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.

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Cumulative germination curves were recorded for carrot (Daucus carota L.) seeds at a range of constant temperatures (T ) and water potentials (Ψ) in the laboratory and under variable soil conditions in 15 seed-bed environments in the field. A single base temperature (T b ), a distribution of base water potentials (Ψ b (G)) for percentiles (G) of the population and the hydrothermal time constant (H g ) were determined from laboratory data. Although less effective at low Ψ, it was possible, using these germination parameters, to satisfactorily describe the effect of T and Ψ on germination rates under constant conditions according to the threshold models of thermal and hydrothermal time. These models were applied to field data with the condition that the germination process ceased if T T b for thermal time and additionally Ψ Ψ b (G) for hydrothermal time.Neither model accurately predicted germination patterns in the field. However, the pattern of germination was adequately described in most situations by a modified threshold model in which the predicted progress towards germination was unaffected by soil Ψ, provided it remained above Ψ b (G), and was therefore more rapid under variable seed bed conditions than hydrothermal time. In this modified threshold model, the condition Ψ Ψ b (G) had to be fulfilled at the initiation of radicle extension before germination occurred. This result implies that the initiation of radicle growth operates as a moisture-sensitive step that can determine germination and seedling emergence timing under variable soil-moisture conditions.Seedling emergence was also recorded in the field and used to determine, separately, the impact of germination and post-germination growth on the variation in seedling emergence patterns. The analysis suggests that delays in seedling emergence occur largely in the germination phase, but that seedling losses and variation in the spread of seedling emergence times within the population occur largely during the post-germination growth phase.

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...

S U M M A R YOsmotic priming of carrot seeds for 2 wk in polyethylene glycol (PEG, -a 0 MPa) at 15 "C led to more rapid and synchronous germination at 20 "C compared to untreated seeds. These responses were enhanced by a 24 h pre-priming soak in water or a change of solution after the first 24 h of priming to remove leachate. The inclusion of 200 mg litre-' N-substituted phthalimide in the pre-priming soak and/or in the PEG further enhanced the results of priming. Leachate removal combined with phthalimide inclusion gave 79% and 86% germination from seeds of two carrot cultivars during the first day in 20°C water following priming. In contrast, cumulative germination of untreated seeds of the same cultivars was 18% and 61% respectively after 3 days in 20 "C water. Seeds primed in PEG containing 200mg phthalimide litre-' with the solution replaced after the first 24 h germinated earlier and more synchronously than untreated seeds over a range of germination temperatures (5, 10, 15, or 20 "C), but the effects of priming were most marked at 5 "C.

S U M M A R YThe relationship between seedling characters and germination rate within a seed lot was studied in cauliflower, leek and onion seeds. Newly germinated seeds were selected after successive days of imbibition at 20°C and placed on slope tests to assess early seedling growth. In all three species seedling length decreased and the coefficient of variation of those seedling lengths increased with increasing number of days of imbibition required for germination. Slow germinating seeds in all three species produced fewer normal healthy seedlings than faster germinating seeds. The relevance of these results to pre-germinated seed sowing techniques is discussed.

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SUMMARY A series of experiments were carried out to study the effects of different drying regimes on the survival of newly‐germinated (radicle emerged 0·5‐1·5 mm through the seed coat) cabbage seeds and to establish a treatment for reducing the moisture content of these germinated seeds. A preliminary surface‐drying step proved necessary to avoid seed agglutination during the main drying stage. Of the drying regimes compared both viability and vigour were most effectively maintained in air, conditioned to between 20 and 30°C and with 80% r.h., flowing through the seeds at 0·25 m s‐1. Under these conditions, the viability and vigour of newly‐germinated seed was maintained during drying to an equilibrium moisture content of c. 14%. At this moisture content further radicle growth was prevented and seeds remained free‐flowing and separate for sowing or for further treatment.