Summary• The effect of temperature on the minimum (base) water potential for seed germination ( Ψ b ) was investigated in Daucus carota and Allium cepa and then described in two hydrothermal threshold models.• Germination was recorded over a wide range of temperatures and water potentials.• At temperatures of 15 ° C and below the base water potential for germination of the 50th percentile ( Ψ b (50)) was constant, but in both species, above a temperature ( T d ) around 16 -19 ° C, Ψ b (50) increased linearly with temperature. Hydrothermal time (HTT) and virtual osmotic potential (VOP) models were altered so that the effective base water potential ( Ψ b ( G,T )) for any percentile of the seed population ( G ), above • Germination response to all temperatures and water potentials can be adequately described in both the HTT and VOP models by incorporating changes in Ψ b ( G,T ) with temperature.

Summary The ability of hydrothermal time (HTT) and virtual osmotic potential (VOP) models to describe the kinetics of maize (Zea mays) and chickpea (Cicer arietinum) seed germination under variable conditions of water potential was investigated with a view to gaining an improved understanding of the impact of on‐farm seed priming on seedling establishment through simulation. Germination and/or imbibition time courses were recorded over a wide range of constant temperatures and water potentials and simple stepwise changes in water potential. Both models adequately described germination under constant environmental conditions, but not conditions of water potential that varied. To test the hypothesis that this inaccuracy resulted from the use of ambient water potential, a parsimonious model of seed imbibition was developed to calibrate the HTT and VOP models (IHTT and IVOP) and drive them with estimates of seed water potential. The IHTT and IVOP models described germination during stepwise changes in water potential more accurately than the conventional models, and should contribute to improved predictions of germination time in the field.

The response of pre-emergent shoots of carrot and onion to mechanical impedance, water stress and suboptimal temperature was studied. We used model laboratory systems in which mechanical impedance and water stress could be varied independently of each other and independently of other complicating factors (e.g. aeration and hydraulic conductivity). Our results showed that mechanical impedance reduced the rate and extent of shoot development in both carrot and onion. Roots of both carrot and onion were less sensitive to mechanical impedance than shoots. The recovery of shoot length following the removal of impedance was studied. The data were used to develop a new model of shoot elongation as a function of mechanical stress, water stress, temperature, shoot length and time. Our results also provide a new insight into the physiology of shoot development in strong soils. We contrast the effect of mechanical impedance on pre-emergent seedling development in carrot and onion.Key-words: carrot; mathematical shoot growth models; mechanical impedance; onion; physiological response; water stress induced by PEG. INTRODUCTIONThe success of seedling emergence from soil under nonoptimal conditions has a large influence on the size and uniformity of plant populations both in crop production and in the natural environment. Seeds exhibit a wide range of species specific mechanisms that control germination, so that they tend to germinate under favourable conditions of temperature and moisture for that species. Modelling approaches to predict germination from nondormant seeds as a function of temperature and water potential are reasonably well developed (Gummerson 1986;Finch-Savage & Phelps 1993;Dahal & Bradford 1994;Finch-Savage, Steckel & Phelps 1998). Finch-Savage & Phelps (1993) used the concepts developed to model the germination of seeds to explain seedling emergence patterns in the field in terms of soil temperature and water potential. However, a weakness in this deterministic approach is that it does not allow any predictions of the percentage of germinated seeds which will eventually emerge.To improve models for seedling emergence we must consider how the development of the shoot is modified by its physical environment. The emphasis of this paper will be to understand the effects of mechanical impedance on seedling development. However, we will also present data to demonstrate the effects of suboptimal temperature and the interaction between mechanical impedance and water stress on shoot development. The effects of mechanical impedance on shoot development have received some attention (Hegarty & Royle 1976;Collis-George & Yoganathan 1985a;Collis-George & Yoganathan 1985b;Souty et al. 1992;Braunack 1995;Nasr & Selles 1995;Wilson & Thurling 1996), but in comparison with seed germination, they are poorly understood. Models to describe shoot development in soils with time dependent strength have received little attention. There are two approaches which are commonly used to provide different conceptual frameworks in w...

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Efecto del cultivo de palma de aceite sobre las propiedades físicas del suelo y su relación con la producción y la pudrición de cogolloEffect of the oil palm plantations on the physical properties of the soil and his relation with the production and the bud rot disease Gelber Norberto Gutiérrez Palacio, MSc. ResumenLa investigación tuvo como objetivo cuantificar la afectación física que se presenta en los suelos dedicados al cultivo de palma de aceite (Elaeis guineensis), para lo cual se llevó a cabo una campaña de mediciones, observaciones y muestreos en la plantación Guaicaramo Ltda., ubicada en el municipio de Barranca de Upía, departamento del Meta, Colombia, sobre un área cultivada de aproximadamente 1890 hectáreas.El análisis cualitativo y cuantitativo de las propiedades físicas de los suelos en las zonas características del cultivo (calle, plato y palera) sirvió como base para la determinación de su estado físico y para establecer las relaciones que se pueden presentar entre estas propiedades, la productividad del cultivo y la enfermedad conocida como pudrición de cogollo (PC). Los resultados obtenidos permitieron identificar el desarrollo de fenómenos de compactación en las distintas zonas del cultivo, los cuales determinaron como consecuencia la reducción de la porosidad, de la capacidad de intercambio gaseoso y de la capacidad de infiltración del suelo; todos ellos, factores determinantes de las condiciones observadas de drenaje inapropiado, excesos de agua, encharcamiento de lotes y en algunos casos de escorrentía superficial excesiva; condiciones que restringen el aprovechamiento eficiente de los nutrientes por parte de las plantas. También se pudo observar la relación existente entre la compactación de los suelos y la PC, donde el suelo actúa como un agente que predispone la planta a contraer la enfermedad y no como un agente causante de la misma. Palabras clave: Labores culturales, compactación, Elaeis guineensis, Phytophthora palmivora. AbstractThe research was to objective quantify physical affectation that occurs in soils for the cultivation of oil palm (Elaeis guineensis), for which it carried out a comprehensive campaign of measurements, observations and sampling in the plantation Guaicaramo Ltda., located in the municipality of Barranca de Upía, department of Meta, Colombia, on a cultivated area of approximately 1890 hectares. The qualitative and quantitative analysis of the physical properties of the soils in the zones typical of the crop (street, plate, and zone sticks) served as the basis for the determination of their physical condition and to establish that can be present between these properties, the productivity of the crop and the disease known as bud rot (BRD). The obtained results allowed to identify the development of compaction phenomena in different zones of the crop, which determined as a consequence the reduction of the porosity, the reduction of the capacity of gaseous exchange and the reduction of the capacity of infiltration of the soil; all of them, determinant factors of ...