Chilling responses of maize (Zea mays L.) seedlings: root hydraulic conductance, abscisic acid, and stomatal conductance.

Maize seedling water relations and abscisic acid (ABA) levels were measured over 24 h of root chilling (5.5 degrees C). At 2.5 h into chilling, leaf ABA levels increased by 40x and stomatal conductance (g(s)) decreased to 20% compared with prechill levels. Despite a rapid g(s) response to root chilling, leaf water potential (Psi(L)) of chilled seedlings decreased to -2.2 MPa resulting in a complete loss of turgor potential (psi(p)). Ineffective g(s) control early in chilling resulted from decreased root hydraulic conductance (L(r)) due to increased water viscosity and factor(s) intrinsic to the roots. After 24 h chilling, Psi(L) and psi(p) of chilled seedlings recovered to control levels due to stomatal control of transpiration and increased L(r). The impact of the temporal changes in g(s) and L(r) on maize seedling water relations during chilling was analysed using a simple, quantitative hydraulic model. It was determined that g(s) is critical to stabilizing Psi(L) at non-lethal levels in chilled seedlings at 2.5 h and 24 h chilling. However, there was also a significant contribution due to increased L(r) at 24 h chilling so that psi(p) increased to control levels. As a first step in determining the factor(s) responsible for the increase in L(r), cDNA microarrays were used to quantify the transcript levels of eight aquaporins obtained from mature root tissue at 24 h chilling. None of these were significantly up-regulated, suggesting that the increase in L(r) was not due to regulation of these aquaporins at the transcriptional level.