Dynamics of Rehydration in Leaf Disks1

Abstract

AbstractThe time course of rehydration by living tissue appears to occur in two distinct phases. Phase I is associated with passive recovery from a water deficit while phase II is associated with the resumption of growth‐related water uptake. However, experimental efforts have been unable to clearly separate the two water absorption mechanisms, and much controversy exists concerning when phase II begins, when phase I ends, and the relative contribution of each phase to the total water uptake. The latter is particularly important in relative turgidity determinations where phase II uptake leads to errors.In order to help answer some of the above questions it was decided to apply the water transport theory of Molz and Ikenberry to water uptake by floating leaf disks of sunflower (Helianthus annuus L.). The theory can be used to predict quantitatively the time course of phases I and II separately and together. From computer solutions of the governing equations, it is concluded that water uptake by a dehydrated disk is initially a passive rehydration process which becomes progressively more complicated by growth‐related water uptake (phase II) as the tissue water potential rises above −4 bars. Because of the operation of phase II, which begins initially at the cut edge of the disk and progresses inward with time, the interiors of leaf disks are prevented from attaining a water potential of zero. Instead, they are maintained at −0.2 to −3 or −4 bars, a factor that could lead to significant errors in relative turgidity determinations. Phase II neither starts immediately at a steady rate (Yemm and Willis hypothesis) nor waits for phase I completion before beginning (Barrs and Weatherley hypothesis). Rather, is predicted to begin immediately at a very slow rate and to increase monotonically until a steady, maximum rate is reached at the same time phase I is completed. For a 0.5‐cm radius disk and a tissue diffusivity around 10−5 cm2sec−1, phase II water uptake accounted for about 10 to 20% of the total water uptake after a floating time of 3 to 4 hours.