Vapour Pressure Deficit Response of Cuticular Conductance in Intact Leaves of Fagus sylvatica L.

The cuticular conductance (g_c) of the astomatous adaxial surfaceof Fagus sylvatica L. leaves was determined under varying vapourpressure deficits at a constant temperature of 20 °C. Cuticularconductance was determined from the weight loss of detachedleaves after both the stomatous abaxial surface and the cutend of the petiole had been sealed using low melting-point paraffinwax. Cuticular conductance was found to decrease as the watervapour pressure was increased in steps. No response was observedwhen vapour pressure deficit was decreased from an initiallyhigh value. It is concluded that these results are consistentwith the hypothesis that cuticular conductance is influencedby the water content of the cuticle and that the apparent hysteresisis likely to be a result of the long time-constant for the processof cuticle rehydration in intact leaves. Key words: Cuticle, cuticular conductance, Fagus sylvatica L., Fagaceae, vapour pressure deficit, VPD     

Variation in stomatal aperture in leaves of Avena fatua L. observed by low-temperature scanning electron microscopy

Abstract. A novel technique to record the variability of stomatal aperture over the leaf surface is described. This combines observations of leaf surfaces using low-temperature scanning electron microscopy (LTSEM), with digital image analysis to produce the most accurate aperture measurements obtained to date. Leaf samples are rapidly immobilized by cryo-fixation in liquid nitrogen and stored in a purpose-built cryo-storage system. Specimens can be collected in the field, remote from the cryopreparation system, and stored for up to several weeks before being examined on the LTSEM. The advantages of this method are that the time frame of the measurements is accurately known, and is identical for all stomatal apertures in a sample, and the precision of the measurements is not limited by the resolving power of the microscope. Measurements of stomatal aperture were obtained from leaves of field grown Avena fatua using the above procedure. Leaf surface conductance (g_sur) was determined by porometry immediately before cryo-fixation of the same region of the leaf. Measurements of aperture size showed a high degree of variability within each specimen, with coefficients of variation similar to those found in previous studies. Stomatal conductance (g_s) was calculated from stomatal dimensions using formulae derived elsewhere. A linear regression between the computed values of g_s and porometric estimates of g_sur showed good agreement with the regression line passing through the origin with a slope of 1.0 (R²=0.96). Applications of the experimental system are discussed.     

Abrasive damage by wind to the needle surfaces of Picea sitchensis (Bong.) Carr. and Pinus sylvestris L.

Abstract. After a period of one week at 11m s⁻¹ in a wind tunnel, the leaf surface of Picea sitchensis and Pinus sylvestris had undergone structural modification. Low-temperature scanning electron microscopy was used to examine these changes. Picea showed flattening and smearing of wax crystals, as well as cracks in some of the wax structures filling the stomatal antechambers. In Pinus , most damage was on the cells surrounding the stomatal antechamber or on needle ridges. Artificially abraded surfaces were of similar appearance. Minimum epidermal conductance to water vapour ( g _e^MIN) was determined gravimetrically. In Picea g _c^MIN was more than doubled by wind treatment and increased eightfold by rubbing. Similar but less extreme increases occurred in Pinus. Neither species showed recovery of g _e^MIN after 1 week at low windspeed.