Influence of Substrate Water Content and Daily Light Integral on Photosynthesis, Water Use Efficiency, and Morphology of Heuchera americana

Two investigations were conducted to determine the morphological and physiological impacts of varying light and substrate water levels on Heuchera americana 'Dale's Strain' (american alum root). Both investigations used a capacitance sensor automated irrigation system to maintain constant substrate volumetric water contents (theta = volume of water/volume of substrate). In the first study, the substrate was maintained at one of eight theta ranging from 0.15 to 0.50 L.L-1. Leaf area of plants grown at the highest theta was more than twice that of plants grown at the lowest theta. Shoot dry weight also responded positively to 0 increasing from 0.15 to 0.35 L.L-1, but plants did not have greater dry weights when maintained at theta higher than 0.35 L.L-1. The second experiment assessed american alumroot's performance under four daily light integrals (DLIs) (7.5, 10.8, 14.9, and 21.8 mol.m(-2).d(-1)) with theta maintained at 0.35 L.L-1. Increasing DLI from 7.5 to 21.8 mol.m(-2).d(-1) caused shoot dry weight, leaf area, maximum width, and leaf count to change quadratically. Dry weight and leaf area reached their maximum at 10.8 mol.m(-2).d(-1), whereas leaf count was greatest at 14.9 mol.m(-2).d(-1). Increasing DLI to 21.8 mol.m(-2).d(-1) negatively impacted leaf area and leaf count but did not lower shoot dry weight. Leaf area ratio and petiole length of the uppermost fully expanded leaf decreased with increasing DLI. Measures of leaf-level net photosynthesis, light response curves, and CO2 response curves indicated no physiological differences among plants grown under different water or light levels. In both studies, long-term, whole crop measures of water use efficiency based on shoot dry weight and water applied (WUEc,) did not reflect the same water use trends as instantaneous, leaf-level measures of WUE based on leaf gas exchange (WUE1). WUE, decreased with increasing theta and DLI, whereas WUE1 was not influenced by Band increased with increasing DLI. WUE1 is often used to provide insight as to how various abiotic and biotic factors influence how efficiently water is used to produce biomass. However, these findings demonstrate that there are limitations associated with making such extrapolations.