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

The response of pre-emergent shoots of carrot and onion to mechanical imped ance, water stress and suboptimal temperature was studied. We used model la boratory systems in which mechanical impedance and water stress could be va ried independently of each other and independently of other complicating fa ctors (e.g. aeration and hydraulic conductivity). Our results showed that m echanical impedance reduced the rate and extent of shoot development in bot h carrot and onion. Roots of both carrot and onion were less sensitive to m echanical impedance than shoots, The recovery of shoot length following the removal of impedance was studied. The data were used to develop a new mode l of shoot elongation as a function of mechanical stress, water stress, tem perature, shoot length and time, Our results also provide a new insight int o the physiology of shoot development in strong soils. We contrast the effe ct of mechanical impedance on pre-emergent seedling development in carrot a nd onion.