© 2007 Heron Publishing—Victoria, Canada
Plasticity in mesophyll volume fraction modulates light-acclimation in needle photosynthesis in two pines
Ülo Niinemets (1–3), Aljona Lukjanova (4), Matthew H. Turnbull (5) and Ashley D. Sparrow (5, 6)
1. Department of Plant Physiology, University of Tartu, Riia 23, Tartu 51010, Estonia / 2. Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 64, Tartu 51014, Estonia / 3. Corresponding author (ylo.niinemets@emu.ee) / 4. Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Kreutzwaldi 5, Tartu 51014, Estonia / 5. School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8020, New Zealand / 6. Department of Natural Resources and Environmental Science, University of Nevada, Mail Stop 186, 1000 Valley Road, Reno, NV
9512, USA / Received September 19, 2006; accepted November 12, 2006; published online May 1, 2007
Summary
Acclimation potential of needle photosynthetic capacity varies greatly among pine species, but the underlying chemical, anatomical
and morphological controls are not entirely understood. We investigated the light-dependent variation in needle characteristics
in individuals of Pinus patula Schlect. & Cham., which has 19–31-cm long pendulous needles, and individuals of P. radiata D. Don., which has shorter (8–17-cm-long) stiffer needles. Needle nitrogen and carbon contents, mesophyll and structural
tissue volume fractions, needle dry mass per unit total area (MA) and its components, volume to total area ratio (V/AT) and needle density (D = MA/(V/AT)), and maximum carboxylase activity of Rubisco (Vcmax) and capacity of photosynthetic electron transport (Jmax) were investigated in relation to seasonal mean integrated irradiance (Qint). Increases in Qint from canopy bottom to top resulted in proportional increases in both needle thickness and width such that needle total to
projected surface area ratio, characterizing the efficiency of light interception, was independent of Qint. Increased light availability also led to larger MA and nitrogen content per unit area (NA). Light-dependent modifications in MA resulted from increases in both V/AT and D, whereas NA changed because of increases in both MA and mass-based nitrogen content (NM) (NA = NMMA). Overall, the volume fraction of mesophyll cells increased with increasing irradiance and V/AT as the fraction of hypodermis and epidermis decreased with increasing needle thickness. Increases in MA and NA resulted in enhanced Jmax and Vcmax per unit area in both species, but mass-based photosynthetic capacity increased only in P. patula. In addition, Jmax and Vcmax showed greater plasticity in response to light in P. patula. Species differences in mesophyll volume fraction explained most of the variation in mass-based needle photosynthetic capacity
between species, demonstrating that differences in plastic adjustments in mass-based photosynthetic activities among these
representative individuals were mainly associated with contrasting investments in mesophyll cells. Greater area-based photosynthetic
plasticity in P. patula relative to P. radiata was associated with larger increases in MA and mesophyll volume fraction with increasing irradiance. These data collectively demonstrate that light-dependent increases
in mass-based nitrogen contents and photosynthetic activities were associated with an increased mesophyll volume fraction
in needles at higher irradiances. They also emphasize the importance of light-dependent anatomical modifications in determining
needle photosynthetic capacity.
Keywords:
needle anatomy, nitrogen content, photosynthetic acclimation, Pinus patula, Pinus radiata, support costs.
