Germination and post‐germination growth to carrot seedling emergence: predictive threshold models and sources of variation between sowing occasions

Abstract

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.

Modelling the germination of Stellaria media using the concept of hydrothermal time

“…Similar lack of fit under conditions of water stress has been noted in previous studies (Finch-Savage et al, 1998). Whether this observed variability under conditions of high water stress could lessen the practical applicability of a weed germination model depends greatly on the ultimate purpose of the modelling exercise.…”

“…A similar approach has also been applied successfully to processes outside seed biology, such as the forecasting of conidial production by sclerotia of Botrytis squamosa (Clarkson et al, 2000). Recently, Finch-Savage et al (1998) have proposed a modification of the hydrothermal time concept to demonstrate that threshold germination models can be used successfully to predict emergence of carrot (Daucus carota) in variable field conditions.…”

“…Relative importance of pre-and postgermination phases in modelling weed emergence Finch-Savage et al (1998) separated the pre-and postgermination phases, and noted that the pregermination phase tends to make the largest contribution towards the exact timing of an emergence event. The postgermination phase tends to affect both the number of seedlings emerging (due to postgermination death) and the spread of emergence.…”

“…In well prepared horticultural seedbeds, moisture and temperature should be the primary determinants of seed germination patterns (Finch-Savage et al, 1998). Hydrothermal time (Gummerson, 1986), provides a basis for describing such patterns, and has been extended to whole populations for lettuce (Lactuca sativa) and tomato (Lycopersicon esculentum) by Bradford (1990) and Dahal & Bradford (1994), respectively.…”

A model of the effects of water stress on seed advancement and germination

“…Finch-Savage & Phelps (1993) have used this approach to describe the emergence of onion seedlings, simplifying its application by assuming that the rates of increase of ∆θ HT below ψ b were zero and above ψ b are determined only by temperature. Finch-Savage et al (1998) used a similar model to describe the germination of carrots under field conditions. Seed priming is used commercially to increase the speed and uniformity of germination.…”