Sensitivity of root and leaf water status in maize (Zea mays) subjected to mild soil dryness

The aim of this work was to test the sensitivity of the water potential (Ψ w ), osmotic potential (Ψ s ) and turgor (Ψ p ) of roots and leaves of maize seedlings (Zea mays L. cv. Carla) subjected to a mild stress in drying soil in a growth chamber. To the best of our knowledge there are no experimental data which describes diurnal courses of Ψ w in soil, roots, and leaves and the parallel changes in the osmotic potential (Ψ s ) and turgor (Ψ p ) of roots and leaves from plants grown in moist and drying soil. Root and leaf Ψ w varied diurnally, the amplitude being much more marked in leaves than in roots. Root and leaf Ψ w did not achieve equilibrium at predawn with the bulk soil matric potential (Ψ m ) but became higher. Our results are at variance with data indicating root Ψ w is a sensitive indicator of soil dryness. Root Ψ w in the well-watered and drought-stressed treatments did not differ, whereas daytime leaf Ψ w in the droughted treatment was lower 6 days after water was withheld. Diurnal changes in Ψ s and Ψ p were more marked in leaves than in roots. Withholding water lowered leaf Ψ p , whereas root Ψ p substantially increased after only 3 days of withholding water. Early mild stress can be more easily and more quickly identified by changes in root Ψ s , increases in root Ψ p , or the divergence in root and leaf Ψ p than by a lower Ψ w of root or leaf. Relative water contents of roots and leaves measured in the light period indicated also sensitively falling Ψ m .