CONTROL OF PRESSURE-CHAMBER AND REHYDRATION-TIME EFFECTS ON PRESSURE-VOLUME DETERMINATION OF WATER-RELATION PARAMETERS

In a study of the seasonal variation of water-relation parameters in f our black spruce (Picea mariana (Mill) BSP) populations, the weekly de rivation of 32 pressure-volume curves with the sap-expression method b etween May and October required the simultaneous use of four pressure chambers and variable sample rehydration periods. Pressure chamber had a significant effect on three of the five characteristics studied: sy mplastic water fraction, maximum modulus of elasticity, and osmotic po tential at full turgor. These effects were constant over the growing s eason. On average, rehydration-time effects were significant for all w ater-relation parameters except osmotic potential at turgor loss point , but they were not constant over sampling dates. Variations in water- relation parameters due to chambers and rehydration times were control led and estimated through an experimental design based on the Latin sq uare, which allowed precise comparisons among populations despite the increase in variability due to these two factors. The efficiency of th e design varied between 110 and 142 % relative to a completely randomi zed design in which neither chambers nor rehydration times would have been controlled for.