- The influence of shading intensity on performance, chlorophyll fluorescence emissions, the slope of the fluorescence induction curve, chlorophyll contents, and stomatal conductance of four Pacific Northwest conifer species [ponderosa pine (Pinus ponderosa Laws.), Douglas-fir
(Pseudotsuga menziesii (Mirb.) Franco), western redcedar (Thujaplicata Donn.), and western hemlock (Tsuga heterophylla (Raf.) Sarg.)] was investigated. Seedlings were grown under shade-cloth tents (supported by
PVC pipe frames) using a repeated measures design for approximately 29 weeks in 1993 at Forest Research Laboratory, Oregon State University, Corvallis, Oregon. Four shade treatments (0, 30, 50, and 70% of full
sunlight) with four replications each were used. Shading significantly influenced morphology, chlorophyll fluorescence, and chlorophyll contents of all four species.
Although seedlings elongated most under 70% shade and least in full sunlight, biomass production significantly decreased with increasing shade. Overall, best performance was achieved with 30% shade. Chlorophyll fluorescence (Fm, F, F / Fm), measured with an integrating fluorometer, significantly decreased over time with increasing shade for all four species. There was a significant decrease in final Fm, F, F / Fm of 27, 48, and 29%, respectively, from 0% to 70% shade. The slope of the fluorescence induction curve between the 60 and 120 second
portion of the curve, regardless of shade treatment, had a significant linear relationship with time and species. Chlorophyll contents of old and new needles were strongly negatively correlated with all chlorophyll fluorescence variables and directly related with increasing shade. Chlorophyll b and total chlorophyll in old needles substantially increased by 188 and 158% from 0% to 70% shade. Stomata! conductance of old needles, regardless of shade treatment, had a linear relationship with time and species while in new needles, this relationship was quadratic. Findings of the study show that further evaluation of fluorescence emissions and the slope of the fluorescence curve is warranted. To make it more meaningful in context with seedling's physiological status, measurements should be made concomitantly with CO2 assimilation.