Seed reserve-dependent growth responses to temperature and water potential in carrot (Daucus carota L.)

Both temperature and soil moisture vary greatly in the surface layers of the soil through which seedlings grow following germination. The work presented studied the impact of these environmental variables on post-germination carrot growth to nominal seedling emergence. The rapid pre-crook downward growth of both the hypocotyl and root was consistent with their requirement for establishment in soil drying from the surface. At all temperatures, both hypocotyl and root growth rates decreased as water stress increased and there was a very distinct temperature optimum that tended to occur at lower temperatures as water stress increased. A model based on the thermodynamics of reversible protein denaturation was adapted to include the effects of water potential in order to describe these growth rate responses. In general, the percentage of seedlings that reached the crook stage (start of upward hypocotyl growth) decreased at the extremes of the temperature range used and was progressively reduced by increasing water stress. A model was developed to describe this response based on the idea that each seedling within a population has lower and upper temperature thresholds and a water potential threshold which define the conditions within which it is able to grow. This threshold modelling approach which applies growth rates within a distribution of temperature and water potential thresholds could be used to simulate seedling growth by dividing time into suitable units.