Shaobing Peng

Opportunities for increased nitrogen-use efficiency from improved resource management in irrigated rice systems

Abstract Research and extension work to improve nitrogen (N) management of irrigated rice has received considerable investment because yield levels presently achieved by Asian farmers depend on large amounts of N fertilizer. Most work has focused on placement, form, and timing of applied N to reduce losses from volatilization and denitrification. In contrast, less emphasis has been given to development of methods to adjust N rates in relation to the amount of N supplied by indigenous soil resources. As a result, N fertilizer recommendations are typically made for districts or regions with the implicit assumption that soil N supply is relatively uniform within these domains. Recent studies, however, document tremendous variation in soil N supply among lowland rice fields with similar soil types or in the same field over time. Despite these differences, rice farmers do not adjust applied N rates to account for the wide range in soil N supply, and the resulting imbalance contributes to low N-use efficiency. A model for calculating N-use efficiency is proposed that explicitly accounts for contributions from both indigenous and applied N to plant uptake and yield. We argue that increased N-use efficiency will depend on field-specific N management tactics that are responsive to soil N supply and plant N status. N fertilizer losses are thus considered a symptom of incongruence between N supply and crop demand rather than a driving force of N efficiency. Recent knowledge of process controls on N cycling, microbial populations, and soil organic matter (SOM) formation and decomposition in flooded soils are discussed in relation to N-use efficiency. We conclude that the intrinsic capacity of wetland rice systems to conserve N and the rapid N uptake potential of the rice plant provide opportunities for significant increases in N efficiency by improved management and monitoring of indigenous N resources, straw residues, plant N status, and N fertilizer.

Increased N-use efficiency using a chlorophyll meter on high-yielding irrigated rice

The chlorophyll meter (SPAD) can be used to monitor leaf N status in rice (Oryza sativa L.). The objective of this study was to compare N-use efficiency of SPAD-based N fertilizer management with recommended fixed-schedule N fertilizer splits applied at key growth stages. Four field experiments with ‘IR72’ were conducted, three at the International Rice Research Institute (IRRI) and one at the Philippine Rice Research Institute (PhilRice). In the SPAD-based N-treatment, N was topdressed when the SPAD value of the topmost fully expanded leaf fell below 35 (corresponding to about 1.4 g N m−2 leaf) from 15 days after transplanting to booting stage. The amount of N applied in each topdressing was determined by expected crop N demand predicted by equations relating N uptake to cumulative thermal units. Yields with SPAD-based management were 93–100% of maximum yields achieved by the best fixed-timing treatment but lower total N rates were used in all SPAD-based N treatments. Increased recovery efficiency from applied N and greater utilization of the acquired N to produce grain contributed to the significantly greater fertilizer-N efficiency of the SPAD-based than of the fixed-timing N treatments. The improved congruence of N supply and crop demand in SPAD-based treatments resulted in fewer unproductive tillers, less leaf senescence at flowering stage and comparable or greater crop growth rates after flowering than the best fixed-timing treatment. It is concluded that increased N fertilizer efficiency at high yield levels is possible using a chlorophyll meter to monitor leaf N status and guide fertilizer-N timing on irrigated rice.

Current Status and Challenges of Rice Production in China

Abstract Rice production in China has more than tripled in the past five decades mainly due to increased grain yield rather than increased planting area. This increase has come from the development of high-yielding varieties and improved crop management practices such as nitrogen fertilization and irrigation. However, yield stagnation of rice has been observed in the past ten years in China. As its population rises, China will need to produce about 20% more rice by 2030 in order to meet its domestic needs if rice consumption per capita stays at the current level. This is not an easy task because several trends and problems in the Chinese rice production system constrain the sustainable increase in total rice production. Key trends include a decline in arable land, increasing water scarcity, global climate change, labor shortages, and increasing consumer demand for high-quality rice (which often comes from low-yielding varieties). The major problems confronting rice production in China are narrow genetic background, overuse of fertilizers and pesticides, breakdown of irrigation infrastructure, oversimplified crop management, and a weak extension system. Despite these challenges, good research strategies can drive increased rice production in China. These include the development of new rice varieties with high yield potential, improvement of resistances to major diseases and insects, and to major abiotic stresses such as drought and heat, and the establishment of integrated crop management. We believe that a sustainable increase in rice production is achievable in China with the development of new technology through rice research.

COMPARISON OF HIGH-YIELD RICE IN TROPICAL AND SUBTROPICAL ENVIRONMENTS. I.DETERMINANTS OF GRAIN AND DRY MATTER YIELDS

Abstract Yields over 13 t ha−1 have been reported for irrigated rice in subtropical environments while maximum yield of only 10 t ha−1 has been achieved in the tropical lowlands. While it is generally accepted that the longer growth duration in subtropical environments mainly contributes to the greater yield potential, comparisons of adapted cultivars in tropical and subtropical climates are lacking and other factors that might be responsible for differences in yield potential have not been identified. Field experiments were conducted in a tropical environment at the International Rice Research Institute (IRRI) in the Philippines and in a subtropical environment at Taoyuan Township, Yunnan, China in 1995 and 1996. Three to five high-yielding rice cultivars were grown in each experiment under optimum crop management to achieve maximum attainable yields. Yield, yield components, plant dry matter and harvest index (HI) were determined at maturity. Growth analyses were conducted at key growth stages to determine crop growth rate (CGR), leaf area index (LAI), and leaf area duration (LAD). Daily radiation and air temperature were monitored. The highest yield of 15.2 t ha−1 was produced at Yunnan by Shanyou 63, a Chinese indica F1 hybrid, whereas maximum yield at IRRI was 9.3 t ha−1. On the average across cultivars, Yunnan produced 33 and 62% greater yields than IRRI in 1995 and 1996, respectively. Sink size (spikelets per m2) was responsible for these yield differences. Larger panicles (spikelets per panicle) contributed mostly to the greater sink size at Yunnan. Biomass production was 42% and 58% greater at Yunnan than at IRRI in 1995 and 1996, respectively, while differences in HI were relatively small. Dry matter accumulation and CGR were significantly greater at Yunnan than at IRRI during vegetative and grain-filling stages. These differences were relatively small and inconsistent across cultivars and years during the reproductive phase. Yunnan had greater LAI and LAD than at IRRI, which might be responsible for greater CGR. These results indicate that further improvement in rice yield potential in the tropics will depend mainly on the ability to increase sink size and biomass production.

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.

Opportunities for increased nitrogen-use efficiency from improved lowland rice germplasm

Understanding of the mechanisms governing the efficient use of N by rice plants—both its acquisition and internal use—is reviewed. Acquisition efficiency is considered in terms of root properties influencing the absorption and assimilation of NH+4 and other N species, and their regulation; root-induced changes in the rhizosphere affecting N mineralization, transformation and transport; and root-associated biological N2 fixation. Efficiency of internal use is considered in terms of the translocation, distribution and remobilization of absorbed N in different plant organs, flag leaf N import/export and leaf senescence patterns, and the efficiency with which N is used in CO2 fixation. Evidence for genetic variation in both acquisition efficiency and internal-use efficiency is given for plants under N-sufficient and N-limited conditions. The possibility of incorporating in rice the machinery for N2 fixation is discussed.

Grain filling pattern and cytokinin content in the grains and roots of rice plants.

Grain filling patterns and their relationships withzeatin (Z), zeatin riboside (ZR), indole-3-acetic acid(IAA) and gibberellin (GA) contents in the grains androots during grain development were examined in sixrice (Oryza sativa L.) genotypes grown in thefield and in water culture. Three grain fillingpatterns based on the filling rate of superior andinferior spikelets were observed, i.e., fastsynchronous: all spikelets started filling early andfast at the early filling stage; slowsynchronous: all spikelets filled slowly at the earlyfilling stage and reached the maximum filling ratelate; and asynchronous: superior spikeletsstarted filling and reached the maximum filling ratemuch earlier than the inferior ones. The order ofgrain filling percentage in the three types of grainfilling patterns was: fast synchronous >asynchronous > slow synchronous. Changes in Z + ZRcontents in the superior and inferior spikelets wereassociated with the grain filling patterns. Grainfilling percentage was significantly correlated withZ + ZR contents in the grains and roots at the earlyand middle grain filling stages. IAA and GA(GA1 + GA3 + GA4)contents in the grains and roots were notsignificantly correlated with grain fillingpercentage. The results suggest that cytokinins in thegrains and roots during the early phase of graindevelopment play an important role in regulating grainfilling pattern and consequently influence grainfilling percentage.

Molecular dissection of seedling-vigor and associated physiological traits in rice.

Abstract.seedling-vigor is important for crop establishment. There have been reported quantitative trait locus (QTL) analyses on seedling-vigor related morphological traits. However, physiological understanding of these detected QTLs is rather limited. In this study, we employed a recombinant inbred population to detect QTLs for seedling-vigor traits and physiological traits related to seedling-vigor. Germination rate and seedling growth were measured to quantify seedling-vigor. Total amylase activity, α-amylase activity, reducing sugar content, root activity and seed weight were determined. Correlations were observed between the seedling-vigor and physiological traits. QTL analysis reveals that the intervals of RG393-C1087-RZ403 on chromosome 3, C246-RM26-C1447 and R830-R3166-RG360-C734b on chromosome 5, and the interval of Waxy on chromosome 6 are the four main chromosomal regions controlling seedling-vigor. Several QTLs for amylase activities, reducing sugar content and root activity were localized in the similar regions as the QTLs for seedling-vigor. The results suggest that these traits were under the control of pleiotropic and/or closely linked QTLs. The implications of the results in the understanding of the physiological basis of seedling-vigor were discussed.

Comparison of high-yield rice in tropical and subtropical environments: II. Nitrogen accumulation and utilization efficiency

Nitrogen requirements to achieve rice grain yields higher than 13 t ha−1 and the associated internal N-utilization efficiency (NUE) have not been documented. The objective of this study was to compare N accumulation and NUE of irrigated rice in tropical and subtropical environments at yield-potential levels in both climates. Field experiments were conducted in 1995 and 1996 at the International Rice Research Institute, Philippines (IRRI, tropical site), and at Taoyuan Township, Yunnan, China (subtropical site). Three to five high-yielding rice cultivars were grown under optimum crop management. Plants were sampled at key growth stages to determine tissue N concentration, plant N accumulation, N harvest index (NHI), N translocation ratio and NUE. Plant N accumulation at maturity was 19 to 30% greater at Yunnan than at IRRI. Most of this difference resulted from greater N accumulation and N uptake rate during the vegetative period at Yunnan than at IRRI. During reproductive and grain-filling periods, N accumulation and N uptake rate were similar or higher at IRRI than at Yunnan. Grain N concentration at maturity was lower and N translocation ratio from straw to grains during grain filling was higher at Yunnan than at IRRI, and these traits contributed to larger NHI and NUE at Yunnan than at IRRI. Cultivars that produced grain yields over 13 t ha−1 at Yunnan required the accumulation of about 250 kg N ha−1 within the crop and had a NUE of 59 to 64 kg grain per kg plant N.

Nitrogen and yield potential of irrigated rice

Yield potential of modern rice varieties and implications for N management were evaluated in a series of field studies that provided data for validation of an eco-physiological simulation model for rice. We tested the hypothesis that N was the major factor limiting yield potential of irrigated rice. The simulation model ORYZA1 was used to evaluate the observed yield differences between varieties grown with different N management and in different environments. The model explained differences in yield of the treatments resonably well on the basis of differences in radiation, temperature, leaf N content and variety coefficients for phenological development. It was demonstrated by the model and experimental data that yield levels of 6 t ha-1 in the wet season and 10 t ha-1 in the dry season can be obtained in the tropics with the current short duration varieties only when the N supply from soil and fertilizer is adequately maintained at key growth stages. Yield probabilities for rice crops were simulated for different environments using long term weather data at two Philippine sites.