Light-induced plasticity in leaf hydraulics, venation, anatomy, and gas exchange in ecologically diverse Hawaiian lobeliads.

Leaf hydraulic conductance (Kleaf ) quantifies the capacity of a leaf to transport liquid water and is a major constraint on light-saturated stomatal conductance (gs ) and photosynthetic rate (Amax ). Few studies have tested the plasticity of Kleaf and anatomy across growth light environments. These provided conflicting results. The Hawaiian lobeliads are an excellent system to examine plasticity, given the striking diversity in the light regimes they occupy, and their correspondingly wide range of Amax , allowing maximal carbon gain for success in given environments. We measured Kleaf , Amax , gs and leaf anatomical and structural traits, focusing on six species of lobeliads grown in a common garden under two irradiances (300/800 µmol photons m(-2)  s(-1) ). We tested hypotheses for light-induced plasticity in each trait based on expectations from optimality. Kleaf , Amax , and gs differed strongly among species. Sun/shade plasticity was observed in Kleaf , Amax, and numerous traits relating to lamina and xylem anatomy, venation, and composition, but gs was not plastic with growth irradiance. Species native to higher irradiance showed greater hydraulic plasticity. Our results demonstrate that a wide set of leaf hydraulic, stomatal, photosynthetic, anatomical, and structural traits tend to shift together during plasticity and adaptation to diverse light regimes, optimizing performance from low to high irradiance.

Senescence-related changes in nitrogen in fine roots: mass loss affects estimation.

The fate of nitrogen (N) in senescing fine roots has broad implications for whole-plant N economies and ecosystem N cycling. Studies to date have generally shown negligible changes in fine root N per unit root mass during senescence. However, unmeasured loss of mobile non-N constituents during senescence could lead to underestimates of fine root N loss. For N fertilized and unfertilized potted seedlings of Populus tremuloides Michx., Acer rubrum L., Acer saccharum Marsh. and Betula alleghaniensis Britton, we predicted that the fine roots would lose mass and N during senescence. We estimated mass loss as the product of changes in root mass per length and root length between live and recently dead fine roots. Changes in root N were compared among treatments on uncorrected mass, length (which is independent of changes in mass per length), calcium (Ca) and corrected mass bases and by evaluating the relationships of dead root N as a function of live root N, species and fertilization treatments. Across species, from live to dead roots, mass decreased 28-40%, N uncorrected for mass loss increased 10-35%, N per length decreased 5-16%, N per Ca declined 14-48% and N corrected for mass declined 12-28%. Given the magnitude of senescence-related root mass loss and uncertainties about Ca dynamics in senescing roots, N loss corrected for mass loss is likely the most reliable estimate of N loss. We re-evaluated the published estimates of N changes during root senescence based on our values of mass loss and found an average of 28% lower N in dead roots than in fine roots. Despite uncertainty about the contributions of resorption, leaching and microbial immobilization to the net loss of N during root senescence, live root N was a strong and proportional predictor of dead root N across species and fertilization treatments, suggesting that live root N alone could be used to predict the contributions of senescing fine roots to whole-plant N economies and N cycling.