The response of carrot (Daucus carota L.) and onion (Allium cepa L.) seedlings to mechanical impedance and water stress at sub‐optimal temperatures

Abstract

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...

The impact of mechanical impedance on the emergence of carrot and onion seedlings

“…A sand table (Whalley et al, 1999) was covered with a layer of dry sand (RH 65 grade silica sand, WBB Minerals, Sandbach, UK, with a particle size distribution by percentage of mass retained of 710 µm, 0.1%; 500 µm, 0.7%; 355 µm, 7.3%; 250 µm, 28.3%; 180 µm, 41.5%; 125 µm, 19.2%; 90 µm, 2.4%; 63 µm, 0.4%; < 63 µm, 0.1%). Seeds were placed on the dry sand and covered with 10 mm of sand.…”

“…However, the strength of the seedbed is also an important constraint to emergence (Whalley et al, 1999). Understanding the impact of mechanical impedance on seedling emergence is important as high soil strength can result from modest soil drying as well as high bulk density.…”

“…Understanding the impact of mechanical impedance on seedling emergence is important as high soil strength can result from modest soil drying as well as high bulk density. For example, Mullins et al (1992) reported that penetrometer pressure can exceed 1 MPa at a matric potential of −0.1 MPa, while Whalley et al (1999) reported that the penetrometer pressure of a sandy soil was 0.57 MPa at a matric potential of only −0.02 MPa. These matric potentials (−0.1 MPa or greater) would have little direct effect on germination or seedling growth Rowse and Finch-Savage, 2003) but the resulting penetrometer pressures would adversely affect shoot emergence.…”

“…Results from the wax discs in the present study are compared with field data. Carrot and onion were selected for this work because they are of economic importance, their developmental physiology is different and because they respond differently to changes in mechanical impedance (Hoffman, 1933;Whalley et al, 1999).…”

Temperatura cardeal e potencial hídrico na germinação de sementes de corda-de-viola (Ipomoea triloba)

“…Assim, a maior habilidade da radícula para crescer em condições mais frias, quando comparada com a parte aérea, facilitaria a fixação das plân-tulas emergentes e a captura e a translocação dos recursos disponíveis para as plântulas crescerem (Roman et al, 1999). A temperatura fora da faixa ótima reduz-se na taxa de crescimento da parte aérea e da parte radicular (Whalley et al, 1999;Roman et al, 2000).…”