Embolisms in the vessels of maize axile roots of different types
were observed directly after rapid freezing of intact,
functioning roots in the field, by cryo-scanning electron microscopy.
Quantification of the degree of embolization
in each root was made by counting empty and full vessels of both the
late and early metaxylem (LMX & EMX),
and expressed as percent embolized vessels of the LMX, and %EMX poles
containing embolized vessels.
Contents of the connecting xylem (CX) at branch root junctions, and
of xylem in branch roots were observed also,
but not systematically quantified. Records of % embolized vessels were
made from dawn to dusk on summer days
in Ottawa under moderate irradiance, and in Canberra under high irradiance.
Measurements in Canberra were
supported by estimates of irradiance, of stomatal conductance, and of
chamber balance pressure of bagged and
unbagged leaves. Soon after sunrise embolisms appeared in all types of
vessel, at balance pressures c. 300–400 kPa,
and increased rapidly with increasing irradiance. During the middle of
day % embolized vessels reached a
maximum (LMX ≈70% in Ottawa, and ≈80% in Canberra). At all times
EMX vessels were less embolized.
The midday maximum was brief in Ottawa, and % embolized vessels fell to
low value during the afternoon. In
Canberra the maximum was prolonged into late afternoon. By dusk nearly
all vessels were once again filled with
sap. The balance pressures measured during vessel refilling in Canberra
ranged from 500 kPa to 1200 kPa. At all
times of the day sap was seen entering some embolized vessels. Almost
all were refilling by mid- to late-afternoon.
Such refilling was especially frequent at junctions of branch roots with
the axile roots. X-ray microanalysis of the
sap entering the vessels, and of the liquid filling or partly
filling vessels, showed the concentration of mineral
solutes present in the sap was below the threshold of detection
(≈12 mM). These results are discussed in relation
to current opinions about embolisms and vessel refilling.