Vol. 24 No. 1

April 1999

International Rice Research Institute IRRI home page: http://www.cgiar.org/irri Riceweb: http://www.riceweb.org Riceworld:http://www.riceworld.org

International Rice Research NotesThe International Rice Research Notes (IRRN) expedites communication among scientists concerned with the development of improved technology for rice and rice-based systems. The IRRN is a mechanism to help scientists keep each other informed of current rice research findings. The concise scientific notes are meant to encourage rice scientists to communicate with one another to obtain details on the research reported. The IRRN is published three times a year in April, August, and December by the International Rice Research Institute.Editorial Board Michael Cohen (pest science and management), Editor-in-Chief Darshan Brar (plant breeding; molecular and cell biology) David Dawe (socioeconomics; agricultural engineering) Achim Dobermann (soil, nutrient, and water management; environment) Baorong Lu (genetic resources) Leonard Wade (crop management and physiology) Production Team Katherine Lopez, Managing Editor Editorial Bill Hardy and Tess Rola Design and layout The CPS Creative Services Team: Albert Borrero, Grant Leceta, Erlie Putungan, Juan Lazaro, Emmanuel Panisales Word processing Arleen Rivera

Contents4 EDITORS NOTEMINI REVIEWS

5

Nitrogen nutrition in hybrid riceX. Yang, J. Zhang, W. Ni, and A. Dobermann

9

A new rice cultivation technology: Plastic film mulching S. Peng,K. Shen, X. Wang, J. Liu, X. Luo, and L. Wu

About the cover A hybrid rice plot near Changsha, Hunan Province, China Cover picture: S.S. Virmani

RESEARCH NOTESPlant breeding 11 Genetics of tolerance for iron toxicity in riceJ. Owusu Nipah, O. Safo-Kantanka, M.P. Jones, and B.N. Singh

16 Evaluation of EMC models for fitting sorption isotherms for rough rice, brown rice, and milled riceL. Nayak and J.P. Pandey

12 Genetics of resistance to brown planthopper (Nilaparvata lugens Stl) in riceV.K. Verma, D.J. Pophaly, R.K. Mishra, and R.K. Sahu

17 Partitioning of high-density grains and its implication in a rice selection programR. Govindarasu, K. Manian, N. Ramamoorthi, and A. Mohamed Hanifa

13 CORH2, a new medium-duration rice hybridM. Rangaswamy, K. Thiyagarajan, P. Rangasamy, P. Jayamani, A.S. Ponnusamy, R. Latha, and P. Vaidyanathan

18 KDML 105, an alternative aromatic rice for the semideepwater ecosystemN.K. Sarma, L. Salkla, P. Salkla, R. Borgohain, H.N. Gogol, and W. Palaklang

13 Evaluation of rice hybrids in different agroclimatic zones of Andhra PradeshR. Vijaya Kumar, P.V. Satyanarayana, M. Subba Rao, and N.S. Reddy

19 Sudhir, a progeny of FR13AS. Mallik, B.K. Mandal, C. Kundu, S.D. Chatterjee, and S.N. Sen

15 Birsa Dhan 107, a high-yielding and early maturing variety for the upland regions of Bihar, IndiaM.P. Singh and D.N. Singh

21 Genetic variability in leaf area and chlorophyll content of aromatic riceU.K. Bansal, R.G. Saini, and A. Kaur

15 Jaldi Dhan 8, an improved and potential source of wide compatibility for hybrid rice breedingS. Kumar and S.N. Chakrabarti

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Pest science & management 22 Nematode resistance of IRRI breeding lines in Assam, IndiaN.K. Sarma, D. Das, S.A. Hussain, B. Barman, and D. Senadhira

26 Multilocational screening of Oryza sativa and O. glaberrima for resistance to African rice gall midge Orseolia oryzivora in West AfricaC.T. Williams, O. Okhidievbie, M.N. Ukwungwu, D. Dakouo, S. Nacro, A. Hamadoun, and S.I. Kamara

23 Identification of Xanthomonas oryzae pv. oryzae by insertion sequence-based polymerase chain reaction (IS-PCR)T. Adhikari, C.M. Vera Cruz, T.W. Mew, and J.E. Leach

27 Resistance to rice stripe virus in differential rice line BL 1K. Ise, K. Ishikawa, C. Li, Q. Yang, and C. Ye

25 Influence of weeds and rice cultivar on nematode population densities in lowland riceD.L. Coyne, D.E. Johnson, M. Jones, and R.A. Plowright

28 Ischaemum rugosuma potential alternate host of rice tungro viruses in West BengalS.C. Mallick, A.K. Chowdhury, and D. Pal

Soil, nutrient, & water management 30 Influence of NPK levels and split N application on grain filling and yield of fine riceM. Asif, F.M. Chaudhary, and M. Saeed

32 Response of rainfed lowland rice varieties to different N levels under two types of water managementC.R. Biswas, A.B. Mandal, and S.C. Pramanik

31 The adaptability of Azolla mexicana in the Ege region of TurkeyM.N. Gevrek

33 Influence of organic, biofertilizer, and inorganic forms of nitrogen on rice qualityM. Hemalatha, V. Thirumurugan, and R. Balasubramanian

Crop management & physiology 34 Effect of grain and forage legumes on the yield of rice- and potato-based cropping systems in NepalB.B. Khatri and G.J. Wells

36 Effective land preparation for wet seeding on a reclaimed saline soil in KoreaK.-S. Lee, J.-K. Nam, and H.-T. Shin

35 Performance of indica/japonica derivatives in wet and boro season in West Bengal, IndiaS.K. Bardhan Roy and D. Senadhira

Agricultural engineering 37 Seed drill for upland rice grown in undulating terrainC.R. Subudhi, P.C. Pradhan, and P.C. Senapati

38 A modified area sampler for aquatic invertebrate assemblages in flooded riceK.G. Schoenly and I. Domingo

37 Rolling marker for a rice fieldT.M. Lando, B. Abidin, and A. Najamuddin

Socioeconomics 40 Medicinal weeds in rice fields of Chhattisgarh, IndiaP. Oudhia

41 43

RESEARCH HIGHLIGHTS NOTES FROM THE FIELD

45 47

NEWS INSTRUCTIONS TO CONTRIBUTORS

IRRN 3

EDITORS NOTE

It is with great pleasure that we present to our readers this first issue of the new IRRN. In this note, we would like to highlight some of the changes in the content, format, and operation of IRRN and provide the rationale for these changes. One of the most significant changes has been the formation of an editorial board composed of IRRI scientists. In the past, editors in IRRIs Communication and Publications Services (CPS) have handled the content and format of IRRN. While these editors have always produced a very useful journal of high quality, we hope that actions of a scientific editorial board will result in a more up-to-date and balanced coverage of developments in rice science, and improved quality and consistency of the IRRN manuscript review process. Equally as important as the establishment of an editorial board has been the creation of the IRRN managing editor position. Our first managing editor is Katherine Lopez, and we would like to express our gratitude to her for the excellent job that she has done. Our thanks also go to other members of CPS who have contributed to the new IRRN, including Gene Hettel, Bill Hardy, and the CPS creative services team. Credit for proposing several of the changes that have taken place in IRRN is due to the IRRI ad hoc Committee on IRRN, whose members were Serge Savary, Swapan Datta, and Carolyn Dedolph. Directions for some of the other changes in format and content are based partly on results of a readership survey that CPS and the IRRN team conducted beginning in October 1998. We would like to thank IRRN readers and others for participating in the survey and for their constructive comments and suggestions on improving the substance and look of IRRN. One of the principal purposes of IRRN is to provide an international forum for rice scientists to publish their work.4 April 1999

The editors and reviewers of IRRN have always faced a difficult challenge in balancing the ability of scientists to publish with the standard of publishing only those papers that are of sufficient quality and interest to be of use to IRRN readers. It is the goal of the editorial board to maintain a high standard for research notes published in IRRN. We also intend to provide authors with better feedback on their manuscripts, and suggest ways in which these could be improved. Prospective authors should carefully read the revised Instructions to Contributors, which appear in every issue. Readers will note several new features in IRRN. The mini reviewsand research highlights are meant to provide readers with up-to-date information on important new scientific developments. Notes from the field is a section introduced in response to requests that IRRN provide a venue for noteworthy developments observed in rice-growing areas, such as pest outbreaks or the adoption of innovative crop management practices. We have also changed the format of the research notes, which can now include up to five references. The goal of several of these changes is to provide IRRN readers with more links to the scientific literature, which they can then consult for more detailed information. The IRRI library is a

valuable resource for rice scientists who wish to obtain copies of scientific papers, including those cited in IRRN. Please consult the announcement from the IRRI library that appears on p 14. The editorial board and CPS staff have also worked to revise and standardize distribution policies for IRRN. We wish to make IRRN available to as many rice researchers as possible, but we are constrained by our financial resources. IRRN is provided free of charge to university libraries and agricultural research institutions engaged in rice research in developing countries. Institutions and individuals from developed countries are charged for subscriptions to IRRN. An important new development in IRRN distribution is that the journal is now available on the World Wide Web. Hundreds of visits were made to the three 1998 issues of IRRN on the Web, and we hope to see the number of hits increase as more and more institutions gain access to the Internet. Finally, we wish to stress that IRRN will continue to evolve in response to the needs of its readers. We encourage you to continue to send your suggestions to the editorial board, via the managing editor. We thank our readers for their longstanding support and enthusiastic response to IRRN. We look forward to hearing from you! The IRRN editorial board

New IRRN features formation of an editorial board composed of IRRI scientists creation of a managing editor position addition of new sections: Mini reviews, Research highlights, Notes from the field improved manuscript review process addition of cited references (from 2 to 5) new categories for Research notes revised instructions to contributors modified distribution policies availability of IRRN on the World Wide Web

MINI REVIEWS

Characteristics of nitrogen nutrition in hybrid riceX.Yang, J. Zhang,W. Ni, Plant Nutrition Institute, College of Natural Resources and Environmental Science, Zhejiang University, Huajiachi Campus 310029, Hangzhou, China; and A. Dobermann, Soil and Water Sciences Division, IRRI

ice hybrids have a mean yield advantage of 10-15% over inbred varieties (Li 1981, Yang and Sun 1988). Growth and development processes associated with higher grain yields of rice hybrids include a more vigorous and extensive root system (Li 1981, Yang and Sun 1988), increased growth rate during vegetative growth (Yamauchi 1994), more efficient sink formation and greater sink size (Kabaki 1993), greater carbohydrate translocation from vegetative plant parts to the spikelets (Song et al 1990), and larger leaf area index (LAI) during the grain-filling period, but the physiological basis for heterosis remains unknown (Peng 1998). Specific characteristics of the uptake and physiology of N in hybrid rice appear to play a key role in this. In this paper, we reviewed the characteristics of N uptake and N use by hybrid rice, a proposed physiological basis for N efficiency, and the role of nitrate nutrition in hybrid rice. All cited papers are comparisons between hybrids and the best conventional cultivars, i.e., not comparisons between the hybrid and its parents.IRRN 5

R

Nitrogen uptake and use The total N uptake by hybrid rice shoots is greater than that of conventional cultivars, especially from transplanting to tillering and from panicle emergence to grain-filling stages (Yang 1987). Hybrid rice takes up about 15-20% of the total amount of N accumulated in the plant after heading and responds well to late application of N at flowering. In the same studies, N uptake after heading of conventional varieties was only 6-7% of total N uptake (Yang 1987). In field experiments, hybrid rice had a greater N efficiency (defined as grain yield per unit N fertilizer applied) than conventional rice (Lin and Yuan 1980, Yang 1987). This increased N efficiency was not due to greater internal N use in dry matter production (defined as unit dry matter produced per unit N accumulated in the plant) (Yang 1987, Yang and Sun 1992). We propose that higher recovery efficiency of applied N (N uptake per unit N applied) because of greater root N absorption potential, greater shoot N-use capacity (N demand by the shoot, i.e., how much and how fast the shoot can use N), and greater N remobilization efficiency (N translocation to the grain, i.e., N har-

vest index) are the major factors that cause higher N efficiency in hybrid rice (Fig. 1). Physiological basis for nitrogen efficiency in hybrid rice Root growth and distribution density in soils, aerobic respiration, oxidizing power, and energy synthetic metabolism are the important traits responsible for higher N absorption potential in hybrid rice (Yang and Sun 1991c). Rice hybrids develop an extensive root system, which is essential for efficient N absorption from the soil and topdressed N fertilizer applications. Under field conditions, hybrids have greater root fresh and dry weights, root volume, and root length density than inbred varieties (Yang and Sun 1988, 1992). The activities of dehydrogenase and cytochrome oxidase, oxidizing power, and ATP content of hybrid roots were much greater than those of conventional cultivars at both early and late growth stages (Yang and Sun 1988). These root morphological and physiological characteristics were positively and significantly correlated with N uptake by rice shoots (Yang 1987).

N-assimilating activity Carbon-assimilating activity Energy synthetic metabolism Tillering power Leaf area

Shoot utilization capacity

N harvest index

Translocation

Root growth and distribution density Root respiration and energy metabolism Km and Vmax N assimilation in root

Root absorption potential

Fig. 1. Physiological parameters associated with N efficiency in hybrid rice. We hypothesized that the higher N efficiency in hybrid rice results from higher root absorption potential, greater shoot use capacity, and more efficient translocation of N and their positive interactions.The major related physiological traits for efficient absorption, use, and translocation of N by rice plants are listed.

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April 1999

The activities of nitrate reductase (NR, a rate-regulating enzyme in converting NO3--N to NH3), glutamate synthase (FdGOGAT, a rate-regulating enzyme in processing NH 3 to glutamate), amino acid synthase (GPT and GOT), and CO2-assimilating enzymes (Rubisco, a rate-regulating enzyme in photosynthesis and a key enzyme in N metabolism) in leaves at different growth stages differed considerably between hybrid rice and conventional cultivars (Yang and Sun 1989). The activities of NR and Fd-GOGAT in hybrid leaves were 40-60% greater than in leaves of conventional rice at the heading stage (Yang and Sun 1989, Yang and Sun 1992) and close correlations were observed between leaf N concentration and activities of these enzymes (Yang 1987). Compared with conventional cultivars, the activity and protein content of Rubisco were 25-40% higher at low N levels and twofold higher at adequate N levels in hybrid rice leaves (Yang and Sun 1991b, 1992). Apparently, the stimulation of activity and level of Rubisco by N is greater in hybrid rice than in inbred cultivars. Higher activities of Rubisco and of the N synthetic metabolism in hybrid leaves result from higher protein levels of the enzymes. Thus, the enhanced N- and C-assimilating metabolism of hybrid rice, combined with its greater tillering capacity and leaf area (Yang 1987), are possibly the major physiological causes of greater N-use capacity in hybrid rice shoots. The interaction between N and C synthetic metabolism further increases shoot N-use capacity in hybrid rice and, in turn, raises root absorption potential. Hybrids tend to have a higher N harvest index than conventional cultivars. After flowering, more N is translocated into hybrid rice panicles than into those of conventional cultivars (Yang and Sun 1990). This higher N translocation efficiency could be one of the major factors responsible for increased N efficiency in hybrid rice.

Nitrate vs ammonium nutrition during reproductive growth Research conducted during the past 10 yr has provided evidence that hybrid rice roots are more efficient in absorbing NO3- than inbred varieties and that a preferential uptake of NO3- during reproductive growth may be one of the causes of the higher yields observed (Yang and Sun 1990, 1991a, 1992). Although NH4+ is the major available N form in flooded soils, nitrate exists in the oxidized surface soil layer, in the irrigation water, and, probably, in the oxidized rhizosphere surrounding rice roots. Rhizosphere oxidation and acidification in hybrid rice exceed those of conventional rice (Yang et al 1997), but there is no information available about the distribution of nitrate in the rhizosphere. A special feature of hybrid rice is the much greater development of fine, dense, superficial roots. These are mainly distributed in the oxidized surface soil layer (Yang and Sun 1988) and appear to (1) enhance surface soil and rhizosphere oxidation and (2) be capable of absorbing NO3- efficiently, particularly after panicle initiation. In nutrient solution experiments, hybrid roots had higher affinity for NO3-, especially at the reproductive growth stages, than conventional cultivars. The uptake of NO3--N by roots of hybrid rice increased linearly with increasing NH4NO3 concentrations in nutrient solution, but did not in the conventional cultivar (Luo et al 1993). At high supply levels (80-120 mg N L-1), the hybrid absorbed more NO3- than NH4+, whereas the conventional cultivar did not, implying that preference for NO3- is genetically controlled and dependent on N supply levels. Studies on NO3- uptake kinetics showed that Km values of NO3- uptake by hybrid roots were 27% lower for 22-d-old seedlings and 62% lower for 63-d-old seedlings than those by roots of conventional cultivars (Yang and Sun 1991c).

Early rice Nitrate mg kg-1 shoot DM 200 160 120 80 40 0 T1 T3 200 160 120 80 40 0

Late rice Nitrate mg kg-1 shoot DM NPK1 120 80 40 B H M Har 0 T1 T3 PI B H M Har NPK1

NPK2

120 80 40

Conventional NPK2 Hybrid

T1

T3

B

H

M

Har

0

T1

T3

PI

B

H

M

Har

Growth stageT1=early tillering, T3=late tillering, B=booting, PI= panicle initiation, H=heading, M=milky stage, Har=harvest

Fig. 2. Nitrate concentrations in the shoots of conventional and hybrid cultivars at different growth stages and different NPK levels. Field measurements conducted at Jinhua, China, 1998.

IRRN

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In pot experiments with topdressing of 15N-labeled NO3--N and NH4+-N fertilizers during reproductive growth, NO3--N application stimulated the growth of superficial roots and increased grain yield more significantly in hybrid rice than in conventional rice, mainly because of improved grain filling (Yang and Sun 1990). Nitrate fertilizer use efficiency by hybrid rice after anthesis was 7.8% higher than that of conventional rice. NO3--N stimulated the vigorous growth of superficial roots, increased the synthesis of cytokinins (mainly zeatin) in roots, and delayed the appearance of the abscisic acid (ABA) peak in both leaves and filling grains. High ratios of zeatin/ABA enhanced the synthesis of RNA, which resulted in protein synthesis for carbon assimilation and transportation (Yang and Sun 1991a). High nitrate reductase activities in both seedlings and functioning leaves of rice plants have been reported (Lin et al 1986, Yang and Sun 1989). Nitrate reductase activity at the seedling stage was negatively related to tolerance of a rice cultivar for high N supply, and was recommended as an indicator or parameter for screening high-N-tolerant cultivars (Lin et al 1986). Nitrate reductase activities were higher and more sensitive to N supply levels in hybrid rice leaves than in conventional rice (Yang and Sun 1989). Under field conditions, shoot NO3- concentrations of both hybrid and conventional rice varied with growth stages and NPK supply levels, but shoot NO3- concentrations in hybrid rice exceeded those of the inbred variety at most growth stages, especially with high amounts of N, P, and K applied (Fig. 2). Similarly, higher grain yields were obtained in hybrid rice than in the conventional rice cultivar, suggesting that NO3- accumulation and nutrition might be associated with high yield in rice. Research needs Our studies on the morphology and physiology of N nutrition in hybrid rice indicated that the greater N efficiency in hybrid rice mainly results from higher root absorption potential, greater shoot N-use capacity, and efficient N translocation, as well as their positive interactions (Fig. 1). More research is needed to understand (1) the influx and compartmentation of NO3- by rice roots, (2) strategies for hybrid rice to absorb more NO3- than an inbred variety, (3) the fate of NO3- from soil to leaves, and (4) the contribution of NO3- to N nutrition as well as to yield formation in rice. A quantitative linkage between form of N nutrition, cytokinin synthesis, delayed appearance of the ABA peak, and other physiological processes with grain yield remains to be established under field conditions to demonstrate the possible contribution of N nutrition to heterosis in rice. Only then will we be able to finetune water and N management practices in hybrid rice.

ReferencesKabaki N. 1993. Growth and yield of japonica-indica hybrid rice. Jpn. Agric. Res. Q. 27:88-94. Li ZB. 1981. Biological basis of heterosis utilization in rice plant. In: Research and practice of hybrid rice. Beijing: Agricultural Science and Technology Press. p 186-287. Lin SC, Yuan LP. 1980. Hybrid rice breeding in China. In: Innovative approaches to rice breeding. Manila (Philippines): International Rice Research Institute. p 35-37. Lin ZW, Ten ZF, Tan YW. 1986. Nitrate reductase as an indicator for rice cultivars to tolerate high N levels [in Chinese]. J. Crop Sci. 12:9-14. Luo AC, Xu JM, Yang X. 1993. Effect of nitrogen (NH4NO3) supply on absorption of ammonium and nitrate by conventional and hybrid rice during reproductive growth. Plant Soil 155/156:395-398. Peng S. 1998. Physiology-based crop management for yield maximization of hybrid rice. In: Virmani SS, Siddiq EA, Muralidharan K, editors. Advances in hybrid rice technology. Proceedings of the 3rd International Symposium on Hybrid Rice, 14-16 Nov 1996, Hyderabad, India. Manila (Philippines): International Rice Research Institute. p 157176. Song X, Agata W, Kawamitsu Y. 1990. Studies on dry matter and grain production of F1 hybrid rice in China. II. Characteristics of grain production. Jpn. J. Crop Sci. 59:29-33. Yamauchi M. 1994. Physiological bases of higher yield potential in F1 hybrids. In: Virmani SS, editor. Hybrid rice technology: new developments and future prospects. Manila (Philippines): International Rice Research Institute. p 71-80. Yang X. 1987. Physiological mechanisms of nitrogen efficiency in hybrid rice. PhD dissertation. Zhejiang Agricultural University, Hangzhou, China. Yang X, Roemheld V, Marschner H, Baligar VC, Martens DV. 1997. Shoot photosynthesis and root growth of hybrid rice and a conventional rice cultivar as affected by N and K levels in the rhizosphere. Pedosphere 7:35-42. Yang X , Sun X. 1988. Physiological characteristics of F1 hybrid rice roots. In: Hybrid rice. Manila (Philippines): International Rice Research Institute. p 159-164. Yang X, Sun X. 1989. Characteristics of hybrid rice N metabolism. Acta Agric. Univ. Zhejiangensis 15(1):87-96. Yang X, Sun X. 1990. Effects of NH4+-N and NO3--N topdressing on the nutrition of hybrid and conventional rice varieties at late growth stage. Acta Agric. Nucl. Sin. 4(2):75-79. Yang X, Sun X. 1991a. The physiological effect of nitrate or ammonium topdressing on hybrid and conventional rice cultivars at the late growth stage. Acta Agron. Sin. 17(4):283-291. Yang X, Sun X. 1991b. Heteroses in photosynthesis of F1 hybrid in relation to N nutrition. Acta Agric. Univ. Zhejiangensis 17:355-359. Yang X, Sun X. 1991c. Kinetics of NH4+ and NO3- uptake by different rice cultivars. Chin. J. Soil Sci. 22(5):222-224. Yang X, Sun X. 1992. Physiological mechanism of varietal difference in rice plant response to low N level. Acta Pedol. Sin. 29:73-79.

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April 1999

A new rice cultivation technology: plastic film mulchingS. Peng, IRRI; K. Shen, X.Wang, J. Liu, Agricultural Bureau of Shiyan, Hubei; X. Luo, Agricultural Bureau of Zhushan County, Hubei; and L. Wu, Zhejiang Agricultural University, Hangzhou, Zhejiang, China

Rice growing under plastic film mulching cultivation in Zhejiang, China, 1998

rowing lowland rice using plastic film mulch under nonflooded conditions was tested extensively in 1993 by the Agricultural Bureau of Shiyan, Hubei Province, China (Shen et al 1997b). This cultivation system has also been evaluated in Liaoning, Anhui, Zhejiang, and Jiangsu provinces. In the mountainous areas of Shiyan, rice yield is greatly limited by low temperature. Film mulching cultivation is one crop management option that effectively alleviates low-temperature stress on rice growth and consequently increases rice production. In some rice-growing areas of Anhui Province, water scarcity is the major constraint to rice production. Film mulching cultivation reduces irrigation water consumption and therefore increases rice production by extending rice-planting areas. In this mini review, we compared the major characteristics of film mulching cultivation with the continuously flooded rice system.

G

Plastic film mulching cultivation In the plastic film mulching cultivation test in Shiyan, Hubei, China (Shen et al 1997a), land was first flooded and then plowed and puddled. During land preparation, N, P, and K fertilizers and farmyard manure were applied and incorporated in the soil. The field was drained and 1.6-m-wide beds were prepared before plastic film was installed. Plastic film, 0.005-mm-thick and 1.7-m-wide, was used to cover the beds. Rice seedlings were transplanted in seven rows in each bed at a spacing of 0.13 0.25 m with two seedlings per hill. During the growing season, foliar N was applied only when rice plants showed N deficiency. Furrow irrigation was done when the field was dry. In control plots without plastic film, the field was continuously flooded. Other crop management practices used were the same for both plastic film mulch and control plots. Increasing soil temperature The plastic film mulch preserves heat and increases soil temperature. Shen et al (1997b) reported that mulching increased the daily mean temperature of the soil surface by 3-6 C and seasonal accumulative heat units byIRRN 9

430-450 C. Nagata et al (1997) found that the temperature of the film surface in the mulched plot was 10.3 C greater than that of the water surface in the control plot. In a pot study, Nagata et al (1994) observed that 0.02-mm-thick plastic film mulch increased soil temperature at a 3-cm depth by up to 5 C during the day and by 1.5 C at night.

whether plastic film mulching can increase grain yield under favorable rice-growing conditions.

Water conservation Plastic film mulching cultivation saves water by reducing water consumption during land preparation and evaporation during the growing season. Shen et al (1997a) reported that water saved by mulching cultivation was 640 m3 ha-1 before crop establishment and 450600 m3 ha-1 from reduced evaporation during the growing season. Compared with the continuously flooded system, plastic film mulch reduced water consumption by almost 50%. In plastic film mulching cultivation, water is often supplied by furrow irrigation. In some ricegrowing areas of China, rainfall provides enough water for rice growth in the plastic film mulch system. Improving fertilizer use efficiency In the continuously flooded rice system, fertilizer N-use efficiency is low due to rapid losses of applied N through volatilization, percolation, runoff, and seepage. N losses through percolation and seepage are greatly reduced in plastic film mulching cultivation due to furrow irrigation. Increased soil temperature in plastic film mulching cultivation also contributes to greater nutrient uptake in low-temperature areas. Experiments conducted in Zhejiang Province indicated that plastic film mulching reduced fertilizer N input by 30-50% (Wu 1997, personal communication). It is unknown whether film mulching reduces N loss through volatilization to a greater extent than the continuously flooded system. Weed control Plastic film mulching probably controls weeds by reducing O2 concentration in the layer between the soil and the film. Shen et al (1997a) reported 28 weed plants m-2 with film mulching and 326 m-2 in the control. Plastic film mulching reduces weeds by 90%; thus, there is no need to apply herbicides in this cultivation system. Increasing rice yield In rice-growing areas where air temperature is low, plastic film mulching cultivation increases rice grain yield. A maximum yield of 10.8 t ha-1 under plastic film mulching cultivation or a 42% increase over the control was reported by Shen et al (1997a). In 1996, plastic film mulching cultivation was demonstrated in many farmers fields in Shiyan (Shen et al 1997a). The average yield from 1,156 ha was 7.3 t ha-1, a 19% increase over the control. Total growth duration was reduced by 5 d. The yield improvement was mainly due to increased panicle number per square meter because high temperature accelerated early tillering. Plastic film mulching cultivation produced larger and more vigorous root systems than those under the continuously flooded system (Shen et al 1997a). It is necessary to determine10 April 1999

Economic analysis Plastic film mulching cultivation is economically acceptable to Chinese farmers because of the low costs of plastic film and labor. About 53 kg of plastic film was needed per hectare, which costs about US$90. Plastic film mulching saved US$25 ha-1 from fertilizer N, US$18 from herbicides, and US$37 from manual weeding in Shiyan (Shen et al 1997b). The savings from irrigation water was not estimated because water was free in Shiyan. The cost for transplanting and film installation was estimated at US$55 ha-1. The increase in grain yield was 1.4 t ha-1 averaged across 1,156 ha, which was equivalent to US$268 ha-1. Farmers net income increased by US$203 ha-1. Future outlook Plastic film mulching cultivation is a promising technology for ricegrowing areas where low temperature and water shortage limit rice production. It is also suitable for hybrid rice production because lowdensity planting and a single seedling per hill have been practiced in growing hybrid rice. The adoption of this technology largely depends on the cost of plastic film and labor costs for film installation and transplanting. Direct dibble seeding coupled with plastic film mulching has been tested in Zhejiang Province. A medium-size machine that can both lay the plastic film and direct seed would promote the extension of this technology. Reduced fertilizer N losses and reduced herbicide application in plastic film mulching cultivation will minimize environmental pollution. Methane emission may also be reduced because of the aerobic condition and film covering. The used plastic film, however, is a potential threat to the environment if it is not removed from the field and if recycling is not properly practiced. Biodegradable plastic film will solve this problem if its price is affordable. Future studies should focus on developing new varieties and a fertilizer management strategy suitable for this cultivation system. Short- and long-term changes in soil quality (soil chemistry and physics) due to plastic film mulching cultivation should be monitored and studied in the context of sustainability. ReferencesNagata M, Hiyoshi K, Okada Y, Umezaki T. 1994. Mulching cultivation system using polyethylene film for early-season culture rice: measurement of paddy soil temperature in pot experiment. Bull. Fac. Agric. Miyazaki Univ. 41:57-64. Nagata M, Hiyoshi K, Okada Y, Umezaki T, Tadeo BD, Shimoda T. 1997. Mulching cultivation system using polyethylene film for early-season culture rice. II. Thermal image analysis of paddy field by thermographic system. Bull. Fac. Agric. Miyazaki Univ. 43:129-136. Shen K, Wang X, Liu J, Luo X. 1997a. Test and demonstration on wetcultivation with film mulching of rice. Hubei Agric. Sci. 5:18-22. Shen K, Wang X, Luo X. 1997b. Preliminary report on wet-cultivation with film mulching of rice. Hubei Agric. Sci. 1:6-8.

Plant breeding

Genetics of tolerance for iron toxicity in riceJ. Owusu Nipah, Coconut Project (OPRI-CSIR) c/o M.O.F.A., P.O. Box 245, Sekondi; O. Safo-Kantanka, University of Science and Technology, Faculty of Agriculture, Crop Science Department, Kumasi, Ghana; M.P. Jones, and B.N. Singh, WARDA/ADRAO 01 BP 2551, Bouak, Cte dIvoire

Frequency (no. of hills) 50 40 30 20 10 0 70 60 50 40 30 20 10 0 1 2 3 4 5 6 Iron toxicity score 7 8 9P 1 P 2 F1 F1

Cross I: CK4/ITA330 at 75 DAT

P1 P2

1

2

3

4

5

6

7

8

9

Cross II: CK73/Bouake 189 at 75 DAT

Distribution and mean of parents, F1 and F2 plants.The solid lines represent the range of the generations about their means (points).

Table 1. Generations of crosses used in the experiment. Total number Basic generations CK4/ITA 330 (I) 48 48 24 120 24 24 CK73/ Bouak 189 (II) 48 48 24 144 24 24

Table 2. Estimation of additive, dominance, and interaction parameters with a sixparameter model of Hayman. Parametera m [d] [h] [i] [j] [l]a

Cross I: CK4/ITA330 Estimate 4.6667 -1.7083 -5.5521 -3.2500 -0.4479 0.6875

Cross II: CK73/Bouak 189 Std. error Probability

Std. error Probability Estimate 0.1180 0.2444 0.6916 0.6793 0.2614 1.1161