Shoot growth in wheat is sensitive to high soil strength, but as high strength and drying tend to occur together it has proved difficult to separate the effects of water stress and mechanical impedance. The results of two field experiments in 2003 and 2004, where soil strength was manipulated by compaction and irrigation, demonstrated that the yield of wheat (Triticum aestivum L.) was sensitive to physical stress in the root zone. We obtained linear relationships between yield and soil strength and between yield and accumulated soil moisture data (accumulation analogous to thermal time), with similar slopes for both seasons. We were unable to detect root-sourced signals of xylem-sap ABA concentration, despite changes in stomatal conductance. When mechanical impedance and matric potential were varied independently in controlled environments, the growth of wheat was sensitive to mechanical impedance, but not to small changes in matric potential. While the response of stomatal conductance to soil drying in the field could be interpreted as evidence of hydraulic signalling, we suggest that the role of high soil strength, in limiting growth rates on moderately dry soil, requires further research.

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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...

Abstract. The in‐field calibration of a dielectric probe to measure soil water content is described. The probe uses an access tube analogous to that of the neutron probe. The dielectric constant was measured at soil depths of 10, 20, 30, 40, 60 and 100 cm. Cores of soil were then taken from the face of pits dug 30 cm from the access tube and their soil water contents determined by oven drying. The dielectric constant values measured by the probe were calibrated against water contents from these cores. We found that sensor depth needed to be included to achieve a good calibration model that explained 72% of the variance. It is argued that depth needs to be included because of artefacts introduced during the installation of the access tube.

The matric potential of soil water is probably the most useful assessment of soil water status. However, the water-filled tensiometer (the benchmark instrument for measuring matric potential) typically only operates in the range 0 to ÿ85 kPa. In this paper, we report the development of a porous-matrix sensor to measure matric potential in the approximate range ÿ50 to ÿ300 kPa. The sensor uses a dielectric probe to measure the water content of a ceramic material with known water retention characteristics. The calculation of matric potential takes into account hysteresis through the application of an appropriate model to measured wetting and drying loops. It is important that this model uses closed, rather than open, scanning loops. The calibrated sensors were tested in the field and the output compared with data from water-filled tensiometers and dielectric measurements of soil water content. These comparisons indicated that conventional tensiometers gave stable but false readings of matric potential when soil dried to matric potentials more negative than ÿ80 kPa. The porous-matrix sensors appeared to give reliable readings of matric potential in soil down to ÿ300 kPa and also responded appropriately to repeated wetting and drying. This porous-matrix sensor has considerable potential to help understand plant responses to drying soil.

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The in-®eld calibration of a dielectric probe to measure soil water content is described. The probe uses an access tube analogous to that of the neutron probe. The dielectric constant was measured at soil depths of 10, 20, 30, 40, 60 and 100 cm. Cores of soil were then taken from the face of pits dug 30 cm from the access tube and their soil water contents determined by oven drying. The dielectric constant values measured by the probe were calibrated against water contents from these cores. We found that sensor depth needed to be included to achieve a good calibration model that explained 72% of the variance. It is argued that depth needs to be included because of artefacts introduced during the installation of the access tube.

The development of a method using wax layers to simulate the effect of strong soil crusts on seedling emergence is described. Wax layers of different strengths were prepared by melting together white soft paraffin and paraffin wax in different proportions. The wax discs were placed above seeds planted in wet, but well-aerated sand in controlled environments. The effect of adding charcoal to the wax discs to prevent transmission of light was also tested. When wax layer penetrometer pressure was 0.2-0.25 MPa, light transmission greatly decreased the final emergence of carrot, but not onion seedlings. Penetrometer pressures above 0.25 MPa greatly decreased emergence through black wax layers in both carrot and onion. The presence of 2-4 mm stones immediately below wax layers (to simulate aggregates) decreased the emergence of onion shoots through wax layers with penetrometer pressures of 0.25 MPa and above. The emergence of carrot and onion seedlings from wax layers was compared to emergence in the field from different soil types and aggregate sizes. In both laboratory and field experiments, carrot gave better emergence than onion when emergence was relatively poor. Results were consistent with the hypothesis that mechanical impedance is a major factor in poor crop emergence in temperate conditions.