Ultrasound acoustic emissions from dehydrating leaves of deciduous and evergreen trees

While drying, detached leaves produced ultrasound acoustic emissions (UAE) comparable to emissions from stem and twig wood. Experiments on Ilex aquifolium L. showed that the main source of these signals was cavitation in the veins, to which conduits and fibres probably both contributed. Regions of the leaf blade with abundant mesophyll and only small veins emitted few signals. More signals were counted on the adaxial side of the midrib than on the abaxial one and on the proximal third than on the distal one, in accordance with the anatomical structure. Sound attenuation was pronounced. Eight species were compared with respect to cavitation behaviour, field water relations and pressure–volume curves, and the data showed differences in cumulative number of events and resistance of leaves to cavitation. Data were generally in good agreement with anatomical structure and habitat preferences. The number of signals per conduit counted on cross-sections was in some leaves much higher than unity, which suggests short xylem elements or an acoustic activity of cells other than conduits. There was no correlation between cavitation threshold or cumulative number of signals and the degree of sclerophylly; unexpectedly, there was a correlation between the cumulative number of signals at a water potential of -1.3 MPa and the bulk modulus of elasticity.     

Ultrasound acoustic emissions from freezing xylem*

Rachides of Juglans regia L. (Juglandaceae) and one‐year‐old twigs of Evonymus latifolia (L.) Mill. (Celastraceae) were cooled in air to ?25 °C, with an ultrasound detector attached to the xylem where peripheral tissues had been peeled off. Ultrasound acoustic emissions started between ?4·5 and ?14·3 °C, as measured with a thermocouple inserted into the xylem. The number of emissions was significantly lower from saturated plant parts than from those frozen at field water potentials. Bench‐drying of saturated samples produced significantly less signals than the freezing protocols. These findings are in accordance with the hypothesis that freezing of xylem under tension induces cavitation events. They corroborate earlier work which tried to provide a logical explanation for the seemingly paradoxical cryo‐scanning electron microscope observations of changing vessel contents during a daycourse in the field.     

A method for inducing xylem emboli in situ: experiments with a field-grown tree

A pressure collar, assembled around 25cm sections of 4-year-old willow twigs, was used to examine cavitation events under field conditions. When the air pressure inside the collar was raised to between 1–8 and 2–8MPa, ultrasound acoustic emission signals were triggered which indicated the breaking of water columns in the xylem. The hydraulic conductivity of the twig portion inside the chamber decreased markedly. As a result, water potentials and conductances in leaves at the end of the twig decreased. Similar changes were induced at comparable pressures in detached twigs. The equipment used is described in detail, and evidence is presented that the mechanism of this artificial production of emboli follows the air-seeding principle hypothesized for natural cavitation events.     

Oleic acid synthesis in the fruit of Persea americana MILL

The tissue slices from the mesocarp of avocado could incorporateradioactive acetate into lipids. Oleic, palmitic and stearicacids were the most labeled fatty acids found to accumulatein triglycerides. The conclusions that oleic acid was formedby way of chain elongation of already unsaturated short-chainfatty acids, and that there was no evidence for the desaturationof stearic acid were based on the following observations: 1)Stearic acid-¹⁴C was incorporated into triglycerides by thetissue slices without formation of oleic acid. 2) The oleicacid synthesized from specifically labeled acetate was not randomlylabeled. The specific radioactivity of azelaic acid moiety ofoleic acid was rapidly increased while that of pelargonic acidmoiety was gradually increased. 3) An unexpected rise of stearicacid was observed among commonly occurring fatty acids in thetissue slices. This was accentuated by anaerobiosis which prevailedduring vacuum infiltration of labeled acetate. ¹Present address: Department of Botany, Faculty of Agriculture,Hokkaido University, Sapporo, Japan. (Received January 13, 1969; )     

Leaf Discs or Press Saps? A Comparison of Techniques for the Determination of Osmotic Potentials in Freeze-Thawed Leaf Material

Two standard methods for the determination of osmotic potentialsin plant organs, thermocouple hygrometry of press saps and offreeze-thawed leaf discs, were compared on leaves of ten species.Strictly parallel samples invariably showed lower (more negative)osmotic potentials for the leaf discs. Evidence is presentedthat osmotically-active solutes are not completely extractedduring the preparation of press saps and that the discs contain,on average, a smaller amount of apoplastic water than the saps.Hydrolysis of macromolecules in the discs was not responsiblefor the differences. The presence of the cutinized epidermisresults in a comparatively long equilibration time for leafdiscs, which may be reduced by appropriate measures. Key words: Thermocouple psychrometry, vapour pressure equilibration, leaf discs, press saps, solute extraction     

Role of the pentose phosphate pathway during callus development in explants from potato tuber

Metabolic change during callus formation in explants from potatotuber (Solarium tuberosum L.) was investigated. Under the presentculture conditions, callus starts exponential growth (estimatedby the fresh weight increase) with a 5-day lag period, at whichtime marked production of DNA occurs. In the lag period, respirationis very much resistant to malonate, and the value of the C6/C1ratio and activities of both G6PDH 6PGDH pass their peaks. Onthe basis of these and relevant findings, glucose metabolismthrough the PP pathway is surmised to play a significant rolein the initial DNA multiplication phase of callus development. (Received October 15, 1969; )     

Leaf hygrometer v. pressure chamber: a comparison of pressure–volume curve data obtained on single leaves by alternating measurements

Abstract. Data for the construction of pressure-volume curves were obtained by measuring water potentials of detached leaves repeatedly and alternately, with a pressure chamber and a leaf hygrometer. Good agreement between the parameters of the two resulting curves was observed. Regression lines on values after the loss of turgor were always more negative for the thermocouple data, with a maximum difference for the osmotic potential at full saturation of 0.25 MPa in Triticum and a minimum of 0.01 MPa in Populus. Neither the slopes of the regression lines nor the intercepts with the axes were statistically different. We see no reason for using one of these two unrelated methods as a standard against which the other is calibrated. Implications for the theory of pressure-volume curves are discussed.     

Rapid Osmotic Adjustment in Detached Wheat Leaves

Osmotic adjustment is induced in detached wheat leaves by rapiddrying to a relative water content below 0·65, followedby re-saturation. Quantitatively, the response to this treatmentis comparable to the maximum of adjustment obtained with pottedplants at the same developmental stage. Low temperatures duringdrying and re-saturation of the leaves reduce the adjustmentresponse. We conclude that drought stress serves as a triggeronly, while the metabolic events lowering the osmotic potentialare favoured by high or intermediate water contents. Triticum durum L., durum wheat, rapid dehydration, osmotic adjustment, pressure-volume curves     

Metabolism of glucose in relation to the lipid synthesis in the fruit of Persea americana MILL.

Studies on the lipogenesis in the fruit of avocado were undertakento elucidate the metabolism of glucose relative to the lipidsynthesis as an important process. The relative participationof the pentose phosphate pathway was determined. Approximately30% of the glucose metabolized by catabolic pathways was directlyoxidized to release carbon dioxide and to provide about 50%of the total reducing power responsible for the fatty acid synthesis.Whereas carbon number 1 of glucose was incorporated into theglyceryl moiety, carbon number 6 was incorporated into the fattyacyl moiety of glycerolipids in the tissue slices of the developingfruit. From the experiments in vitro, it may be concluded that,besides glucose-6-phosphate dehydrogenase, malic enzyme andisocitrate dehydrogenase participate in the provision of NADPH₂for the fatty acid synthesis in the mesocarp of avocado fruit. ¹Present address: Department of Botany, Faculty of Agriculture,Hokkaido University, Sapporo 060, Japan (Received January 13, 1969; )