Whole-plant Production Research Articles

Carbon allocation dynamics of Spartina alterniflora in Georgia saltmarsh, USA

We developed a phenology-based growth model(PG model) for Spartina alterniflora that incorporates the effects of light, temperature, and salinity on plant production. The PG model is the first to quantify carbon translocation between both above- and below-ground biomass across three phenological periods: growth, senescence, and dormancy periods. This model, fitted to field data from short, medium, and tall S. alterniflora types, estimates physiological parameters such as mass-specific rates of carbon translocation. Once parameterized, the model is applied in forward mode to predict whole-plant production, growth, respiration, mortality, and translocation. Model results reveals that short forms allocate 82 % of photosynthate to below-ground biomass during the growing season, compared to tall (52 %) and medium (22 %) types. However, tall forms, with extensive above-ground biomass, show the highest absolute carbon translocation to below-ground tissues during growth(ave. 3940 g dry weight m−2) and senescence(ave. 265 g dry weight m−2) period. An average mortality rate of 52 % of net production in the tall form below-ground biomass throughout the year indicates a substantial contribution to organic carbon sequestration within the habitat sediment. Model results also reveal that the carbon translocation from below- to above-ground tissues may not be required for survival during winter in milder climate like Sapelo Island, Georgia.

Interspecific variation in leaf traits, photosynthetic light response, and whole-plant productivity in amaranths (Amaranthus spp. L.).

Photosynthetic light response curve parameters help us understand the interspecific variation in photosynthetic traits, leaf acclimation status, carbon uptake, and plant productivity in specific environments. These parameters are also influenced by leaf traits which rely on species and growth environment. In accessions of four amaranth species (Amaranthus. hybridus, A. dubius, A. hypochondriacus, and A. cruentus), we determined variations in the net photosynthetic light response curves and leaf traits, and analysed the relationships between maximum gross photosynthetic rate, leaf traits, and whole-plant productivity. Non-rectangular hyperbolae were used for the net photosynthesis light response curves. Maximum gross photosynthetic rate (Pgmax) was the only variant parameter among the species, ranging from 22.29 to 34.21 μmol m–2 s–1. Interspecific variation existed for all the leaf traits except leaf mass per area and leaf inclination angle. Stomatal conductance, nitrogen, chlorophyll, and carotenoid contents, as well as leaf area correlated with Pgmax. Stomatal conductance and leaf nitrogen explained much of the variation in Pgmax at the leaf level. At the plant level, the slope between absolute growth rate and leaf area showed a strong linear relationship with Pgmax. Overall, A. hybridus and A. cruentus exhibited higher Pgmax at the leaf level and light use efficiency at the whole-plant level than A. dubius, and A. hypochondriacus. Thus, A. hybridus and A. cruentus tended to be more efficient with respect to carbon assimilation. These findings highlight the correlation between leaf photosynthetic characteristics, other leaf traits, and whole plant productivity in amaranths. Future studies may explore more species and accessions of Amaranthus at different locations or light environments.

Candidate regulators and target genes of drought stress in needles and roots of Norway spruce.

Drought stress impacts seedling establishment, survival and whole-plant productivity. Molecular responses to drought stress have been most extensively studied in herbaceous species, mostly considering only aboveground tissues. Coniferous tree species dominate boreal forests, which are predicted to be exposed to more frequent and acute drought as a result of ongoing climate change. The associated impact at all stages of the forest tree life cycle is expected to have large-scale ecological and economic impacts. However, the molecular response to drought has not been comprehensively profiled for coniferous species. We assayed the physiological and transcriptional response of Picea abies (L.) H. Karst seedling needles and roots after exposure to mild and severe drought. Shoots and needles showed an extensive reversible plasticity for physiological measures indicative of drought-response mechanisms, including changes in stomatal conductance (gs), shoot water potential and abscisic acid (ABA). In both tissues, the most commonly observed expression profiles in response to drought were highly correlated with the ABA levels. Still, root and needle transcriptional responses contrasted, with extensive root-specific down-regulation of growth. Comparison between previously characterized Arabidopsis thaliana L. drought-response genes and P. abies revealed both conservation and divergence of transcriptional response to drought. In P. abies, transcription factors belonging to the ABA responsive element(ABRE) binding/ABRE binding factors ABA-dependent pathway had a more limited role. These results highlight the importance of profiling both above- and belowground tissues, and provide a comprehensive framework to advance the understanding of the drought response of P. abies. The results demonstrate that a short-term, severe drought induces severe physiological responses coupled to extensive transcriptome modulation and highlight the susceptibility of Norway spruce seedlings to such drought events.

Deciphering the main determinants of O3 tolerance in Euramerican poplar genotypes

Tropospheric ozone (O3) is the main secondary pollutant and considered to be the most damaging for growth and productivity. O3 is well known to induce oxidative stress and Reactive Oxygen Species accumulation in leaf tissues. Several mechanisms have been suggested to enable trees to cope with such stress; however, their relative contribution to O3 tolerance is still unclear. Here, ten Euramerican poplar genotypes (Populus deltoides × nigra) were investigated regarding their response to 120 ppb of O3 for 3 weeks in order to determine main mechanisms and identify the key traits and strategies linked to a better tolerance to O3-induced oxidative stress. Results showed that ascorbate peroxidase and ascorbate regeneration through monodehydroascorbate reductase are the main determinants of O3 tolerance in Euramerican poplar, in protecting photosynthesis capacity from oxidative stress and therefore, maintaining growth and productivity. Besides, stomatal closure was harmful in sensitive genotypes, suggesting that avoiding strategy can be further deleterious under chronic ozone. Finally, O3-induced early senescence appeared essential when up scaling leaf-level mechanistic response to whole-plant productivity, in fine-tuning resource reallocation and photosynthesis area.

The prospects for constraining productivity through time with the whole-plant physiology of fossils.

Anatomically preserved fossils allow estimation of hydraulic parameters, potentially providing constraints on interpreting whole-plant physiology. However, different organ systems have typically been considered in isolation - a problem given common mismatches of high and low conductance components coupled in the hydraulic path of the same plant. A recent paper addressed the issue of how to handle resistance mismatches in fossil plant hydraulics, focusing on Carboniferous medullosan seed plants and arborescent lycopsids. Among other problems, however, a fundamental error was made: the transpiration stream consists of resistances in series (where resistances are additive and the component with the largest resistance can dominate the behavior of the system), but emphasis was instead placed on the lowest resistance, effectively treating the system as resistances in parallel (where the component with the smallest resistance will dominate the behavior). Instead of possessing high assimilation capacities to match high specific stem conductances, it is argued here that individual high conductance components in these Paleozoic plants are nonetheless associated with low whole-plant productivity, just as can be commonly seen in living plants. Resolution of how to handle these issues may have broad implications for the Earth system including geobiological feedbacks to rock weathering, atmospheric composition, and climate.

The hidden season: growing season is 50% longer below than above ground along an arctic elevation gradient.

There is compelling evidence from experiments and observations that climate warming prolongs the growing season in arctic regions. Until now, the start, peak, and end of the growing season, which are used to model influences of vegetation on biogeochemical cycles, were commonly quantified using above-ground phenological data. Yet, over 80% of the plant biomass in arctic regions can be below ground, and the timing of root growth affects biogeochemical processes by influencing plant water and nutrient uptake, soil carbon input and microbial activity. We measured timing of above- and below-ground production in three plant communities along an arctic elevation gradient over two growing seasons. Below-ground production peaked later in the season and was more temporally uniform than above-ground production. Most importantly, the growing season continued c. 50% longer below than above ground. Our results strongly suggest that traditional above-ground estimates of phenology in arctic regions, including remotely sensed information, are not as complete a representation of whole-plant production intensity or duration, as studies that include root phenology. We therefore argue for explicit consideration of root phenology in studies of carbon and nutrient cycling, in terrestrial biosphere models, and scenarios of how arctic ecosystems will respond to climate warming.

CONSUMER CONTROL OF GRASSLAND PLANT PRODUCTION

The effect of herbivory on grassland whole-plant production is poorly un- derstood. Herbivores can increase grassland aboveground productivity, and laboratory ex- periments suggest that herbivory should reduce grass root growth. However, few field studies have directly measured the response of grassland root production to herbivores. We ex- amined the effect of native migratory ungulates on grassland primary production by com- paring aboveground (NAP) and belowground (NBP) production in grazed vs. ungrazed (fenced) grassland at nine variable sites in Yellowstone National Park. NBP was determined with minirhizotrons to account for root turnover. Grazers stimulated aboveground, below- ground, and whole-grassland productivity by 21%, 35%, and 32%, respectively. Root pro- duction was stimulated seven times more (217 g/m2) than shoot production (30 g/m2), indicating that the major effect of herbivores in this system was a positive feedback on root growth. Results refute the prevailing notion that grassland herbivory leads to a reduction in root productivity, and a concomitant decline in soil carbon content, and provide a potential explanation for how organic-rich soil developed in grassland that was grazed throughout millennia.

Dry matter yield and quality evaluation at two phenological stages of forage triticale grown in the Po Valley and Sardinia, Italy

Autumn-sown cereals are a very important source of nutrients for livestock. Moreover, the harvesting of the entire plant at milk-dough stage makes a double-cropping approach possible resulting in higher yields per unit area. Of the various winter cereals, triticale is particularly interesting due to its quality–quantity potential as whole-plant silage. Given the broad range of triticale cultivars available today and the important role played by the environment on the nutritional value of forages, the present study was undertaken to establish the adaptability of modern cultivars to different locations for high quality whole-plant production at two growing stages. Our results indicate the yield potential of triticale as silage both in irrigated or in rainfed Mediterranean areas. Regarding quality of the forage the best performance was recorded in Sardinian environments where plant growth was characterised by a gradual temperature increase. Thus, in Mediterranean areas, triticale could represent a viable source of livestock feed during the summer period offsetting the dearth of fresh forage typical in these environments.

‘Daleks’ and ‘Whirligig’ used to uncover the fate of intercepted light by an apple tree canopy

Canopy light interception and distribution has a major influence on CO2 fixation and the growth and productivity of fruit trees. Spatial and temporal variations in microclimate alter the gas exchange from the leaves and other organs (fruit, branch, etc.). The variable nature of the light environment within the tree crown, and other factors such as leaf age and various stresses (water, diseases, pests) can often impair our ability to extrapolate whole-canopy net carbon exchange (NCE) and transpiration from a small sample of single-leaf measurements. This paper compares two methods for estimating whole-canopy gas exchange. An open-top, through-flow whole-canopy cuvette (Wunsche & Palmer, 1997, HortScience, 32:653-658) was used to enclose the above-ground portion of a 4 year-old apple tree (Malus domestica Borkh. cv. `Braeburn? on M.9 rootstock). NCE and transpiration from the whole canopy was measured over a one week period in mid-season. Measurements of light interception were made on a neighbouring apple tree of the same age and of a similar growth habit using a `whole-plant? radiometer known as the Whirligig (McNaughton et al., 1992, Plant Cell Environ., 17:1061-1068). Data from the Whirligig were used to estimate the distribution of PAR radiation over the total leaf area of the tree. This PAR distribution was then combined with leaf stomatal conductance and leaf net photosynthesis response functions to estimate the rates of whole-canopy NCE and transpiration. The Whirligig calculations of transpiration were in good agreement with data from the whole-canopy cuvette, and with independent measurements of sap flow recorded using heat-pulse sensors located in the trunk of the cuvette tree. The Whirligig estimates of NCE were also comparable to values obtained from the whole-canopy curvette. We conclude that both instruments provide an integrated approach for whole-plant productivity studies

Differential growth and recovery rates following defoliation in related deciduous and evergreen trees

Deciduous larches,Larix spp., and evergreen pines,Pinus spp., are sympatric Pinaceae conifers. Adjacent monocultures of 10-year-oldLarix decidua Mill, andPinus resinosa Ait. were subjected to single-season artificial defoliation by clipping from 0% to 99% of each needle. Survival, above-ground productivity, and architecture were measured for 36 months.P. resinosa andL. decidua exhibited differential relationships with defoliation intensity and recovery time. Two months after treatment, defoliation reduced larch height growth but had no effect on radial growth. By contrast,P. resinosa stem radial growth was reduced immediately, but height growth was not decreased until the following year. Pine leader growth and above-ground biomass following 66% defoliation never recovered to control values or 33% defoliated pines. Conversely, defoliated larch quickly recovered from an initial growth loss to eliminate all treatment effects on biomass. The plasticity in architectural response found in larch, but not pine, might partially account for defoliation tolerance. BothP. resinosa andL. decidua exhibited non-linear responses to defoliation. These patterns may be caused partially by the uneven distribution of nutrients within needles, rather than a simple function of leaf area lost to defoliators. Concentrations of 13 nutrients inP. resinosa were highest either in the mid-(Ca, Mg, S, Zn, B, Mn, Fe, Al and Na) or basal-(N, P, K, and Cu) section. The relatively low nutrient content in needle tips may contribute to similar biomass productivity between trees defoliated 33% and controls. Removal of needle mid-sections significantly reduced whole-plant productivity. In contrast,L. decidua nutrients are concentrated in the distal sections. Nutrient concentrations were generally highest in larch. Our results agree with an emergent prediction of the carbon/ nutrient balance theory that defoliation more severely reduces growth of evergreen than deciduous species. These results are discussed within the physiological, ecological and evolutionary context of allocation theory, with implications for natural resource management and plant-insect interaction theory.

Effect of Enhanced UV-B Radiation of Pollen Quantity, Quality, and Seed Yield in Brassica rapa (Brassicaceae)

We conducted three experiments to examine the influence of ultraviolet-B radiation (UV-B; 280–320 nm) exposure on reproduction in Brassica rapa (Brassicaceae). Plants were grown in a greenhouse under three biologically effective UV-B levels that simulated either an ambient stratospheric ozone level (control), 16% (“low enhanced”), or 32% (“high enhanced”) ozone depletion levels at Morgantown, WV, USA in mid-March. In the first experiment, we examined whether UV-B level during plant growth influenced in vivo pollen production and viability, and flower production. Pollen production and viability per flower were reduced by ≈50% under both enhanced UV-B levels relative to ambient controls. While plants under high-enhanced UV-B produced over 40% more flowers than plants under the two lower UV-B treatments, whole-plant production of viable pollen was reduced under high-enhanced UV-B to 17% of that of ambient controls. Whole-plant production of viable pollen was reduced under low-enhanced UV-B to 34% of ambient controls. In the second experiment, we collected pollen from plants under the three UV-B levels and examined whether source-plant UV-B exposure influenced in vitro pollen germination and viability. Pollen from plants under both enhanced-UV-B treatments had initially lower germination and viability than pollen from the ambient level. After in vitro exposure to the high-enhanced UV-B levels for 6 h, viability of the pollen from plants grown under ambient UV-B was reduced from 65 to 18%. In contrast, viability of the pollen from plants grown under both enhanced UV-B treatments was reduced to a much lesser extent: only from ≈43 to 22%. Thus, ambient source-plant pollen was more sensitive to enhanced UV-B exposure. In the third experiment, we used pollen collected from source plants under the three UV-B levels to fertilize plants growing under ambient-UV-B levels, and assessed subsequent seed production and germination. Seed abortion rates were higher in plants pollinated with pollen from the enhanced UV-B treatments, than from ambient UV-B. Despite this, seed yield (number and mass) per plant was similar, regardless of the UV-B exposure of their pollen source. Our findings demonstrate that enhanced UV-B levels associated with springtime ozone depletion events have the capacity to substantially reduce viable pollen production, and could ultimately reduce reproductive success of B. rapa.

Eelgrass ( Zostera marina L.) transplants in San Francisco Bay: Role of light availability on metabolism, growth and survival

Survival, metabolism and growth of Zostera marina L. transplants were examined along depth gradients in Keil Cove and Paradise Cove in the extremely turbid San Francisco Bay estuary. Water transparency was unusually high throughout 1989–1990 for San Francisco Bay. Transplant survival was strongly depth-dependent at Paradise Cove but not at Keil Cove. All transplants were lost below − 1.0 m depth within 1 year at Paradise Cove, but survived to depths of − 1.5 m at Keil Cove. Half the transplants growing in shallow water survived the first year at both sites. Shoot photosynthesis, respiration, growth, and sugar content did not differ between sites. Daily periods of irradiance-saturated photosynthesis ( H sat) were over 6 h all year. Seasonal photosynthetic acclimation to light availability maintained long H sat periods and high ratios of daily whole-plant production to respiration through the winter, indicating a potential for net carbon gain throughout the year. Winter growth was 50% of the summer rate. Despite high initial losses, surviving transplants have persisted at both sites through 1994. Although eelgrass transplants can succeed in San Francisco Bay given sufficient light availability, the role of carbon reserves and transplant timing may influence transplant survival.

Nighttime exposure to ozone reduces whole-plant production in Betula pendula

For 20 weeks during the growing season, cuttings of one birch clone (Betula pendula Roth.) were exposed in the Birmensdorf fumigation chambers to O(3)-free air (control) or 75 nl O(3) l(-1). Ozone was supplied either from 1900 until 0700 h (nighttime regime), from 0700 until 1900 h (daylight regime), or all day (24-h regime). By autumn, reductions in whole-plant biomass production, root/shoot biomass and stem weight/length ratios were evident in all three O(3) regimes. The reductions in cuttings receiving the 24-h O(3) treatment were about twofold larger than in cuttings receiving the daylight O(3) treatment. Stomata were open at night, and stomatal conductance was about 50% of its maximum daytime value. We calculated that the rate of O(3) uptake into leaves in the dark approached 4 nmol m(-2) s(-1). Whole-plant production and carbon allocation were more sensitive to O(3) during the night than during the day; however, O(3) exposure caused similar visible leaf injury in both of the 12-h regimes, although the leaves exposed to O(3) at night exhibited delayed O(3)-induced shedding. Overall, changes in production and carbon allocation were determined by the external O(3) dose rather than by the kind of O(3) exposure, indicating that, at the seasonal scale, the internal dose of ozone that was physiologically effective was a constant fraction of the external O(3) dose. We conclude that nighttime O(3) exposures should be included in the daily time period for determining critical concentrations of O(3) causing injury in trees.

Whole-plant growth and leaf formation in ozonated hybrid poplar ( Populus × euramericana)

Seasonal growth was studied in potted cuttings of hybrid poplar (one clone of Populus × euramericana) either exposed to ozone in filtered air (0 = control, 0·05, 0·10 μl litre −1) or in ambient air (mean = 0·03 μl litre −1). Only at 0·10 μl litre −1 was biomass production reduced and related to leaf loss rather than leaf formation, since the latter was similar in all treatments. Stem length at 0·10 μl litre −1 approached that of the control, whereas starch concentration in the green stem bark tended to be reduced, as were the ratios of stem weight/length and root/shoot biomass. The changes in carbon allocation and biomass production gradually became established during the second half of the growing season. At the altered carbon allocation at 0·10 μl litre −1, the ratio of whole-plant production/attached foliage area resembled that of the other O 3 regimes. However, the latter ratio was strongly reduced at 0·10 μl litre −1 when calculated on the basis of the potential foliage area, as compensated for the O 3-induced leaf loss. Thus the carbon return/cost balance of the totally formed foliage was low, although the relative-growth rate of ozonated plants temporarily reached that of the control. The relation between leaf differentiation under ozonation (lowered stomatal density) and whole-plant production remains uncertain. The plant behavior found is discussed in terms of passive response or acclimatization to O 3 stress.

Seasonal growth, ?13C in leaves and stem, and phloem structure of birch (Betula pendula) under low ozone concentrations

The growth of potted birch cuttings (one clone of Betula pendula) was studied under low O3 concentrations (0, 0.050, 0.075, 0.100 μl l-1) throughout an entire growing season. With increasing O3 dose, 20–50% of all leaves formed were prematurely shed, while 40–70% of the remaining foliage displayed advanced discoloration by the end of the season. Ozonation affected the S, P and N concentration of leaves and increased δ13C in leaves and stem, while the CO2 assimilation rate declined with increasing CO2 concentration in mesophyll intercellulars. While whole-plant production correlated negatively with the O3 dose, ozone increased the specific leaf weight (i.e. leaf weight/leaf area, SLW) but decreased the ratios of stem weight/stem length and root/shoot biomass. Neither the latter ratio nor SLW changed in experimentally defoliated control plants, whereas in ozonated plants starch accumulated along leaf veins and phloem tissue was deformed in the leaf petioles and the stem. Only in early summer was the relative growth rate higher in the ozonated than in the control plants. The ratio of whole-plant biomass production versus total foliage area formed was lowered under O3 stress. However, when relating biomass to the actual foliage area present due to leaf loss, this ratio did not differ between treatments. Similarly the ratio of actual foliage area versus basal stem area in cross-section did not differ. Overall, whole-plant production was strongly determined by O3-caused changes in crown structure and began to be limited at O3 doses (approximately 180 μl l-1 h) similar to those of rural sites in Central Europe.

Ecological conditions that determine when grazing stimulates grass production

We report the results of a pot experiment that examined the effects of three ecologically important factors controlling plant growth rates in savanna grasslands: defoliation, soil nitrogen and soil water availability. The experiment was conducted in the Amboseli region in east Africa, and was designed to simulate natural conditions as far as possible, using local soils and a grass species that is heavily grazed by abundant large herbivores. Productivity by different plant components was reduced, stimulated or unchanged by defoliation, depending on specific watering and fertilization treatments. Total above-ground production was stimulated by defoliation and was maximized at moderate clipping intensities, but this was statistically significant only when plants were watered infrequently (every 8 days), and most important, periods between clipping events were extended (at least 24 days). Under these conditions, plant growth rates were limited by water availability at the time of clipping, and soil water conserved in clipped, compared to unclipped plants. Within a given fertilization treatment, whole-plant production was never stimulated by defoliation because root growth was unaffected or inhibited by clipping. However, when fertilization was coupled to defoliation, as they are in the field, whole-plant production by fertilized and moderately clipped plants exceeded production by infertilized, unclipped plants. Under this interpretation, maximum whole-plant production coincided with optimum conditions for herbivores (maximum nitrogen concentration in grass leaves) when watering was frequent, and plants were moderately defoliated. However, these conditions were not the same as those that maximized relative above-ground stimulation of growth (infrequent watering and clipping).The results indicate that above-ground grass production can be stimulated by grazing, and when that is likely to occur. However, the results emphasize that plant production responses to defoliation can vary widely, contigent upon a complex interaction of ecological factors.

Photosynthetic characteristics of giant kelp (Macrocystis pyrifera) determined in situ

Rates of net photosynthesis and nocturnal respiration by individual blades of the giant kelp Macrocystis pyrifera (L.) C. Agardh in southern California, were determined in situ by measuring oxygen production in polyethylene bags during spring/summer of 1983. Mature blades from different depths in the water column exhibited different photosynthetic characteristics. Blades from the surface canopy (0 to 1 m depth) exhibited higher photosynthetic capacity under saturating irradiance and higher photosynthetic efficiency at low irradiances than blades from 3 to 5 or 7 to 9 m depths. Saturating irradiance was lower for canopy blades than for deeper blades. Canopy blades showed no short-term photoinhibition, but photosynthetic rates of deeper blades were significantly reduced during 1 to 2 h incubations at high irradiances. Results of 1 to 2 wk acclimation experiments indicated that differences between photosynthetic characteristics of blades from different depths were primarily attributable to acclimation light conditions. Vertical displacement of blades within the kelp canopy occurred on a time-scale of 1 min to 1 h. Blades continually moved between the unshaded surface layer and deeper, shaded layers. Vertical movement did not maximize photosynthesis by individual blades; only a small proportion of blades making up a dense surface canopy maintained light-saturated photosynthetic rates during midday incubations. The relatively high photosynthetic rates exhibited by canopy blades over the entire range of light conditions probably resulted from acclimation to intermittent high and low irradiances, a consequence of vertical displacement. Vertical displacement also reduced the afternoon depression in photosynthesis of individual canopy blades. The overall effect of vertical displacement was optimization of total net photosynthesis by the kelp canopy and, therefore, optimization of whole-plant production.