Can meristematic activity determine variation in leaf size and elongation rate among four Poa species? A kinematic study

We studied inherent variation in final leaf size among four Poa spp. that live at different elevations. The average final length of leaf 7 of the main stem of the smallest species (Poa alpina) was only one-half that of the largest species (Poa trivialis); it was correlated with leaf elongation rate, but not with the duration of leaf elongation. A faster rate of leaf elongation rate was associated with (a) larger size of the zone of cell expansion, and (b) faster rates of cell production (per cell file) in the meristem, which in turn were due to greater numbers of dividing cells, whereas average cell division rates were very similar for all species (except Poa annua). Also we found that the proliferative fraction equaled 1 throughout the meristem in all species. It was remarkable that rates of cell expansion tended to be somewhat higher in the species with slower growing leaves. We discuss the results by comparing the spatial and material viewpoints, which lead to different interpretations of the role of cell division. Although the presented data do not strictly prove it, they strongly suggest a regulatory role for cell division in determining differences in growth rate among the present four Poa spp.     

Effects of leaf shape plasticity on leaf surface temperature北大核心CSCD

Aims The shape plasticity of plant leaves is an important survival strategy to high temperature and drought in arid region, yet reliable evidences are insufficient to validate the fundamental concepts. Our objective was to demonstrate the specific effects of leaf morphology on leaf surface temperature. Methods Infrared thermal images were processed to determine the leaf temperature and shape parameters of simulated and actual leaf shape. Microclimatic conditions were recorded using a automatic weather station near the sampling plot, including wind speed, radiation and air temperature. Important findings Under the drought and high temperature, the plasticity of leaf shape appeared an important measure to regulate leaf temperature, except leaf transpiration. The exchange rates of matter and energy between leaves and the environment were enhanced by smaller leaves that effectively decreased leaf temperature. With low wind speed and high temperature, leaf surface temperature decreased 2.1 °C per 1 cm reduction in leaf width. However, leaf surface temperature of a simulated leaf decreased 0.60–0.86 °C per 1 cm reduction in leaf width. Results from this study will help us to understand plant adaptability and survival strategy in arid region. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All rights reserved.     

Vulnerability to cavitation of leaf minor veins: any impact on leaf gas exchange?

Vulnerability to cavitation of leaf minor veins and stems of Laurus nobilis L. was quantified together with that of leaflets, rachides and stems of Ceratonia siliqua L. during air‐dehydration of 3‐year‐old branches. Embolism was estimated by counting ultrasound acoustic emissions (UAE) and relating them to leaf water potential (Ψ_L). The threshold Ψ_L for cavitation was less negative in L. nobilis than in C. siliqua according to the known higher drought resistance of the latter species. Leaf minor vein cavitation was also quantified by infiltrating leaves with fluorescein at different dehydration levels and observing them under microscope. Distinct decreases in the functional integrity of minor veins were observed during leaf dehydration, with high correlation between the two variables. The relationship between leaf conductance to water vapour (g_L) and Ψ_L showed that stomata of L. nobilis closed in response to stem and not to leaf cavitation. However, in C. siliqua, g_L decreased in coincidence to the leaf cavitation threshold, which was, nevertheless, very close to that of the stem. The hypothesis that stem cavitation acts as a signal for stomatal closure was confirmed, while the same role for leaf cavitation remains an open problem.     

In vitro response of leaf tissues from Lolium multiflorum — a comparison with leaf segment position,leaf age and in vivo mitotic activity

Summary Immature gramineous leaves provide a convenient system for comparing the response of cells in culture with their state of differentiation. Callusing frequency is compared with leaf segment position, leaf age and in vivo mitotic activity in Lolium multiflorum. (1) In a succession of one millimeter sections from the immature leaf base, callus was formed from the first and second sections but not the third or subsequent sections. The frequency of those explants callusing decreased with distance from the base of the leaf and with leaf age (or leaf extension growth). (2) In vivo, the proportion of cells in mitosis declined from around 10–14% at the base of young leaves to zero at 5 mm from the base and beyond. Mitotic activity also declined in leaves as they aged, and dividing cells were not observed in leaves 30 days from initiation or older. (3) A high frequency of callus formation was associated with a high mitotic index in the explant. But for corresponding mitotic indices, cells further away from the leaf base were less responsive in culture. (4) It is proposed that cells are becoming differentiated even in highly meristematically active regions of the leaf and concomitantly losing their ability to respond in culture.     

Does leaf quality mediate the stimulation of leaf breakdown by phosphorus in Neotropical streams?

1. Lowland tropical streams have a chemically diverse detrital resource base, where leaf quality could potentially alter the effect of high nutrient concentrations on leaf breakdown. This has important implications given the extent and magnitude of anthropogenic nutrient loading to the environment. 2. Here, we examine if leaf quality (as determined by concentrations of cellulose, lignin and tannins) mediates the effects of high ambient phosphorus (P) concentration on leaf breakdown in streams of lowland Costa Rica. We hypothesised that P would have a stronger effect on microbial and insect processing of high‐ than of low‐quality leaves. 3. We selected three species that represented extremes of quality as measured in leaves of eight common riparian species. Species selected were, from high‐ to low‐quality: Trema integerrima > Castilla elastica > Zygia longifolia. We incubated single‐species leaf packs in five streams that had natural differences in ambient P concentration (10–140 μg soluble reactive phosphorus (SRP) L^?1), because of variable inputs of solute‐rich groundwater and also in a stream that was experimentally enriched with P (approximately 200 μg SRP L^?1). 4. The breakdown rate of all three species varied among the six streams: T. integerrima (k‐values range: 0.0451–0.129 day^?1); C. elastica (k‐values range: 0.0064–0.021 day^?1); and Z. longifolia (k‐values range: 0.002–0.008 day^?1). Both ambient P concentration and flow velocity had significant effects on the breakdown rate of the three species. 5. Results supported our initial hypothesis that litter quality mediates the effect of high ambient P concentration on leaf processing by microbes and insects. The response of microbial respiration, fungal biomass and invertebrate density to high ambient P concentration was greater in Trema (high quality) than in Castilla or Zygia (low quality). Variation in flow velocity, however, confounded our ability to determine the magnitude of stimulation of breakdown rate by P. 6. Cellulose and lignin appeared to be the most important factors in determining the magnitude of P‐stimulation. Surprisingly, leaf secondary compounds did not have an effect. This contradicts predictions made by other researchers, regarding the key role of plant secondary compounds in affecting leaf breakdown in tropical streams.     

Do pH changes in the leaf apoplast contribute to rapid inhibition of leaf elongation rate by water stress? Comparison of stress responses induced by polyethylene glycol and down‐regulation of root hydraulic conductivity

We have dissected the influences of apoplastic pH and cell turgor on short-term responses of leaf growth to plant water status, by using a combination of a double-barrelled pH-selective microelectrodes and a cell pressure probe. These techniques were used, together with continuous measurements of leaf elongation rate (LER), in the (hidden) elongating zone of the leaves of intact maize plants while exposing roots to various treatments. Polyethylene glycol (PEG) reduced water availability to roots, while acid load and anoxia decreased root hydraulic conductivity. During the first 30 min, acid load and anoxia induced moderate reductions in leaf growth and turgor, with no effect on leaf apoplastic pH. PEG stopped leaf growth, while turgor was only partially reduced. Rapid alkalinization of the apoplast, from pH 4.9 ± 0.3 to pH 5.8 ± 0.2 within 30 min, may have participated to this rapid growth reduction. After 60 min, leaf growth inhibition correlated well with turgor reduction across all treatments, supporting a growth limitation by hydraulics. We conclude that apoplastic alkalinization may transiently impair the control of leaf growth by cell turgor upon abrupt water stress, whereas direct hydraulic control of growth predominates under moderate conditions and after a 30-60 min delay following imposition of water stress.     

Is leaf dry matter content a better predictor of soil fertility than specific leaf area?

Background and Aims

Specific leaf area (SLA), a key element of the ‘worldwide leaf economics spectrum’, is the preferred ‘soft’ plant trait for assessing soil fertility. SLA is a function of leaf dry matter content (LDMC) and leaf thickness (LT). The first, LDMC, defines leaf construction costs and can be used instead of SLA. However, LT identifies shade at its lowest extreme and succulence at its highest, and is not related to soil fertility. Why then is SLA more frequently used as a predictor of soil fertility than LDMC?

Methods

SLA, LDMC and LT were measured and leaf density (LD) estimated for almost 2000 species, and the capacity of LD to predict LDMC was examined, as was the relative contribution of LDMC and LT to the expression of SLA. Subsequently, the relationships between SLA, LDMC and LT with respect to soil fertility and shade were described.

Key Results

Although LD is strongly related to LDMC, and LDMC and LT each contribute equally to the expression of SLA, the exact relationships differ between ecological groupings. LDMC predicts leaf nitrogen content and soil fertility but, because LT primarily varies with light intensity, SLA increases in response to both increased shade and increased fertility.

Conclusions

Gradients of soil fertility are frequently also gradients of biomass accumulation with reduced irradiance lower in the canopy. Therefore, SLA, which includes both fertility and shade components, may often discriminate better between communities or treatments than LDMC. However, LDMC should always be the preferred trait for assessing gradients of soil fertility uncoupled from shade. Nevertheless, because leaves multitask, individual leaf traits do not necessarily exhibit exact functional equivalence between species. In consequence, rather than using a single stand-alone predictor, multivariate analyses using several leaf traits is recommended.     

Axioms and axes in leaf formation?

Formation of leaves and floral organs involves down-regulation of meristem-specific homeobox genes, and de novo expression of genes for organ identity, growth and patterning. Genes required for all these aspects of organ formation have been identified. The challenge now is to establish how they interact to direct organogenesis.     

Specific leaf area estimation model building based on leaf dry matter content of Cunninghamia lanceolata北大核心CSCD

Aims With progresses of leaf functional traits study, there is an increasing demand to explore the life history strategy and trade-offs in plants, as well as estimate stand productivity, by employing easy and simple leaf parameters. For instance, the interconversion between leaf dry matter content (LDMC) and specific leaf area (SLA) just fit the bill. Cunninghamia lanceolata serves as one of the most important afforestation evergreen needle species in subtropical zone. Building the SLA estimation model based on LDMC could provide a new approach to estimate SLA, and establish a connection path between mechanism explanation and productivity evaluation. Moreover, it could also build a bridge between individual level and large-scale, as well as between actuarial and estimation. Methods Leaf samples were collected from two sampling sites located in C. lanceolata growing region: Huitong County of Hunan Province and Xinyang City of Henan Province. The samples covered fundamentally different niches (aspect, slope position, and canopy depth), and different life history (stand age and leaf age). SLA and LDMC were determined along leaf age gradients, and their value distributions in linkage to different factors were discussed. A general model based on LDMC of C. lanceolata was built to estimate SLA, and the impact of leaf age on the model was explored. Important findings The SLA of C. lanceolata was (103.15 ± 69.54) cm2·g–1, while LDMC was 0.39 ± 0.11. The LDMC and SLA of C. lanceolata can be estimated by nonlinear model (R2 = 0.718 4, p < 0.001), which meets the estimation requirements. One-year-old leaves showed the best fitting model (R2 = 0.889, p < 0.001), while old leaves (more than 2-year-old) showed the worst (R2 = 0.100 1, p < 0.001). Old leaves with a lower SLA (52.28–75.74 cm2·g–1) might imply the relative independence among the variation of LDMC. The model based on LDMC to evaluate SLA is credible and effective. The effects on LDMC and SLA along leaf age gradients indicate leaf sensitivity, life history strategies and trade-offs. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All rights reserved.     

Photosynthesis and stomatal conductance of potato leaves—effects of leaf age,irradiance, and leaf water potential

Potatoes (Solanum tuberosum L., cv. Bintje) were grown in a naturally lit glasshouse. Laboratory measurements on leaves at three insertion levels showed a decline with leaf age in photosynthetic capacity and in stomatal conductance at near saturating irradiance. Conductance declined somewhat more with age than photosynthesis, resulting in a smaller internal CO₂ concentration in older relative to younger leaves. Leaves with different insertion number behaved similarly. The changes in photosynthesis rate and in nitrogen content with leaf age were closely correlated. When PAR exceeded circa 100 W m^–2 the rate of photosynthesis and stomatal conductance changed proportionally as indicated by a constant internal CO₂ concentration. The photosynthesis-irradiance data were fitted to an asymptotic exponential model. The parameters of the model are AMAX, the rate of photosynthesis at infinite irradiance, and EFF, the slope at low light levels. AMAX declined strongly with leaf age, as did EFF, but to a smaller extent. During drought stress photosynthetic capacity declined directly with decreasing water potential (range –0.6 to –1.1 MPa). Initially, stomatal conductance declined faster than photosynthetic capacity.Abbreviations LNx leaf number x, counted in acropetal direction - DAP days after planting - DALA days after leaf appearance - C_i CO₂ concentration in the leaf - C_a CO₂ concentration in ambient air - LWP leaf water potential - OP osmotic potential - PAR photosynthetically active radiation     

How do leaf wetting events affect gas exchange and leaf lifespan of plants from seasonally dry tropical vegetation?

• Foliar uptake of dew is likely an important mechanism of water acquisition for plants from tropical dry environments. However, there is still limited experimental evidence describing the anatomical pathways involved in this process and the effects of this water subsidy on the maintenance of gas exchange and leaf lifespan of species from seasonally dry tropical vegetation such as the Brazilian caatinga.
• We performed scanning electron, bright‐field and confocal microscopic analyses and used apoplastic tracers to examine the foliar water uptake (FWU) routes in four woody species with different foliar phenology and widely distributed in the caatinga. Leaves of plants subjected to water stress were exposed to dew simulation to evaluate the effects of the FWU on leaf water potentials, gas exchange and leaf lifespan.
• All species absorbed water through their leaf cuticles and/or peltate trichomes but FWU capacity differed among species. Leaf wetting by dew increased leaf lifespan duration up to 36 days compared to plants in the drought treatment. A positive effect on leaf gas exchange and new leaf production was only observed in the anisohydric and evergreen species.
• We showed that leaf wetting by dew is relevant for the physiology and leaf lifespan of plants from seasonally dry tropical vegetation, especially for evergreen species.

     

Is leaf ADP-glucose pyrophosphorylase an allosteric enzyme?

Barley leaf ADP-glucose pyrophosphorylase (AGPase), a key enzyme of starch synthesis in the chloroplast stroma, was analysed, in both directions of the reaction, with respect to details of its regulation by 3-phosphoglycerate (PGA) and inorganic phosphate (Pi) which serve as activator and inhibitor, respectively. AGPase was found to catalyse a close-to-equilibrium reaction, with the K(eq) value of approximately 0.5, i.e. slightly favouring the pyrophosphorolytic direction. When the enzyme was analysed by substrate kinetics, PGA acted either as a linear (hyperbolic response) 'non-competitive' activator (forward reaction) or a linear near-'competitive' activator (reverse reaction). When the activation and inhibition patterns with PGA and Pi, respectively, were studied in detail by Dixon plots, the response curves to effectors also followed hyperbolic kinetics, with the experimentally determined K(a) and K(i) values on the order of micromolar. The results suggest that the regulation of AGPase proceeds via a non-cooperative mechanism, where neither of the effectors, when considered separately, induces any allosteric response. The evidence, discussed in terms of an overall kinetic mechanism/regulation of leaf AGPase, prompts caution in classifying the protein as an 'allosteric enzyme'.     

Do holly leaf spines really deter herbivory?

Summary Although spinose teeth of holly leaves have been widely cited as an example of a physical defense against herbivores, this assumption is based largely on circumstantial evidence and on general misinterpretation of a single, earlier experiment. We studied the response of third and fifth instar larvae of the fall webworm, Hyphantria cunea Drury, a generalist, edge-feeding caterpillar, to intact American holly leaves and to leaves that had been modified by blunting the spines, by removing sections of leaf margin between the spines, or by removing the entire leaf margin. The results suggest that the thick glabrous cuticle and tough leaf margin of Ilex opaca are more important than the spinose teeth in deterring edge-feeding caterpillars. Microscopic examination of mature leaves revealed that the epidermis is thickened at the leaf margin, and that the leaf is cirucumscribed by a pair of fibrous veins. In simple choice tests neither domesticated rabbits nor captive whitetailed deer discriminated between spinescent holly foliage and foliage from which spines were removed. Nevertheless, we found little evidence of herbivory by mammals in the field, either on small experimental trees or in the forest understory. While it is possible that spinose teeth contribute to defense by reducing acceptibility of holly relative to other palatable plant species, we suggest that the high concentrations of saponins and poor nutritional quality of holly foliage may be more important than spines in deterring vertebrate herbivores. The degree of leaf spinescence and herbivory was compared at different heights with the tree canopy to test the prediction that lower leaves should be more spinescent as a deterrent to browsers. Leaves on lower branches of mature forest trees were slightly more spinescent than were upper leaves, and juvenile trees were slightly more spinescent than were mature trees. However, there was no relationship between degree of spinescence and feeding damage. The greater spinescence of holly leaves low in the canopy is probably an ontogenetic phenomenon rather than a facultative defense against browsers.The investigation reported in this paper (No. 87-7-8-77) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Dirctor     

The influence of leaf size and shape on leaf thermal dynamics: does theory hold up under natural conditions?

Laboratory studies on artificial leaves suggest that leaf thermal dynamics are strongly influenced by the two‐dimensional size and shape of leaves and associated boundary layer thickness. Hot environments are therefore said to favour selection for small, narrow or dissected leaves. Empirical evidence from real leaves under field conditions is scant and traditionally based on point measurements that do not capture spatial variation in heat load. We used thermal imagery under field conditions to measure the leaf thermal time constant (τ) in summer and the leaf‐to‐air temperature difference (?T) and temperature range across laminae (T_range) during winter, autumn and summer for 68 Proteaceae species. We investigated the influence of leaf area and margin complexity relative to effective leaf width (w_e), the latter being a more direct indicator of boundary layer thickness. Normalized difference of margin complexity had no or weak effects on thermal dynamics, but w_e strongly predicted τ and ?T, whereas leaf area influenced T_range. Unlike artificial leaves, however, spatial temperature distribution in large leaves appeared to be governed largely by structural variation. Therefore, we agree that small size, specifically w_e, has adaptive value in hot environments but not with the idea that thermal regulation is the primary evolutionary driver of leaf dissection.     

The relationship between leaf water potential ψ leaf and the levels of abscisic acid and ethylene in excised wheat leaves

The amount of diffusible ethylene from excised wheat leaves (Triticum aestivum L. cv. Eclipse) increased when they were subjected to water stress. The quantity of ethylene produced was related to the severity of the stress, reaching a maximum at a leaf water potential _leaf of approximately-12 bars. Irrespective of the severity of the stress, the maximum rate of ethylene production usually occurred between 135–270 min after applying the stress and then the rate declined. Part of the decline may have been due to an oxygen deficiency in the leaf chambers. In excised water-stressed leaves there was a sigmoid relationship between increasing ethylene and abscisic acid (ABA) levels and decreasing leaf water potential values. The two curves were displaced from each other by approximately 1 bar, with ethylene evolution leading that of ABA accumulation. The maximum rate of increase in ethylene occurred between-8 and-9 bars and for ABA between-9 and-10 bars. A significant increase in the levels of these two plant growth regulators was found when the _leaf decreased outside the normal diurnal _leaf range by 1 bar for ethylene and 2 bars for ABA. Because of the sigmoid nature of the curves there was no distinct threshold _leaf value triggering-off an increase in ethylene or ABA, but with ABA the curve became very steep at a _leaf value of-9.3 bars and this could be looked upon as a kind of threshold value.It seems unlikely that the stress-induced ethylene evolution in excised wheat leaves stimulated the accumulation of ABA, because when the leaves were subjected to a substantial water stress (e.g. _leaf bars) ABA increased immediately and at a faster rate than ethylene.Abbreviations ABA abscisic acid - GLC gas-liquid chromatography - RWC relative water content - TLC thin-layer chromatography - _leaf leaf water potential     

Possible link between photosynthesis and leaf modulus of elasticity among vascular plants: a new player in leaf traits relationships?

A compromise between carbon assimilation and structure investment at the leaf level is broadly accepted, yet the relationship between net assimilation per area (A_n) and leaf mass per area has been elusive. We propose bulk modulus of elasticity (ε) as a suitable parameter to reflect both leaf structure and function, and an inverse relationship between ε and A_n and mesophyll conductance (g_m) is postulated. Using data for A_n, g_m and ε from previous studies and new measurements on a set of 20 species covering all major growth forms, a negative relationship between A_n or g_m and ε was observed. High ε was also related to low leaf capacitance and higher diffusive limitations to photosynthesis. In conclusion, ε emerges as a key trait linked with photosynthetic capacity across vascular plants, and its relationship with g_m suggests the existence of a common mechanistic basis, probably involving a key role of cell walls.     

Does the direct effect of atmospheric CO2 concentration on leaf respiration vary with temperature? Responses in two species of Plantago that differ in relative growth rate

The objective of this study was to investigate the direct effect of elevated atmospheric CO2 concentrations on leaf respiration in darkness (R) over a broad range of measurement temperatures. Our aim was to further elucidate the underlying mechanism(s) of the often-reported inhibition of leaf R by a doubling of the atmospheric CO2 concentration. Experiments were conducted using two species of Plantago that differed in maximum relative growth rate (fast-growing Plantago lanceolata L. and the slow-growing P. euryphylla Briggs, Carolin & Pulley). Rates of leaf respiration (R) were measured at atmospheric CO2 concentrations ranging from 75 to 2000 &mgr;mol mol-1 at temperatures from 12 to 42 degrees C. R was measured as CO2 release with a portable gas exchange system with infrared gas analysers. Our hypothesis was that the changes in temperature alter the flux coefficient (i.e. the extent to which changes in potential enzyme activity has an effect on the rate of a reaction) of enzymes potentially affected by CO2. Initial analysis of our results suggested that R was inhibited by elevated CO2 in both species, with the apparent degree of inhibition being greatest at low temperature. Moreover, the apparent degree of inhibition following a doubling of atmospheric CO2 concentration from 350 to 700 &mgr;mol mol-1 was similar to that reported by several previous studies (approximately 14% and 26% for P. lanceolata and P. euryphylla, respectively) at a temperature equal to the mean of the previous studies. However, subsequent correction for diffusion leaks of CO2 across the gas exchange's cuvette gaskets revealed that no significant inhibition had occurred in either species, at any temperature. The inhibitory effect of elevated CO2 on leaf respiratory CO2 release reported by previous studies may therefore have been overestimated.