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Leaf photosynthetic capacity (light-saturated net assimilation rate, A A ) increases from bottom to top of plant canopies as the most prominent acclimation response to the conspicuous within-canopy gradients in light availability. Light-dependent variation in A A through plant canopies is associated with changes in key leaf structural (leaf dry mass per unit leaf area), chemical (nitrogen (N) content per area and dry mass, N partitioning between components of photosynthetic machinery), and physiological (stomatal and mesophyll conductance) traits, whereas the contribution of different traits to within-canopy A A gradients varies across sites, species, and plant functional types. Optimality models maximizing canopy carbon gain for a given total canopy N content predict that A A should be proportionally related to canopy light availability. However, comparison of model expectations with experimental data of within-canopy photosynthetic trait variations in representative plant functional types indicates that such proportionality is not observed in real canopies, and A A vs. canopy light relationships are curvilinear. The factors responsible for deviations from full optimality include stronger stomatal and mesophyll diffusion limitations at higher light, reflecting greater water limitations and more robust foliage in higher light. In addition, limits on efficient packing of photosynthetic machinery within leaf structural scaffolding, high costs of N redistribution among leaves, and limited plasticity of N partitioning among components of photosynthesis machinery constrain A A plasticity. Overall, this review highlights that the variation of A A through plant canopies reflects a complex interplay between adjustments of leaf structure and function to multiple environmental drivers, and that A A plasticity is limited by inherent constraints on and trade-offs between structural, chemical, and physiological traits. I conclude that models trying to simulate photosynthesis gradients in plant canopies should consider co-variations among environmental drivers, and the limitation of functional trait variation by physical constraints and include the key trade-offs between structural, chemical, and physiological leaf characteristics.

Leaf area growth determines the light interception capacity of a crop and is often used as a surrogate for plant growth in high-throughput phenotyping systems. The relationship between leaf area growth and growth in terms of mass will depend on how carbon is partitioned among new leaf area, leaf mass, root mass, reproduction, and respiration. A model of leaf area growth in terms of photosynthetic rate and carbon partitioning to different plant organs was developed and tested with Arabidopsis thaliana L. Heynh. ecotype Columbia (Col-0) and a mutant line, gigantea-2 (gi-2), which develops very large rosettes. Data obtained from growth analysis and gas exchange measurements was used to train a genetic programming algorithm to parameterize and test the above model. The relationship between leaf area and plant biomass was found to be non-linear and variable depending on carbon partitioning. The model output was sensitive to the rate of photosynthesis but more sensitive to the amount of carbon partitioned to growing thicker leaves. The large rosette size of gi-2 relative to that of Col-0 resulted from relatively small differences in partitioning to new leaf area vs. leaf thickness.

CONTENTS: Summary 30 I. Allocation in perspective 31 II. Topics of this review 32 III. Methodology 32 IV. Environmental effects 33 V. Ontogeny 36 VI. Differences between species 40 VII. Physiology and molecular regulation 41 VIII. Ecological aspects 42 IX. Perspectives 45 Acknowledgements 45 References 45 Appendices A1–A4 49 SUMMARY: We quantified the biomass allocation patterns to leaves, stems and roots in vegetative plants, and how this is influenced by the growth environment, plant size, evolutionary history and competition. Dose–response curves of allocation were constructed by means of a meta‐analysis from a wide array of experimental data. They show that the fraction of whole‐plant mass represented by leaves (LMF) increases most strongly with nutrients and decreases most strongly with light. Correction for size‐induced allocation patterns diminishes the LMF‐response to light, but makes the effect of temperature on LMF more apparent. There is a clear phylogenetic effect on allocation, as eudicots invest relatively more than monocots in leaves, as do gymnosperms compared with woody angiosperms. Plants grown at high densities show a clear increase in the stem fraction. However, in most comparisons across species groups or environmental factors, the variation in LMF is smaller than the variation in one of the other components of the growth analysis equation: the leaf area : leaf mass ratio (SLA). In competitive situations, the stem mass fraction increases to a smaller extent than the specific stem length (stem length : stem mass). Thus, we conclude that plants generally are less able to adjust allocation than to alter organ morphology.

Hot days are becoming hotter and more frequent, threatening wheat yields worldwide. Developing wheat varieties ready for future climates calls for improved understanding of how elevated CO₂ (eCO₂) and heat stress (HS) interactively impact wheat yields. We grew a modern, high-yielding wheat cultivar (Scout) at ambient CO₂ (aCO₂, 419 μl l−1) or eCO₂ (654 μl l−1) in a glasshouse maintained at 22/15 °C (day/night). Half of the plants were exposed to HS (40/24 °C) for 5 d at anthesis. In non-HS plants, eCO₂ enhanced (+36%) CO₂ assimilation rates (Asat) measured at growth CO₂ despite down-regulation of photosynthetic capacity. HS reduced Asat (−42%) in aCO₂- but not in eCO₂-grown plants because eCO₂ protected photosynthesis by increasing ribulose bisphosphate regeneration capacity and reducing photochemical damage under HS. eCO₂ stimulated biomass (+35%) of all plants and grain yield (+30%) of non-HS plants only. Plant biomass initially decreased following HS but recovered at maturity due to late tillering. HS equally reduced grain yield (−40%) in aCO₂- and eCO₂-grown plants due to grain abortion and reduced grain filling. While eCO₂ mitigated the negative impacts of HS at anthesis on wheat photosynthesis and biomass, grain yield was reduced by HS in both CO₂ treatments.

Coffee (Coffea spp.), a globally traded commodity, is a slow -growing tropical tree species that displays an improved photosynthetic performance when grown under elevated atmospheric CO2 concentrations ([CO2]). To investigate the mechanisms underlying this response, two commercial coffee cultivars (Catuaí and Obatã) were grown using the first free-air CO2 enrichment (FACE) facility in Latin America. Measurements were conducted in two contrasting growth seasons, which were characterized by the high (February) and low (August) sink demand. Elevated [CO2] led to increases in net photosynthetic rates (A) in parallel with decreased photorespiration rates, with no photochemical limitations to A. The stimulation of A by elevated CO2 supply was more prominent in August (56% on average) than in February (40% on average). Overall, the stomatal and mesophyll conductances, as well as the leaf nitrogen and phosphorus concentrations, were unresponsive to the treatments. Photosynthesis was strongly limited by diffusional constraints, particularly at the stomata level, and this pattern was little, if at all, affected by elevated [CO2]. Relative to February, starch pools (but not soluble sugars) increased remarkably (>500%) in August, with no detectable alteration in the maximum carboxylation capacity estimated on a chloroplast [CO2] basis. Upregulation of A by elevated [CO2] took place with no signs of photosynthetic downregulation, even during the period of low sink demand, when acclimation would be expected to be greatest.

• Photosynthetic stimulation by elevated [CO₂] (e[CO₂]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N₂-fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO₂]. However, drought not only constrains photosynthesis but also the size and activity of sinks, and little is known about the interaction of e[CO₂] and drought on carbon sink strength of nodules and other organs. • To compare carbon sink strength, faba bean was grown under ambient (400 ppm) or elevated (700 ppm) atmospheric [CO₂] and subjected to well-watered or drought treatments, and then exposed to 13C pulse-labelling using custom-built chambers to track the fate of new photosynthates. • Drought decreased 13C uptake and nodule sink strength, and this effect was even greater under e[CO₂], and was associated with an accumulation of amino acids in nodules. This resulted in decreased N₂ fixation, and increased accumulation of new photosynthates (13C/sugars) in leaves, which in turn can feed back on photosynthesis. • Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO₂], and legumes may only be able to achieve greater C gain if nodule activity is maintained.

Summary 565 I. LMA in perspective 566 II. LMA in the field 567 III. Inherent differences 568 IV. Relation with anatomy and chemical composition 570 V. Environmental effects 572 VI. Differences in space and time 577 VII. Molecular regulation and physiology 579 VIII. Ecological consequences 580 IX. Conclusions and perspectives 582 Acknowledgements 582 References 582 Appendices 587

Plants in their natural environment are often exposed to fluctuating light because of self-shading and cloud movements. As changing frequency is a key characteristic of fluctuating light, we speculated that rapid light fluctuation may induce rapid photosynthetic responses, which may protect leaves against photoinhibition. To test this hypothesis, maize seedlings were grown under fluctuating light with various frequencies (1, 10, and 100 cycles of fluctuations/10 h), and changes in growth, chlorophyll content, gas exchange, chlorophyll a fluorescence, and P700 were analyzed carefully. Our data show that though the growth and light-saturated photosynthetic rate were depressed by rapidly fluctuating light, photosynthesis induction was clearly speeded up. Furthermore, more rapid fluctuation of light strikingly reduced the chlorophyll content, while thermal dissipation was triggered and enhanced. The chlorophyll a fluorescence induction kinetics and P700 absorption results showed that the activities of both photosystem II and photosystem I decreased as the frequency of the fluctuating light increased. In all treatments, the light intensities of the fluctuating light were kept constant. Therefore, rapid light fluctuation frequency itself induced the acceleration of photosynthetic induction and the enhancement of photoprotection in maize seedlings, which play important roles in protecting photosynthetic apparatus against fluctuating high light to a certain extent.

Plants continually adjust the photosynthetic functions in their leaves to fluctuating light, thereby optimizing the use of photosynthetic nitrogen (Nph ) at the canopy level. To investigate the complex interplay between external signals during the acclimation processes, a mechanistic model based on the concept of protein turnover (synthesis and degradation) was proposed and parameterized using cucumber grown under nine combinations of nitrogen and light in growth chambers. Integrating this dynamic model into a multi-layer canopy model provided accurate predictions of photosynthetic acclimation of greenhouse cucumber canopies grown under high and low nitrogen supply in combination with day-to-day fluctuations in light at two different levels. This allowed us to quantify the degree of optimality in canopy nitrogen use for maximizing canopy carbon assimilation, which was influenced by Nph distribution along canopy depth or Nph partitioning between functional pools. Our analyses suggest that Nph distribution is close to optimum and Nph reallocation is more important under low nitrogen. Nph partitioning is only optimal under a light level similar to the average light intensity during acclimation, meaning that day-to-day light fluctuations inevitably result in suboptimal Nph partitioning. Our results provide insights into photoacclimation and can be applied to crop model improvement.

Background Light deficit in shaded environment critically impacts the growth and development of turf plants. Despite this fact, past research has predominantly concentrated on shade avoidance rather than shade tolerance. To address this, our study examined the photosynthetic adjustments of Bermudagrass when exposed to varying intensities of shade to gain an integrative understanding of the shade response of C4 turfgrass. Results We observed alterations in photosynthetic pigment-proteins, electron transport and its associated carbon and nitrogen assimilation, along with ROS-scavenging enzyme activity in shaded conditions. Mild shade enriched Chl b and LHC transcripts, while severe shade promoted Chl a, carotenoids and photosynthetic electron transfer beyond Q A − (ET 0 /RC, φE 0 , Ψ 0 ). The study also highlighted differential effects of shade on leaf and root components. For example, Soluble sugar content varied between leaves and roots as shade diminished SPS , SUT1 but upregulated BAM . Furthermore, we observed that shading decreased the transcriptional level of genes involving in nitrogen assimilation (e.g. NR ) and SOD, POD, CAT enzyme activities in leaves, even though it increased in roots. Conclusions As shade intensity increased, considerable changes were noted in light energy conversion and photosynthetic metabolism processes along the electron transport chain axis. Our study thus provides valuable theoretical groundwork for understanding how C4 grass acclimates to shade tolerance.