FREEZING REGIME AND TRADE-OFFS WITH WATER TRANSPORT EFFICIENCY GENERATE VARIATION IN XYLEM STRUCTURE ACROSS DIPLOID POPULATIONS OF LARREA SP (ZYGOPHYLLACEAE)

Premise of the study: The impact of changing temperature regime on plant distributions may depend on the nature of physiological variation among populations. The arid-land genus Larrea spans habitats with a range of freezing frequency in North and South America. We hypothesized that variation in xylem anatomy among populations and species within this genus is driven by plasticity and trade-offs between safety from freeze-thaw embolism and water transport efficiency. Methods: We measured vessel density and diameter distributions to predict freeze-thaw embolism and water transport capacity for high and low latitude populations of three Larrea species grown in the field and a greenhouse common garden. Key results: Among field-grown L. divaricata, low latitude plants had larger mean vessel diameter and greater predicted freeze-thaw embolism, but higher water transport capacity compared with high latitude plants. Though high latitude L. tridentata and L. nitida had abundant smaller vessels, these plants also produced very large vessels and had semi ring-porous wood structure. Thus, their predicted embolism and water transport capacity were comparable to those of low latitude plants. Differences among field-grown and common-garden-grown plants demonstrate that plasticity contributes to population differentiation in xylem characters, though high latitude L. divaricata exhibited relatively lower plasticity. Conclusions: Our results indicate that a trade-off between transport safety and efficiency contributes substantially to variation in xylem structure within the genus Larrea. In addition, we suggest that xylem plasticity may play a role in negotiating these trade-offs, with implications for responses to future climate change.