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Notes: Dispersion of elastic waves in a bonded plate has been analyzed using the exact solution as well as by two approximate models: a spring model that neglects the inertia effects and a density model that does not account for the finite stiffness of the bond layer. It is shown that the two model predictions agree with the exact solution in two overlapped regions of the density and stiffness ratios. However, they fail to predict the actual behavior in certain regions of the parameter domain.

Notes: The response of upland rice (Oryza sativa L.) cultivars Kinandang Patong and UPLRi-7, and the C4 weeds Echinochloa colonum (L.) Link, Eleusine indica (L.) Gaertn., and Rottboellia cochinchinensis (Lour.) W.D. Clayton to low light intensity of 150, 250, and 400 μmol m−2 s-1 photosynthetically active radiation (PAR) at different development stages was studied in a combined growth chamber and glasshouse experiment. The weed species had higher net CO2 exchange rates (CER) than rice at all light intensities and growth phases. The response of CER to light intensity was greatest at the early vegetative stage and became less pronounced at later stages in all species. Low light intensity reduced CER and growth rates at an early vegetative phase, particularly in rice, while Rottboellia maintained the highest CER. Differences in low-light-induced growth reduction between the species became less marked with plant age. Although prolonged exposure to low light intensity increased CER for all species, it was least for Eleusine, indicating acclimation of the photosynthetic system. All three weed species had a lower leaf conductance, low transpiration rate, and higher water use efficiency than rice. Both rice and weeds recovered from low-light treatment, particularly if the latter was imposed early. Continuous exposure to 150 (μmol m−2 s−1 PAR strongly inhibited growth of Eleusine and Echinochloa, had an intermediate effect on rice, and affected Rottboellia least. It is concluded that Rottboellia has superior growth and assimilation compared to rice under both low and high intensity light, whereas Echinochloa and, to an even greater extent, Eleusine, are more susceptible to shading. Possible physiological causes of these differences are discussed. Réponse physiologique du riz et des adventices à une faible intensité lumineuse à différents stades de croissance La réponse du riz hautes terres (Oryza sativa L.) cultivars Kinandang Patong et UPLRi-7, et des adventices C4 (Echinochloa colonum (L.) Link, Elsine indica (L.) Gaertn, et Rottboelliacochinchinensis (Lour.) W.D. Clayton, a de faibles intensité lumineuses de radiation photo-synthétiquement active (PAR) de 150, 250 et 400 (μmol m−2 s−1à différents stades de croissance a étéétudié dans une expérimentation combinée en chambre climatique et en serre. Les espéces adventices avaient des taux nets d'échange en CO2 (CER) plus élevés que le riz a toutes les intensités lumineuses et phases de croissance. La réponse du CER à 1'intensité lumineuse était la plus élevée aux stades végé-tatifs précoces et devenait moins é1evée aux stades plus tardifs pour toutes les espéces. La faible intensité lumineuse a réduit le CER et les taux de croissance à un stade végétatif précoce, particuliérement chez le riz, tandis que Rottboellia maintenait le CER élevé. Les diffe'rences dans la réduction de croissance induite par faible lumiére entre les espéces devenaient moins marquées avec 1'âge des plantes. Bien qu'une exposition à une faible intensité lumineuse ait augmenté le CER pour toutes les espèces, cela était moindre pour Eleusine, démontrant une acclimatation du systéme photosynthétique. Les 3 espèces d'adventices avaient une conductance foliaire plus faible, un taux de transpiration plus faible et une efficacité de l'utilisation d'eau plus élevé que le riz. Le riz et les adventices se remettaient du traitement de faible lumière surtout s'il était imposé tôt. Une exposition continue à 150 μmol m−2 s−1 a fortement inhibé la croissance d'Eleusine et d'Echinochloa, a moyennement affecté le riz de façon et de moinde façon, le Rottboeillia. II est conclu que Rottboellia a une croissance et une assimilation supérieures au riz à la fois sous faible et sous forte in tensité lumineuse, tandis qu'Echinochloa et même encore plus Eleusine sont plus sensibles à l'ombrage. Les causes physiologiques possibles de ces différences sont discutées. Die physiologische Reaktion von Reis und Unkräutem in verschiedenen Entwicklungsstadien auf geringe Lichtintensität Die Reaktion von Bergreissorten (Oryza sativa L. ‘Kinandang Patong’ und ‘UPLRi-7’) und der C4-Unkräuter Echinochloa colonum (L.) Link, Eleusine indica (L.) Gaertn. und Rottboellia cochinchinensis (Lour.) W.D. Clayton in verscbiedenen Entwicklungsstadien auf geringe Lichtintensität von 150, 250 und 400 μmol m−2 s−1 wurde in Pflanzenwuchskammer- und Gewächshausversuchen untersucht. Die Unkrautarten hatten bei alien Lichtintensitäten und in alien Entwicklungsstadien höhere Netto-CO2-Austauschraten (CER) als der Reis. Die CER-Reaktion auf die Lichtintensität war in den frühen Entwicklungsstadien am größten und danach bei allen Arten immer weniger deutlich. Niedrige Lichtintensität verringerte die CER und die Wachstumsraten in frühen Entwicklungsphasen, besonders bei Reis, während Rottboellia die höchste CER beibehielt. Unterschiede in der durch geringes Licht hervorgerufenen Wachstumsreduktion unter den Arten wurden mit zunehmendem Alter weniger deutlich. Obwohl durch eine längere Exposition bei geringer Lichtintensität die CER bei alien Arten zunahm, war dies bei Eleusine am geringsten, worin sich die Akklimation des Photosynthesesystems zeigte. Alle 3 Unkrautarten hatten ein geringeres Blattleitungsvermögen, geringere Transpirationsraten und eine höhere Assimilationsleistung als Reis. Sowohl Reis als auch die Unkräuter erholten sich von der Dunkellichtzeit, besonders wenn sie ihm früh ausgesetzt worden waren. Ständige Exposition auf eine Lichtintensität von 150 μmo m−2 s−1 unterdrückte das Wachstum von Eleusine und Echinochloa erheblich, den Reis weniger und Rottboellia am wenigsten. Daraus wurde geschlossen, daßRottboellia sowohl bei niedriger als auch hoher Lichtintensität im Vergleich zum Reis über stärkeren Wucbs und bessere Assimilation verfügt, während Echinochloa und, mehr noch, Eleusine gegen Beschattung empfindlicher sind. Die möglichen physiologischen Gründe für dieseUnterschiede wurden diskutiert.

Notes: The effects of concentrated sulphuric acid, dry heat, hot water and NaOCl treatments on the germinability of dormant large and small aerial Commelina benghalensis L. seeds were evaluated. Concentrated sulphuric acid and NaOCl treatments were more effective than the dry heat and hot water treatments in breaking the dormancy of C. benghalensis aerial seeds. Treatments increased germination of both seed types due to its effect on the seed coat integrity. A scanning electron microscope revealed that changes as a result of scarification occurred in the hilum region of the seed and in the seed coat surface. The large aerial seeds were affected by all treatments more than the small aerial seeds. Difference in germinability between the two seed types was related to the difference in their seed vigour.

Notes: Responses of upland rice (Oryza saliva L.) and the weeds Amaranthus spinosus L., Cyperus rotundus L., Eleusine indica (L.) Gaertn., and Rottboellia cochinchinensis (Lour.) W. D. Clayton to nitrogen (N) and time of N application were studied in the glasshouse both in monoculture and in mixture. The N response of two rice cultivais and R. cochinchinensis in monoculture levelled off between 69 and 103 mg N kg−1 air-dried soil, whereas the other weed species continued to respond beyond 103 mg N kg−1 air-dried soil. At high N application rates. N uptake by weeds was higher than that by rice; weeds used N more efficiently. Weeds, which had high dry matter production at high N application rates in monoculture, were most competitive at low N and least sensitive to low applied N. Nitrogen application levels sub-optimal to rice reduced the competitive ability of rice when in mixture with E. indica, R. cochinchinensis, and A spinosus, but not with C. rotundus. Although N response of a plant species generally declined with age, interspecific differences were observed. Timing of N application can be exploited in rice-weed management. Les effets d'applications d'azote sur la croissance, l'efficacité de l'utilisation de l'azote et l'interaction riz-mauvaises herbes Les réponses du riz pluvial (Oryza sativa L.) et des mauvaises herbes Amaranthus spinosus L., Cyperus rotundus L., Eleusine indica (L.) Gaertn., et Rottboellia cochinchinensis (Lour.) W. D. Clayton à des applications d'azote, et à la période de ces applications, ont étéétudiées en serre, en cultures pures et en associations. Les réponses à l'azote de deux variétés de riz et de R. cochinchinensis en culture pure montrent un plateau entre 69 et 103 mg N kg−1 de sol séchéà l'air, alors que les autres mauvaises herbes continuaient de répondre au delà da cette dernière dose. Aux doses élevées d'azote, les mauvaises herbes prélevaient davantage d'azote que le riz et elles l'utilisaient plus efficacement. Les mauvaises herbes qui, en culture pure, montraient une forte production de matière sèche aux doses d'azote élevées, étaient les plus compétitives à faibles doses d'azote et les moins affectées par les faibles doses. Les doses d'azote sub-optimales pour le riz réduisaient son aptitude à la concurrence quand il était associéà A. spinosus, E. indica et R. cochinchinensis, mais non àC. rotundus. Bien que chez toutes les espèces la réponse à l'azote déclinait généralement avec l'âge des plantes, des différences interspécifiques ont été observées. La période d'application de l'azote pourrait être exploitée dans la maîtrise des mauvaises herbes du riz. Wirkung der Stickstoffdüngung auf das Wachstum, die Stickstoff-Assimilation und die Konkurrenz zwischen Unkraut und Reis Die Wirkung von Stickstoff (N) und des Zeitpunkts der Stickstoffdüngung auf Hochlandreis (Oryzasativa L.) und die Unkrautarten Amaran thus spinosus L., Cyperus rotundus L., Eleusine indica (L.) Gaertn. und Rottboellia cochinchinensis (Lour.) W. D. Clayton in Rein- sowie Mischbeständen wurde im Gewächschaus untersuchte. Die Reaktion auf die Stickstoffgaben begann bei 2 Reissorten und Rottboellia co-chinchinensis bei 69 und 103 mg N kg−1 lufttrockenen Boden, während die anderen Unkrautarten bei mehr als 103 mg N kg−1 lufttrockenen Boden reagierten. Bie hohen N-Gaben war die N-Auf-nahme der Unkräuter stärker als beim Reis, und die Unkräuter setzten den Stickstoff besser um. Unkräuter mit hoher Trockenmasseproduktion in Reinkulture bei hohen N-Gaben waren bei niedrigen N-Gaben am konkurrenzstärksten. Suboptimale N-Gaben verminderten die Konkurrenzkraft des Reises in Mischbeständen mit Eleusine indica, Rottboellia cochinchinensis, Amaranthus spinosus, aber nicht mit Cyperus rotundus. Obwohl die N-Wirkung bei den Pflanzenarten allgemein mit dem Alter abnahm, lieβen sich zwischen den Arten Unterschiede beobachten. Der Zeitpunkt der N-Düngung kann für die Steuerung der Konkurrenz der Unkräuter mit Reis genutzt werden.

Notes: Light and nitrogen (N) interaction effects on rice (Oryza sativa L., cv. UPLRi-7) were studied in the growth chamber, and effects on its competition with Rottboellia cochinchinensis (Lour.) W.D. Clayton and Echinochloa colonum (L.) Link were studied in the field. In the field, N availability increased the canopy light absorption coefficient, and reduced the sunlit leaf area index of rice. In growth chambers, rice plants grown under conditions of low photosynthetically active radiation (PAR) had higher shoot N concentration than when grown at higher PAR. The photosynthetic rate was strongly correlated with leaf N content per unit leaf area. When no N was applied there was no difference in dry matter yield between plants grown at 150 and 400 μE m−2s−1. At various N levels, dry matter increased with increasing light intensity. A reduction in light intensity did not give a proportionate decrease in plant growth. The results suggest that an increase in shoot N concentration is an adaptive mechanism of shade plants. The limited plant response to N under shade and acclimation of rice to reduced irradiance could be significant factors in light and N interaction on rice-weed competition. Effets de la lumière, de l'azote et de leur interaction sur la dynamique de la compétition riz/mauvaises herbes Les effets de l'interaction de la lumière et de l'azote (N) sur le riz (Oryza sativa L.), cv UPLRi-7) ont étéétudiés en chambre climatique et les effets sur sa compétition avec Rottboeillia cochinchinensis (Lour) WD Clayton et Echinochloa colonum (L.) Link ont étéétudiés au champ. Au champ, la disponibilité en azote a augmenté le coefficient d'absorption de la lumière par la canopée et a réduit l'index de la surface foliaire ensoleillée du riz. En chambre climatique, les plants de riz cultivés à une faible radiation photosynthétiquement active (PAR) avaient une concentration en azote de leur tige plus élevée qu'à une forte PAR. Le taux photosynthétique a été fortement correlé avec la teneur en azote foliaire par unité de surface foliaire. Quand aucune azote a été appliquée, il n'y a pas de différence dans les rendements en matière sèche entre les plantes cultivées de 150 à 500 μE m−2 s−1; A des doses variées d'azote, la matière sèche a augmenté avec l'augmentation de l'intensité lumineuse. Une réduction de l'intensité lumineuse n'a pas entraîné une réduction proportionnelle de la croissance de la plante. Les résultats donnent à penser que l'augmentation de la concentration en azote des tiges est un mécanisme d'adaptation des plantes à l'ombre. La réponse des plantes à l'azote limitée sous l'ombre et l'acclimation du riz à une irradiation réduite pourraient être des facteurs significatifs dans l'interaction lumière-azote sur la compétition riz-adventices. Wirkungen und Wechselwirkungen von Licht und Stickstoffdüngung auf die Konkurrenz zwischen Reis und Unkraut Der Einfluß des Lichts und der Stickstoffdüngung (N) auf Reis (Oryza sativa L. ‘UPLRi-7’) wurde in der Pflanzenwuchskammer und die Wirkungen auf die Konkurrenz mit Rottboellia cochinchinensis (Lour.) W.D. Clayton und Echinochloa colonum (L.) Link im Freiland untersucht. Die Verfügbarkeit von N erhöhte im Feld den Licht-Absorptionskoeffizienten und reduzierte den Blattflächenindex beim Reis. Bei der Anzucht in Pflanzenwuchs-kammern hatten Reispflanzen bei niedriger Beleuchtungsstärke eine hohere N-Konzentration als bei stärkerer Beleuchtung. Die Photosyntheserate war deutlich mit dem N-Gehalt, bezogen auf die Blattfläche, korreliert. Ohne N-Düngung traten bei 150 und 400 μE m−2 s−1 keine Trockenmassenunterschiede auf. Bei verscbiedenen N-Düngungsstufen nahm die Bildung von Trockenmasse mit der Lichtintensität zu. Eine Verminderung der Lichtintensität führte nicht zu einer proportionalen Abnahme des Pflanzenwuchses. Aus den Ergebnissen wurde geschlossen, daß die Zunahme der N-Konzentration im Sproß ein adaptiver Mechanismus von Schattenpflanzen ist. Die begrenzte Reaktion der Pflanzen auf Stickstoff im Schatten und die Akklimation von Reis auf vermindertes Licht könnten signifikante Faktoren für die Wechselwirkungen von Licht und Stickstoffdüngung auf die Konkurrenz zwischen Reis und Unkraut sein.

Notes: The germination and growth of three strains of gooseweed (Sphenoclea zeylanica Gaertn.) that showed susceptibility, moderate susceptibility, and tolerance for foliar-applied 2,4-D isobutyl ester were studied in the laboratory and greenhouse. The plants were isolated from samples collected from major rice (Oryza sativa L.) growing areas in the Philippines. The seeds of all three strains possessed dormancy; light stimulated germination but did not completely overcome dormancy. Without N application, stem elongation and leaf production did not differ among the strains. With 60 kg N ha−1, the susceptible and moderately susceptible strains responded better than the tolerant strain, the difference being apparent at 6 weeks after emergence. Measurements of the leaf cuticle showed significant differences among the strains starting at the 8- to the 10-leaf stage, a trend that paralleled their response to 2,4-D. Adding a surfactant to the spray formulation nullified the differential responses. Implications for managing the weed and correlations with current field observations are discussed.

Notes: Summary Although denitrification has long been considered a major loss mechanism for N fertilizer applied to lowland rice (Oryza sativa L.) soils, direct field measurements of denitrification losses from puddled rice soils in the tropics have only been made recently. This paper summarizes the results of direct measurement and indirect estimation of denitrification losses from puddled rice fields and reviews the status of research methodology for measurement of denitrification in rice fields. The direct recovery of (N2+N2O)-15N from 15N-enriched urea has recently been measured at sites in the Philippines, Thailand, and Indonesia. In all 12 studies, recoveries of (N2+N2O)-15N ranged from less than 0.1 to 2.2% of the applied N. Total gaseous N losses, estimated by the 15N-balance technique, were much greater, ranging from 10 to 56% of the applied urea-N. Denitrification was limited by the nitrate supply rather than by available C, as indicated by the values for water-soluble soil organic C, floodwater (nitrate+nitrite)-N, and evolved (N2+N2O)-15N from added nitrate. In the absence of runoff and leaching losses, the amount of (N2+N2O)-15N evolved from 15N-labeled nitrate was consistently less than the unrecovered 15N in 15N balances with labeled nitrate, which presumably represented total denitrification losses. This finding indicates that the measured recoveries of (N2+N2O)-15N had underestimated the denitrification losses from urea. Even with a probable two-or threefold underestimation, direct measurements of (N2+N2O)-15N failed to confirm the appreciable denitrification losses often estimated by the indirect difference method. This method, which determines denitrification losses by the difference between total 15N loss and determined ammonia loss, is prone to high variability. Measurements of nitrate disappearance and 15N-balance studies suggest that nitrification-denitrification occurs under alternate soil drying and wetting conditions both during the rice cropping period and between rice crops. Research is needed to determine the magnitude of denitrification losses when soils are flooded and puddled for production of rice.

Notes: Summary The relative importance of ammonia volatilization and denitrification as loss processes following the application of urea to flooded rice by the traditional method was assessed at four sites with different characteristics in the Philippines. The effect of reducing ammonia loss on denitrification and total N loss was also studied. The total N loss was determined by a 15N-balance method and ammonia volatilization was assessed by a bulk aerodynamic method following the application of urea to small plots (4.8×5.2 m). As run-off was prevented and leaching losses were negligible, the denitrification loss was assessed as the difference between total N loss and ammonia loss. When urea was broadcast into the floodwater at transplanting, the ammonia loss varied from 10% to 56% of the applied N. Loss was smallest at Aguilar where wind speeds were low and the greatest at Mabitac where floodwater pH values and temperatures were high and the winds were strong. The ammonia loss was reduced at all sites by incorporating the urea into the soil by harrowing. However, the reduction achieved varied markedly between sites, with the largest reduction (from 56% to 7% loss of the applied N) being observed at Mabitac. The total N lost from the basal application into the floodwater ranged from 59% to 71% of the applied N. Incorporating the urea by harrowing reduced the total N loss at two sites, increased the total N loss at the third site, and had no effect at the fourth site. The denitrification losses ranged widely (from 3% to 50% of the applied N) when urea was broadcast into the floodwater at the four sites. The denitrification loss was low when the ammonia loss was high (Mabitac) and high when the ammonia loss was low (Aguilar). Reducing ammonia losses by incorporating the urea into the flooded soil resulted in increased denitrification losses at three of the sites and appeared to have no effect on denitrification at the fourth site. The results show that reducing the ammonia loss by incorporating urea into the soil does not necessarily result in reduced total N loss, and suggest that the efficiency of fertilizer N will be improved only when both N-loss processes are controlled simultaneously.

Notes: Abstract The use of N fertilizer in Asia has increased from 24 to 39% of the world's total consumption between 1973 and 1987/88. Approximately 60% of the N fertilizer consumed in Asia is used on rice (Oryza sativa L.). However, the N applied to rice, primarily as urea, is not effectively utilized by the crop. Ammonia volatilization is recognized as a major mechanism of N loss, causing ineffective N utilization. Basal incorporation of urea without standing water; deep placement of urea; and modification of urea with algicides, urea inhibitors or coatings are strategies to reduce ammonia loss. Loss of N by nitrification-denitrification may be a serious problem particularly when soil is dried between rice crops, then flooded for the subsequent rice crop. The use of organic N sources, such as green manure and organic manures, as partial substitutes of inorganic N fertilizer is receiving renewed research interest. The use of P fertilizers for rice is most necessary on Oxisols and Ultisols with high P-fixing capacity. Phosphate rock and partially acidulated phosphate rock are alternatives to soluble P sources used on these soils. Response to K is normally highest on light-textured soils. The limited available information suggests that in lowland rice-upland crop rotations, K fertilizers should be applied to the non-rice crop. Zinc deficiency can be overcome through (a) use of varieties more tolerant to zinc deficiency, (b) application of zinc sulfate, and (c) dipping seedling roots in a zinc oxide suspension. Increasing use of S-free fertilizers, intensive cropping, and use of high yielding rice varieties have led to S deficiency in many rice growing countries. Sulfur deficiency can be corrected by applying S-containing materials even with elemental S. Residual effects have also been reported even at a low rate of 20 kg S/ha. Thus, S does not need to be applied every season. To address the unresolved integrated nutrient management issues, both strategic and applied research are required on interacting soil-plant-water-nutrient-climate processes. Long-term sustainability is one of the parameters that must be considered in evaluating the desirability of alternative rice technologies.

Notes: Abstract Field experiments were conducted in the 1984 and 1985 wet seasons to determine the effect of N fertilizer application method on15N balances and yield for upland rice (Oryza sativa L.) on an Udic Arguistoll in the Philippines. The test cultivars were ‘IR43’ and ‘UPLRi-5’ in 1984 and ‘IR43’ in 1985. Unrecovered15N in15N balances for 70 kg applied urea-N ha−1, which represented N fertilizer losses as gases and movement below 0.5 m soil depth, ranged from 11–58% of the applied N. It was lowest (11–13%) for urea split applied at 30 days after seeding (DS) and at panicle initiation (PI), and highest (27–58%) for treatments receiving basal urea in the seed furrows. In all treatments with basal-applied urea, most N losses occurred before 50 DS. Heavy rainfall in 1985 before rice emergence resulted in large losses of native soil N and fertilizer N by leaching and possibly by denitrification. During the week of seeding, when rainfall was 492 mm, 91 kg nitrate-N ha−1 disappeared from the 0.3-m soil layer in unfertilized plots. Although rainfall following the basal N application was less in 1984 than in 1985, the losses from basal applied urea-N were comparable in the two years. Daily rainfall of 20–25 mm on 3 of the 6 days following basal N application in 1984 may have created a moist soil environment favorable for ammonia volatilization. In both years, highest grain yield was obtained for urea split-applied at 30 DS and at PI. Delayed rather than basal application of N reduced losses of fertilizer N and minimized uptake of fertilizer N by weeds.