Cyclic Variations in Plant Properties under Constant Environmental Conditions

Abstract

Cyclic fluctuations in stomatal aperture, transpiration rate and leaf water potential under constant environmental conditions have been investigated in intact plants of cotton, pepper, and sunflower. Stomatal aperture and transpiration rate were least when leaf water potential was high and were greatest when leaf water potential was low. Lowest leaf water potential values lagged behind the occurrence of highest transpiration rates, and high overall resistance to water flow occurred in cycling plants. Both of these are considered essential for the occurrence of persistent cyclic behaviour. Hydropassive opening of stomates as the leaves wilted facilitated cycling in cotton and pepper, but not in sunflower, where hydropassive opening did not occur. The roots were identified as the site of the major resistance to water flow in the plant and further experiments directly showed the importance of this root resistance in initiating cycling by causing water stress in the leaves as the stomates opened. Root resistance varied diurnally, becoming increasingly important at night. Root resistance naturally rose to high levels in cotton. High levels were induced in pepper or sunflower by having the roots in deionized water for several days or by anoxia. Quantitative measurements of overall plant resistance were made from leaf water potential and transpiration rate data. The results are discussed and it suggested that plant resistance may indirectly be of importance in the movement of water from the plant to the air.

Water Relations of Vitis vinifera L. cv. Sunred Seedless in Response to Soil Water Depletion Before Harvest

“…Shallow, shorter oscillation periods are generally associated with control of CO2 status, whereas deeper, longer periods are associated with water status control (Barrs, 1971 and references therein). The generally accepted explanation is the following: as stomata open in the light, transpiration increases, leaf water content falls, and the associated water deficit causes partial stomatal closure, leaves regain turgor upon water inflow in response to the steepened water potential gradient, and finally Ψl rises again, permitting stomata to re-open, thus re-commencing the cycle (Barrs & Klepper, 1968). It was shown that stomatal oscillations in sunflower disappeared when Ψm fell below -0.1 MPa (Cox, 1968).…”

“…Cyclic stomatal behaviour has been observed in many species, including cotton, pepper and sunflower (Barrs & Klepper, 1968), citrus trees (Levy & Kaufmann, 1976), rose bushes (Rose & Rose, 1994) and grapevines (Düring, 2000;Düring & Stoll, 1996). These oscillations can be the result of instability in either a CO 2 control system or a water control system, or a combination thereof (Barrs, 1971).…”

“…The difference between air and leaf temperature (Cox, 1968), as well as CO2 exchange rate (Rose & Rose, 1994), follow the cyclic stomatal behaviour. Increases in xylem potential (Levy & Kaufmann, 1976) and Ψl (Barrs & Klepper, 1968) generally lag behind partial stomatal closure. The length of the lag period seems to increase due to increased root resistance at lower soil temperatures, i.e.…”

Oscillations in Stomatal Conductance

“…When the ambient humidity was further reduced, at B, the system became unstable; the initial fluctuation was followed by successively larger fluctuations. When instability of this kind occurs, the oscillations tend to become steady and to be maintained for prolonged periods of time (2,3,(7)(8)(9) It is easy to appreciate that the environmental gain increases as the ambient humidity is decreased, because the difference in humidity across the leaf epidermis is increased. Thus we would explain the way in which the behavior in Figure 1 was brought about as being a consequence of the enhanced feedback which accompanied increase of environmental gain (3).…”

“…A xgigo E = +gb gl + gb (2) Use of the appropriate value for the conductance of the boundary layer enables the leaf conductance to be automatically computed. Leaf conductance is the reciprocal of the leaf resistance commonly used in plant physiological literature.…”