Symbiotic nitrogen fixation of two field grown soybean [Glycine max (L.) Merr.] varieties, 'Chippewa 64' and 'Clay,' was measured at 7 to 10 day intervals starting at flowering, using the acetylene reduction assay. At he end of flowering, treatments were applied which were designed to alter the relationship between the photosynthetic source and sink components in the plants. The treatments were supplemental light, 25% shade, 50% depodding, 60% defoliation, and control.Total acetylene reduction activity per plant increased in both varieties during the flowering period, reached a maximum near the end of flowering, and then declined markedly during the early podfilling stage. The activity decline was due to a decline in the specific activity of the nodules, and occurred immediately prior to the time when the growth rate of the pods (including seed) exceeded that of the total plant tops in the respective varieties. Treatments designed to enhance he photosynthetic source/sink ratio (supplemental light and depodding) maintained nodule activity well above the control in both varieties. Conversely, treatments designed to reduce the source/sink ratio (shading and defoliation) decreased nodule activity below the level of the control. Treatment effects on total plant protein recovered at maturity closely reflected the treatment effects on total nodule activity. The results of this study are interpreted as evidence that symbiotic nitrogen fixation in these varieties declined during podfilling as the result of inadequate assimilate supply to the nodules.Additional index words: Glycine max (L.) Merr., Acetylene reduction, Protein yield. P RELIMINARY studies at the University of Minnesota indicated that senescence of soybean [Glycine max (L.) Merr.] nodules commenced during the early podfilling stage, causing a marked decline

Despite numerous altempts, the development of generalized models capable of accurate predictions of the times from sowing to flowering (f) of crop plants in field environments has remained elusive. Models which seek to corrclatcy\vith various states of environmental factors such as photoperiod, P, and temperature, T, using formal statistical procedures arc often complex. Here, we describe a family of photothermal responses (involving unambiguous parameters and limits) which quantify the linear, non-interacting effects of P and Tnot on^but on \/f, i.e. on the rate of progress towards (lowering. Based on these relations we suggest that the modelling of crop phenology will be simplified, more reliable and more biologically plausible. R. J. Summerficld el a/.: Prediction del tiempo defloration. RESUMENA pesar dc los numcrosos intcntos, aun cs imposiblc dcsarrollar modclos gcneralizados capaccs de predceir con exactitud los pcrfodos cntrc la sicmbra y la fioracion dc la plantas dc cultivo en ambicntes dc campo. Los modclos quc procuran corrclacionar la fioracion a los divcrsos cstados de los factorcs del medio ambicnte, tal como el fotopen'odo y la tempcratura, utili/.ando proccdimicntos cstadi'sticos formalcs, a menudo resultan muy complejos. En cstc articulo sc describe una familia dc rcspucslas fototcrmicas (quc implican paramctros y limitcs no ambiguos) quc cuantifican los cfectos lineales no intcractivos del fotopen'odo y la tempcratura, no en la fioracion sino en una lloracion, o sea, la velocidad del avancc hacia la fioracion. En base a estas rclaciones, sc sugicrc quc el modclado de la fenologfa de los cullivos podn'a scr simplificado, mas confiablc y mas plausible desde el punto de vista biologico.

Eleven genotypes of soyabean (Glycine max) of tropical, sub-tropical and temperate origin and one accession of G. soja were grown in six locations in Australia during 1986-88, and at one location in Australia and two in Taiwan during 1989-91. Dates of sowing were varied within and among locations so as to expose plants to as many as 32 environments of widely different diurnal temperature and daylength. Times from sowing to flowering (/) were recorded, from which rates of progress towards flowering (1//) were calculated. These derived data were then related to mean pre-flowering values of temperature (T) and photoperiod (P) using a threeplane linear model developed from controlled environment data. Among genotypes, mean values of/ varied between 24-49 d and between 134-291 d in the most-and least-inductive environments, respectively. These differences were associated with variations in P from about 11 to 16 h d~', in daily mean maximum temperatures from about 17° to 36°C, in daily mean minimum temperatures from about 5° to 25°C, and in T from about 11° to 30°C, that is, a very wide range of photothermal regimes. The relations of l / / t o T and P can be described in photoperiod-insensitive genotypes by a thermal plane defined by two constants, a and />, and additionally by a photothermal plane defined by three constants, a', V and c'', in the more numerous photoperiod-sensitive genotypes. If photoperiod-sensitive genotypes are grown in sufficiently long days then a third photoperiod and temperature-insensitive plane is exposed, defined by a constant, d'\ this plane indicates the maximum delay in flowering of which the Present addresses: §Queensland Department of Primary Industries, PO Box 591, Ayr, Queensland 4807, Australia; "11 Edward Crescent, Byford, Western Australia 6201. 254 R. J. SUMMERFIELD et al.genotype is capable. The constants a', b', c' and d' define the delay in flowering caused by photoperiod-sensitivity genes. The two intercepts between the three planes define, respectively, the critical photoperiod, P c , above which increase in daylength delays flowering, and the ceiling photoperiod, P ce , above which there is no further delay. The values of the six constants for any genotype can be estimated from observations of/in several natural environments. Comparisons between years in Australia and between Australia and Taiwan show that these genotypic constants can predict \/f, and so the time taken to flower, given data on latitude, sowing date and daily values of maximum and minimum air temperatures. This model is more accurate than an alternative logistic model; we also believe that all six constants in the three-plane rate model described here have biological meaning. They indicate separate genetic control of flowering responses to P and T and could form a rational basis for the genetic characterization and analysis of these responses in the soyabean germplasm. Pronostico del momento defloration II RESUMENSe cultivaron once genotipos de poroto de soja (Glyane max) de origen tropical, subtropical y te...

Approaches using breeding, physiology and modelling for evaluating adaptation of plant genotypes to target environments are discussed and methods of characterizing the target environments outlined. Traditional approaches, and their limitations, to evaluation ofgenotypic adaptation using statistical and classificatory techniques with a phenotypic model are discussed. It is suggested that a simple biological model is the most appropriate framework in which to integrate physiology and modelling with plant breeding. Methods by which physiology and modelling may contribute to assessment of adaptive traits and to selection for adaptation in a breeding programme are considered.Mejora en la adaptation genotipica de las cosechas: un trabajopara los reproductores, psicologosy modeladores RESUMEN En este articulo se discuten los enfoques que utilizan reproduccion, psicologi'a y modelado para la evaluacion de la adaptacion de los genotipos de las plantas a ciertos ambientes-objetivo, al igual que los metodos de caracterizacion de dichos ambientes-objetivo. Tambien se tratan los enfoques tradicionales respecto de la evaluacion de la adaptacion genotipica-y sus limitaciones -que utilizan tecnicas estadisticas y clasificatorias con un modelo fenotipico. Se sugiere que un modelo biologico simple resulta el cuadro mas apropiado para integrar la psicologi'a, el modelado y la reproduccion de las plantas. Tambien se consideran los metodos por los cuales la psicologia y el modelado podrian contribuir a la evaluacion de las caracteristicas adaptativas y de la selection para la adaptacion dentro de un programa de reproduccion.

Growth and development of the tropical grain legumes are generally highly sensitive to photothermal regime, so that seasonal and regional effects on phenology and yield potential can be large. Yet failure adequately to recognize and fully exploit the consequences of genotype X latitude/sowing date X density interactions has frequently constrained both agronomic and genetic advance with these species. Thus there is opportunity for short term productivity improvements through agronomic strategies which accept the implications of phenological plasticity, and seek to optimize management practices such as sowing date and sowing density, to exploit more effectively the yield potential and broaden the adaptation of existing cultivars. The greatest physiological potential for genetic improvement in the productivity of the tropical grain legumes lies not with increasing total biomass, but with increasing the proportion of biomass partitioned into seed, i.e. with improved harvest index (HI). There are difficulties in selecting for higher HI, including its association with phenology, although the latter provides the most powerful tool for manipulating HI in the short term. As with the cereals, more productive genotypes are likely to be characterized by reduced sensitivity to photothermal conditions, shorter growth duration, a more synchronous reproductive ontogeny, and greater HI, than is typical of traditional cultivars. Accordingly, optimal sowing densities will be higher, and crops will need more inputs, better management and more effective protection, than afforded to them in subsistence systems of production. Moreover, the increases in productivity will be achieved, at least partially, at the expense of the potential for yield homeostasis in adverse environments, and of non-seed components such as forage, fuel or tubers -attributes that are often highly valued in subsistence agriculture where many of the tropical food legumes are presently grown. Improvement programmes will therefore need to adopt strategies in particular situations which reflect local perceptions of the importance of productivity improvement relative to that of minimizing inputs and risk.R. J. Lawn: Limitaciones agronomicas y fisioldgicas en la productividad de granos leguminosos tropicales y perspective para el mejoramiento. RESUMENPor lo general, el crecimento y el desarrollo de los granos leguminosos tropicales son sumamente susceptibles al regimen fototermico, de modo que los efectos estacionales y regionales sobre la fenologia y el potencial de rendimiento son grandes. Sin embargo, el no reconocer bien y explotar al maximo las consecuencias de las interacciones genotipo X latitud/fecha de siembra X densidad ha limitado con frecuencia los avances agronomicos y geneticos en estas especies. Existe pues la oportunidad de mejoras en la productividad a corto plazo mediante estrategias agronomicas que acepten las implicaciones de la plasticidad fenologica y que busquen perfeccionar las practicas de gestion tales como la fecha y densidad de siembra, para...