Plant-centered biosystems in space environments: technological concepts for developing a plant genetic assessment and control system.

Plants will play an essential role in providing life support for any long-term space exploration or habitation. We are evaluating the feasibility of an adaptable system for measuring the response of plants to any unique space condition and optimizing plant performance under those conditions. The proposed system is based on a unique combination of systems including the rapid advances in the field of plant genomics, microarray technology for measuring gene expression, bioinformatics, gene pathways and networks, physiological measurements in controlled environments, and advances in automation and robotics. The resulting flexible module for monitoring and optimizing plant responses will be able to be inserted as a cassette into a variety of platforms and missions for either experimental or life support purposes. The results from future plant functional genomics projects have great potential to be applied to those plant species most likely to be used in space environments. Eventually, it will be possible to use the plant genetic assessment and control system to optimize the performance of any plant in any space environment. In addition to allowing the effective control of environmental parameters for enhanced plant productivity and other life support functions, the proposed module will also allow the selection or engineering of plants to thrive in specific space environments. The proposed project will advance human exploration of space in the near- and mid-term future on the International Space Station and free-flying satellites and in the far-term for longer duration missions and eventual space habitation.

Photosynthetic differences between saplings and adult trees: an integration of field results by meta-analysis.

Ontogenetic changes in gas exchange parameters provide both insight into mechanisms underlying tree growth patterns, and data necessary to scale environmental impacts on young trees to predict responses of older trees. We present a quantitative review and meta-analysis of field measurements of gas exchange parameters in saplings and mature trees of 35 tree species (seven conifers, seven temperate deciduous trees, and 21 tropical evergreen trees). Data for saplings were obtained in both understory environments and open areas or large gaps. We also present data on ontogenetic changes in photosynthesis for Pseudotsuga menziesii (Mirb.) Franco and Tsuga heterophylla (Raf.) Sarg., species of particular interest because of their large maximal heights and long life-spans. Among tree species, there is evidence for both ontogenetic increases and ontogenetic decreases in photosynthetic capacity on a leaf area basis (A(area)). Overall, A(area) is generally higher for upper-canopy leaves of adult trees than for saplings, especially in temperate deciduous trees. However, the pattern for photosynthetic capacity on a leaf mass basis (A(mass)) is the reverse of that observed for A(area). Saplings of both conifers and broad-leaved trees, even when acclimated to low-light conditions, characteristically have a higher A(mass) than adult trees. This pattern is driven largely by an ontogenetic increase in leaf mass per unit area (LMA), as found in 100% of studies reviewed. Data for Pacific Northwest conifers, although including measurements on some of the tallest trees studied, did not differ greatly from patterns found in other tree species. We conclude that ontogenetic changes in LMA are the single most consistent difference between saplings and adult trees, and that changes in LMA and related aspects of leaf morphology may be critical to understanding both variation in gas exchange during tree growth, and stage-dependent responses of trees to environmental change.

Foliage physiology and biochemistry in response to light gradients in conifers with varying shade tolerance.

To examine the predictability of leaf physiology and biochemistry from light gradients within canopies, we measured photosynthetic light-response curves, leaf mass per area (LMA) and concentrations of nitrogen, phosphorus and chlorophyll at 15-20 positions within canopies of three conifer species with increasing shade tolerance, ponderosa pine [Pinus ponderosa (Laws.)], Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], and western hemlock [Tsuga heterophylla (Raf.) Sarg.]. Adjacent to each sampling position, we continuously monitored photosynthetically active photon flux density (PPFD) over a 5-week period using quantum sensors. From these measurements we calculated FPAR: integrated PPFD at each sampling point as a fraction of full sun. From the shadiest to the brightest canopy positions, LMA increased by about 50% in ponderosa pine and 100% in western hemlock; Douglas fir was intermediate. Canopy-average LMA increased with decreasing shade tolerance. Most foliage properties showed more variability within and between canopies when expressed on a leaf area basis than on a leaf mass basis, although the reverse was true for chlorophyll. Where foliage biochemistry or physiology was correlated with FPAR, the relationships were non-linear, tending to reach a plateau at about 50% of full sunlight. Slopes of response functions relating physiology and biochemistry to ln(FPAR) were not significantly different among species except for the light compensation point, which did not vary in response to light in ponderosa pine, but did in the other two species. We used the physiological measurements for Douglas fir in a model to simulate canopy photosynthetic potential (daily net carbon gain limited only by PPFD) and tested the hypothesis that allocation of carbon and nitrogen is optimized relative to PPFD gradients. Simulated photosynthetic potential for the whole canopy was slightly higher (<10%) using the measured allocation of C and N within the canopy compared with no stratification (i.e., all foliage identical). However, there was no evidence that the actual allocation pattern was optimized on the basis of PPFD gradients alone; simulated net carbon assimilation increased still further when even more N and C were allocated to high-light environments at the canopy top.

Effects of O3 on alder photosynthesis and symbiosis with Frankia.

Alnus serrulata (Aiton) Willdenow seedlings with and without root nodules formed by the nitrogen-fixing actinomycete Frankia were exposed to clean filtered air or ozone (O3 ) at 0.12 µl l-1 for 27 d (approximately 164 h total exposure). Gas exchange measurements on leaves and transmission electron micrographs of root nodule cells were made to detect any O3 effects on the functioning of leaves and the root symbiont. Photosynthesis, stomatal conductance, and internal CO2 , concentration were calculated for all plants in clean O3 -free air more than three weeks after the fumigations began. Significant positive correlations between photosynthesis and conductance were found for leaves of control nodulated and unnodulated alders and O3 -treated nodulated alders. There was a weak positive correlation between photosynthesis and conductance for unnodulated O3 -treated seedlings measured in clean air. When O3 -treated leaves were measured during fumigation with O3 , no positive correlation between photosynthesis and conductance was found for either nodulated or unnodulated seedlings. Photosynthetic rates of leaves having the highest stomatal conductance values were decreased by O3 for both nodulated and unnodulated plants. Transmission electron microscopy (TEM) revealed that after a 27 d exposure of shoots to O3 , host root cells of nodules from O3 -treated plants lacked organelles and showed extensive cytoplasmic breakdown. Hyphae and N2 -fixing vesicles of Frankia appeared normal. The Frankia endophyte seems to be more resistant to O3 than is the host root nodule cell. These results show that ambient levels of O3 may reduce photosynthesis and bring about associated degradation in rhizosphere symbiosis.

Ecology of SO2 resistance : V. effects of volcanic SO2 on native Hawaiian plants.

Plant species reflected SO2-stress gradients that existed with increased distance from Hawaiian volcano vents which emit SO2. These changes relate, in part at least, to species differences in stomatal responses to SO2. The sensitive leaves do not close their stomata when exposed to elevated atmospheric SO2 concentrations.

THE EFFECTS OF THE MOUNT ST. HELENS ERUPTION CLOUD ON FIR (ABIES SP.) NEEDLE CUTICLES: ANALYSIS WITH SCANNING ELECTRON MICROSCOPY.

Dead fir needles were collected from standing trees of Abies amabilis in five sites at the margin of the blast zone and located progressively farther from the eruption crater of Mount St. Helens. Scanning electron microscope techniques were used to determine patterns of cuticular melting for these needles and for Abies grandis needles which were heated at specific temperatures for 2 min. Comparisons between cuticular appearances of oven-heated needles and needles from Mount St. Helens were made to determine air temperatures at the collection sites at the time of the eruption. Air temperatures at these sites are estimated to have ranged from about 50 C to about 250 C. Analysis of cuticular sulfur content showed these needles adsorbed little or no volcanic S02 . Conifer needles provided a record of maximum air temperatures during the eruption, and helped reveal the pattern of heat distribution from the eruption cloud. This technique may prove useful for ecological studies in other heat-stressed habitats.

Ecology of SO2 resistance: III. Metabolic changes of C3 and C4 Atriplex species due to SO2 fumigations.

The photosynthetic processes of two ecologically-matched, herbaceous Atriplex species differed in their response to SO2 fumigations. Atriplex triangularis, a C3 species, was more sensitive than the C4 species, A. sabulosa. This difference in sensitivity can be attributed in part to the higher conductance of the C3 species in normal air and saturating light as well as greater stimulation of stomatal opening following exposure to SO2. In addition, photosynthetic mechanisms of the C3 species had higher intrinsic SO2 sensitivity than the C4 species. Differences between photosynthetic responses of these two species may also reflect differences in morphological configuration of mesophyll tissues and greater SO2 sensitivity of the initial photosynthetic carboxlating enzyme of the C3 species. It is likely that certain of the differences in photosynthetic SO2 sensitivity of these contrasting C3 and C4 Atriplex species are characteristic of C3 and C4 plants in general.

Ecology of SO2 resistance: I. Effects of fumigations on gas exchange of deciduous and evergreen shrubs.

A unique gas exchange system is described in which photosynthesis, transpiration, and stomatal conductance can be measured on leaves during SO2 fumigations. SO2 concentrations can be continuously monitored and manipulated between 0 and 2.0 ppm. Rates of total SO2 uptake and SO2 absorption through stomates of a fumigated leaf can also be determined.Using this system we compared the effects of SO2 on the gas exchange rates of two shrub species that co-occur in the Califormian chaparral. Diplacus aurantiacus, a deciduous shrub, was more sensitive to SO2 fumigation than Heteromeles arbutifolia, an evergreen shrub. The differences in photosynthetic sensitivity could be attributed, in large part, to differential SO2 absorption rates.

Ecology of SO2 resistance: II. Photosynthetic changes of shrubs in relation to SO2 absorption and stomatal behavior.

In an effort to predict SO2 sensitivity of plants from their morphological and physiological features, the effects of SO2 on photosynthesis were partitioned between stomatal and nonstomatal components for a drought deciduous shrub, Diplacus aurantiacus, and an evergreen shrub, Heteromeles arbutifolia. As predicted, the drought deciduous shrub had the higher gas conductance, and hence SO2 absorptance. However, nonstomatal components also play a role in determining SO2 sensitivity. Apparently a plant with a high intrinsic photosynthetic capacity will be more sensitive to SO2 than one with a lower capacity.

Stable sulfur isotope analysis of SO2 pollution impact on vegetation.

The δ34S value of SO2 emitted by natural gas refineries is about +25, which is higher than that for non-industrial sulfur sources in our study areas. Terrestrial mosses absorb SO2 from the atmosphere and have a δ34S value which is directly related to the degree of SO2 stress to which they are subjected. The δ34S values for conifer needles are lower than for mosses at the same collection site, which indicates that trees obtain sulfur from both atmospheric and soil sources.Potted conifers were transferred to sites differing in their degree of SO2 stress. This difference is reflected by the change of δ34S values of their needles. SO2 absorbant pot covers, such as charcoal and moss, reduce the amount of airborne sulfur which is available to tress. Moss also may reduce SO2 absorbed by soils in forest stands. We have used analysis of δ34S values to (1) help define SO2 dispersion patterns; (2) reveal the rates at which plants accumulate this pollutant; and (3) associate suspected SO2 injury more closely to an emission source.

Contrasts between bryophyte and vascular plant synecological responses in an SO2-stressed white spruce association in central Alberta.

Canopy coverage analysis was used to examine the synecological changes exhibited by vascular plants and terrestrial mosses in a white spruce association exposed to SO2 fumigation. Both these understory components were found to decline in coverage as SO2 stress increased, but mosses were more sensitive to SO2 in the more heavily stressed areas. This was observed along both an angle-dependent and a distance-dependent gradient of pollution stress. Diversity steadily declined with increasing SO2 stress along the angle-dependent gradient but some localized increases in diversity occurred with increasing stress along the distance-dependent gradient. This was due to invasion of openings resulting from attrition of SO2-sensitive species by weedy angiosperms and by vegetative growth of moss species more tolerant of pollution stress. Conclusions have been drawn about the reproductive strategy of vascular plants and mosses subjected to increasing concentrations of SO2. We have elucidated the ecological consequences for community structure of the systematic removal of pollution-sensitive understory species from an otherwise stable vegetation unit.

Terrestrial mosses as bioindicators of SO2 pollution stress : Synecological analysis and the index of atmospheric purity.

The turf structure of terrestrial mosses was analysed in the understory of white spruce associations along two SO2 stress gradients associated with a single SO2 source. Changes in coverage, turf depth, biomass and percent living moss were followed. SO2 stress initially caused changes in turf structure in the vertical plane (green turf depth, percentage and biomass) whereas changes in the horizontal plane (coverage) were most noticeable in areas of severe stress. The Index of Atmospheric Purity (IAP) reflected SO2 stress only in these latter areas. Intrusion of weeds and increased prominence of subordinate species occurred in lightly stressed areas, but these perturbations were not reflected by the IAP. The effect of Q i, frequency, and coverage factors in the IAP are discussed and the criteria of the Index of Community Vigor are proposed.