▪ Abstract  The use of stable isotope techniques in plant ecological research has grown steadily during the past two decades. This trend will continue as investigators realize that stable isotopes can serve as valuable nonradioactive tracers and nondestructive integrators of how plants today and in the past have interacted with and responded to their abiotic and biotic environments. At the center of nearly all plant ecological research which has made use of stable isotope methods are the notions of interactions and the resources that mediate or influence them. Our review, therefore, highlights recent advances in plant ecology that have embraced these notions, particularly at different spatial and temporal scales. Specifically, we review how isotope measurements associated with the critical plant resources carbon, water, and nitrogen have helped deepen our understanding of plant-resource acquisition, plant interactions with other organisms, and the role of plants in ecosystem studies. Where possible we also introduce how stable isotope information has provided insights into plant ecological research being done in a paleontological context. Progress in our understanding of plants in natural environments has shown that the future of plant ecological research will continue to see some of its greatest advances when stable isotope methods are applied.

No abstract

Partial mycoheterotrophy, a newly discovered form of mixotrophy in plants, has been described in at least two major lineages of angiosperms, the orchids and ericaceous plants in the tribe Pyroleae. Partial mycoheterotrophy entails carbon gains both directly from photosynthesis and via symbiotic mycorrhizal fungi, but determining the degree of plant dependence on fungal carbon is challenging. The purpose of this study was to determine if two chlorophyllous species of Pyroleae, Chimaphila umbellata and Pyrola picta, were receiving carbon via mycorrhizal networks and, if so, if their proportional dependency on fungal carbon gains increased under reduced light conditions. This was accomplished by a field experiment that manipulated light and plants' access to mycorrhizal networks, and by using the stable carbon isotope composition (δ(13)C) of leaf soluble sugars as a marker for the level of mycoheterotrophy. Based on leaf soluble sugars δ(13)C values, we calculated a site-independent isotope enrichment factor as a measure of fungal contributions to plant C. We found that, under each treatment and over time, the two test species demonstrated different isotopic responses caused by their different intrinsic physiologies. Our data, along with previously published studies, suggest that Chimaphila umbellata is primarily an autotrophic understory plant, while Pyrola picta may be capable of partial mycoheterotrophy. However, in this study, a 50% decrease in light availability did not significantly change the relative dependency of P. picta on carbon gains via mycoheterotrophy.

Arbuscular mycorrhizal fungi (AMF) can help mitigate plant responses to water stress, but it is unclear whether AMF do so by indirect mechanisms, direct water transport to roots, or a combination of the two. Here, we investigated if and how the AMF Rhizophagus intraradices transported water to the host plant Avena barbata, wild oat.We used two-compartment microcosms, isotopically labeled water, and a fluorescent dye to directly track and quantify water transport by AMF across an air gap to host plants.Plants grown with AMF that had access to a physically separated compartment containing 18 O-labeled water transpired almost twice as much as plants with AMF excluded from that compartment. Using an isotopic mixing model, we estimated that water transported by AMF across the air gap accounted for 34.6% of the water transpired by host plants. In addition, a fluorescent dye indicated that hyphae were able to transport some water via an extracytoplasmic pathway.Our study provides direct evidence that AMF can act as extensions of the root system along the soil-plant-air continuum of water movement, with plant transpiration driving water flow along hyphae outside of the hyphal cell membrane.

Stomatal response to leaf water status was experimentally manipulated by pressurizing the soil and roots of potted common bean plants enclosed in a custom-built root pressure chamber. Gas exchange was monitored using a wholeplant cuvette and plant water status using in situ leaf psychrometry. Bean plants re-opened their stomata upon pressurization, but the extent of re-opening was strongly dependent on the time of day when the soil was pressurized, with maximum re-opening in the morning hours and limited re-opening in the afternoon. Neither leaf nor xylem abscisic acid concentrations could explain the reduced response to pressurization in the afternoon. The significance of this phenomenon is discussed in the context of circadian rhythms and of other recent findings on the 'apparent feed-forward response' of the stomata of some species to vapour pressure deficit.

To further our understanding of the greater susceptibility of apical kernels in maize inflorescences to water stress, abscisic acid (ABA) catabolism activity was evaluated in developing kernels with chirally separated (+)-[(3)H]ABA. The predominant pathway of ABA catabolism was via 8'-hydroxylase to form phaseic acid, while conjugation to glucose was minor. In response to water deficit imposed on whole plants during kernel development, ABA accumulated to higher concentrations in apical than basal kernels, while both returned to control levels after rewatering. ABA catabolism activity per gram fresh weight increased about three-fold in response to water stress, but was about the same in apical and basal kernels on a fresh weight basis. ABA catabolism activity was three to four-fold higher in placenta than endosperm, and activity was higher in apical than basal kernels. In vitro incubation tests indicated that glucose did not affect ABA catabolism. We conclude that placenta tissue plays an important role in ABA catabolism, and together with ABA influx and compartmentation, determine the rate of ABA transport into endosperms.

No abstract

Summary1. Seasonal and inter-annual variability in ecosystem carbon dioxide (CO 2 ) exchange is attributed to numerous climate drivers. However, climate effects on metabolism often override ecological functions. This study seeks insight into which biological and ecological processes influence temporal patterns of ecosystem productivity in natural ecosystems. 2. The specific objectives of this study are to (i) identify seasonal and inter-annual patterns of ecosystem-level photosynthesis in relation to climatic conditions, (ii) examine and compare seasonal and inter-annual variations in leaf traits for annual grasses and oak trees across multiple years, and (iii) explore interactions among leaf traits and ecosystem-level photosynthesis across multiple seasons and years. 3. We conducted this study in a woody savanna and open grassland in California, USA. Ecosystem-level photosynthetic rates of annual grasses (A grass ) and oak tree canopy (A canopy ) were deduced from eddy covariance measurements over a 7-year period (2001 and 2007). In conjunction, we sampled grass and oak leaves at weekly to monthly intervals and constructed a multiyear time series of leaf nitrogen concentration (N), leaf mass per unit area (LMA), leaf carbon concentration (C), and leaf carbon stable isotope discrimination (D). 4. Given the same grass community age or tree canopy age, inter-annual variations of the photosynthetic rates were up to 1-2 gC m )2 day )1 for annual grasses and oak trees while the two types of vegetation were exposed to different, wide ranges of inter-annual climate fluctuations: up to 5°C in daily mean soil temperature, 15% in soil moisture, and 10 mol m )2 day )1 in photosynthetically active radiation. 5. While both grass and oak leaf traits varied seasonally and inter-annually, they experienced temporal patterns and seasonal peaks that were distinct from one another. Multi-year means of grass leaf N, C, D, and LMA were 2AE3%, 40AE8%, 22AE6& and 71AE3 g m )2 , respectively; multi-year means of oak leaf N, C, D, and LMA were 1AE9%, 45AE1%, 20AE5& and 132 g m )2 , respectively. 6. Based on the analysis of variance, seasonal and inter-annual terms were associated with A grass or A canopy up to 90% or 81%. On the other hand, variations in leaf N, LMA, C, D, and their interactions could statistically explain about 53% and 26% of variations in A grass and A canopy, respectively. 7. We discussed possible biological and ecological processes involved in regulating seasonal and inter-annual variability in ecosystem-level photosynthesis. Clearly, seasonal and inter-annual variation in ecosystem photosynthesis was strongly associated with the dynamics of leaf traits.