|MadSci Network: Botany|
I'm not sure I have the right answer but can provide some possibilities. Both potassium ion and sucrose concentrations in guard cells correlate with stomatal opening. (Talbott and Zeiger, 1996). Lua et al. (1995) found that the sucrose concentration in guard cells was highest when stomata were open. Outlaw and Vlieghere-He (2001) found that with a high transpiration rate, sucrose concentration in the guard cell walls of bean plants increased and caused the stomatal aperature to decrease. I'm not sure exactly how you were measuring transpiration but assume it was some kind of potometer. With potometers that use volume or flow measurements, you cannot differentiate between water that is absorbed and retained by the twig and water that is absorbed and transpired. (Potometers that measure weight loss do not have this problem). Twigs soaked in a sucrose solution would absorb a sucrose solution that had a lower water potential than the tap water. When the twigs were later placed in tap water, they could have absorbed more tap water because of their lower water potential due to the sucrose solution they absorbed. Thus, the twigs soaked in the sucrose solution might appear to be transpiring more but in reality they could have simply been absorbing additional water because of their lower water potential. References Lua, P., Zhangb, S.Q., Outlaw, W.H. Jr. and Riddlea, K.A. 1995.Sucrose: a solute that accumulates in the guard-cell apoplast and guard-cell symplast of open stomata. FEBS Lett. 362(2): 180-184. Outlaw, W.H. Jr. and Vlieghere-He, X.D. 2001. Transpiration Rate. An Important Factor Controlling the Sucrose Content of the Guard Cell Apoplast of Broad Bean. Plant Physiology 126: 1716-1724. Talbott, L.D.and Zeiger, E. 1996. Central Roles for Potassium and Sucrose in Guard-Cell Osmoregulation. Plant Physiology 111(4): 1051–1057. The Clickable Guard Cell: Electronically linked Model of Guard Cell Signal Transduction Pathway
Try the links in the MadSci Library for more information on Botany.