In order to avoid excess water loss during drought, plants close their stomata, but the stomatal closure point differs between species. Hydraulic theory suggests that the stomatal closure point is linked with xylem anatomy and vulnerability to embolism, but it also has implications for phloem transport and anatomy. Plants that withstand lower leaf water potentials (anisohydric) should have smaller xylem conduits and/or fewer and smaller inter-conduit pits, but larger or more abundant phloem conduits to compensate for the increase in sap viscosity, relative to plants that close their stomata even during mild drought (isohydric). To test these hypotheses, we compared the anatomy of two co-occurring trees that differ in their response to drought: piñon pine, which is considered isohydric and vulnerable to xylem embolism, and one-seed juniper, which is considered anisohydric and embolism-resistant. We grew them in three different treatments: drought, ambient, and irrigation and then imaged branch samples using a phase contrast synchrotron radiation X-ray tomographic microscope at the TOMCAT beam line of Swiss Light Source, Paul Scherrer Institute. These images were used to analyze xylem and phloem conduit size, and the abundance and size of inter-conduit pits. Light microscopy was used to analyze total xylem and phloem area.
Our analysis revealed one major anatomical difference between the two species: pine trees had a xylem-to-phloem ratio of almost 2-to-1 across all precipitation treatments, whereas junipers had very nearly the opposite ratio. In theory, a greater phloem area, which allows for greater phloem transport capacity, is necessary to ensure that phloem flow continues at low leaf water potentials. Xylem and phloem tend to be at hydraulic equilibrium, so when water tension increases in the xylem, phloem adjusts osmotically to match. In turn, as leaf water potential drops, the viscosity of the phloem sap increases. That anisohydric junipers had a greater phloem proportion than isohydric pines supports this theory. Interestingly, none of the other parameters we analyzed explained any differences in drought response or xylem vulnerability to embolism between the species. These results suggest that, for at least these two species, xylem-to-phloem ratio reflects the differences in stomatal closure point, but there are no anatomical differences to explain contrasting vulnerabilities to xylem embolism.