Machado, E. C. | bioRxiv Science

bioRxiv2022

Dynamic changes in gas solubility of xylem sap reiterate the enigma of plant water transport under negative pressure

Despite a long research history, we do not fully understand why plants are able to transport xylem sap under negative pressure without constant failure. Microbubble formation via direct gas entry is assumed to cause hydraulic failure, while the concentration of gas dissolved in xylem sap is traditionally supposed to be constant, following Henrys law. Here, the concentration of soluble gas in xylem sap was estimated in vivo using well-watered Citrus plants under varying levels of air temperature and photoperiodic exposure, and compared to modelled data. The gas concentration in xylem sap showed non-equilibrium curves, with a minimum over- or undersaturation of 5% compared to gas solubility based on Henrys law. A similar diurnal pattern was obtained from the gas concentration in the cut-open conduits and discharge tube, and oversolubility was strongly associated with decreasing xylem water potentials during transpiration. Although our model did not explain the daily changes in gas solubility for an anisobaric situation, oversolubility characterises nanoconfined liquids, such as sap inside cell walls. Thus, plants are able to transport sap under negative pressure with relatively high amounts of dissolved gas, providing them with a buffering capacity to prevent hydraulic failure, despite diurnal changes in pressure and temperature.

plant biology↗

bioRxiv2019

Enhanced nitric oxide synthesis through nitrate supply improves drought tolerance of sugarcane plants

Nitric oxide (NO) is an important signaling molecule associated with many biochemical and physiological processes in plants under stressful conditions. Nitrate reductase (NR) not only mediates the reduction of NO3- to NO2- but also reduces NO2- to NO, a relevant pathway for NO production in higher plants. Herein, we hypothesized that sugarcane plants supplied with more NO3- as a source of N would produce more NO under water deficit. Such NO would reduce oxidative damage and favor photosynthetic metabolism and growth under water limiting conditions. Sugarcane plants were grown in nutrient solution and received the same amount of nitrogen, with varying nitrate:ammonium ratios (100:0 and 70:30). Plants were then grown under well-watered or water deficit conditions, in which the osmotic potential of nutrient solution was -0.15 and -0.75 MPa, respectively. Under water deficit, plants exhibited higher root [NO3-] and [NO2-] when supplied with 100% NO3-. Accordingly, the same plants also showed higher root NR activity and root NO production. We also found higher photosynthetic rates and stomatal conductance in plants supplied with more NO3-, which improved root growth. ROS accumulation was reduced due to increases in the activity of catalase in leaves and superoxide dismutase and ascorbate peroxidase in roots of plants supplied with 100% NO3- and facing water deficit. Such positive responses to water deficit were offset when a NO scavenger was supplied to the plants, thus confirming that increases in leaf gas exchange and plant growth were induced by NO. Concluding, NO3- supply is an interesting strategy for alleviating the negative effects of water deficit on sugarcane plants, increasing drought tolerance through enhanced NO production. Our data also provide insights on how plant nutrition could improve crop tolerance against abiotic stresses, such as drought. HighlightsO_LINitrate supply improves sugarcane growth under water deficit. C_LIO_LINitrate supply stimulated nitrate reductase activity and NO synthesis in sugarcane roots facing water deficit. C_LIO_LILeaf gas exchange was increased by nitrate supply as well as root growth under water limiting conditions. C_LIO_LIAntioxidant responses were also improved in plants supplied exclusively with nitrate. C_LIO_LINitrogen management may be an interesting strategy for improving drought tolerance in sugarcane fields. C_LI

plant biology↗