Effect of Mid-day Shading on Stem Diameter, Xylem Pressure Potential, Leaf Surface Resistance, and Net Assimilation Rate in a White Oak Sapling

Abstract

The diurnal relationship between xylem pressure potential and stem diameter in white oak (Quercusalba L.) resulted in a hysteresis loop on clear days where, for a given level of xylem pressure potential, the stem was thinner upon rehydration than during dehydration. Cloudy days complicated this pattern by forming a loop within the main loop. This additional loop was caused by the same factor which caused the main loop, the delayed reaction or lag between changes in stem diameter and xylem pressure potential. Both net assimilation rate and leaf surface resistance were closely linked and responded rapidly to changes in radiation. Hydroactive increases in leaf surface resistance were observed as xylem pressure potential decreased below −23 bars. A discussion of the relationship between stem diameter, radiation, leaf surface resistance, and xylem pressure potential is presented.