Wheat Maintains Stem Water Potential During Drought Stress Despite Declining Osmotic Potential

Plant water potential is one of the most frequently measured variables of plant water status. Stem water potential, often approximated by wrapping the leaves, is assumed to be more stable and a better measure of drought stress than leaf water potential. In wheat (Triticum aestivum L.), the stems cannot be seen as merely water transporting organs; rather, they store high amounts of osmotically active carbohydrates, which are postulated to affect water uptake and storage. This study compared the true stem water potential with the wrapped leaf water potential. One drought-sensitive cultivar (Viking) and two resistant cultivars (Impala and Servus) were subjected to different levels of drought stress. Osmotic potential and water content were also measured to study and compare the hydraulic responses to drought stress in the different cultivars. We found that the wrapped leaf water potential does not match the stem water potential in wheat. Instead, wheat maintains its stem water potential during drought stress. Despite the low osmotic potential in the stem parenchyma, the water potential in the stem xylem did not decline as drought stress progressed. This paradox can be explained by recent findings that not only water potential-driven flow, but also turgor-driven flow occurs in wheat stems. This hypothesis suggests that the carbohydrates in stem parenchyma induce an influx of water, but that this water is transported back out under a hydrostatic gradient, redirecting the water to the developing ear.