics under both well-watered and water-deficit conditions. Generally, the two parents showed significant differences in plant height (PH), maximum root length (MRL), shoot fresh weight (SFW), root fresh weight (RFW), number of roots and root:shoot ratio based on fresh weight under both well-watered and water-deficit conditions. For two parents, PH and SFW were significantly inhibited under water deficit. However, the other four traits had larger values under water deficit. Among 26 and 34 QTLs detected for the six traits studied under the well-watered and water-deficit conditions, respectively, a total of 14 QTLs for SFW, PH, RFW and MRL (23% of total QTLs detected) were detected in similar or tight linkage regions in both conditions. Among 11 intervals on 7 chromosomes identified to harbor multiple QTLs, 8 intervals were found to affect related traits under the two water supply conditions and 3 intervals were observed to be water supply-specific regions and had effects only under well-watered conditions, suggesting that water supply-specific regions or QTLs may be closely associated with the responses of lines to water deficit in the study. Several regions for the traits studied were also found to affect the root-related traits in previous studies and might be used in marker-assisted selection for drought-resistant rice in breeding programs.

Drought stress condition increases root to shoot ratio via alteration of carbohydrate partitioning and enzymatic activity in rice seedlings

To understand the underlying mechanism for plasticity in root to shoot ratio (R/S) in response to drought stress, two rice cultivars, Zhenshan97 (drought susceptible) and IRAT109 (drought resistant), were grown hydroponically, and R/S, carbohydrate concentration and partitioning, and activities of enzymes for sucrose conversion in seedlings exposed to drought stress condition (DS) imposed by polyethylene glycol 6000 were investigated. The R/S significantly increased under DS in comparison with that under well-watered condition. The proportion of dry matter and soluble sugar of roots markedly increased under DS. The R/S was negatively correlated with proportion of soluble sugar in stems, and positively with the proportions of soluble sugar and starch in roots. Drought stress condition significantly increased leaf sucrose-phosphate synthase (EC 2.4.1.14) activity and root acid and neutral/alkaline invertase (EC 3.2.1.26) activity. The R/S was positively correlated with leaf sucrose-phosphate synthase and root acid invertase activity, and negatively with leaf sucrose synthase activity in the cleavage direction. Our results indicate that the increase in R/S in response to DS is closely associated with the higher proportion of dry matter and soluble sugar in roots, and this occurs via an increase in leaf sucrose-phosphate synthase and root invertase activity, and thus more sucrose is available for transport from leaves to roots.

Improving Nitrogen Fertilization in Rice by Site-Specific N Management

Excessive nitrogen (N) application to rice (Oryza sativa L.) crop in China causes environmental pollution, increases the cost of rice farming, reduces grain yield and contributes to global warming. Scientists from the International Rice Research Institute have collaborated with partners in China to improve rice N fertilization through site-specific N management (SSNM) in China since 1997. Field experiments and demonstration trials were conducted initially in Zhejiang province and gradually expanded to Guangdong, Hunan, Jiangsu, Hubei and Heilongjiang provinces. On average, SSNM reduced N fertilizer by 32% and increased grain yield by 5% compared with farmers’ N practices. The yield increase was associated with the reduction in insect and disease damage and improved lodging resistance of rice crop under the optimal N inputs. The main reason for poor fertilizer N use efficiency of rice crop in China is that most rice farmers apply too much N fertilizer, especially at the early vegetative stage. We observed about 50% higher indigenous N supply capacity in irrigated rice fields in China than in other major rice-growing countries. Furthermore, yield response of rice crop to N fertilizer application is low in China, around 1.5 t ha − 1 on average. However, these factors were not considered by rice researchers and extension technicians in determining the N fertilizer rate for recommendation to rice farmers in China. After a decade of research on SSNM in China and other Asian rice-growing countries, we believe SSNM is a matured technology for improving both fertilizer N use efficiency and grain yield of rice crop. Our challenges are to further simplify the procedure of SSNM and to convince policy-makers of the effectiveness of this technology in order to facilitate a wider adoption of SSNM among rice farmers in China.

Relationships of non-structural carbohydrates accumulation and translocation with yield formation in rice recombinant inbred lines under two nitrogen levels.

Stem non-structural carbohydrates (NSCs) and its relationship with yield formation was investigated under low nitrogen (LN) and normal nitrogen (NN) treatments, using 46 recombinant inbred lines from Zhenshan 97 × Minghui 63 (Oryza sativa). Apparent contribution of transferred NSC to grain yield (AC(NSC) ) ranged from approximately 1 to 28% under LN and from 1 to 15% under NN. Concentration and total mass of NSC in stem (TM(NSC) ) at heading, apparent transferred mass of NSC (ATM(NSC) ) and AC(NSC) were larger under LN compared with NN. However, there was no significant difference in the apparent ratio of transferred NSC from stems to grain (AR(NSC) ). ATM(NSC) was positively correlated with grain yield, 1000-grain weight and AC(NSC) under both nitrogen levels, whereas AR(NSC) was highly correlated with harvest index and AC(NSC) . Leaf area contributed more strongly to grain yield compared with ATM(NSC) under both LN and NN. ATM(NSC) showed larger direct effects on grain yield under LN compared with NN. TM(NSC) at heading, small vascular bundles (SVBs) and spikelets per m(2) under LN had positive direct effects on ATM(NSC) . SVB and spikelets per m(2) under LN had larger and positive direct effects, and large vascular bundles had negative direct effects on AR(NSC) . TM(NSC) at heading and SVB under LN had positive direct effects on AC(NSC) . In brief, LN supply increased stem NSC accumulation and translocation to developing grain. Components of the source-sink-flow system showed different effects on NSC translocation and contribution to yield formation, depending on genotype and nitrogen level.

Benefits of rice seed priming are offset permanently by prolonged storage and the storage conditions

Seed priming is a commercially successful practice, but reduced longevity of primed seeds during storage may limit its application. We established a series of experiments on rice to test: (1) whether prolonged storage of primed and non-primed rice seeds for 210 days at 25°C or −4°C would alter their viability, (2) how long primed rice seed would potentially remain viable at 25°C storage, and (3) whether or not post-storage treatments (re-priming or heating) would reinstate the viability of stored primed seeds. Two different rice cultivars and three priming agents were used in all experiments. Prolonged storage of primed seeds at 25°C significantly reduced the germination (>90%) and growth attributes (>80%) of rice compared with un-stored primed seeds. However, such negative effects were not observed in primed seeds stored at −4°C. Beneficial effects of seed priming were maintained only for 15 days of storage at 25°C, beyond which the performance of primed seeds was worse even than non-primed seeds. The deteriorative effects of 25°C storage were related with hampered starch metabolism in primed rice seeds. None of the post-storage treatments could reinstate the lost viability of primed seeds suggesting that seeds become unviable by prolonged post-priming storage at 25°C.

Alleviating soil sickness caused by aerobic monocropping: Responses of aerobic rice to various nitrogen sources

Abstract Yield decline resulting from continuous cropping of aerobic rice is a constraint to the widespread adoption of aerobic rice technology. Shifts in water management from flooded to aerobic conditions are known to influence the availability and form of N present in the soil and might require a different approach to N management in aerobic rice. The present study was conducted to determine the effects of different N sources on the plant growth and grain yield of aerobic rice. Four pot experiments were conducted in which rice was aerobically grown in soil that was taken from fields where aerobic rice has been cultivated for 11 consecutive seasons and an adjacent field where flooded rice has been grown continuously. Nitrogen was applied as ammonium sulfate, urea, ammonium chloride, ammonium nitrate and potassium nitrate at four N rates of 0.3, 0.6, 0.9 and 1.2 g N pot−1. Two unfertilized controls consisting of soil that was either untreated or oven heated at 120°C for 12 h were also included. Plants were sampled during the vegetative stage or at maturity to measure plant growth, N uptake, grain yield and the yield components. Growth of aerobic rice in aerobic soil was generally better with the application of ammonium-N than nitrate-N. Potassium nitrate decreased plant growth and caused plant death at the high N rate. Ammonium sulfate was more effective in improving the vegetative plant growth, N nutrition and grain yield of aerobic rice than urea at the high N rates. The application of ammonium sulfate achieved the same and even better plant growth than the soil oven-heating treatment. These results suggest that there is a possibility of reversing the yield decline observed in the continuous aerobic rice system by using the right source of N fertilizer at the optimal rate.