Research Branch Report No. 141
Diurnal changes in water stress in young Pinus radiata D. Don under plantation conditions. P. W. Farrell and R. O. Squire. July 1979. 14 pp. (unpubl.)
This study investigated the diurnal variation in xylem pressure potential in young Pinus radiata growing on first and second rotation sites at Rennick. Xylem pressure potential, measured as the force required to extract water from needle fascicles, is an index of plant water stress; low xylem pressure potential indicating high water stress, and high xylem pressure potential indicating low water stress.
Over two periods of 24 hours, one of high and one of low surface soil moisture availability, xylem pressure potential was measured periodically, along with vapor pressure deficit, air temperature and solar radiation.
Results indicated that under conditions of high surface soil moisture availability no significant differences in xylem pressure potential existed between the two rotations during daylight hours. Significant differences occurred at night and were probably related to microclimatic variations. Under conditions of low surface soil moisture availability, significant differences in xylem pressure potential occurred during the major portion of the 24 hour period and were directly related to differences in surface soil moisture availability on the two sites, the first rotation site containing less available surface soil moisture. As vapor pressure deficit, air temperature and solar radiation decreased, xylem pressure potential on the second rotation site steadily increased to pre-dawn levels whereas xylem pressure potential on the first rotation site decreased sharply before increasing to pre-dawn levels.
Multiple linear models relating xylem pressure potential to the meteorological variables vapor pressure deficit, air temperature and solar radiation were developed for each diurnal period x rotation, and were found to explain a large percentage of the observed variation in xylem pressure potential. The combining of data between periods and rotations was not as successful.