► The intention of the present review is to update the consensus knowledge on gm. ► Gaps in knowledge and research priorities are indicated for the near future. ► In particular, how has gm evolved ...among phylogenetically distant groups? ► Can gm be uncoupled from the water path regulation? ► Mechanistic models of gm incorpored in photosynthesis models are needed.
Mesophyll diffusion conductance to CO2 is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of gm, and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance.
Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.
The appearance of agriculture in the Fertile Crescent propelled the development of Western civilization. Here we investigate the evolution of agronomic conditions in this region by reconstructing ...cereal kernel weight and using stable carbon and nitrogen isotope signatures of kernels and charcoal from a set of 11 Upper Mesopotamia archaeological sites, with chronologies spanning from the onset of agriculture to the turn of the era. We show that water availability for crops, inferred from carbon isotope discrimination (Δ(13)C), was two- to fourfold higher in the past than at present, with a maximum between 10,000 and 8,000 cal BP. Nitrogen isotope composition (δ(15)N) decreased over time, which suggests cultivation occurring under gradually less-fertile soil conditions. Domesticated cereals showed a progressive increase in kernel weight over several millennia following domestication. Our results provide a first comprehensive view of agricultural evolution in the Near East inferred directly from archaeobotanical remains.
Xerophilic subshrubs exhibit multiple functional types and frequently show hydrological niche segregation. In the poorly studied Irano-Turanian gypsum deserts, knowledge of the ecohydrological ...strategies of different plant species is essential to understand community complexity in these vulnerable ecosystems.
We studied the ecohydrological strategies of five co-existing subshrub members of Caryophyllales, ascertaining if their rooting architecture, gypsum affinity or photosynthetic pathway determined their water uptake, and if gypsum crystallisation water could be a relevant water source for plants in different seasons.
We conducted soil and xylem sampling for isotope analyses in spring and summer and extracted water by cryogenic vacuum distillation. Oxygen and hydrogen isotope compositions were determined and compared with visual representation and Bayesian Mixing Models to determine species ecohydrological strategies.
Species - season interactions were related to differences in xylem sap isotopic composition. Three basic strategies relying on contrasting the use of free topsoil moisture and deep soil water could be detected and were in part explained by rooting architecture. Plant gypsum affinity and photosynthetic pathways did not have a significant effect on the water sources used by the plants.
Ecohydrological niche segregation was explained partly by rooting architecture and species-specific traits. Gypsum crystallisation water was not used in summer by the studied species.
Carbon isotope discrimination (Δ13C) in charred grains from archaeological sites provides reliable information about water availability of ancient crops. However, as cereals are cultivated plants, ...they may reflect not only climatic fluctuations, but also the effect on water status of certain agronomic practices, such as sowing in naturally wet soils or irrigation. In this work, we propose a methodological approach to combine Δ13C data from different plant species, in order to discriminate between climate-derived and anthropogenic effects on ancient crops. We updated previous models for estimating water inputs from Δ13C of cereal grains of Hordeum vulgare and Triticum aestivum/durum, and we applied them to published data from several archaeological sites, including samples from the Neolithic to the present day in northeast and southeast Spain, as well as from the Neolithic site of Tell Halula (northwest Syria). We found an important decrease in water availability from the Neolithic to the present time in the three areas of study, especially clear for the two driest areas (southeast Spain and northwest Syria). Potential differences in water management practices between wheat and barley, as well as between cereal and legume crops (Vicia faba and Lens culinaris), are also discussed on the basis of the comparison of Δ13C values across several archaeological sites.
Data relating to grain weight of cereal crops cultivated in the past could be useful to track early genetic and/or agronomic improvements, facilitating the comparison of archaeological data with ...current agronomic studies. However, archaeological grains are usually preserved by a process of carbonization, during which there is a considerable reduction in mass and dimension changes. The aim of this study was to develop a model for the estimation of original grain weight from the dimensions of charred grains, taking into account the effect of carbonization. For this purpose, extant grains of wheat and barley were experimentally carbonized at three temperature levels (200, 250, 300 °C) and under two atmospheric conditions (oxidant, reducing). Weight, length (L), breadth (B) and thickness (T) were measured before and after carbonization. The products L×B and L×T provided the best estimation of grain weight (
r
2=0.80–0.86) and were relatively stable across treatments. As a case study, we applied grain weight models to wheat and barley grains gathered from a range of archaeological sites in the Segre and Cinca Valley (Catalonia, NE Spain, ca. 3900–2200 cal. BP). In this region, we observed a relatively constant increment in grain weight during the first half of I millennium BCE, probably explained by the increased water availability indicated by palaeoenvironmental studies. In contrast, the increased grain weight of current samples, compared with that of archaeological grains, is largely attributable to more recent genetic improvements.
The mechanistic understanding of isotope fractionation processes is increasing but we still lack detailed knowledge of the processes that determine the isotopic composition of the tree-ring archive ...over the long term. Especially with regard to the path from leaf photosynthate production to wood formation, post-assimilation fractionations/processes might cause at least a partial decoupling between the leaf isotope signals that record processes such as stomatal conductance, transpiration and photosynthesis, and the wood or cellulose signals that are stored in the paleophysiological record. In this review, we start from the rather well understood processes at the leaf level such as photosynthetic carbon isotope fractionation, leaf water evaporative isotope enrichment and the issue of the isotopic composition of inorganic sources (CO2 and H2O), though we focus on the less explored 'downstream' processes related to metabolism and transport. We further summarize the roles of cellulose and lignin as important chemical constituents of wood, and the processes that determine the transfer of photosynthate (sucrose) and associated isotopic signals to wood production. We cover the broad topics of post-carboxylation carbon isotope fractionation and of the exchange of organic oxygen with water within the tree. In two case studies, we assess the transfer of carbon and oxygen isotopic signals from leaves to tree rings. Finally we address the issue of different temporal scales and link isotope fractionation at the shorter time scale for processes in the leaf to the isotopic ratio as recorded across longer time scales of the tree-ring archive.
We review the relevance and use of stable isotopes for the study of plant community succession. Stable isotope measurements provide information on the origin of resources acquired by plants, the ...processes governing resource uptake and transformation, and the physiological and environmental conditions of plant growth. When combined with measurements of the stable isotope ratio values of soil microbial biomass, soil organic matter and plant litter, isotope measurements of plants can indicate effects of successional changes on ecosystem processes. However, their application to questions of plant succession and ecosystem change is limited by the degree to which the underlying assumptions are met in each study, and complementary measures may be required, depending upon the question of interest. First, we discuss the changes that occur in the stable isotope composition of plants and ecosystems with ontogeny and species replacements, as well as their potential evolutionary significance. Second, we discuss the imprints of plant competition and facilitation on leaf and wood tissue, as well as how stable isotopes can provide novel insights on the mechanisms underlying plant interactions. Finally, we discuss the capacity for stable isotope measurements to serve as a proxy record for past disturbances such as fire, logging and cyclones.
The carbon (δ13C) and oxygen (δ18O) stable isotope composition is widely used to obtain information on the linkages between environmental drivers and tree physiology over various time scales. The ...tree-ring archive can especially be exploited to reconstruct inter- and intra-annual variation of both climate and physiology. There is, however, a lack of information on the processes potentially affecting δ13C and δ18O on their way from assimilation in the leaf to the tree ring. As a consequence, the aim of this study was to trace the isotope signals in European beech (Fagus sylvatica L.) from leaf water (δ18O) and leaf assimilates (δ13C and δ18O) to tree-ring wood via phloem-transported compounds over a whole growing season. Phloem and leaf samples for δ13C and δ18O analyses as well as soil water, xylem water, leaf water and atmospheric water vapour samples for δ18O analysis were taken approximately every 2 weeks during the growing season of 2007. The δ13C and δ18O samples from the tree rings were dated intra-annually by monitoring the tree growth with dendrometers. δ18O in the phloem organic matter and tree-ring whole wood was not positively related to leaf water evaporative enrichment and δ18O of canopy organic matter pools. This finding implies a partial uncoupling of the tree-ring oxygen isotopic signal from canopy physiology. At the same time, internal carbon storage and remobilization physiology most likely prevented δ13C in tree-ring whole wood from being closely related to intra-annual variation in environmental drivers. Taking into account the post-photosynthetic isotope fractionation processes resulting in alterations of δ13C and δ18O not only in the tree ring but also in phloem carbohydrates, as well as the intra-annual timing of changes in the tree internal physiology, might help to better understand the meaning of the tree-ring isotope signal not only intra- but also inter-annually.
Identifying how physiological responses are structured across environmental gradients is critical to understanding in what manner ecological factors determine tree performance. Here, we investigated ...the spatiotemporal patterns of signal strength of carbon isotope discrimination (Δ(13)C) and oxygen isotope composition (δ(18)O) for three deciduous oaks (Quercus faginea (Lam.), Q. humilis Mill. and Q. petraea (Matt.) Liebl.) and one evergreen oak (Q. ilex L.) co-occurring in Mediterranean forests along an aridity gradient. We hypothesized that contrasting strategies in response to drought would lead to differential climate sensitivities between functional groups. Such differential sensitivities could result in a contrasting imprint on stable isotopes, depending on whether the spatial or temporal organization of tree-ring signals was analysed. To test these hypotheses, we proposed a mixed modelling framework to group isotopic records into potentially homogeneous subsets according to taxonomic or geographical criteria. To this end, carbon and oxygen isotopes were modelled through different variance-covariance structures for the variability among years (at the temporal level) or sites (at the spatial level). Signal-strength parameters were estimated from the outcome of selected models. We found striking differences between deciduous and evergreen oaks in the organization of their temporal and spatial signals. Therefore, the relationships with climate were examined independently for each functional group. While Q. ilex exhibited a large spatial dependence of isotopic signals on the temperature regime, deciduous oaks showed a greater dependence on precipitation, confirming their higher susceptibility to drought. Such contrasting responses to drought among oak types were also observed at the temporal level (interannual variability), with stronger associations with growing-season water availability in deciduous oaks. Thus, our results indicate that Mediterranean deciduous and evergreen oaks constitute two clearly differentiated functional groups in terms of their carbon and water economies, despite co-existing in a wide range of environments. In contrast, deciduous oaks form a rather homogeneous group in terms of climate sensitivity.
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