Summary 1.Litter decomposition recycles nutrients and causes large fluxes of carbon dioxide into the atmosphere. It is typically assumed that climate, litter quality and decomposer communities determine litter decay rates, yet few comparative studies have examined their relative contributions in tropical forests. 2. We used a short-term litterbag experiment to quantify the effects of litter quality, placement and mesofaunal exclusion on decomposition in 23 tropical forests in 14 countries. Annual precipitation varied among sites (760-5797 mm). At each site, two standard substrates ( Raphia farinifera and Laurus nobilis ) were decomposed in fine-and coarse-mesh litterbags both above and below ground for approximately 1 year. 3. Decomposition was rapid, with >95% mass loss within a year at most sites. Litter quality, placement and mesofaunal exclusion all independently affected decomposition, but the magnitude depended upon site. Both the average decomposition rate at each site and the ratio of above-to below-ground decay increased linearly with annual precipitation, explaining 60-65% of among-site variation. Excluding mesofauna had the largest impact on decomposition, reducing decomposition rates by half on average, but the magnitude of decrease was largely independent of climate. This suggests that the decomposer community might play an important role in explaining patterns of decomposition among sites. Which litter type decomposed fastest varied by site, but was not related to climate. 4. Synthesis . A key goal of ecology is to identify general patterns across ecological communities, as well as relevant site-specific details to understand local dynamics. Our pan-tropical study shows that certain aspects of decomposition, including average decomposition rates and the ratio of above-to below-ground decomposition are highly correlated with a simple climatic index: mean annual precipitation. However, we found no relationship between precipitation and effects of mesofaunal exclusion or litter type, suggesting that site-specific details may also be required to understand how these factors affect decomposition at local scales.

The C4 photosynthetic pathway uses water more efficiently than the C3 type, yet biogeographical analyses show a decline in C4 species relative to C3 species with decreasing rainfall. To investigate this paradox, the hypothesis that the C4 advantage over C3 photosynthesis is diminished by drought was tested, and the underlying stomatal and metabolic mechanisms of this response determined. The effects of drought and high evaporative demand on leaf gas exchange and photosynthetic electron sinks in C3 and C4 subspecies of the grass Alloteropsis semialata were examined. Plant responses to climatic variation and soil drought were investigated using a common garden experiment with well-watered and natural rainfall treatments, and underlying mechanisms analysed using controlled drying pot experiments. Photosynthetic rates were significantly higher in the C4 than the C3 subspecies in the garden experiment under well-watered conditions, but this advantage was completely lost during a rainless period when unwatered plants experienced severe drought. Controlled drying experiments showed that this loss was caused by a greater increase in metabolic, rather than stomatal, limitations in C4 than in the C3 leaves. Decreases in CO2 assimilation resulted in lower electron transport rates and decreased photochemical efficiency under drought conditions, rather than increased electron transport to alternative sinks. These findings suggest that the high metabolic sensitivity of photosynthesis to severe drought seen previously in several C4 grass species may be an inherent characteristic of the C4 pathway. The mechanism may explain the paradox of why C4 species decline in arid environments despite high water-use efficiency.

Intrinsic water use efficiency (WUE(intr)), the ratio of photosynthesis to stomatal conductance to water, is often used as an index for crop water use in breeding projects. However, WUE(intr) conflates variation in these two processes, and thus may be less useful as a selection trait than knowledge of both components. The goal of the present study was to determine whether the contribution of photosynthetic capacity and stomatal conductance to WUE(intr) varied independently between soybean genotypes and whether this pattern was interactive with mild drought. Photosynthetic capacity was defined as the variation in WUE(intr) that would occur if genotypes of interest had the same stomatal conductance as a reference genotype and only differed in photosynthesis; similarly, the contribution of stomatal conductance to WUE(intr) was calculated assuming a constant photosynthetic capacity across genotypes. Genotypic differences in stomatal conductance had the greatest effect on WUE(intr) (26% variation when well watered), and was uncorrelated with the effect of photosynthetic capacity on WUE(intr). Thus, photosynthetic advantages of 8.3% were maintained under drought. The maximal rate of Rubisco carboxylation, generally the limiting photosynthetic process for soybeans, was correlated with photosynthetic capacity. As this trait was not interactive with leaf temperature, and photosynthetic capacity differences were maintained under mild drought, the observed patterns of photosynthetic advantage for particular genotypes are likely to be consistent across a range of environmental conditions. This suggests that it is possible to employ a selection strategy of breeding water-saving soybeans with high photosynthetic capacities to compensate for otherwise reduced photosynthesis in genotypes with lower stomatal conductance.

*: These authors contributed equally 24 25One-sentence summary: 26The gas phase of mesophyll conductance is impacted by the 3D traits tortuosity, path 27 lengthening and airspace connectivity, in addition to porosity. 2 JME, GTR, and CRB conceived the study and developed the methods, with 31 contributions from MEG and AJM; JME and GTR acquired and analyzed the data; ABR 32 performed the phylogenetic analyses; JME and GTR wrote the manuscript; ABR, MEG, 33 AJM, and CRB complemented the writing. 3 ABSTRACT 42The leaf intercellular airspace (IAS) is generally considered to have high conductance to 43 CO 2 diffusion relative to the liquid phase. While previous studies accounted for leaf-level 44 variation in porosity and mesophyll thickness, they omitted 3D IAS traits that potentially 45 influence IAS conductance (g IAS ). Here we re-evaluated the standard equation for g IAS 46 by incorporating tortuosity, lateral path lengthening, and IAS connectivity. We measured 47 and spatially mapped these geometric IAS traits for 19 Bromeliaceae species with CAM 48 or C3 photosynthetic pathways using X-ray microCT imaging and a novel computational 49 approach. We found substantial variation in porosity (0.04-0.73 m 3 m -3 ), tortuosity 50 -2 s -1 bar -1 ) plants due to a coordinated decline in these IAS traits. Our re-evaluated 54 equation also generally predicted lower g IAS values than the former one. Moreover, we 55 observed high spatial heterogeneity in these IAS geometric traits throughout the 56 mesophyll, especially within CAM leaves. Our data show that IAS traits that better 57 capture the 3D complexity of leaves strongly influence g IAS and that the impact of the 58 IAS on mesophyll conductance should be carefully considered with respect to leaf 59 anatomy. We provide a simple function to estimate tortuosity and lateral path 60 lengthening in the absence of access to imaging tools such as X-ray microCT or other 61 novel 3D image-processing techniques. INTRODUCTION 63By volume, as little as 3% and up to 73% (this study) of 64 the inside of a leaf is composed of air. Such a wide range of values results from the 65 multiple roles that mesophyll cells play in leaf function, the degree of reticulation of the 66 embedded vein network, and cell size and shape, all reflecting the various adaptations 67 plants have made in colonizing nearly every terrestrial habitat on Earth. From an 68 evolutionary perspective, the transition from oceans to land exposed plant tissues to air, 69 which dramatically lowered the resistance for CO 2 diffusion to chloroplasts by ~10,000-70fold. Evolutionary development of the leaf intercellular airspace (IAS) is therefore 71 considered a key innovation to profit from that lowered diffusion resistance (Ligrone et 72 al., 2012). Yet, terrestrial inhabitation also exposed leaves to the risk of desiccation. 73Plants presumably navigated this trade-off by developing a complex spatial cellular 74 arrangement in order to produce a more or less tortuous 3D IAS network that rapidly 75 delivered CO 2 t...

Bundle sheath extensions (BSEs) are key features of leaf structure with currently little-understood functions. To test the hypothesis that BSEs reduce the hydraulic resistance from the bundle sheath to the epidermis (r be ) and thereby accelerate hydropassive stomatal movements, we compared stomatal responses with reduced humidity and leaf excision among 20 species with heterobaric or homobaric leaves and herbaceous or woody life forms. We hypothesized that low r be due to the presence of BSEs would increase the rate of stomatal opening (V) during transient wrong-way responses, but more so during wrong-way responses to excision (V e ) than humidity (V h ), thus increasing the ratio of V e to V h . We predicted the same trends for herbaceous relative to woody species given greater hydraulic resistance in woody species. We found that V e , V h , and their ratio were 2.3 to 4.4 times greater in heterobaric than homobaric leaves and 2.0 to 3.1 times greater in herbaceous than woody species. To assess possible causes for these differences, we simulated these experiments in a dynamic compartment/resistance model, which predicted larger V e and V e /V h in leaves with smaller r be . These results support the hypothesis that BSEs reduce r be . Comparison of our data and simulations suggested that r be is approximately 4 to 16 times larger in homobaric than heterobaric leaves. Our study provides new evidence that variations in the distribution of hydraulic resistance within the leaf and plant are central to understanding dynamic stomatal responses to water status and their ecological correlates and that BSEs play several key roles in the functional ecology of heterobaric leaves.

Within the Panicoid grasses, C4 (NADP-ME) species are metabolically more sensitive to drought than C3 species and recover more slowly from drought.

Maintaining high rates of photosynthesis in leaves requires efficient movement of CO 2 from the atmosphere to the mesophyll cells inside the leaf where CO 2 is converted into sugar. CO 2 diffusion inside the leaf depends directly on the structure of the mesophyll cells and their surrounding airspace, which have been difficult to characterize because of their inherently three-dimensional organization. Yet faster CO 2 diffusion inside the leaf was probably critical in elevating rates of photosynthesis that occurred among angiosperm lineages. Here we characterize the three-dimensional surface area of the leaf mesophyll across vascular plants. We show that genome size determines the sizes and packing densities of cells in all leaf tissues and that smaller cells enable more mesophyll surface area to be packed into the leaf volume, facilitating higher CO 2 diffusion. Measurements and modelling revealed that the spongy mesophyll layer better facilitates gaseous phase diffusion while the palisade mesophyll layer better facilitates liquid-phase diffusion. Our results demonstrate that genome downsizing among the angiosperms was critical to restructuring the entire pathway of CO 2 diffusion into and through the leaf, maintaining high rates of CO 2 supply to the leaf mesophyll despite declining atmospheric CO 2 levels during the Cretaceous.

Key messageThis study identifies a small distal region of the 1RS chromosome from rye that has a positive impact on wheat yield.AbstractThe translocation of the short arm of rye (Secale cereale L.) chromosome one (1RS) onto wheat (Triticum aestivum L.) chromosome 1B (1RS.1BL) is used in wheat breeding programs worldwide due to its positive effect on yield, particularly under abiotic stress. Unfortunately, this translocation is associated with poor bread-making quality. To mitigate this problem, the 1RS arm was engineered by the removal and replacement of two interstitial rye segments with wheat chromatin: a distal segment to introduce the Glu-B3/Gli-B1 loci from wheat, and a proximal segment to remove the rye Sec-1 locus. We used this engineered 1RS chromosome (henceforth 1RSWW) to develop and evaluate two sets of 1RS/1RSWW near isogenic lines (NILs). Field trials showed that standard 1RS lines had significantly higher yield and better canopy water status than the 1RSWW NILs in both well-watered and water-stressed environments. We intercrossed the 1RS and 1RSWW lines and generated two additional NILs, one carrying the distal (1RSRW) and the other carrying the proximal (1RSWR) wheat segment. Lines not carrying the distal wheat region (1RS and 1RSWR) showed significant improvements in grain yield and canopy water status compared to NILs carrying the distal wheat segment (1RSWW and 1RSRW), indicating that the 1RS region replaced by the distal wheat segment carries the beneficial allele(s). NILs without the distal wheat segment also showed higher carbon isotope discrimination and increased stomatal conductance, suggesting that these plants had improved access to water. The 1RSWW, 1RSWR and 1RSRW NILs have been deposited in the National Small Grains Collection.Electronic supplementary materialThe online version of this article (doi:10.1007/s00122-014-2408-6) contains supplementary material, which is available to authorized users.