Studies On The Embolism Repair In The Tracheids Of Gymnosperm Samples

Cavitation may be initiated during water stress by the entry of air through conduit pit membranes, or by bubbles formed during freezing and thawing of xylem sap. The conduit quickly becomes filled with water vapor and air, resulting in an embolism within the conduit. Embolism is of importance to trees because it results in reduced hydraulic conductivity, which could lead to increased tension and the possibility of further runaway cavitation if transpiration continues at the same rate.Following daily cavitation the refilling of embolized conduits is common in both woody and herbaceous plants even when the water in neighboring conduits is under tension. Condensation of water vapor, dissolution of gas and expulsion of gas, as possible mechanisms for the repair of cavitated conduits had been suggested. Air dissolution is a primary mechanism for embolism repair in daily life.We did experiments like Lewis'(1994), but report different kinetics of embolus collapse in the tracheids of stem segments of several kinds of gymnosperm. Radial wood sections were trimmed. They were observed under a microscope, while segments were dehydrated and rehydrated. And we measured the contact angle of the tracheids.As there were some phenomena, which were not mentioned in Lewis' experiments, the air dissolution hypothesis ran into difficulty min our experiments. We found that the bigger bubbles under coverslip and above slide were getting larger and larger and the smaller ones were getting smaller and smaller during the collapse of the elongated bubbles in tracheids. We calculated total amount of the gas within all tracheids and all spherical bubbles under coverslip and above slide, the results indicated that the amounts of the gas in all bubbles were nearly the same both at the beginning and at the middle time, and at the final time of the collapse of elongated bubbles in a field of view. For the experiment phenomena, we did the qualitative analysis and quantitative count by physical theory, which repeated in the several kinds of gymnosperm very well. Therefore, we suggest that all the gas in elongated bubbles might move into the spherical bigger bubbles which were under coverslip and above slide .And we also suggest that there are a close relation between the contact angle and the embolism repair of trees.