The reliability of cryoSEM for the observation and quantification of xylemembolisms and quantitative analysis of xylem sap in situ

The reliability of cryoSEM for visualizing gas embolisms in xylem vessels o f intact, functioning roots is examined and discussed. The possibility that these embolisms form as a result of freezing water columns under tension i s discounted by a double-freeze experiment. Two regions of the same root, o ne frozen under tension, the other isolated from the tension by the first f reeze, had the same percentage of embolisms, as did also long pieces of roo t frozen simultaneously along their length. The reliability of energy-dispe rsive X-ray analysis to measure xylem sap concentration in situ in frozen t issue was established by measurement of KCl standard solution frozen on stu bs, and within xylem vessels. Solute heterogeneity within the vessels varie d with freezing procedure; deep-freeze>LN2>cryopliers>liquid ethane, but on ly the deep-freeze method gave unsatisfactory estimates of concentration fo r the standard solution. It is concluded that cryoanalytical SEM is useful for direct observation of gas and liquid-filled compartments, and for solut e analyses at depth within intact plant organs.