Leaf rosette closure in the alpine rock species Saxifraga paniculata Mill.: significance for survival of drought and heat under high irradiation

The significance of leaf rosette closure for survival of drought and heat under high irradiation on alpine rock sites was investigated in the cushion forming rosette plant, Saxifraga paniculata Mill. With decreasing water content the leaves fold over the rosette centre reducing reversibly the evaporative leaf surface area by 80%. Internal water redistribution driven by an osmotic gradient from older to younger leaves occurs. The oldest leaves dry out to promote the survival of the individual. Leaf temperatures above 45 °C (which match heat tolerance limits 45–57 °C; LT₅₀) co-occurred with low substrate water potentials (less than – 0·5 MPa) on 11·3% of summer days. Shading by leaves can be crucial to surviving high temperatures as it keeps the rosette centre up to 10 °C colder. Mutual shading prevented sustained drought-induced photoinhibition in upper leaf surfaces at relative water contents below 60%. In exposed lower leaf surfaces restoration of photosystem II took several days. Leaf temperatures above 40 °C (21·3% of summer days) induced photoinhibition in situ. Periods with sufficient water supply can be fully utilized as rehydration is fast ( < 12 h) and exposes the upper leaf surfaces that showed only minor photoinhibition. By reversible leaf rosette closure environmental extremes that otherwise could exceed tolerance are efficiently avoided.     

The 'hydrology' of leaves: co-ordination of structure and function in temperate woody species

The hydraulic conductance of the leaf lamina (K_lamina) substantially constrains whole‐plant water transport, but little is known of its association with leaf structure and function. K_lamina was measured for sun and shade leaves of six woody temperate species growing in moist soil, and tested for correlation with the prevailing leaf irradiance, and with 22 other leaf traits. K_lamina varied from 7.40 × 10^?5 kg m^?2 s^?1 MPa^?1 for Acer saccharum shade leaves to 2.89 × 10^?4 kg m^?2 s^?1 MPa^?1 for Vitis labrusca sun leaves. Tree sun leaves had 15–67% higher K_lamina than shade leaves. K_lamina was co‐ordinated with traits associated with high water flux, including leaf irradiance, petiole hydraulic conductance, guard cell length, and stomatal pore area per lamina area. K_lamina was also co‐ordinated with lamina thickness, water storage capacitance, 1/mesophyll water transfer resistance, and, in five of the six species, with lamina perimeter/area. However, for the six species, K_lamina was independent of inter‐related leaf traits including leaf dry mass per area, density, modulus of elasticity, osmotic potential, and cuticular conductance. K_lamina was thus co‐ordinated with structural and functional traits relating to liquid‐phase water transport and to maximum rates of gas exchange, but independent of other traits relating to drought tolerance and to aspects of carbon economy.     

Contribution of trehalose biosynthetic pathway to drought stress tolerance of Capparis ovata Desf

Trehalose and the trehalose biosynthetic pathway are important contributors and regulators of stress responses in plants. Among recent findings for trehalose and its metabolism, the role of signalling in the regulation of growth and development and its potential for use as a storage energy source can be listed. The xerophytic plant Capparis ovata (caper) is well adapted to drought and high temperature stress in arid and semi‐arid regions of the Mediterranean. The contribution of trehalose and the trehalose biosynthetic pathway to drought stress responses and tolerance in C. ovata are not known. We investigated the effects of PEG‐mediated drought stress in caper plants and analysed physiological parameters and trehalose biosynthetic pathway components, trehalose‐6‐phosphate synthase (TPS), trehalose‐6‐phosphate phosphatase (TPP), trehalase activity, trehalose and proline content in drought stress‐treated and untreated plants. Our results indicated that trehalose and the trehalose biosynthetic pathway contributed to drought stress tolerance of C. ovata. Overall growth and leaf water status were not dramatically affected by drought, as both high relative growth rate and relative water content were recorded even after 14 days of drought stress. Trehalose accumulation increased in parallel to induced TPS and TPP activities and decreased trehalase activity in caper plants on day 14. Constitutive trehalose levels were 28.75 to 74.75 μg·g·FW^?1, and drought stress significantly induced trehalose accumulation (385.25 μg·g·FW^?1 on day 14) in leaves of caper. On day 14 of drought, proline levels were lower than on day 7. Under drought stress the discrepancy between trehalose and proline accumulation trends might result from the mode of action of these osmoprotectant molecules in C. ovata.     

The role of leaf and canopy-level gas exchange in the replacement of Quercus virginiana (Fagaceae) by Juniperus ashei (Cupressaceae) in semiarid savannas

Photosynthesis, transpiration, and leaf area distribution were sampled in mature Quercus virginiana and Juniperus ashei trees to determine the impact of leaf position on canopy-level gas exchange, and how gas exchange patterns may affect the successful invasion of Quercus communities by J. ashei. Sampling was conducted monthly over a 2-yr period in 12 canopy locations (three canopy layers and four cardinal directions). Photosynthetic and transpiration rates of both species were greatest in the upper canopy and decreased with canopy depth. Leaf photosynthetic and transpiration rates were significantly higher for Q. virginiana (4.1–6.7 μmol CO₂·m⁻²·s⁻¹ and 1.1–2.1 mmol H₂O·m⁻²·s⁻¹) than for J. ashei (2.1–2.8 μmol CO₂·m⁻²·s⁻¹ and 0.7–1.0 mmol H₂O·m⁻²·s⁻¹) in every canopy level and direction. Leaves on the south and east sides of both species had higher gas exchange rates than leaves on the north and west sides. Although Quercus had a greater mean canopy diameter than Juniperus (31.3 vs. 27.7 m²), J. ashei had significantly greater leaf area (142 vs. 58 m²/tree). A simple model combining leaf area and gas exchange rates for different leaf positions demonstrated a significantly greater total canopy carbon dioxide uptake for J. ashei compared to Q. virginiana (831 vs. 612 g CO₂·tree⁻¹·d⁻¹, respectively). Total daily water loss was also greater for Juniperus (125 vs. 73 Ltree⁻¹·d⁻¹). Differences in leaf gas exchange rates were poor predictors of the relationship between the invasive J. ashei and the codominant Q. virginiana. Leaf area and leaf area distribution coupled with leaf gas exchange rates were necessary to demonstrate the higher overall competitive potential of J. ashei.     

Apoplastic water fraction and rehydration techniques introduce significant errors in measurements of relative water content and osmotic potential in plant leaves

Relative water content (RWC) and the osmotic potential (π) of plant leaves are important plant traits that can be used to assess drought tolerance or adaptation of plants. We estimated the magnitude of errors that are introduced by dilution of π from apoplastic water in osmometry methods and the errors that occur during rehydration of leaves for RWC and π in 14 different plant species from trees, grasses and herbs. Our data indicate that rehydration technique and length of rehydration can introduce significant errors in both RWC and π. Leaves from all species were fully turgid after 1–3 h of rehydration and increasing the rehydration time resulted in a significant underprediction of RWC. Standing rehydration via the petiole introduced the least errors while rehydration via floating disks and submerging leaves for rehydration led to a greater underprediction of RWC. The same effect was also observed for π. The π values following standing rehydration could be corrected by applying a dilution factor from apoplastic water dilution using an osmometric method but not by using apoplastic water fraction (AWF) from pressure volume (PV) curves. The apoplastic water dilution error was between 5 and 18%, while the two other rehydration methods introduced much greater errors. We recommend the use of the standing rehydration method because (1) the correct rehydration time can be evaluated by measuring water potential, (2) overhydration effects were smallest, and (3) π can be accurately corrected by using osmometric methods to estimate apoplastic water dilution.     

Variable agronomic practices, cultivar, strain source and initial contamination dose differentially affect survival of Escherichia coli on spinach

Aims: Greenhouse and field trials were conducted under different agronomic practices and inoculum doses of environmental Escherichia coli and attenuated E. coli O157:H7, to comparatively determine whether these factors influence their survival on leaves and within the rhizosphere. Methods and Results: Hydroponic conditions: E. coli spray‐inoculated at log 4 CFU ml^?1 was recovered from leaf surfaces at a mean population of 1·6 log CFU g^?1 at 15 days. E. coli O157:H7 sprayed at log 2 or 4 CFU ml^?1 levelled off on spinach leaf surfaces at a mean average population of 1·4 log CFU g^?1 after 14 days, regardless of initial dose. Quantitative recovery was inconsistent across leaf developmental age. Field conditions: Average populations of E. coli O157:H7 spray‐inoculated at log 1·45 or 3·4 CFU m^?2 levelled off at log 1·2 CFU g^?1 over a 14‐day period. Pathogen recovery from leaves was inconsistent when compared to regularly positive detection on basal shoot tissue. Pathogen recovery from soil was inconsistent among sampling locations. Moisture content varied up to 40% DW and was associated with 50% (P < 0·05) decrease in positive locations for E. coli O157:H7 but not for E. coli. Conclusions: Overall, similar populations of environmental E. coli and E. coli O157:H7 were recovered from plants despite differences in inoculum dose and agronomic conditions. Strain source had a significant impact on the quantitative level and duration of survival on leaves and in soil. Water availability appeared to be the determinant factor in survival of E. coli and E. coli O157:H7; however, E. coli showed greater environmental fitness. Significance and Impact of the Study: Persistence of surrogate, indicator E. coli and E. coli O157:H7, irrespective of variable growing conditions in spinach is predominantly limited by water availability, strain source and localization within the plant. These findings are anticipated to ultimately be adopted into routine and investigative pathogen testing protocols and mechanical harvest practices of spinach.     

Through-flow of water in leaves of a submerged plant is influenced by the apical opening

Submerged aquatic higher plants maintain acropetal water transport to the young leaves in active growth to satisfy their demand for nutrients and hormones derived from the roots. We here present the first measurements of hydraulic properties for a submerged plant, the monocotyledon Sparganium emersum Rehman. The hydraulic conductance per unit length, K_h, was measured in leaf segments without the leaf tip and shown to be greater in old, fully developed leaves (1.5 · 10⁻¹⁰ · m⁴ · MPa⁻¹ · s⁻¹) than in young leaves (1.0 · 10⁻¹⁰ · m⁴ · MPa⁻¹ · s⁻¹). In leaves with intact leaf tips, however, K_h was significantly greater in the youngest leaves, which suggests that the leaf tip with the hydathode influences resistance and thus flow. Microscopy confirmed that the hydathodal area, which is an apical opening, undergoes structural changes with leaf age; a matrix of microorganisms develops in the older leaves and probably restricts water flow by clogging the hydathodes. The leaf specific conductivity expressing transport capacity relative to the leaf area supplied, of S. emersum (0.1 · 10⁻⁸ to 9 · 10⁻⁸ · m² MPa⁻¹· s⁻¹) was within the same range as for various species of terrestrial ferns, vines and trees. This finding does not support the traditional concept of functionally reduced vascular transport in Received: 15 July 1996 / Accepted: 30 November 1996     

Survival and colonization of Escherichia coli O157:H7 on spinach leaves as affected by inoculum level and carrier, temperature and relative humidity

Aims: To determine survival and colonization of Escherichia coli O157:H7 on spinach leaves as affected by inoculum level and carrier, temperature and relative humidity (r.h.). Methods and Results: Spinach leaves were inoculated with suspensions of E. coli O157:H7 in distilled water (DW) and 0·1% peptone water (PW) and incubated at 4, 12 and 25°C and 43, 85 and 100% r.h. The number of E. coli O157:H7 on leaves (5·6 or 1·9 log CFU per leaf) inoculated using DW as a carrier medium increased significantly at 25°C and 100% r.h. within 120 h but remained constant or decreased significantly under other test conditions. E. coli O157:H7 on leaves (5·4 log CFU per leaf) inoculated using PW as a carrier increased significantly within 72 and 24 h, respectively, at 12 or 25°C and 100% r.h.; counts using a low inoculum (2·2 log CFU per leaf) increased significantly within 24 h at 25°C. Conclusions: Escherichia coli O157:H7 can colonize on spinach leaves at 12 or 25°C in a 100% r.h. environment. Organic matter in the inoculum carrier may provide protection and nutrients which enhance survival and colonization. Significance and Impact of the Study: Colonization of E. coli O157:H7 on spinach leaves as affected by organic matter in the inoculum, temperature and r.h. was determined. These observations will be useful when developing strategies to prevent growth of E. coli O157:H7 on pre‐ and postharvest spinach.     

BENDING STIFFNESS OF CYLINDRICAL PLANT ORGANS WITH A ‘CORE-RIND’ CONSTRUCTION: EVIDENCE FROM JUNCUS EFFUSUS LEAVES

The mechanical behavior of leaves of Juncus effusus L. in bending was investigated in terms of a closed-form analytical solution derived to predict the bending stiffness of a cylindrical sandwich beam consisting of an outer ‘rind’ (sclerenchyma and chlorenchyma) and an inner ‘core’ (aerenchyma). The elastic moduli (E_TOTAL) of intact leaves was measured by means of multiple resonance frequency spectra and compared to that of leaves for which the aerenchymatous core was surgically destroyed. Based on ten leaves, E_TOTAL = 22.33 × 10⁴ ± 5.37 ± 10⁴ kg · cm^–2 while the elastic modulus of the ‘rind’ was 22.29 × 10⁴ ± 5.69 × 10⁴ kg · cm^–2. The elastic modulus of the ‘core’ was estimated at 3.12 × 10⁴ ± 1.42 × 10⁴ kg · cm^–2. Load-deflection curves for three leaf segments indicated leaves were linearly elastic within the range of loading and could be predicted with considerable accuracy based on the closed-form solution. The aerenchymatous core was found to contribute very little to the bending stiffness of leaves, although its contribution appeared to increase as leaf diameter decreased. Leaves mechanically failed by Brazier buckling when excessively loaded and were best considered to mechanically operate as hollow tubes. Nonetheless, the analytical solution for bending stiffness could be applied and, in theory, can be used to predict the behavior of other plant organs with a ‘corerind’ construction.     

Marginal and laminar hydathode-like structures in the leaves of the desiccation-tolerant angiosperm Myrothamnus flabellifolius Welw.

The fan-shaped leaves of the resurrection plant Myrothamnus flabellifolius Welw. fold during episodes of drought and consequent desiccation of the tissue. The leaf teeth of M. flabellifolius have several features characteristic of hydathodes. Tracheary elements from the three vein endings that converge in each leaf tooth subtend and extend into a cluster of cells significantly smaller than those of the adjacent mesophyll. The stomata overlying this putative epithem are larger than the other stomata on the leaf surface. Crystal violet is absorbed via these stomata in non-transpiring leaves, suggesting that they are water pores. Two to four such water pores occur per hydathode and are readily distinguished in desiccated leaves. Laminar hydathodes apparently also occur in the leaves of M. flabellifolius. Branched vein endings that terminate in short, wide tracheary elements subtend the outer edges of the abaxial leaf ridge, which otherwise lack stomata, and coincide with regions of crystal violet uptake. Guttation could not be induced in M. flabellifolius. However, desiccated leaves readily absorb liquid water through the leaf surface. The use of Calcafluor White to trace the pathway of apoplastic water movement suggests a role for both types of hydathode in foliar water uptake during rehydration while the accumulation of Sulphorhodamine G (indicating solute retrieval from the apoplast) in the epithem of transpiring plants suggests the hydathodes may be a pathway of water loss in the desiccating leaf.     

Stomatal and photosynthetic responses of olive (Olea europaea L.) leaves to water deficits

The leaf gas exchange of mature olive trees (Olea europaea L.) was characterized over a wide range of water deficits in the field during 1998, in Cordoba, Spain. Leaf photosynthesis (A) and stomatal conductance (g_l) responded diurnally and seasonally to variations in tree water status and evaporative demand. In the absence of water stress, A and g_l were generally high during autumn and low in days of high vapour pressure deficits (VPD). Leaf A varied between 0 and 2 µmol m^?2 s^?1 under severe water deficits that lowered the stem water potential (Ψ_x) to ?8·0 MPa, but recovered rapidly following rehydration. Transpiration efficiency (TE) was curvilinearly related to VPD and not influenced by water deficits except in cases of severe water stress, where low TE values were observed at Ψ_x below ?4 MPa. Three models of leaf conductance were calibrated and validated with the experimental data; two were based on the model proposed by Leuning (L) and the other was derived from the widely used Jarvis (J) model. The L models performed better than the J model in two validation tests. The scatter of the predictions and the limited accuracy of all three models suggest that, in addition to the physiological and environmental variables considered, there are additional endogenous factors influencing the g_l of olive leaves.     

Diurnal oscillatory movements of growing leaves of tobacco

The analysis of diurnal oscillatory movements of tobacco leaves was used in the diagnosis of viral infection of plants. The oscillatory helices circumscribed by a growing leaf of a healthy plant were regular, but some deviations, particularly in the transition points, were recorded. In order to make clear the cause of these irregularities of trajectory, the course of elongation of leaf petiole and blade in relation to localization and shift of zones of elongation during ontogenesis was analysed. The present analysis is similar to that described by the author's earlier experiments with pea roots. Oscillatory curves circumscribed by petiole, projected on a horizontal plane, were compared with curves circumscribed by the blade tip. The analysis of the leaves of different ages enabled us to study this process in dependence on growth rate. It was confirmed that oscillations are a result of elongation; the extent of oscillations is quantitatively dependent on the growth rate. An analysis of the zones of growth showed that in petiole the active meristems are localized near to its base while in the leaf lamina they move gradually during the ontogenesis from the apical to the basal part of the leaf blade. Active meristems of young rapidly growing leaves are localized approximately in the middle of the blade while those of old leaves were found in close proximity to the base of the lamina. The growth rate of petiole can be expressed in hundreds of mm per hour (4.8×10^?2 mm h^?1); half of this value was recorded for its apical part. The growth rate of leaf blade was found approximately ten times higher (3.2×10^?1 mm h^?1). The oscillatory movements of growing leaf consists of two integrate components: of oscillations originating in the base of the petiole and of oscillations of leaf blade the centrum of which is localized in the basal third of the blade. The arrangement of the experiments did not enable us to determine to what extent the phototropic response of leaf blade participates in leaf movements. The movements of leaves of an intact plant are evidently affected by rhythmic stem oscillations. Stem is an integral part of a system which participates in the transfer of information in plants.     

Long‐term effect of salinity on plant quality,water relations,photosynthetic parameters and ion distribution in Callistemon citrinus

The effect of saline stress on physiological and morphological parameters in Callistemon citrinus plants was studied to evaluate their adaptability to irrigation with saline water. C. citrinus plants, grown under greenhouse conditions, were subjected to two irrigation treatments lasting 56 weeks: control (0.8 dS·m^?1) and saline (4 dS·m^?1). The use of saline water in C. citrinus plants decreased aerial growth, increased the root/shoot ratio and improved the root system (increased root diameter and root density), but flowering and leaf colour were not affected. Salinity caused a decrease in stomatal conductance and evapotranspiration, which may prevent toxic levels being reached in the shoot. Net photosynthesis was reduced in plants subjected to salinity, although this response was evident much later than the decrease in stomatal conductance. Stem water potential was a good indicator of salt stress in C. citrinus. The relative salt tolerance of Callistemon was related to storage of higher levels of Na⁺ and Cl^? in the roots compared with the leaves, especially in the case of Na⁺, which could have helped to maintain the quality of plants. The results show that saline water (around 4 dS·m^?1) could be used for growing C. citrinus commercially. However, the cumulative effect of irrigating with saline water for 11 months was a decrease in photosynthesis and intrinsic water use efficiency, meaning that the interaction of the salinity level and the time of exposure to the salt stress should be considered important in this species.     

Oviposition preference of Anthocoris nemorum and A. nemoralis for apple and pear

Anthocoris nemorum L. and Anthocoris nemoralis Fabricius (Heteroptera: Anthocoridae) are important predators of insect pests in pome fruit. Females insert their eggs in leaf tissue. The females’ choice of oviposition site is important for the subsequent distribution of nymphs on host plants. Oviposition preference for apple and pear leaves was tested in the laboratory in four experiments (experiments 1–4). In three experiments it was tested whether simulated insect damage to leaves (experiments 5 and 6) or the presence of prey (experiment 7) influenced oviposition preference. The effect of the presence of prey was only tested for A. nemorum on apple leaves. There was a highly significant anthocorid species × plant interaction for the number of eggs laid on apple and pear leaves. Anthocoris nemorum laid more eggs on apple than on pear leaves, while A. nemoralis preferred pear. Anthocoris nemorum's preference for apple increased over the 6‐week period in which experiments 1–4 were performed, from 66% to 91% eggs laid on apple leaves. No change over time in preference was found for A. nemoralis. Across experiments 1–4, the majority of A. nemorum eggs were laid near leaf margins, whereas eggs of A. nemoralis were more commonly found in the leaf centre, 5 mm or more from the margin, with a highly significant leaf region × species interaction. There was no significant difference in preference for leaf side between A. nemorum and A. nemoralis, but there was a highly significant plant × leaf side × experiment interaction. Thus, more eggs were laid on the ventral than on the dorsal side of pear leaves in experiment 4, while significantly more eggs were laid on the dorsal side of apple leaves in experiments 3 and 4. Choice tests between damaged and healthy leaves showed that A. nemorum laid significantly more eggs on the damaged leaves, while A. nemoralis preferred healthy leaves. Anthocoris nemorum showed a near‐significant preference for ovipositing on leaves with eggs of Operophtera brumata (Lepidoptera: Geometridae). The oviposition preferences found correspond to the natural distribution of these predators in apple and pear orchards. The preference of A. nemorum for leaf margins, and of A. nemoralis for the leaf centre as an oviposition site, supports earlier observations. A preference for leaf side for oviposition site has not been reported earlier. Preference for damaged leaves could help A. nemorum to locate prey in a field situation.     

Abscisic acid introduced into the transpiration stream accumulates in the guard-cell apoplast and causes stomatal closure

Petioles of water‐sufficient intact Vicia faba L. plants were infused with 1 µm abscisic acid (ABA) to simulate the import of root‐source ABA. This protocol permitted quantitative ABA delivery, up to 300 pmol ABA over 60 min, to the leaf without ambiguities associated with perturbations in plant–water status. The ABA concentrations in whole‐leaf samples and in apoplastic sap increased with the amount infused; ABA degradation was not detected. The ABA concentration in apoplastic sap was consistent with uptake of imported ABA into the leaf symplast, but this interpretation is qualified. Our focus was quantitative cellular compartmentation of imported ABA in guard cells. Unlike when leaves are stressed, the guard‐cell symplast ABA content did not increase because of ABA infusion (P = 0·48; 3·0 ± 0·5 versus 4·0 ± 1·2 fg guard‐cell‐pair^?1). However, the guard‐cell apoplast ABA content increased linearly (R² = 0·98) from ?0·2 ± 0·5 to 3·1 ± 1·3 fg guard‐cell‐pair^?1 (≈ 3·1 µm ) and was inversely related to leaf conductance (R² = 0·82). Apparently, xylem ABA accumulates in the guard‐cell wall as a result of evaporation of the apoplast solution. This mechanism provides for integrating transpiration rate and ABA concentration in the xylem solution.     

Gravity can induce epinastic bending of isolated leaves

Abstract Isolated leaves of Plectranthus fruticosus were grown in cubic plastic cuvettes, and were supplied via their cut petioles with nutrient solution and indole-3-acetic acid (10^?6m ). Holes bored in the cuvette walls allowed the petioles to be oriented at approximately 60°, 90° or 120° to the vertical. Growth of the leaves initially oriented at angles of 60° and 90°, which simulated the situation in the intact plant, did not result in epinastic bending of the petiole. Inversion of the leaves (adaxial surface of the petiole downwards) and orientation of the adaxial/abaxial surfaces of the horizontal petiole parallel to the gravity vector, however, yielded strong epinastic bending of the petioles. In the latter case, this bending was not in the direction of the gravity vector (evidence for point (iii), below). Furthermore, epinastic bending occurred, when the isolated leaves were rotated on a clinostat (petioles parallel to the rotation axis or inclined to the rotation axis at an angle of 30°; 3 r.p.m.). Since a possible influence of the shoot was excluded, it is concluded that (i) perception and response are restricted to the leaf, (ii) gravity alone is sufficient to induce epinasty, (iii) a gravitropic component of the response can probably be excluded. The clinostat induced epinasty may not have been caused by nullifying the effect of gravity but due to continuous gravistimulation of the leaf.     

Miscanthus × giganteus leaf senescence,decomposition and C and N inputs to soil

Energy crops are currently promoted as potential sources of alternative energy that can help mitigate the climate change caused by greenhouse gases (GHGs). The perennial crop Miscanthus × giganteus is considered promising due to its high potential for biomass production under conditions of low input. However, to assess its potential for GHG mitigation, a better quantification of the crop's contribution to soil organic matter recycling under various management systems is needed. The aim of this work was to study the effect of abscised leaves on carbon (C) and nitrogen (N) recycling in a Miscanthus plantation. The dynamics of senescent leaf fall, the rate of leaf decomposition (using a litter bag approach) and the leaf accumulation at the soil surface were tracked over two 1‐year periods under field conditions in Northern France. The fallen leaves represented an average yearly input of 1.40 Mg C ha^?1 and 16 kg N ha^?1. The abscised leaves lost approximately 54% of their initial mass in 1 year due to decomposition; the remaining mass, accumulated as a mulch layer at the soil surface, was equivalent to 7 Mg dry matter (DM) ha^?1 5 years after planting. Based on the estimated annual leaf‐C recycling rate and a stabilization rate of 35% of the added C, the annual contribution of the senescent leaves to the soil C was estimated to be approximately 0.50 Mg C ha^?1yr^?1 or 10 Mg C ha^?1 total over the 20‐year lifespan of a Miscanthus crop. This finding suggested that for Miscanthus, the abscised leaves contribute more to the soil C accumulation than do the rhizomes or roots. In contrast, the recycling of the leaf N to the soil was less than for the other N fluxes, particularly for those involving the transfer of N from the tops of the plant to the rhizome.     

贵州特有植物美丽红山茶水分胁迫的生理响应

采用水分抗逆性指标主成分分析法,选择4年实生苗,测定和评价了连续30 d水涝和干旱胁迫的生理响应.结果表明：(1)水涝处理的叶片生长旺盛,光泽度和观赏性无明显变化,叶片相对含水量、MDA含量、SOD活性、POD活性、CAT活性的变化差异在5％以内,土壤含水量增加3．47％,细胞膜透性减少1．59％,短期水涝对美丽红山茶伤害不明显;(2)随着干旱时间延长,新叶卷缩、老叶脱落,土壤含水量、叶片相对含水量随时间呈负相关缓慢减少,细胞膜透性呈正相关缓慢增长,但复水10 d 后可恢复正常生长,无明显生理变化响应;(3)SOD活性和MDA含量分别增加了0．66％和5．31％,POD活性随干旱时间上下波动而增加6．47％, CAT活性随干旱时间延长而增加,20 d增加33．33％达最大值20．8 U??g-1??min-1.在抗性生理中,维持膜稳定性的叶片含水量、POD和CAT的变化对抗旱能力大小起主要作用.     

Wound and insect‐induced jasmonate accumulation in carnivorous Drosera capensis: two sides of the same coin

Carnivorous sundew plants catch and digest insect prey for their own nutrition. The sundew species Drosera capensis shows a pronounced leaf bending reaction upon prey capture in order to form an ‘outer stomach’. This formation is triggered by jasmonates, phytohormones typically involved in defence reactions against herbivory and wounding. Whether jasmonates still have this function in D. capensis in addition to mediating the leaf bending reaction was investigated here. Wounded, insect prey‐fed and insect‐derived oral secretion‐treated leaves of D. capensis were analysed for jasmonates (jasmonic acid, JA; jasmonic acid‐isoleucine conjugate, JA‐Ile) using LC‐MS/MS. Prey‐induced jasmonate accumulation in D. capensis leaves was persistent, and showed high levels of JA and JA‐Ile (575 and 55.7 pmol·g·FW^?1, respectively), whereas wounding induced a transient increase of JA (maximum 500 pmol·g·FW^?1) and only low (3.1 pmol·g·FW^?1) accumulation of JA‐Ile. Herbivory, mimicked with a combined treatment of wounding plus oral secretion (W+OS) obtained from Spodoptera littoralis larvae induced both JA (4000 pmol·g·FW^?1) and JA‐Ile (25 pmol·g·FW^?1) accumulation, with kinetics similar to prey treatment. Only prey and W+OS, but not wounding alone or OS, induced leaf bending. The results indicate that both mechanical and chemical stimuli trigger JA and JA‐Ile synthesis. Differences in kinetics and induced jasmonate levels suggest different sensing and signalling events upon injury and insect‐dependent challenge. Thus, in Drosera, jasmonates are still part of the response to wounding. Jasmonates are also employed in insect‐induced reactions, including responses to herbivory and carnivory.     

The Effect of Leaf Water Potential, Leaf Temperature and Light Intensity on Leaf Diffusion Resistance and the Transpiration of Leaves of Malus sylvestris

The relationship between leaf resistance to water vapour diffusion and each of the factors leaf water potential, light intensity and leaf temperature was determined for leaves on seedling apple trees (Malus sylvestris Mill. cv. Granny Smith) in the laboratory. Leaf cuticular resistance was also determined and transpiration was measured on attached leaves for a range of conditions. Leaf resistance was shown to be independent of water potential until potential fell below — 19 bars after which leaf resistance increased rapidly. Exposure of leaves to CO₂-free air extended the range for which resistance was independent of water potential to — 30 bars. The light requirement for minimum leaf resistance was 10 to 20 W m^?2 and at light intensities exceeding these, leaf resistance was unaffected by light intensity. Optimum leaf temperature for minimum diffusion resistance was 23 ± 2°C. The rate of change measured in leaf resistance in leaves given a sudden change in leaf temperature increased as the magnitude of the temperature change increased. For a sudden change of 1°C in leaf temperature, diffusion resistance changed at a rate of 0.01 s cm^?1 min^?1 whilst for a 9°C leaf temperature change, diffusion resistance changed at a rate of 0.1 s cm^?1 min^?1. Cuticular resistance of these leaves was 125 s cm^?1 which is very high compared with resistances for open stomata of 1.5 to 4 s cm^?1 and 30 to 35 s cm^?1 for stomata closed in the dark. Transpiration was measured in attached apple leaves enclosed in a leaf chamber and exposed to a range of conditions of leaf temperature and ambient water vapour density. Peak transpiration of approximately 5 × 10^?6 g cm^?2 s^?1 occurred at a vapour density gradient from the leaf to the air of 12 to 14 g m^?3 after which transpiration declined due presumably to increased stomatal resistance. Leaves in CO₂-free air attained a peak transpiration of 11 × 10^?6 g cm^?2 s^?1 due to lower values of leaf resistance in CO₂ free air. Transpiration then declined in these leaves due to development of an internal leaf resistance (of up to 2 s cm^?1). The internal resistance was masked in leaves at normal CO₂ concentrations by the increase in stomatal resistance.