• Energy Research

To test if some leaf parameters are predictors of productivity in a range of Populus deltoides (Bartr.) Marsh. x P. nigra L. clones, we assessed leaf traits and productivity in 2-month-old rooted cuttings from 31 clones growing in 4-l pots in a greenhouse, under conditions of controlled temperature and optimal irrigation. We evaluated four groups of variables describing (1) productivity (total biomass), (2) leaf growth (total leaf number increment and total leaf area increment rate), (3) leaf structure (specific leaf area and nitrogen and carbon contents) and (4) carbon isotope discrimination (delta), which is negatively correlated with time-integrated water-use efficiency. High-yielding clones did not necessarily display high leaf growth rates, but they displayed a larger total leaf area, lower specific leaf area and lower leaf nitrogen concentration than clones with low productivity. Total leaf area was mainly controlled by maximal individual leaf area and total leaf area increment rate (r = 0.51 and 0.56, respectively). Carbon isotope discrimination did not correlate with total biomass, but it was associated with total number of leaves and total leaf area increment rate (r = 0.39 and 0.45, respectively). Therefore, leaf area and specific leaf area were better indicators of productivity than leaf growth traits. The observed independence of delta from biomass production provides opportunities for selecting poplar clones combining high productivity and high water-use efficiency.

We assessed the irradiance-related plasticity of hydraulic architecture in saplings of Betula pendula Roth., a pioneer species; Acer pseudoplatanus L., Fraxinus excelsior L. and Quercus robur L., which are post-pioneer light-requiring species; and Quercus petraea Matt. Liebl. and Fagus sylvatica L. Plants were grown in pots in 36%, 16% and 4% of full sunlight. Hydraulic conductance was measured with a high-pressure flow-meter in entire, in situ root systems and in excised shoots. Leaf-specific whole-plant conductance (LSC) increased with irradiance, due, in part, to an effect of irradiance on plant size. In addition, there was a size-independent effect of irradiance on LSC due, in part, to an increase in root hydraulic conductance paralleled by an increase in root biomass scaled to leaf area. Changes in shoot conductivity also contributed to the size-independent plasticity of LSC. Vulnerability to cavitation measured in current-year twigs was much larger in shade-grown plants. Betula pendula had the highest whole-plant, root and shoot conductances and also the greatest vulnerability to cavitation. The other species were similar in LSC, but showed some variation in root conductance scaled to biomass, with Q. robur, Q. petraea and F. sylvatica having the lowest root conductance and susceptibility to cavitation. All species showed a similar irradiance-related plasticity in LSC.

Mesophyll diffusion conductance to CO(2) 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 g(m), 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.

AbstractIn agroforestry systems, shade trees strongly affect the physiology of the undergrown crop. However, a major paradigm is that the reduction in absorbed photosynthetically active radiation is, to a certain extent, compensated by an increase in light‐use efficiency, thereby reducing the difference in net primary productivity between shaded and non‐shaded plants. Due to the large spatial heterogeneity in agroforestry systems and the lack of appropriate tools, the combined effects of such variables have seldom been analysed, even though they may help understand physiological processes underlying yield dynamics.In this study, we monitored net primary productivity, during two years, on scales ranging from individual coffee plants to the entire plot. Absorbed radiation was mapped with a 3D model (MAESPA). Light‐use efficiency and net assimilation rate were derived for each coffee plant individually.We found that although irradiance was reduced by 60% below crowns of shade trees, coffee light‐use efficiency increased by 50%, leaving net primary productivity fairly stable across all shade levels.Variability of aboveground net primary productivity of coffee plants was caused primarily by the age of the plants and by intraspecific competition among them (drivers usually overlooked in the agroforestry literature) rather than by the presence of shade trees.

Le Hêtre est considéré comme une essence tolérante à l'ombrage, au moins dans son jeune âge. Il est néanmoins capable de tirer profit rapidement de trouées dans les couverts pour présenter une remarquable accélération de sa croissance. Dans cet article, nous comparons les traits fonctionnels liés à la photosynthèse et à l'interception de lumière, de semis de Hêtre cultivés sous différents niveaux de lumière à ceux d'essences accompagnatrices. Nous montrons qu'aucun de ces traits pris séparément ne suffit à caractériser le niveau de tolérance à l'ombrage de ces essences. Par contre, il semble que, pour permettre la survie des individus sous faible lumière, il soit indispensable que le bilan annuel de carbone à l'échelle de ces individus (en incluant les pertes par respiration de tous les organes) soit positif. Des traits composites comme le rapport entre la surface foliaire et la biomasse totale, ou mieux encore comme l'efficacité d'interception de la lumière par unité de biomasse peuvent, sous un éclairement donné, être des indicateurs de ce bilan et donc du degré de tolérance à l'ombre. Ces deux indicateurs diminuent cependant avec les dimensions des plants, et ce d'autant plus rapidement que la croissance est active. Dans ces conditions, une croissance lente est en soi une caractéristique de tolérance à l'ombrage. Dans cet article, nous développons cette idée et la confrontons à un ensemble de données issues de nos expériences. Beech is considered to be a shade-tolerant species, at least while it is juvenile. It is nonetheless quick to take advantage of clearings in the canopy to grow significantly faster. In this article, the authors compare the functional features connected with photosynthesis and light interception of cultivated beech seedlings subjected to various levels of light with those of accompanying species. They show that none of these features taken alone is able to account for these species’ degree of shade-tolerance. However, for individuals to be able to survive in weak light conditions, it is essential for the annual carbon balance on the scale of those individuals (including losses by all organs through respiration) to be positive. Combined features suc as ratio of the leaf area to total biomass, or moreover, the per biomass unit light interception efficiency for a given illumination, are indicators of that balance and hence of the degree of shade-tolerance. The value o these two indicators however decreases as a function of seedling size and at an increasing rate when growt is intense. On this basis, slow growth is in itself a characteristic of shade-tolerance. In the article, the authors develop this idea and match it up against a set of data derived from their experiments.

We examined the relationships among productivity, water use efficiency (WUE) and drought tolerance in 29 genotypes of Populus x euramericana (Populus deltoides x Populus nigra), and investigated whether some leaf traits could be used as predictors for productivity, WUE and drought tolerance. At Orléans, France, drought was induced on one field plot by withholding water, while a second plot remained irrigated and was used as a control. Recorded variables included stem traits (e.g. biomass) and leaf structural (e.g. leaf area) and functional traits [e.g. intrinsic water use efficiency (Wi) and carbon isotope discrimination (Delta)]. Productivity and Delta displayed large genotypic variability and were not correlated. Delta scaled negatively with Wi and positively with stomatal conductance under moderate drought, suggesting that the diversity for Delta was mainly driven by stomatal conductance. Most of the productive genotypes displayed a low level of drought tolerance (i.e. a large reduction of biomass), while the less productive genotypes presented a large range of drought tolerance. The ability to increase WUE in response to water deficit was necessary but not sufficient to explain the genotypic diversity of drought tolerance.