MODELING PREEMERGENT MAIZE SHOOT GROWTH .2. HIGH-TEMPERATURE STRESS CONDITIONS

High soil temperatures are a significant constraint to crop establishm ent, Exposure of preemergent maize (Zea mays L.) to temperatures in ex cess of 40 degrees C stops shoot elongation and may also damage the se edling, reducing subsequent shoot growth in the physiological temperat ure range (9 to 40 degrees C). At present, there is little quantitativ e understanding of the effects of high temperature stress on preemerge nt maize shoot growth. Consequently, existing maize emergence or preem ergent maize shoot growth models are restricted to tile physiological temperature range. The work described in this paper quantified the eff ect of supramaximum temperatures (temperatures greater than 40 degrees C) on maize shoot growth, and in particular on its component parts, t he coleoptile and first internode, The major impact of supramaximum te mperatures on shoot growth was due to a severe reduction in the first internode growth rate, which was correlated with the maximum temperatu re reached during the day. We also found that, under diurnally varying temperature conditions, coleoptile growth ceased only at 45 degrees C , rather than at 40 degrees C as found previously in constant temperat ure experiments. Inhibition of shoot growth due to high temperature da mage was alleviated when seedlings were grown in the physiological tem perature range for 33 h. Based on these findings, a model was develope d that can predict shout growth of seedlings exposed to supramaximum t emperatures, The model successfully predicted shoot growth of seedling s exposed to supramaximum temperatures as a part of the diurnal cycle. The model was unable, however, to predict seedling recovery from high temperature damage.