The transmission of gas pressure to xylem fluid pressure when plants are inside a pressure bomb

In earlier work tobacco leaves were placed in a Scholander-Hammel pressure bomb and the end of the petiole sealed with a pressure transducer in order to measure pressure transmission from the compressed gas (P-g) in the bomb to the xylem fluid (P-x). Pressure bomb theory would predict a 1 : 1 relati onship for P-g:P-x when tobacco leaves start at a balance pressure of zero. Failure to observe the expected 1 :1 relationship has cast doubt on the pr essure-bomb technique in the measurement of the xylem pressure of plants. The experimental and theoretical relationship between P-x and P-g was inves tigated in Tsuga canadensis (L) branches and Nicotiana rustica (L) leaves i n this paper. It is concluded that the non 1 : 1 outcome was due to the com pression of air bubbles in embolized xylem vessels, evaporation of water fr om the tissue, and the expansion of the sealed stem segment (or petiole) pr otruding beyond the seal of the pressure bomb. The expected 1 : 1 relations hip could be obtained when xylem embolism was eliminated and stem expansion prevented. It is argued that the non 1 : 1 relationship in the positive pr essure range does not invalidate the Scholander pressure bomb method of mea suring xylem pressure in plants because P-x never reaches positive values d uring the determination of the balance pressure.