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The Kepler data show that habitable small planets orbiting Red Dwarf stars (RDs) are abundant, and hence might be promising targets to look at for biomarkers and life. Planets orbiting within the habitable zone of RDs are close enough to be tidally locked. Some recent works have cast doubt on the ability of planets orbiting RDs to support life. In contrast, it is shown that temperatures suitable for liquid water and even for organic molecules may exist on tidally locked planets (TLPs) of RDs for a wide range of atmospheres. We chart the surface temperature distribution as a function of the irradiation, greenhouse factor and heat circulation. The habitability boundaries and their dependence on the atmospheric properties are derived. By extending our previous analyses of TLPs, we find that tidally locked as well as synchronous (not completely locked) planets of RDs and K-type stars may support life, for a wider range of orbital distance and atmospheric conditions than previously thought. In particular, it is argued that life clement environments may be possible on tidally locked and synchronously orbiting planets of RDs and K-type stars, with conditions supporting oxygenic photosynthesis, which on Earth was a key to complex life. Different climate projections and the biological significance of tidal locking on putative complex life are reviewed. We show that when the effect of continuous radiation is taken into account, the photo-synthetically active radiation available on TLPs, even of RDs, could produce a high-potential plant productivity, in analogy to mid-summer growth at high latitudes on Earth. Awaiting the findings of TESS and JWST, we discuss the implications of the above arguments to the detection of biomarkers such as liquid water and oxygen, as well as to the abundance of biotic planets and life.
Photosynthesis offers a convenient means of sustaining biospheres. We quantify the constraints for photosynthesis to be functional on the permanent nightside of tidally locked rocky exoplanets via reflected light from their exomoons. We show that the exomoons must be at least half the size of Earth's moon in order for conventional oxygenic photosynthesis to operate. This scenario of photosynthesis is unlikely for exoplanets around late-type M-dwarfs due to the low likelihood of large exomoons and their orbital instability over long timescales. Subsequently, we investigate the prospects for photosynthesis on habitable exomoons via reflected light from the giant planets that they orbit. Our analysis indicates that such photosynthetic biospheres are potentially sustainable on these moons except those around late-type M-dwarfs. We conclude our analysis by delineating certain physiological and biochemical features of photosynthesis and other carbon fixation pathways, and the likelihood of their evolution on habitable planets and moons.
The physiology of mesophotic Scleractinia varies with depth in response to environmental change. Previous research has documented trends in heterotrophy and photosynthesis with depth, but has not addressed between-site variation for a single species. Environmental differences between sites at a local scale and heterogeneous microhabitats, because of irradiance and food availability, are likely important factors when explaining the occurrence and physiology of Scleractinia. Here, 108 colonies of Agaricia lamarcki were sampled from two locations off the coast of Utila, Honduras, distributed evenly down the observed 50 m depth range of the species. We found that depth alone was not sufficient to fully explain physiological variation. Pulse Amplitude-Modulation fluorometry and stable isotope analyses revealed that trends in photochemical and heterotrophic activity with depth varied markedly between sites. Our isotope analyses do not support an obligate link between photosynthetic activity and heterotrophic subsidy with increasing depth. We found that A. lamarcki colonies at the bottom of the species depth range can be physiologically similar to those nearer the surface. As a potential explanation, we hypothesize sites with high topographical complexity, and therefore varied microhabitats, may provide more physiological niches distributed across a larger depth range. Varied microhabitats with depth may reduce the dominance of depth as a physiological determinant. Thus, A. lamarcki may ‘avoid’ changes in environment with depth, by instead existing in a subset of favourable niches. Our observations correlate with site-specific depth ranges, advocating for linking physiology and abiotic profiles when defining the distribution of mesophotic taxa.
Cyanobacteria and plants carry out oxygenic photosynthesis. They use water to generate the atmospheric oxygen we breathe and carbon dioxide to produce the biomass serving as food, feed, fibre and fuel. This paper scans the emergence of structural and mechanistic understanding of oxygen evolution over the past 50 years. It reviews speculative concepts and the stepped insight provided by novel experimental and theoretical techniques. Driven by sunlight photosystem II oxidizes the catalyst of water oxidation, a hetero-metallic Mn4CaO5(H2O)4 cluster. Mn3Ca are arranged in cubanoid and one Mn dangles out. By accumulation of four oxidizing equivalents before initiating dioxygen formation it matches the four-electron chemistry from water to dioxygen to the one-electron chemistry of the photo-sensitizer. Potentially harmful intermediates are thereby occluded in space and time. Kinetic signatures of the catalytic cluster and its partners in the photo-reaction centre have been resolved, in the frequency domain ranging from acoustic waves via infra-red to X-ray radiation, and in the time domain from nano- to milli-seconds. X-ray structures to a resolution of 1.9 Å are available. Even time resolved X-ray structures have been obtained by clocking the reaction cycle by flashes of light and diffraction with femtosecond X-ray pulses. The terminal reaction cascade from two molecules of water to dioxygen involves the transfer of four electrons, two protons, one dioxygen and one water. A rigorous mechanistic analysis is challenging because of the kinetic enslaving at millisecond duration of six partial reactions (4e−, 1H+, 1O2). For the time being a peroxide-intermediate in the reaction cascade to dioxygen has been in focus, both experimentally and by quantum chemistry. Homo sapiens has relied on burning the products of oxygenic photosynthesis, recent and fossil. Mankind's total energy consumption amounts to almost one-fourth of the global photosynthetic productivity. If the average power consumption equalled one of those nations with the highest consumption per capita it was four times greater and matched the total productivity. It is obvious that biomass should be harvested for food, feed, fibre and platform chemicals rather than for fuel.
Lichens are one of the common dominant biota in biological soil crusts (biocrusts), a community that is one of the largest in extent in the world. Here we present a summary of the main features of the lifestyle of soil crust lichens, emphasizing their habitat, ecophysiology and versatility. The soil crust is exposed to full light, often to high temperatures and has an additional water source, the soil beneath the lichens. However, despite the open nature of the habitat the lichens are active under shady and cooler conditions and avoid climate extremes of high temperature and light. In temperate and alpine habitats they can also be active for long periods, several months in some cases. They show a mixture of physiological constancy (e.g. similar activity periods and net photosynthetic rates) but also adaptations to the habitat (e.g. the response of net photosynthesis to thallus water content can differ for the same lichen species in Europe and the USA and some species show extensive rhizomorph development). Despite recent increased research, aspects of soil crust ecology, for example under snow, remain little understood.
Chilling injury is an important natural stress that can threaten cotton production, especially at the sowing and seedling stages in early spring. It is therefore important for cotton production to improve chilling tolerance at these stages. The current work examines the potential for glycine betaine (GB) treatment of seeds to increase the chilling tolerance of cotton at the seedling stage. Germination under cold stress was increased significantly by GB treatment. Under low temperature, the leaves of seedlings from treated seeds exhibited a higher net photosynthetic rate (PN), higher antioxidant enzyme activity including superoxide dismutase, ascorbate peroxidase and catalase, lower hydrogen peroxide (H2O2) content and less damage to the cell membrane. Enzyme activity was correlated negatively with H2O2 content and degree of damage to the cell membrane but correlated positively with GB content. The experimental results suggested that although GB was only used to treat cotton seed, the beneficial effect caused by the preliminary treatment of GB could play a significant role during germination that persisted to at least the four-leaf seedling stage. Therefore, it is crucial that this method is employed in agricultural production to improve chilling resistance in the seedling stage by soaking the seeds in GB.
The citrus leafminer (CLM), Phyllocnistis citrella Stainton causes injury to citrus and related species in the Rutaceae family. The damage that the CLM larvae can cause is significant in citrus plantations. We tested two citrus cultivars — ‘Kinnow’ (Citrus reticulata Blanco) and ‘Fairchild’ (a hybrid of Citrus reticulata Clementine x Orlando Tangelo) — to quantify CLM larvae infestation and effect on the physiology of the citrus cultivars. We then compared the CLM larval weight with its associated damage. To calculate infestation level, mine area and total leaf area, we used the image analysis technique. The infestation level of CLM was higher in ‘Fairchild’ than in ‘Kinnow’ cultivar of citrus. For both cultivars, larval weight of CLM was directly proportional to the amount of mines generated. Taken together, the results of this study suggest that the mines that CLM larvae generate pose significant effect on the net photosynthetic rates and water use efficiency of citrus nursery plants. These results will help improve our understanding of the interaction between CLM and citrus nursery plants and effect of the pest on the yield potential of the crop.
Hemiparasitic plants obtain water and solutes from their hosts, but much remains to be learned about these transfers. We used a forest girdling experiment to investigate how leaf gas exchange, carbon and nitrogen cycling in the root hemiparasite Melampyrum lineare Desr. responded to disturbance and changes in physiology of potential host trees. By preventing belowground C allocation by 35% of the canopy, girdling decreased the starch and soluble sugar contents of bulk forest floor fine roots. Photosynthetic rates of M. lineare were statistically significantly lower in the girdled plot, but their hypothesized drivers (foliar N, stomatal conductance and transpiration) had no statistically significant differences between girdled and non-girdled plots. However, M. lineare in the girdled plot had higher foliar C concentrations and Δ14C than in the control plot, suggesting possible photosynthetic down-regulation in the girdled plot due to influx of older (e.g., host-derived) C into the leaves of M. lineare. Within the girdled plot (but not the control plot), M. lineare foliar C concentrations were positively correlated with foliar Δ14C and δ15N, suggesting that M. lineare may respond to changes in both C and N biogeochemistry during the decline of dominant canopy species.
Within Antarctica there are large gradients both in climate and in vegetation which offer opportunities to investigate links between the two. The activity (% total time active) of lichens and bryophytes in hydric and xeric environments was monitored at Livingston Island (62°39'S). This adds a northern site with a maritime, cloudy climate to previous studies in the southern Antarctic Peninsula and the Dry Valleys (78°S). Annual activity increases northwards from less than 1% to nearly 100%. There is a major and consistent difference between hydric sites which, with snow melt, can be 100% active in summer months even in the Dry Valleys, and xeric sites which, depending on precipitation, rarely exceed 80% activity even at Livingston Island. Mosses dominate hydric sites and lichens the xeric sites all along the gradient. Mean temperatures when active are 2–4°C at all sites, as liquid water is required. Light is a potential major stress reaching 880 µmol m-2 s-1 when active in continental sites. The lack of extremes in temperatures and light combined with high activity levels means that summer at Livingston Island is one of the better sites for lichen and bryophyte growth in the world.
Could oxygenic and/or anoxygenic photosynthesis exist on planet Proxima Centauri b? Proxima Centauri (spectral type – M5.5 V, 3050 K) is a red dwarf, whereas the Sun is type G2 V (5780 K). The light regimes on Earth and Proxima Centauri b are compared with estimates of the planet's suitability for Chlorophyll a (Chl a) and Chl d-based oxygenic photosynthesis and for bacteriochlorophyll (BChl)-based anoxygenic photosynthesis. Proxima Centauri b has low irradiance in the oxygenic photosynthesis range (400–749 nm: 64–132 µmol quanta m−2 s−1). Much larger amounts of light would be available for BChl-based anoxygenic photosynthesis (350–1100 nm: 724–1538 µmol quanta m−2 s−1). We estimated primary production under these light regimes. We used the oxygenic algae Synechocystis PCC6803, Prochlorothrix hollandica, Acaryochloris marina, Chlorella vulgaris, Rhodomonas sp. and Phaeodactylum tricornutum and the anoxygenic photosynthetic bacteria Rhodopseudomonas palustris (BChl a), Afifella marina (BChl a), Thermochromatium tepidum (BChl a), Chlorobaculum tepidum (BChl a + c) and Blastochloris viridis (BChl b) as representative photosynthetic organisms. Proxima Centauri b has only ≈3% of the PAR (400–700 nm) of Earth irradiance, but we found that potential gross photosynthesis (Pg) on Proxima Centauri b could be surprisingly high (oxygenic photosynthesis: earth ≈0.8 gC m−2 h−1; Proxima Centauri b ≈0.14 gC m−2 h−1). The proportion of PAR irradiance useable by oxygenic photosynthetic organisms (the sum of Blue + Red irradiance) is similar for the Earth and Proxima Centauri b. The oxygenic photic zone would be only ≈10 m deep in water compared with ≈200 m on Earth. The Pg of an anoxic Earth (gC m−2 h−1) is ≈0.34–0.59 (land) and could be as high as ≈0.29–0.44 on Proxima Centauri b. 1 m of water does not affect oxygenic or anoxygenic photosynthesis on Earth, but on Proxima Centauri b oxygenic Pg is reduced by ≈50%. Effective elimination of near IR limits Pg by photosynthetic bacteria (<10% of the surface value). The spectrum of Proxima Centauri b is unfavourable for anoxygenic aquatic photosynthesis. Nevertheless, a substantial aerobic or anaerobic ecology is possible on Proxima Centauri b. Protocols to recognize the biogenic signature of anoxygenic photosynthesis are needed.
Tomato grafting is practiced worldwide as an innovative approach to manage stress from drought, waterlogging, insects, and diseases. Metribuzin is a commonly used herbicide in tomato but has potential to cause injury after application if plants are under stress. The influence of metribuzin on grafted tomato under drought-stress has not been studied. Greenhouse experiments were conducted in Raleigh, NC to determine the tolerance of drought-stressed grafted and non-grafted tomato to metribuzin. The tomato cultivar ‘Amelia’ was used as the scion in grafted tomato, and for the non-grafted control. Two hybrid tomato ‘Beaufort’ and ‘Maxifort’ were used as rootstocks for grafted plants. Drought-stress treatments included: no drought-stress; 3 d of drought-stress before metribuzin application with no drought-stress after application (3 d DSB); and 3 d of drought-stress before metribuzin application with 3 d of drought-stress after application (3 d DSBA). Metribuzin was applied at 550 g ai ha−1. No difference in injury from metribuzin was observed in grafted and non-grafted plants. However, at 7 and 14 d after metribuzin treatment (DMT), less injury was observed on tomato in the 3 d DSBA treatment (5 and 2% injury, respectively) than on plants in the 3 d DSB treatment (15 and 8% injury, respectively) or those that were never drought-stressed (18 and 11% injury, respectively). Photosynthesis and stomatal conductance measured prior to metribuzin application were reduced similarly in grafted and non-grafted tomato subjected to drought-stress. Photosynthesis and stomatal conductance of grafted and non-grafted tomato at 7 DMT was not different among drought-stress treatments or metribuzin treatments. Grafted and non-grafted tomato plants under drought-stress exhibit similar tolerance to metribuzin. The risk of metribuzin injury to grafted tomato under drought-stress is similar to non-grafted tomato.
A greater relative allocation of phosphorus (P) to photosynthetically active cells functions to maintain a rapid photosynthesis under P limitation, and may be a key mechanism of plants to use P efficiently. This mechanism has not been studied in tropical trees despite the productivity of tropical forests often being limited by P. In this study, the spatial distribution of P among tissues on a cross-section of leaf lamina was analysed for 13 tree species from P-limited sites on Mount Kinabalu, Borneo. Most species showed greater P concentration in palisade mesophyll than in spongy mesophyll and epidermal tissues, suggesting that tropical trees under P limitation localize foliar P in photosynthetic palisade mesophyll.
pCO2/pH perturbation experiments were carried out under two different pCO2 levels to evaluate effects of CO2-driven ocean acidification on semi-continuous cultures of the marine diatom Skeletonema pseudocostatum CSA48. Under higher pCO2/lowered pH conditions, our results showed that CO2-driven acidification had no significant impact on growth rate, chlorophyll-a, cellular abundance, gross photosynthesis, dark respiration, particulate organic carbon and particulate organic nitrogen between CO2-treatments, suggesting that S. pseudocostatum is adapted to tolerate changes of ~0.5 units of pH under high pCO2 conditions. However, dissolved organic carbon (DOC) concentration and DOC/POC ratio were significantly higher at high pCO2, indicating that a greater partitioning of organic carbon into the DOC pool was stimulated by high CO2/low pH conditions. Total fatty acids (FAs) were significantly higher under low pCO2 conditions. The composition of FAs changed from low to high pCO2, with an increase in the concentration of saturated and a reduction of monounsaturated FAs. Polyunsaturated FAs did not show significant differences between pCO2 treatments. Our results lead to the conclusion that the balance between negative or null effect on S. pseudocostatum ecophysiology upon low pH/high pCO2 conditions constitute an important factor to be considered in order to evaluate the global effect of rising atmospheric CO2 on primary productivity in coastal ocean. We found a significant decrease in total FAs, however no indications were found for a detrimental effect of ocean acidification on the nutritional quality in terms of essential fatty acids.
Tropical montane cloud forests (TMCFs) are dynamic ecosystems defined by frequent, but intermittent, contact with fog. The resultant microclimate can vary considerably over short spatial and temporal scales, affecting the ecophysiology of TMCF plants. We synthesized research to date on TMCF carbon and water fluxes at the scale of the leaf, plant and ecosystem and then contextualized this synthesis with tropical lowland forest ecosystems. Mean light-saturated photosynthesis was lower than that of lowland forests, probably due to the effects of persistent reduced radiation leading to shade acclimation. Scaled to the ecosystem, measures of annual net primary productivity were also lower. Mean rates of transpiration, from the scale of the leaf to the ecosystem, were also lower than in lowland sites, likely due to lower atmospheric water demand, although there was considerable overlap in range. Lastly, although carbon use efficiency appears relatively invariant, limited evidence indicates that water use efficiency generally increases with altitude, perhaps due to increased cloudiness exerting a stronger effect on vapour pressure deficit than photosynthesis. The results reveal clear differences in carbon and water balance between TMCFs and their lowland counterparts and suggest many outstanding questions for understanding TMCF ecophysiology now and in the future.
We review the latest findings on extra-solar planets and their potential of having environmental conditions that could support Earth-like life. Focusing on planets orbiting red dwarf (RD) stars, the most abundant stellar type in the Milky Way, we show that including RDs as potential life supporting host stars could increase the probability of finding biotic planets by a factor of up to a thousand, and reduce the estimate of the distance to our nearest biotic neighbour by up to 10. We argue that binary and multiple star systems need to be taken into account when discussing habitability and the abundance of biotic exoplanets, in particular RDs in such systems. Early considerations indicated that conditions on RD planets would be inimical to life, as their habitable zones would be so close to the host star as to make planets tidally locked. This was thought to cause an erratic climate and expose life forms to flares of ionizing radiation. Recent calculations show that these negative factors are less severe than originally thought. It has also been argued that the lesser photon energy of the radiation of the relatively cool RDs would not suffice for oxygenic photosynthesis (OP) and other related energy expending reactions. Numerous authors suggest that OP on RD planets may evolve to utilize photons in the infrared. We however argue, by analogy to the evolution of OP and the environmental physiology and distribution of land-based vegetation on Earth, that the evolutionary pressure to utilize infrared radiation would be small. This is because vegetation on RD planets could enjoy continuous illumination of moderate intensity, containing a significant component of photosynthetic 400–700 nm radiation. We conclude that conditions for OP could exist on RD planets and consequently the evolution of complex life might be possible. Furthermore, the huge number and the long lifetime of RDs make it more likely to find planets with photosynthesis and life around RDs than around Solar type stars.
Several monodominant rain-forest trees in New Caledonia have population size structures suggesting establishment following large-scale disturbance, with eventual replacement by shade-tolerant species predicted in the absence of future disturbance. Links of dominance and population dynamics to leaf-level photosynthesis were investigated in seedlings of 20 tree species from these forests, grown in experimental sun and shade conditions. In particular, we tested whether episodically regenerating (ER) species, including monodominants, have higher assimilation rates at high irradiances and lower tolerance of shade than continuously regenerating species (CR). ER species had higher maximum net assimilation rates (Amax-area) in sun plants (9.6 ± 0.4 μmol m−2 s−1) than CR species (6.2 ± 0.3 μmol m−2 s−1) and high plasticity, typical of shade-intolerant species, but monodominant species did not differ from other ER species. CR species had leaf-level traits consistent with shade tolerance, including lower dark respiration rates (Rd-area = 0.47 ± 0.03μmol m−2 s−1; Rd-mass = 7 ± 1 nmol g−1 s−1) than ER species (Rd-area = 0.63 ± 0.06 μmol m−2 s−1; Rd-mass = 11 ± 2 nmol g−1 s−1) in shade plants. Hence leaf-level assimilation traits were largely consistent with regeneration patterns, but do not explain why some shade-intolerant species can achieve monodominance.
A ‘habitable zone’ of a star is defined as a range of orbits within which a rocky planet can support liquid water on its surface. The most intriguing question driving the search for habitable planets is whether they host life. But is the age of the planet important for its habitability? If we define habitability as the ability of a planet to beget life, then probably it is not. After all, life on Earth has developed within only ~800 Myr after its formation – the carbon isotope change detected in the oldest rocks indicates the existence of already active life at least 3.8 Gyr ago. If, however, we define habitability as our ability to detect life on the surface of exoplanets, then age becomes a crucial parameter. Only after life had evolved sufficiently complex to change its environment on a planetary scale, can we detect it remotely through its imprint on the atmosphere – the so-called biosignatures, out of which the photosynthetic oxygen is the most prominent indicator of developed (complex) life as we know it. Thus, photosynthesis is a powerful biogenic engine that is known to have changed our planet's global atmospheric properties. The importance of planetary age for the detectability of life as we know it follows from the fact that this primary process, photosynthesis, is endothermic with an activation energy higher than temperatures in habitable zones, and is sensitive to the particular thermal conditions of the planet. Therefore, the onset of photosynthesis on planets in habitable zones may take much longer time than the planetary age. The knowledge of the age of a planet is necessary for developing a strategy to search for exoplanets carrying complex (developed) life – many confirmed potentially habitable planets are too young (orbiting Population I stars) and may not have had enough time to develop and/or sustain detectable life. In the last decade, many planets orbiting old (9–13 Gyr) metal-poor Population II stars have been discovered. Such planets had had enough time to develop necessary chains of chemical reactions and may carry detectable life if located in a habitable zone. These old planets should be primary targets in search for the extraterrestrial life.
Genetic inter-communication of the nucleic-organellar dual in eukaryotes is dominated by DNA-directed phenomena. RNA regulatory circuits have also been observed in artificial laboratory prototypes where gene transfer events are reconstructed, but they are excluded from the primary norm due to their rarity. Recent technical advances in organellar biotechnology, genome engineering and single-molecule tracking give novel experimental insights on RNA metabolism not only at cellular level, but also on organismal survival. Here, I put forward a hypothesis for RNA's involvement in gene piece transfer, taken together the current knowledge on the primitive RNA character as a biochemical modulator with model organisms from peculiar natural habitats. It is proposed that RNA molecules of special structural signature and functional identity can drive evolution, integrating the ecological pressure of environmental oscillations into genome imprinting by buffering-out epigenetic aberrancies.
We develop a polarimetry-based remote-sensing method for detecting and identifying life forms in distant worlds and distinguishing them from non-biological species. To achieve this we have designed and built a bio-polarimetric laboratory experiment BioPol for measuring optical polarized spectra of various biological and non-biological samples. Here we focus on biological pigments, which are common in plants and bacteria that employ them either for photosynthesis or for protection against reactive oxygen species. Photosynthesis, which provides organisms with the ability to use light as a source of energy, emerged early in the evolution of life on Earth. The ability to harvest such a significant energy resource could likely also develop on habited exoplanets. Thus, we investigate the detectability of biomolecules that can capture photons of particular wavelengths and contribute to storing their energy in chemical bonds. We have carried out laboratory spectropolarimetric measurements of a representative sample of plants containing various amounts of pigments such as chlorophyll, carotenoids and others. We have also measured a variety of non-biological samples (sands, rocks). Using our lab measurements, we have modelled intensity and polarized spectra of Earth-like planets having different surface coverage by photosynthetic organisms, deserted land and ocean, as well as clouds. Our results demonstrate that linearly polarized spectra provide very sensitive and rather unambiguous detection of photosynthetic pigments of various kinds. Our work paves the path towards analogous measurements of microorganisms and remote sensing of microbial ecology on the Earth and of extraterrestrial life on other planets and moons.
Continuous ingestion of the phloem sap of plants by aphids can remove a significant amount of photoassimilates. Based on our earlier works, we hypothesized that due to the reduced aphid feeding time caused by antibiosis, wheat plants may achieve growth tolerance to aphids. We tested this hypothesis using three wheat cultivars, XY22 (Xiaoyan22), AK58 (Bainongaikang58) and XN979 (Xinong979) and the grain aphid, Sitobion avenae. In the choice test, S. avenae did not show any preference among the three wheat cultivars. However, S. avenae had a lower body weight and a lower intrinsic rate of increase when feeding on XY22 than on AK58 and XN979. The electrical penetration graph results indicated that S. avenae had significantly shorter mean and total phloem ingestion periods on XY22 than on AK58 or XN979. The aphids required a similar time to reach the phloem sap on the three wheat cultivars, but required more time to establish sustained phloem ingestion on XY22. These results suggest that the resistance factors of XY22 may be phloem based. Moreover, XY22 suffered less biomass loss in response to aphid infestation compared with XN979, suggesting that XY22 also had a better growth tolerance to S. avenae than XN979. Wheat resistance level to S. avenae was partially correlated with plant photosynthetic rates, and peroxidase activities. These results confirmed that the limitation in aphid feeding from plant phloem in wheat cultivar XY22 was related to antibiosis but not antixenosis, which caused XY22 tolerance to S. avenae.