PARENT SESSION
Oral Session 10: Physiology I: Temperature, Light, and Growth.
Presiding: C Knight
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room C 123.
Evaluating the possible causes of parenchyma cell death during the transition from sapwood to heartwood.
Spicer, Rachel*,1, Holbrook, N. Michele1, 1 Harvard University, Cambridge, MA
ABSTRACT- Conversion of sapwood to the physiologically inactive heartwood is defined by the death of xylem parenchyma cells, but what actually causes this death is not known. Recent work demonstrating the role of transpiration in the supply of oxygen to internal stem tissue has raised the possibility that anoxia may be responsible for cell death, whereas others have proposed that a build-up of respiratory carbon dioxide may be involved. Instead, we have found that parenchyma cell respiration is only slightly inhibited at the reduced oxygen levels found within stems, and is also surprisingly robust to high levels of carbon dioxide. Species studied included two conifers (Tsuga canadensis, Pinus strobus), two ring-porous angiosperms (Quercus rubra, Fraxinus americana) and one diffuse-porous angiosperm (Acer rubrum). Respiration rates were measured for fresh xylem tissue from the outermost (youngest) and innermost (oldest) sapwood following equilibration to four combinations of O2 and CO2 (shown respectively, with mole fraction represented as a percent): 10% + 0%, 10% + 10%, 5% + 0%, 5% + 10%. In general, inhibition by reduced O2 was far greater than inhibition by elevated CO2. All species showed reduced respiration at low O2 (average inhibition across species was just 25%), whereas only two species showed inhibition by high CO2. In these cases, there was a significant interaction between the gases such that inhibition by CO2 was reduced at low O2. The effect of gas treatment was the same for both old and young sapwood. Our results suggest that gas concentrations within stems are unlikely to play a role in the transition from sapwood to heartwood. Instead, our work on the diversity of patterns of parenchyma cell death, as well as changes in nuclear morphology and cellular ultrastructure during this transition, suggest that this is a form of programmed cell death and should be viewed as an active stage in development.
Key words: heartwood, anoxia, sapwood, respiration