RESUMO
A computer-controlled whole-plant cuvette system is described that allows precise and independent control of temperature (+/- 0.05 degrees C), vapor pressure (+/- 0.02 kPa), CO(2) concentration (+/- 2 micro mol mol(-1)) and photosynthetic photon flux density (+/- 5 micro mol m(-2) s(-1)), and allows the continuous measurement of net photosynthesis and transpiration rates. Vapor pressure is controlled by circulating chamber air through a CaSO(4) desiccant column supported on a digital balance. Transpiration rate is calculated from the change in desiccant mass with time. Photosynthesis rate is measured by integrating the output of a mass flow controller used to inject CO(2) into the chamber to compensate for that assimilated by the plant. The control system can be driven by set points that can be varied, for example, as a function of time, or held constant. We were able to simulate weather data obtained from climate stations and accurately follow, in real time, the output of sensors measuring outside conditions. Experiments on well-watered one- and two-year-old nursery-raised western red cedar (Thuja plicata Donn.) and white spruce (Picea glauca (Moench) Voss) seedlings showed that if the mean daily temperature was increased from 20 to 22 degrees C with vapor pressure remaining constant at 1 kPa, CO(2) concentrations must almost double to compensate for the decrease in net photosynthesis rate.
RESUMO
Changes in the water relations parameters of developing somatic embryogenic and xygotic European larch (Larix decidua) were studied. Water release curves were generated by suspending tissue samples over unsaturated NaCl solutions until they reached vapor equilibration with the surrounding air. Twenty solutions were used whose water potentials ranged from -0.05 to -10 MPa. Water release curves were obtained by plotting paired values of tissue relative water content (RWC) and solution potential. Curves were derived for embryonic larch at various stages of development and for hypocotyls and roots from germinated zygotic and somatic embryos. The ability to resist dehydration increased markedly with development. Stage 1 tissue, which consisted of clusters of loosely associated nonchlorophyllous cells, had extremely low bulk elastic modulus (epsilon) (1.91 MPa) and apoplastic water content (A) (0.023), relatively high osmotic potential (Psi(pi)) (-0.53 MPa), and lost turgor at 0.56 RWC. In contrast, mature embryoids with primary roots, hypocotyl, and cotyledons (stage 3) had an almost 4-fold increase in A (0.089), significantly higher epsilon (3.49 MPa), and lower Psi(pi) (-0.88 MPa) and lost turgor at 0.66 RWC. Hypocotyl tissue from germinated somatic embryos lost turgor at 0.74 RWC and had higher epsilon, A, and solute accumulation than pregerminated tissue. Hypocotyl tissue resisted dehydration more strongly than root tissue, and differences between root and hypocotyl water relation parameters were more pronounced in xygotic than in somatic seedlings. Highest dehydration resistance was in zygotic hypocotyls. The characterization of the water relations of tissue cultures should allow the development of more consistent and reliable desiccation protocols to induce maturation of embryos and produce synchronously germinating seed.
