DAILY EMBOLISM AND REFILLING OF XYLEM VESSELS IN THE ROOTS OF FIELD-GROWN MAIZE

Embolisms in the vessels of maize axile roots of different types were observed directly after rapid freezing of intact, functioning roots in the held, by cryo-scanning electron microscopy. Quantification of the degree of embolization in each root was made by counting empty and fu ll vessels of both the late and early metaxylem (LMX & EMX), and expre ssed as percent embolized vessels of the LMX, and %EMX poles containin g embolized vessels. Contents of the connecting xylem (CX) at branch r oot junctions, and of xylem in branch roots were observed also, but no t 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 s upported by estimates of irradiance, of stomatal conductance, and of c hamber balance pressure of bagged and unbagged leaves. Soon after sunr ise embolisms appeared in all types of vessel, at balance pressures c. 300-400 kPa, and increased rapidly with increasing irradiance. During the middle of the day % embolized vessels reached a maximum (LMX simi lar to 70 % in Ottawa, and similar to 80 % in Canberra). At all times the EMX vessels were less embolized. The midday maximum was brief in O ttawa, and % embolized vessels fell to a low value during the afternoo n. In Canberra the maximum was prolonged into late afternoon. By dusk nearly all vessels were once again filled with sap. The balance pressu res measured during vessel refilling in Canberra ranged from 500 kPa t o 1200 kPa. At all times of the day sap was seen entering some emboliz ed vessels. Almost all were refilling by mid-to late-afternoon. Such r efilling 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 concentra tion of mineral solutes present in the sap was below the threshold of detection (similar to 12 mM). These results are discussed in relation to current opinions about embolisms and vessel refilling.