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 degreesC). At 2.5 h into chilling, leaf ABA levels increased by 40 x 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 gs and L-r on maize seedling water relations during chilling was analysed using a simple, quantitative hydraulic model. It was determined that gs 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.