Xylem Embolism Occurrence In Wheat Plants Relating To Physiological Basis For Water Saving And Drought Resistance

By ultrasonic acoustic emissions (AE) monitoring technique, leaf xylem cavitation and embolism were continuously monitored in field-grown winter wheat plants from jointing to late grain filling stage, and in potted wheat plants over an intense soil drought and rewatering cycle as well. The study emphasized on characteristics of leaf xylem embolism occurrence under water stress and well-watered conditions, and on genotypic variations in xylem cavitation resistance in leaves of field-grown wheat plants as well. For a basic understanding of the relevance of leaf xylem embolism to physiological basis of drought resistance and water saving of wheat plants, relations between leaf xylem embolism and gas exchange, as well as other parameters relating to plant water relations were also analyzed. The major results are as follows.1. Diurnal changing patterns in leaf water potential and AE from xylem of wheat leaves The typical diurnal changing courses of leaf water potential (Ψl) followed shape of the letter"U", consisting of 4 major periods:"steep declining period","low level plateau","steep recovering period"and"slow recovering period". AE signals occurred on a diurnal basis, with diurnal patterns in close association with that ofΨl. AE generally started around 6-8 AM asΨl began to decline. AE activity was relatively weak around midday whileΨl was in period of"low level plateau". AE rate generally peaked between 8 AM and 1 PM whileΨl was steadily decreasing, and/or around sunset asΨl was rising from its midday minimum. AE signals generally stopped between 9 PM - 3 AM after the leaf had been well rehydrated. Considerable variations in day-to-day diurnal patterns existed for AE signals. However, The overall averaged diurnal course of AE tended to show similar patterns between years and treatments. Averaged across both years, AEs recorded per hour in the period of 17:00-22:00 accounted for 7% of the total of 24 h AEs, followed by 4.2% in 10:00-17:00, 3.0-3.5 % in 5:00-10:00, 3.0-3.5 % in 22:00-3:00, and 1.4% in 3:00-5:00, respectively. Large variations among days were observed in the percentage of AEs during period of decliningΨl to the total of 24 h AEs, which averaged 55% in year 2004 and 36% in 2005, respectively. The leaves emitted very few AEs under well-watered condition in potted wheat plants in the laboratory condition. The diurnal AE occurring pattern in droughted leaves from potted plants was similar to that of field-grown ones. Most of the AE waveform physical parameters varied within very broad...