Aims Xylem embolism is a physiological response of tree species to adverse environmental factors, such as water deficit. However there is little research on relationships between phosphorus and xylem embolism. Our objectives were to 1) explore whether phosphorus could increase xylem embolism occurrence and 2) test the hypothesis of “drought tolerance by restricting sap flow in xylem".
Methods The tree species Acer truncatum and Ligustrum lucidum were chosen to study the response of xylem embolism (measured as percentage loss of hydraulic conductivity, PLC) to phosphorus addition under drought-stressed and well-watered treatments (soil water content of about 30%-40% and 70%-80% of field moisture capacity, respectively).
Important findings Daily PLC ranged from 60.7% to 70.7% in A. truncatum and 43.9%-72.3% in L. lucidum, indicating that occurrence and refilling of xylem embolism was a usual event in these woody plants. Phosphorus addition raised PLC significantly for A. truncatum at each water status and L. lucidum was subjected to drought. A. truncatum had more xylem embolism occurrence with drought than when well-watered, but there was no significant difference within two water status in L. lucidum. Drought could increase the embolism vulnerability of A. truncatum, causing higher percentage loss of hydraulic conductivity than in L. lucidum under the same water stress. Consequently, drought-resistant trees are not always invulnerable to water deficit. This might be a strategy for a water-limited environment. This study suggests that xylem embolism might be a “hydraulic signal" in plants. Once a plant is under water stress, its xylem conduits become cavited or embolized, resulting in decreased hydraulic conductivity and closure of stomata to maintain water balance by reducing water transportation and consumption. This study supports the hypothesis of “drought tolerance by restricting sap flow in xylem".