Water manometers were connected to fruits of tomato (Lycopersicon esculentum Mill.) and pepper (Capsicum annuum L.), and then fruits were submerged in water baths providing initial temperature gradients between fruit and water of 0 to 19C. Apple (Malus domestics Borkh.) fruits, carrot (Daucus carota L.) roots, witloof chicory (Cichorium intybus L.) roots, rhubarb Rheum rhabarbarum L.) petioles, and pokeweed (Phytolacca americana L.) stems were subjected to water bath temperature gradients of 5C. Internal partial vacuums developed in all organs within minutes of imposing the gradients. The maximum partial vacuums in tomato and pepper fruits increased with increasing temperature gradients. Uptake of water accompanied changes in internal pressure reaching maxima of 17% (w/w) and 2% (w/w) of pepper and tomato fruits, respectively, after 22 hours. Maximum pressure changes achieved in bulky organs deviated from those predicted by the ideal gas law, possibly due to concomitant changes in gas pressure upon replacement of intercellular spaces with water and dissolution of CO2. Partial vacuums also developed in pepper fruits, rhubarb petioles, and pokeweed stems following exposure to air 15C cooler than initial organ temperatures. Results point to the role of temperature gradients in the transport of liquids and gases in plant organs.

This action of CHI on oat leaf senescence has since been extended to many other species (5-8, 10, 12, 14) and to other inhibitors ofthe synthesis ofRNA and protein (1 1, 14). However, we subsequently discovered an additional aspect of this action, namely that the delay of senescence by CHI was accompanied by the opening of the stomata in darkness (9). The full significance of this second effect was not appreciated at the time, because several other agents, not known to affect protein synthesis, had comparable effects on stomatal opening. But reconsideration of the early evidence of Yoshida ( 13), that the presence of the nucleus promotes senescence in the cells of Elodea, focused our attention on the role of protein synthesis not only on leaf senescence itself but also on the changes in the stomatal opening that seem to accompany it. As a result a study has been made of the action of a number of inhibitors of protein and RNA synthesis, both on senescence proper and on stomatal aperture, and the results lead to a rather unexpected conclusion. 3Abbreviations: CHI, cycloheximide; RIF, rifampicin. MATERIALS AND METHODS Seeds

When leaves of plants with C 3 metabolism are detached and held in darkness, they senesce and the stomata close. Because the relation of senescence and stomatal closure is very close, if not actually causal, the question arose as to whether in the leaves of plants with Crassulacean acid metabolism whose stomata open at night the relationship to senescence would be reversed. Detached leaves of four species of Hoya, floated on water in constant darkness or constant light, were found to show no large differences in stomatal aperture (measured as diffusion resistance) between those in the light or dark, but the aperture changed in a regular circadian rhythm. In some leaves the rhythm was simple, in others the peak showed small secondary peaks, but in all cases the values were nearly the same in the light as in the dark, throughout the cycle. Previous culture ofthe intact plants under normal day/night conditions gave results similar to those with plants that had had prolonged culture under constant light or darkness. In those cases when the stomata were more open in the dark, the chlorophyll content was greater than when the stomata were more open in the light; but when they were more open in the light, the chlorophyll content showed little difference between light and dark. When the leaves had only their petioles in water they showed greater senescence in the light than in the dark, and the stomata were more tightly closed in the light, especially at the apical ends. All four species of Hoya gave similar results. We deduce that senescence of these leaves is modified by stomatal aperture, and generally in the same direction as in C 3 leaves, but that in continuous light or darkness the primary control over the aperture is the endogenous cycle.

No abstract

When leaves of plants with C3 metabolism are detached and held in darkness, they senesce and the stomata close. Because the relation of senescence and stomatal closure is very close, if not actually causal, the question arose as to whether in the leaves of plants with Crassulacean acid metabolism whose stomata open at night the relationship to senescence would be reversed. Detached leaves of four species of Hoya, floated on water in constant darkness or constant light, were found to show no large differences in stomatal aperture (measured as diffusion resistance) between those in the light or dark, but the aperture changed in a regular circadian rhythm. In some leaves the rhythm was simple, in others the peak showed small secondary peaks, but in all cases the values were nearly the same in the light as in the dark, throughout the cycle. Previous culture of the intact plants under normal day/night conditions gave results similar to those with plants that had had prolonged culture under constant light or darkness. In those cases when the stomata were more open in the dark, the chlorophyll content was greater than when the stomata were more open in the light; but when they were more open in the light, the chlorophyll content showed little difference between light and dark. When the leaves had only their petioles in water they showed greater senescence in the light than in the dark, and the stomata were more tightly closed in the light, especially at the apical ends. All four species of Hoya gave similar results. We deduce that senescence of these leaves is modified by stomatal aperture, and generally in the same direction as in C3 leaves, but that in continuous light or darkness the primary control over the aperture is the endogenous cycle.