Water relations of giant cacti of the Sonoran Desert

As climate change continues, understanding the water use strategies and water relations of cacti becomes even more important in conservation. Cacti are not only one of the most threatened taxonomic groups but also ecologically important to desert ecosystems. Water conservation strategies vary among species of columnar cacti as a tradeoff between photosynthetic and water storage capacities, such as the different volume-to-surface-area ratios in Carnegiea gigantea and Stenocereus thurberi. These variations in water and growth relations could be associated with the basipetal xylem vessel widening pattern that has been observed in many woody plant species, and most recently in cacti as well. This phenomenon provides a buffer to the accumulation of hydrodynamic resistance in xylem vessels as the plant stem elongates, and in cacti, stem water storage tissues (cortex and pith) also provide a buffer. This thesis investigates the rate of basipetal xylem conduit widening in Carnegiea gigantea and Stenocereus thurberi, with the expectation that columnar cacti will show similar rates of widening as other plants. I found that while the xylem conduits in both species widened at significantly different rates, the rate of widening was much lower than expected. While there are a few possible explanations, such as buffering from the succulent cortex tissue, more research on cactus xylem anatomy and its reflection in plant water conduction strategies is needed.