Was begrenzt das Leben? Kleidon, Axel
Physik in unserer Zeit,
09/2021, Volume:
52, Issue:
5
Journal Article
ZusammenfassungSeit langer Zeit ist gut dokumentiert, dass die Photosynthese mit einem sehr geringen Wirkungsgrad von weniger als 3 % aus Sonnenlicht chemische Energie gewinnt. Aber warum ist der ...Prozess so ineffizient? Schließlich hatte die Photosynthese mehr als drei Milliarden Jahre in der Erdgeschichte Zeit, sich zu optimieren. Diese Frage wird hier mit dem geringen, aber thermodynamisch limitierten Gasaustausch zwischen Pflanzen und Atmosphäre erklärt. Während des Tages müssen Pflanzen der Atmosphäre Kohlendioxid entziehen, den Rohstoff für die Photosynthese. Dabei verlieren sie unvermeidbar große Mengen an Wasserdampf, verbunden mit ihrer Transpiration. Folglich sind Kohlendioxidaufnahme und Wasserdampfabgabe eng miteinander verbunden. Eine einfache Abschätzung zeigt, dass dieser Massenaustausch zwischen Erdoberfläche und Atmosphäre thermodynamisch limitiert ist. Sie kann die geringe Effizienz der Photosynthese schlüssig erklären.
The oxygen isotope signature of water is a powerful tracer of water movement from plants to the global scale. However, little is known about the short‐term variability of oxygen isotopes leaving the ...ecosystem via transpiration, as high‐frequency measurements are lacking. A laser spectrometer was coupled to a gas‐exchange chamber directly estimating branch‐level fluxes in order to evaluate the short‐term variability of the isotopic composition of transpiration (δE) and to investigate the role of isotopic non‐steady‐state transpiration under natural conditions in cork‐oak trees (Quercus suber) during distinct Mediterranean seasons. The measured δ¹⁸O of transpiration (δE) deviated from isotopic steady state throughout most of the day even when leaf water at the evaporating sites was near isotopic steady state. High agreement was found between estimated and modeled δE values assuming non‐steady‐state enrichment of leaf water. Isoforcing, that is, the influence of the transpirational δ¹⁸O flux on atmospheric values, deviated from steady‐state calculations but daily means were similar between steady state and non‐steady state. However, strong daytime isoforcing on the atmosphere implies that short‐term variations in δE are likely to have consequences for large‐scale applications, for example, partitioning of ecosystem fluxes or satellite‐based applications.
The ratio of plant carbon gain to water use, known as water use efficiency (WUE), has long been recognized as a key constraint on crop production and an important target for crop improvement. WUE is ...a physiologically and genetically complex trait that can be defined at a range of scales. Many component traits directly influence WUE, including photosynthesis, stomatal and mesophyll conductances, and canopy structure. Interactions of carbon and water relations with diverse aspects of the environment and crop development also modulate WUE. As a consequence, enhancing WUE by breeding or biotechnology has proven challenging but not impossible. This review aims to synthesize new knowledge of WUE arising from advances in phenotyping, modeling, physiology, genetics, and molecular biology in the context of classical theoretical principles. In addition, we discuss how rising atmospheric CO
concentration has created and will continue to create opportunities for enhancing WUE by modifying the trade-off between photosynthesis and transpiration.
The effect of contrasting environmental growth conditions (in vitro tissue culture, ex vitro acclimatisation, climate chamber, greenhouse and outdoor) on leaf development, cuticular wax composition, ...and foliar transpiration of detached leaves of the Populus × canescens clone 84 K were investigated. Our results show that total amounts of cuticular wax increased more than 10‐fold when cultivated in different growth conditions, whereas qualitative wax composition did not change. With exception of plants directly taken from tissue culture showing rapid dehydration, rates of water loss (residual foliar transpiration) of intact but detached leaves were constant and independent from growth conditions and thus independent from increasing wax amounts. Since cuticular transpiration measured with isolated astomatous P. × canescens cuticles was identical to residual foliar transpiration rates of detached leaves, our results confirm that cuticular transpiration of P. × canescens leaves can be predicted with high accuracy from residual transpiration of detached leaves after stomatal closure. Our results convincingly show that more than 10‐fold increased wax amounts in P. × canescens cuticles do not lead to decreased rates of residual (cuticular) transpiration.
Stomatal pores in many species are separated from the atmosphere by different anatomical obstacles produced by leaf epidermal cells, especially by sunken stomatal crypts, stomatal antechambers and/or ...hairs (trichomes). The evolutionary driving forces leading to sunken or 'hidden' stomata whose antechambers are filled with hairs or waxy plugs are not fully understood. The available hypothetical explanations are based mainly on mathematical modelling of water and CO2 diffusion through superficial vs. sunken stomata, and studies of comparative autecology. A better understanding of this phenomenon may result from examining the interactions between the leaf cuticle and stomata and from functional comparisons of sunken vs. superficially positioned stomata, especially when transpiration is low, for example at night or during severe drought.
I review recent ideas as to why stomata are hidden and test experimentally whether hidden stomata may behave differently from those not covered by epidermal structures and so are coupled more closely to the atmosphere. I also quantify the contribution of stomatal vs. cuticular transpiration at night using four species with sunken stomata and three species with superficial stomata.
Partitioning of leaf conductance in darkness (gtw) into stomatal and cuticular contributions revealed that stomatal conductance dominated gtw across all seven investigated species with antechambers with different degrees of prominence. Hidden stomata contributed, on average, less to gtw (approx. 70 %) than superficial stomata (approx. 80 %) and reduced their contribution dramatically with increasing gtw. In contrast, species with superficial stomata kept their proportion in gtw invariant across a broad range of gtw. Mechanisms behind the specific behaviour of hidden stomata and the multipurpose origin of sunken stomata are discussed.
Water saving under drought stress is assured by stomatal closure driven by active (ABA-mediated) and/or passive (hydraulic-mediated) mechanisms. There is currently no comprehensive model nor any ...general consensus about the actual contribution and relative importance of each of the above factors in modulating stomatal closure in planta. In the present study, we assessed the contribution of passive (hydraulic) vs active (ABA mediated) mechanisms of stomatal closure in V. vinifera plants facing drought stress. Leaf gas exchange decreased progressively to zero during drought, and embolism-induced loss of hydraulic conductance in petioles peaked to ~50% in correspondence with strong daily limitation of stomatal conductance. Foliar ABA significantly increased only after complete stomatal closure had already occurred. Rewatering plants after complete stomatal closure and after foliar ABA reached maximum values did not induced stomatal re-opening, despite embolism recovery and water potential rise. Our data suggest that in grapevine stomatal conductance is primarily regulated by passive hydraulic mechanisms. Foliar ABA apparently limits leaf gas exchange over long-term, also preventing recovery of stomatal aperture upon rewatering, suggesting the occurrence of a mechanism of long-term down-regulation of transpiration to favor embolism repair and preserve water under conditions of fluctuating water availability and repeated drought events.
•Limited transpiration at elevated vapor pressure deficit confers drought tolerance.•Trait associated with low hydraulic conductivity in plant.•Aquaporin expression related to hydraulic conductivity ...and limited transpiration.•Genotypic variability for limited transpiration identified in major crop species.•Success in basic physiological research being used in improved crops.
Water deficit under nearly all field conditions is the major constraint on plant yields. Other than empirical observations, very little progress has been made in developing crop plants in which specific physiological traits for drought are expressed. As a consequence, there was little known about under what conditions and to what extent drought impacts crop yield. However, there has been rapid progress in recent years in understanding and developing a limited-transpiration trait under elevated atmospheric vapor pressure deficit to increase plant growth and yield under water-deficit conditions. This review paper examines the physiological basis for the limited-transpiration trait as result of low plant hydraulic conductivity, which appears to be related to aquaporin activity. Methodology was developed based on aquaporin involvement to identify candidate genotypes for drought tolerance of several major crop species. Cultivars of maize and soybean are now being marketed specifically for arid conditions. Understanding the mechanism of the limited-transpiration trait has allowed a geospatial analyses to define the environments in which increased yield responses can be expected. This review highlights the challenges and approaches to finally develop physiological traits contributing directly to plant improvement for water-limited environments.
Recent decades have been characterized by increasing temperatures worldwide, resulting in an exponential climb in vapor pressure deficit (VPD). VPD has been identified as an increasingly important ...driver of plant functioning in terrestrial biomes and has been established as a major contributor in recent drought-induced plant mortality independent of other drivers associated with climate change. Despite this, few studies have isolated the physiological response of plant functioning to high VPD, thus limiting our understanding and ability to predict future impacts on terrestrial ecosystems. An abundance of evidence suggests that stomatal conductance declines under high VPD and transpiration increases in most species up until a given VPD threshold, leading to a cascade of subsequent impacts including reduced photosynthesis and growth, and higher risks of carbon starvation and hydraulic failure. Incorporation of photosynthetic and hydraulic traits in ‘next-generation’ land-surface models has the greatest potential for improved prediction of VPD responses at the plant- and global-scale, and will yield more mechanistic simulations of plant responses to a changing climate. By providing a fully integrated framework and evaluation of the impacts of high VPD on plant function, improvements in forecasting and long-term projections of climate impacts can be made.
Plants grow and transpire water during the day and night. Recent work highlights the idea that night-time transpirational water loss is a consequence of allowing respiratory CO2 to escape at ...sufficiently high rates through stomata. Respiration fuels night-time leaf expansion and requires carbohydrates produced during the day. As carbohydrate availability and growth are under the control of the plants’ internal clock, so is night-time transpiration. The cost of night-time transpiration is that water is lost without carbon being gained, the benefit is a higher efficiency of taken up water for use in leaf expansion. This could provide a stress acclimation process.
Plants loose water at significant rates during the night through ‘night-time transpiration’.
Night-time transpirational water loss is most likely the consequence of having respiratory CO2 escape at sufficiently high rates through stomata.
As night-time respiration fuels growth and depends on daytime provision of storage carbohydrates, there must exist some regulation, involving the internal clock, between daytime photosynthesis and growth, and night-time transpiration and growth.
Night-time growth presents a more efficient use of taken up water for leaf cell expansive growth compared with daytime growth, and could represent an overlooked stress acclimation process.
• Plants grow and transpire during the night. The aim of the present work was to assess the relative flows of carbon, water and solutes, and the energy involved, in sustaining night-time ...transpiration and leaf expansive growth under control and salt-stress conditions.
• Published and unpublished data were used, for barley plants grown in presence of 0.5–1 mM NaCl (control) and 100 mM NaCl.
• Night-time leaf growth presents a more efficient use of taken-up water compared with day-time growth. This efficiency increases several-fold with salt stress. Night-time transpiration cannot be supported entirely through osmotically driven uptake of water through roots under salt stress. Using a simple three- (root medium/cytosol/vacuole) compartment approach, the energy required to support cell expansion during the night is in the lower percentage region (0.03–5.5%) of the energy available through respiration, under both, control and salt-stress conditions. Use of organic (e.g. hexose equivalents) rather than inorganic (e.g. Na⁺, Cl⁻, K⁺) solutes for generation of osmotic pressure in growing cells, increases the energy demand by orders of magnitude, yet requires only a small portion of carbon assimilated during the day.
• Night-time transpiration and leaf expansive growth should be considered as a potential acclimation mechanism to salinity.
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