Initiation of vessel formation and vessel maturation indicated by secondary wall deposition have been compared in eleven deciduous broadleaved tree species. In ring-porous species the first vessel element formation in the current growth ring was initiated two to six weeks prior to the onset of leaf expansion, and secondary wall deposition on the vessel elements was completed from one week before to three weeks after leaf expansion. In diffuse-porous species, the first vessel element formation was initiated two to seven weeks after the onset of leaf expansion, and secondary wall deposition was completed four to nine weeks after leaf expansion. These results suggest that early maturation of the first vessel elements in the ring-porous species will serve for water conduction in early spring. On the contrary, the late maturation of the first vessel elements in the diffuse-porous species indicates that no new functional vessels exist at the time of the leaf expansion.

The role of vascular tissue in conducting light was analysed in 21 species of woody plants. Vessels, fibres (both xylem and phloem fibres) and tracheids in woody plants are shown to conduct light efficiently along the axial direction of both stems and roots, via their lumina (vessels) or cell walls (fibres and tracheids). Other components, such as sieve tubes and parenchyma cells, are not efficient axial light conductors. Investigation of the spectral properties of the conducted light indicated that far-red light was conducted most efficiently by vascular tissue. Light gradients in the axial direction were also investigated and revealed that conducted light leaked out of the light-conducting structures to the surrounding living tissues. These properties of the conducted light suggest a close relationship with metabolic activities mediated by phytochromes. The results therefore indicate not only that signals from the external light environment can enter the interior of stems above ground and are conducted by vascular tissue towards roots under ground, but also that the light conducted probably contributes directly to photomorphogenic activities within them.

Dielectric soil moisture sensors have the potential for nondestructive and real-time monitoring of the stem water content (q st ) of living trees. This study was conducted to investigate the water use characteristics of trees in drylands through monitoring of q st using newly developed capacitance sensors (GS3). The plants used for data collection were Prosopis juliflora (Sw.) DC. (mesquite, invasive) in Sudan and Tamarix ramosissima Ledeb. (tamarisk, invasive) and Prosopis pubescens Benth.(screwbean mesquite, native) in the United States. The GS3 probes were installed into the trunks of two trees for each species. Stem-specific calibration equations and temperature calibration equations were derived through laboratory experiments and analysis of field observation data. The temperature calibration equations reduced inappropriate variations of q st caused by daily fluctuations in stem temperature, suggesting that these are essential for correct interpretation of monitoring data of q st in arid environments. The q st of the mesquite trees in Sudan clearly increased after heavy rainfall events and started decreasing when the soil water content became close to the wilting point. These findings indicate that mesquite trees use soil water in rainy seasons, even though they are generally considered to use groundwater through deep tap roots. The q st of neither species in the United States responded clearly to rainfall events, indicating that they depend on shallow saline groundwater. The q st of the tamarisk decreased monotonically throughout the monitoring period, apparently in response to feeding damage caused by the tamarisk leaf beetle (Diohabda sp.), which had been released for biological control of tamarisk.

A new type of dendrometer, comprising two reflection type photointerruptors and flexible mirrors, was developed to record precisely the diameter changes of tree trunks, and the practicality of this device was examined. The maximum resolution of the detector was 600 nm, due to the quantization noise of A /D conversion. Measurement in the laboratory at a constant temperature demonstrated fluctuations in the output of only three bits of A /D conversion, corresponding to a change of c. 3.6 μm in diameter. There was no thermal drift of the device during measurements. Diameter changes of sapling stems and /or a tree trunk of Castanospermum australe (Leguminosae) and Zelkova serrata (Ulmaceae) were measured by this device. In addition to steadily thickening growth and diurnal diameter changes, all the plants showed spikelike diameter changes of the order of minutes, the discovery of which illustrates the superior performance of this new type of dendrometer.

Optical properties of stems in woody plants were investigated in the winter–spring period, and compared with those in the summer–autumn period. In both periods light could enter the interior of the stems and was conducted efficiently along the axial direction. Vessels, fibers and tracheids were all involved in this axial light conduction. However, spectral properties of the light conducted by stems differed in different periods. The light conducted in stems of the winter–spring period showed higher relative intensity ratios at wavelengths around 825 nm and 920 nm than that in stems of the summer–autumn period. Furthermore, in the winter– spring period, stems of deciduous species conducted light at a wavelength around 825 nm at a higher relative intensity ratio than those of evergreen species. These seasonal variations and inter-specific differences in spectral properties of woody stems suggest a close relationship with rhythms of growth, substance metabolism and photomorphogenesis in plants.