Xylem vessel length and distribution: does analysis method matter? A study using Acacia

Knowledge of xylem vessel length is important for several reasons, including the accurate calculation and comparison of hydraulic conductivity studies in excised stems. Vessel length data and distributions are also relevant in some anatomical, ecological, evolutionary, pathological and compatible hydraulic studies. However, determining vessel length is tedious, so is often either avoided or undertaken arbitrarily in hydraulic conductivity studies. We examined four injection media (paints and inks) under transmission electron microscopy to ascertain which was most suitable for determining vessel length. Hunt’s Speedball India ink, with evenly distributed, uniform spherical carbon particles of 33 nm, would avoid premature vessel blockage and, therefore, coupled with the fact that it is non-toxic, is the preferred medium to determine vessel length in Acacia amoena Wendl. terminal stems. The longest vessel was 10 cm, which accounted for 0.4% of vessels. Vessel length distributions were then determined and compared using the same dataset and four established methods. All four methods produced distributions which indicated that the most common vessel length class was short (0–2 cm), and no method was significantly different from the other; however, for ease of calculation, the Christman et al. (2009) method is recommended. Whether vessel length or distribution is necessary for hydraulic-conductivity studies will depend on whether or not merely indicative rates of flow are required, but to provide comparative information for global datasets, then they are needed.