Measurements of the time to refill embolized vessels

The time taken to refill embolized xylem vessels in field-grown roots of maize (Zea mays L.) was determined after transpiration was stopped by detopping the plants, or after individual roots were isolated from the plant. At various times after transpiration-driven flow was stopped, root segments were frozen in situ and the percentage of large, late metaxylem vessels that were embolized was determined by examination in a cryo-SEM. Control roots frozen before interruption of the transpiration stream had a variable percentage of the vessels embolized, but no embolized vessels were found by about 20 min after isolation. Individual roots and vessels showed large variances in time to refill. In detopped root systems, the mean percentage of embolized vessels showed an exponential decay with a half-time of ∼2 min. The variance in refilling time is ascribed to two causes: the various proportions of gas and liquid present in an individual vessel at the time of initial measurement, and the balance in composition between water vapour and air in the gas phase. Time to produce exudate from the cut surface of the isolated roots was variable and unrelated to the percentage of vessel embolism. It is proposed that the refilling of the vessels in these isolated roots is by water pushed radially into the vessels by living cells, and that embolized vessels throughout intact transpiring plants are similarly refilled.