Xylem embolism repair in Valencia Orange

Background/Question/Methods

Plants normally transport water under negative pressure, which make their hydraulic systems, the xylem, vulnerable to embolism formation if there is a water shortage in the soil.  Air can be drawn into xylem conduits through pit membranes, and the resulting embolisms impede water flow, which can lead to branch and leaf mortality.  Recent studies have shown that in some species, embolisms form and are repaired diurnally during transpiration and even under negative xylem pressure, but the mechanisms remain unknown.  Little is known about the vulnerability of fruit trees to embolism formation or their capability of embolism repair.  If fruit trees grown in semi-arid climates were capable of diurnal embolism repair then this could have implications for efficient irrigation scheduling, as watering could potentially be timed to occur when it is most beneficial for hydraulic recovery.  The objective of this study was to test for diurnal embolism formation and repair in Valencia Orange (Citrus x sinensis) grown in Southern California.  Plant water relations parameters, including leaf temperature, stomatal conductance, leaf water potentials, stem hydraulic conductance, and percent loss of conductance (PLC) were monitored over 24 hours periods at 4 hour intervals on nine young dwarf orange trees from fall to spring 2011/12.

Results/Conclusions

Orange stems were highly embolized, with 40 to 60% loss of conductance, in fall and winter, but there was no indication during these periods of embolism repair, even though the trees were irrigated regularly.  Nighttime leaf water potentials during these periods ranged between -0.5 and -0.7 MPa; and no relationship was found during this time between water potentials and the degree of xylem embolism.  Findings from a warm day in spring 2012 were substantially different.  Xylem embolisms formed during the day to the same levels as previously observed (40 to 60% PLC) and were repaired during the night down to less than 25% PLC at leaf water potentials of about -0.3 to -0.6 MPa, corresponding to stem water potentials of about -0.1 to -0.4 MPa.  In spring, there was a significant relationship between percent loss of conductance and water potential, with a 50% PLC estimated at -1.1 MPa.  The findings suggest that orange trees may require high nocturnal temperatures and high water potentials for embolism repair.  The finding that about half of the vessels appear to be almost permanently embolized in dwarf Valencia Orange trees is hypothesized to be a dwarfing root stock effect.