Seasonal, diurnal and rehydration‐induced variation of pressure‐volume relationships in Pseudotsuga menziesii

Seasonal and diurnal variation and rehydration effects of pressure‐volume parameters in Pseudotsuga menziesii (Mirb.) Franco from a plantation in central Pennsylvania, USA, were evaluated during May‐September, 1989. Predawn elastic modulus was lowest in overwintering and newly expanded shoots in May and June, respectively, whereas predawn osmotic potentials at full and zero turgor were lowest in May and in early September, following an August drought. Seasonal variation in predawn relative water content at zero turgor was highly correlated with increases and decreases in elastic modulus and osmotic potential. Diurnal osmotic adjustment resulted in nearly constant turgor pressure, despite decreases in bulk shoot water potential. Elastic modulus decreased diurnally on 1 August and increased on 3 September. Decreases in osmotic potential and/or elastic modulus on 24 June and 1 August lowered the relative water content at zero turgor. Plateaus in pressure‐volume data caused by excess apoplastic water, were present in 67% of naturally rehydrated shoots and in all of the shoots artificially rehydrated for 3, 6, 12 and 24 h, and they increased in volume with rehydration time. Plateaus represented 80–95% of the excess apoplastic water lost during pressure‐volume analysis. Correcting for plateaus via linear regression had no significant effect on osmotic potential at full turgor; however, uncorrected elastic modulus and relative water content at zero turgor were often significantly lower than the plateau‐corrected values, particularly in artificially rehydrated shoots. Plateau‐corrected osmotic potential at full turgor and osmotic potential at zero turgor were significantly higher in most artificially rehydrated shoots than in those naturally rehydrated as the result of loss of symplastic solutes. Corrected elastic modulus decreased following 12 and 24 h of rehydration and corrected relative water content at zero turgor increased in as little as 3 h of rehydration. These results indicate that seasonal and diurnal patterns of tissue‐water parameters in Pseudotsuga menziesii vary with plant phenology and drought conditions, and that the length of rehydration period is an important consideration for pressure‐volume studies.