Exploring the relationship between osmoregulation of wood parenchyma and xylem embolism repair

Background/Question/Methods

                Drought-stressed plants are at considerable risk of forming embolisms when the need for water exceeds the amount of soil moisture available. Plants in dry environments must either resist embolism formation and invest heavily into constructing dense wood and thick conduit walls or invest in mechanisms for embolism repair. One such mechanism may be to utilize living cells embedded within the wood as a source of water and energy to refill embolized vessels. These living cells can rapidly adjust their osmotic potential by converting long-chain polysaccharides into short-chain polysaccharides, and vice versa. A widely accepted theory suggests that these cells may exude osmotica into embolized vessels, allowing water to flow in via a concentration gradient. The purpose of this research is to better understand the relationship between cellular osmoregulation of vessel-associated cells and embolism repair. Xylem sap from both filled and embolized vessels, cellular and wood samples were collected from California chaparral shrubs. Samples were analyzed for sugar and starch content using sugar assays and confocal microscopy. Additionally, these data were compared to physiological data such as sap flow and xylem/whole tissue water potential.

Results/Conclusions

                When plants were under little to no water stress, carbohydrates were found to be crystalized as starch grains in plastids within living cells. Densely-packed starch has less surface area free to interact with water and thus has almost no osmotic effect. Conversely, when plants were under water stress, fewer and smaller starch grains and higher sugar contents indicated that the cells were adjusting their osmotic potential. Little sugar was present in xylem sap from either filled or embolized vessels, but osmotic content was consistently higher in embolized vessels (when it could be collected). However, this elevated osmotic content in embolized vessels was not enough to produce a concentration gradient from wood parenchyma to embolized vessel, as wood parenchyma was found to have a much higher concentration of sugars than samples collected from vessels. It may be that vessel-associated cells experience an increase in hydrostatic pressure when long-chain polysaccharides are dissolved, and thus water may be pushed into embolized vessels under positive pressure. This process would be less energetically costly than exuding, and subsequently disposing of, osmotica into xylem vessels. Further research will focus on more coherently parsing out the functions of vessel-associated cells during xylem embolism repair.