Variations of cuticular wax in mulberry trees and their effects on gas exchange and post-harvest water loss

Though mulberry (Morus alba) tree shows great adaptations to various climate conditions, their leaf water status and photosynthesis are sensitive to climate changes. In the current study, seven widely planted mulberry cultivars in Chongqing, Southwest China, were selected to analyze leaf cuticular wax characteristics, gas exchange index, post-harvest leaf water status and their relationships, aiming to provide new theory in screening high resistant mulberry cultivars. Mulberry trees formed rounded cap-type idioblasts on the adaxial leaf surface. Film-like waxes and granule-type wax crystals covered leaf surfaces, varying in crystal density among cultivars. The stomatal aperture on the abaxial surface of cultivars with high wax amount was smaller than that of cultivars with low wax amount. The amount of total wax was negatively correlated with the net photosynthetic rate (P _N), transpiration rate (E) and stomatal conductance (g _s) and positively correlated with the moisture retention capacity. It suggested that both cuticular wax and stomatal factor might be involved in regulating water loss in mulberry leaves under field conditions. The variability in moisture retention capacity and cuticular wax characteristics might be important in evaluating and screening mulberry cultivars for increasing silk quality and silkworm productivity.     

Leaf epidermal characters related with plant's passive resistance to pathogens vary among accessions of wild beans Phaseolus vulgaris var. aborigineus (Leguminosae–Phaseoleae)

We report here whether accessions of Phaseolus vulgaris var. aborigineus, collected from the southernmost part of the Andean domestication center of common bean, presented differences among leaf epidermal characters. Epidermal microcharacters such as deposition of wax crystalloids, cuticle thickness, trichome types, size and density, stomata types and size, stomatal density and index were examined on the surface of 66 leaves of 11 accessions of P. vulgaris var. aborigineus by means of light and scanning electron microscopy. Unlike the leaves of cultivated beans, those of P. vulgaris var. aborigineus were hypostomatous. Among epidermal characters only the stomatal density, stomatal index, and the number of trichomes on the abaxial leaf epidermis were different between bean accessions. The stomatal density and index of the accessions were inversely related to the altitude of the area of the wild bean collection. This is the first report describing the presence of diversity among epidermal characters (e.g. hypostomatous leaves) in P. vulgaris var. aborigineus that might contribute to plant resistance to pathogens.     

Salt tolerance analysis of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> NOK2 accession under saline conditions and potassium supply

The response to salt treatment and K⁺ provision of two Arabidopsis thaliana accessions grown for 17 days in the presence of 50 mM NaCl was investigated. Leaf and root dry weight deposition was restricted by salt, more in Col accession than in NOK2 accession. In both accessions, the growth inhibition induced by salinity was associated with a decrease in total leaf surface area, which resulted from diminished leaf number, but not from restriction of individual leaf surface area. Comparing the effects of salt on dry matter production and total leaf surface area revealed large difference between Col and NOK2 for net assimilation rate (the amount of whole plant biomass produced per unit leaf surface area), which was augmented by salt and K⁺ in NOK2 but not in Col. This result, which suggested a better capacity of NOK2 to preserve its photosynthetic machinery against salt stress, was in agreement with the effect of NaCl on photosynthetic pigments. Indeed, salt significantly reduced chlorophyll and carotenoid content in Col leaves but had no impact on NOK2 leaf pigment content. Since K⁺ provision had only marginal effects on these responses to salt stress, leaf mineral unbalance was unlikely. Guaiacol peroxidase activity was augmented by salt treatment in leaves and roots of both accessions. Salinity decreased the catalase activity in Col leaves and in roots, and increased this activity in NOK2 organs. In conclusion, when aggressed by salt, NOK2 was able (1) to produce more leaves than Col, and (2) to efficiently protect its photosynthetic apparatus, perhaps by developing more efficient antioxidative defense through increased catalase and peroxidase activities. Consequently, the overall photosynthetic activity was higher and more robust to salt aggression in NOK2 than in Col.     

Leaf Water Repellency as an Adaptation to Tropical Montane Cloud Forest Environments

Adaptations that reduce water retention on leaf surfaces may increase photosynthetic capacity of cloud forests because carbon dioxide diffuses slower in water than air. Leaf water repellency was examined in three distinct ecosystems to test the hypothesis that tropical montane cloud forest species have a higher degree of leaf water repellency than species from tropical dry forests and species from temperate foothills-grassland vegetation. Leaf water repellency was measured by calculating the contact angle of the leaf surface and the line tangent to a water droplet through the point of contact on the adaxial and the abaxial surface. Leaf water repellency was significantly different between the three study areas. The hypothesis that leaf water repellency is higher in cloud forest species than tropical dry forests and temperate foothills-grassland vegetation was not confirmed in this study. Leaf water repellency was lower for cloud forest species (adaxial surface = 50.8°; abaxial surface = 82.9°) than tropical dry forest species (adaxial surface = 74.5°; abaxial surface = 87.3°) and temperate foothills-grassland species (adaxial surface = 77.6°; abaxial surface = 95.8°). The low values of leaf water repellency in cloud forest species may be influenced by presence of epiphylls and loss of epicuticular wax on the leaf surfaces.     

Effects of tussock size and soil water content on whole plant gas exchange in Stipa tenacissima L.: Extrapolating from the leaf versus modelling crown architecture

In this study we evaluated daily whole plant transpiration and net photosynthetic rates in Stipa tenacissima L. (Poaceae) tussocks of different sizes subjected to three levels of soil moisture. The crown architecture of 12 tussocks was reconstructed with the 3D computer model Yplant taking into account the morphology and physiology of the leaves determined at different soil moisture levels. We also calculated whole plant transpiration by extrapolating leaf transpiration in different senescence conditions measured with a diffusion porometer. This extrapolated transpiration overestimated transpiration, particularly when the soil moisture level was high (>15% of volumetric soil water content). At this high level of soil moisture, large tussocks (>60 cm in diameter), which were sexually mature and had a large leaf surface area, were the most efficient with regard to daily water use efficiency (whole plant net photosynthesis/whole plant transpiration). Whole plant water use efficiency decreased with tussock size primarily because small tussocks exhibited high transpiration rates. Small tussocks were more sensitive to soil drying than large and intermediate ones, presenting a faster rate of leaf senescence as water deficit increased. Leaf acclimation to irradiance, which was significantly influenced by the degree of mutual shading among neighbouring leaves, along with the ontogeny of the tussock and its effect upon leaf senescence were found to be the main mechanisms involved in the different responses to water limitations found in whole plant gas exchange variables. Our results show that the size of each individual plant must be taken into account in processes of scaling-up of carbon gain and transpiration from leaf to stand, as this is a particularly relevant aspect in estimating water use by semiarid vegetation.     

Impact of the leaf miner<Emphasis Type="Italic"> Cameraria ohridella</Emphasis> on photosynthesis,water relations and hydraulics of<Emphasis Type="Italic"> Aesculus hippocastanum</Emphasis> leaves

The mining of leaves of Aesculus hippocastanum caused by the larvae of Cameraria ohridella leads to precocious defoliation of trees. Damage to plant productivity was estimated in terms of the photosynthetic performance as well as of leaf water relations and hydraulics of increasingly mined leaves from infested plants in comparison with the same variables measured in non-mined leaves (controls). Electron microscopy and photosynthesis measurements revealed that chloroplasts within the green portions of mined leaves were still intact and photosynthesis of these areas was close to that of non-mined leaves, i.e. damage to functional integrity of the photosynthetic system did not extend beyond the mines. Stomata below the mines were functional as they maintained their physiological kinetics but most chloroplasts in the spongy parenchyma below the mines were degraded so that a 1:1 relationship existed between photosynthesis loss and loss of leaf green areas. Leaf conductance to water vapour and transpiration rate were 60% lower in mined leaf areas but equal to controls in green portions of mined leaves. Leaf water potential was insensitive to the amount of mined leaf area and so was leaf hydraulic conductance. Anatomical observations of leaf minor veins revealed that they were structurally and functionally intact even in leaves with 90% mined surface area. Our conclusion was that the actual damage to A. hippocastanum plants in terms of loss of photosynthates and water and nutrient transport was less than that visually estimated in recent studies.     

西安市常见绿化植物叶片润湿性能及其影响因素

利用接触角测定仪测定了西安市21种常见绿化植物叶片表面的接触角,探讨了叶片表面特性如蜡质、绒毛、气孔对接触角的影响。结果表明,植物叶片正背面、物种间的接触角差异均显著,叶片正面和背面接触角大小在40°～140°。接触角大小与变异系数呈负相关,可能由于接触角小的润湿叶片在不同的生境和位置下,受到环境条件的影响较大而出现大的变异;接触角较大的非润湿性叶片,环境物质持留时间较短,对叶片形态和组成影响较小,因而出现小的变异。植物叶片表面的接触角随蜡质含量的升高而增大。表皮蜡质去除后大部分叶片接触角明显降低,尤其是疏水性较强的银杏(Ginkgo biloba)、月季(Ro-sa chinensis)和紫叶小檗(Berberis thunbergii)。女贞(Ligustrum lucidum)正背面、加杨(Popu-lus canadensis)背面等亲水型的叶片蜡质去除后接触角反而增大。叶片绒毛的多少及其形态、分布方式对接触角具有重要的影响,不同的作用方式表现出润湿和不润湿的特征,人为将其去除可以增加叶片的润湿性。背面气孔密度与气孔长度、保卫细胞长度呈负相关;接触角则与气孔密度呈负相关,与气孔长度呈正相关。     

Internal water balance of barley under soil moisture stress

Leaf water potential, leaf relative water content, and relative transpiration of barley were determined daily under greenhouse conditions at 3 growth stages: tillering to boot, boot to heading, and heading to maturity. The leaf moisture characteristic curve (relative water content versus leaf water potential) was the same for leaves of the same age growing in the same environment for the first 2 stages of growth, but shifted at the heading to maturity stage to higher leaf relative water content for a given leaf water potential. Growth chamber experiments showed that the leaf moisture characteristic curve was not the same for plants growing in different environments.

Relative transpiration data indicated that barley stomates closed at a water potential of about −22 bars at the 3 stages studied.

The water potential was measured for all the leaves on barley to determine the variation of water potential with leaf position. Leaf water potential increased basipetally with plant leaf position. In soil with a moisture content near field capacity a difference of about 16.5 bars was observed between the top and bottom leaves on the same plant, while in soil with a moisture content near the permanent wilting point the difference was only 5.6 bars between the same leaf positions.

     

Morphological factors of the central whorl leaf associated with leaf surface wetness and resistance in sorghum to shoot fly,Atherigona soccata*

Earlier studies showed that leaf surface water on the central whorl leaf of sorghum seedlings is associated with resistance to shoot fly. In this study, the results of an experiment to determine if leaf surface wetness (LSW) originates from atmospheric condensation or from the plant are described. Morphological structures: trichomes, stomata, leaf cuticle and quantity of surface wax of the central whorl leaf were also examined for their role in LSW production. The results suggest that LSW of the central whorl leaf originates from the plant and is not due to condensation of atmospheric moisture. The presence of trichomes was indirectly associated with LSW and resistance to shoot fly but stomatal density was not associated with LSW production. The amount of wax extracted per 100 mg of fresh weight varied significantly between genotypes and seedling age. It was more in susceptible than in resistant genotypes; however, cuticular thickness was not associated with resistance. It is suggested that LSW could be the result of some form of cuticular movement of water to the leaf surface.     

Determinants of some leaf characteristics of Australian mangroves

Leaf characteristics reflecting the size, lifespan (longevity), moisture content (degree of succulence) and complexity of structure of 20 mangrove species were studied over several years at 13 locations along the tropical and subtropical Australian coast. These characteristics were found to fall generally within the ranges of those for woody species from other ecosystems. With the exception of one species, it was found that leaf longevity was related inversely to leaf moisture content, increasing from nearly 6 months in more succulent species to over 2 years in less succulent species. This suggested that more succulent leaves are less complex in their structure because they have less well‐developed ability to compartmentalize salt. There was a tendency also for leaf longevity to increase in species with larger leaves. These findings were consistent with the general view for land plants that leaf longevity is greater in species that have developed tolerance to environmental stress, salt particularly in the case of mangroves. Leaf tissue in such species is more robust or complex and requires greater metabolic resources in its construction; the plant is then advantaged by retaining the tissue for longer periods. Classification of the species considered here, based on their leaf longevity, moisture content and complexity, identified phylogenetically related species groupings that reflected these leaf longevity effects.     

空气湿度与土壤水分胁迫对紫花苜蓿叶表皮蜡质特性的影响

选用2个抗旱性不同的紫花苜蓿品种,敖汉(强抗旱)和三得利(弱抗旱),设置空气湿度(45%-55%和75%-85%)和土壤水分胁迫(75%和35%田间持水量)处理,分析紫花苜蓿叶表皮蜡质含量、组分及晶体结构、气体交换参数、水势及脯氨酸含量的变化规律。结果表明,单独土壤水分胁迫时,紫花苜蓿叶表皮蜡质晶体结构及蜡质总量无显著变化;敖汉蜡质组分中烷类、酯类含量增加,醇类含量下降;三得利醇类含量下降,烷类、酯类含量变化不显著。低空气湿度胁迫时,两品种蜡质总量无显著变化,烷类和酯类含量显著增加,醇类含量显著下降,叶表皮片状蜡质晶体结构熔融呈弥漫性,扩大了对叶表面积的覆盖,其蒸腾速率显著低于正常湿度。复合胁迫处理时,叶表皮片状蜡质晶体结构继续呈弥漫性,烷类、酯类、未知蜡质组分含量均高于单独胁迫处理,醇类含量最低,而蜡质总量除三得利显著高于对照外,其余均无显著差异。紫花苜蓿叶表皮蜡质各组分含量(除醇类)及蜡质总量与光合速率呈显著负相关,与蒸腾速率无显著相关关系。蜡质总量与叶水势呈显著正相关。总体上,敖汉蜡质总量显著高于三得利,蜡质组分中烷类物质的增加有助于提高植株的抗旱性。在复合胁迫下,强抗旱品种主要通过气孔因素控制水分散失,而弱抗旱品种通过气孔和非气孔因素共同控制植物水分散失。     

Structural and functional traits of Quercus ilex in response to water availability

Water potential and morpho-anatomical parameters were measured, during the course of 1 year, on leaves of Quercus ilex trees growing in two coastal stands in Tuscany (Central Italy) with different conditions of water availability: Colognole (CL, mesic site) and Cala Violina (CV, xeric site). Morpho-anatomical measurements included: general leaf features and sclerophylly indices (surface area, thickness, mass per area and density), leaf moisture indices (water content, relative water content, succulence) and histochemical analysis (detection and localization of cutine and tannins in the leaves and starch reserves in the twigs). During the warmest and driest period (August) pre-dawn water potential (ψ_pd) in Holm-oak leaves reached −2.7 MPa at CV and −0.6 MPa at CL. Leaf surface was lower (−34%) and total leaf thickness (+10%), as well as spongy-palisade parenchyma ratio (+20%) were higher at CV. The sclerophylly parameters (leaf mass per area and leaf tissue density) were higher at CV than at CL (+24% leaf mass per area and +19% leaf tissue density). Among the moisture parameters, water content was higher at CL (+8%) and succulence was higher at CV (+13%). No differences in relative water content were observed between the two sites. All the parameters considered were substantially stable during the study period, with the exception of relative water content at CL, that fluctuated within the year. Histochemical analysis revealed a greater thickness of the upper cuticular layer at CV, whereas there were no differences in tannin distribution and content between the two sites. Differences in starch storage were detected in branchlets: it was abundant in CV but very scarce at CL. The strategies of Quercus ilex to cope with water stress were discussed at morpho-structural level.     

LEAF FOLDING IN MIMOSA PUDICA (FABACEAE): A NUTRIENT CONSERVATION MECHANISM?

Ten plants of Mimosa pudica L. were exposed to artificial rain following anesthesia of the leaves of half of the plants. Anesthetized plants, which no longer displayed the seismonastic response of this species, were hypothesized to show greater rates of foliar leaching of nutrients than their unanesthetized counterparts. Leachate was collected and analyzed for levels of K, Na and Ca. Levels were corrected for the amount of leaf area exposed to rainfall. No differences were noted between treatments for rainfall durations of either 6 or 20 min. Leaf surface morphology was examined and lattice-like extrusions of epicuticular wax were found. We feel that the leaf surface structure may act to keep raindrops from coming in contact with the surface of the leaf thereby minimizing foliar leaching. It appears that the seismonastic response of the leaves plays only a minor role in nutrient conservation.     

The influence of UV-B radiation on the physicochemical nature of tobacco (Nicotiana tabacum L.) leaf surfaces

Relationships between leaf wettability and surface physicochemicalcharacteristics were examined in two genotypes of tobacco (Nicotianatabacum L. cv. Samsun) grown under controlled conditions atthree different levels of biologically effective ultraviolet-B(UV-B_BE; 280–320 nm) radiation; 0 (control), 4.54 and5.66 kJ m^–2d^–1. Leaf wettability, assessed by measuringleaf-water droplet contact angles, was positively correlatedwith epicuticular wax chemical composition and trichome density,but not the amount of wax on the surface of leaves. Tobaccowax comprised a mixture of C₁₉–C₃₃ n-alkanes (59%) withhomologues containing an odd number of carbon atoms predominating,C₂₈–C₃₂ br-alkanes (38%), and a small quantity (3%) offree C_l6–C₁₈ fatty acids. Significant effects of UV-Bradiation upon wax production and chemical composition wererestricted to the adaxial surface of leaves. Enhanced UV-B radiationreduced the quantity of epicuticular wax in the more sensitivegenotype [GR32-3], assessed from effects on dry matter accumulation,partitioning and changes in leaf morphology, and resulted inmarked changes in wax composition and homologue distributionsin both genotypes. UV-B-induced increases in branching, andshifts toward the synthesis of shorter-chain homologues providedevidence for a fundamental effect of UV-B radiation on wax biosynthesis,with the observed effects consistent with a highly specificand direct effect of UV-B radiation on microsomal-based elongasesin the epidermis. UV-B radiation also reduced the density oftrichomes on the adaxial leaf surface, whilst increasing thenumber of trichomes on the abaxial leaf surface. Changes inwax composition and trichome density induced by UV-B radiationwere associated with increases in leaf surface wettability whichwere particularly pronounced on the adaxial surface. The subtle,though possibly far-reaching, physiological consequences ofsuch UV-B-induced changes in surface wettability are discussedin the light of other recent findings. Key words: Epicuticular wax chemistry, wax quantity, leaf wettability, trichome density, ultraviolet-B radiation     

Ontogenetic variation in chemical and physical characteristics of adaxial apple leaf surfaces

The reaction of plants to environmental factors often varies with developmental stage. It was hypothesized, that also the cuticle, the outer surface layer of plants is modified during ontogenesis. Apple plantlets, cv. Golden Delicious, were grown under controlled conditions avoiding biotic and abiotic stress factors. The cuticular wax surface of adaxial apple leaves was analyzed for its chemical composition as well as for its micromorphology and hydrophobicity just after unfolding of leaves ending in the seventh leaf insertion. The outer surface of apple leaves was formed by a thin amorphous layer of epicuticular waxes. Epidermal cells of young leaves exhibited a distinctive curvature of the periclinal cell walls resulting in an undulated surface of the cuticle including pronounced lamellae, with the highest density at the centre of cells. As epidermal cells expanded during ontogenesis, the upper surface showed only minor surface sculpturing and a decrease in lamellae. With increasing leaf age the hydrophobicity of adaxial leaf side decreased significantly indicated by a decrease in contact angle. Extracted from plants, the amount of apolar cuticular wax per area unit ranged from only 0.9 microgcm(-2) for the oldest studied leaf to 1.5 microgcm(-2) for the youngest studied leaf. Differences in the total amount of cuticular waxes per leaf were not significant for older leaves. For young leaves, triterpenes (ursolic acid and oleanolic acid), esters and alcohols were the main wax components. During ontogenesis, the proportion of triterpenes in total mass of apolar waxes decreased from 32% (leaf 1) to 13% (leaf 7); absolute amounts decreased by more than 50%. The proportion of wax alcohols and esters, and alkanes to a lesser degree, increased with leaf age, whereas the proportion of acids decreased. The epicuticular wax layer also contained alpha-tocopherol described for the first time to be present also in the epicuticular wax. The modifications in the chemical composition of cuticular waxes are discussed in relation to the varying physical characteristics of the cuticle during ontogenesis of apple leaves.     

Toxic Effects of Cadmium on Morus alba L. and Bombyx moril L.

A 3-year micro-plot experiment of mulberry cultivation with Cd-polluted soil and silkworm breeding experiments by feeding with exogenous or endogenous –Cd-polluted mulberry leaves were conducted to evaluate the toxic effects of Cd on mulberry and silkworms. There was no apparent harmful effect on mulberry plant growth at a soil Cd content of 8.49 mg/kg. At a soil Cd content of 75.8 mg/kg, a yield reduction in the leaves became apparent, whereas at 145 mg Cd/kg soil, the plants exhibited marginal growth. There was a significant decrease of the ingestive rate of leaves at an exogenous Cd content of 1.98 mg Cd/kg leaf, whereas the digestive rate decreased significantly at an exogenous Cd content of 0.50 mg/kg. The endogenous Cd content that significantly affected the ingestive and digestive rates was 1.66 mg/kg. The influence of exogenous and endogenous Cd on the biomass of the silkworms decreased with increase of the instar stage. The weight of cocoons and rate of silk reeling was significantly reduced at exogenous and endogenous Cd contents of 5.17 mg/kg and 1.66 mg/kg, respectively.     

Comparative Leaf Surface Morphology and the Glossy Characteristic of Sorghum, Maize, and Pearl Millet

Leaf surfaces of seven genotypes of Sorghum bicolor, two ofmaize, Zea mays, and two pearl millet, Pennisetum americanum,were examined by scanning electron microscopy for possible morphologicaldifferences. Leaves 1, 3, 5 and 7 were photographed and printswere used to estimate waxiness, hairiness or pubescence andstomatal density. Glossiness was determined by spraying water,which adhered to the glossy leaves. Cuticular transpirationof detached third and fifth leaves was estimated from the rateof water loss after abscisic acid induced stomatal closure.Sorghum lines SC283, CSM63, CSM90, and pearl millets Souna andTiotioni (all from Mali), were non-glossy, well covered withwax, and exhibited variable hairiness. Older leaves of sorghumvarieties Martin and Redlan were glossy and, like older leavesof the other glossy lines SC1096 and SC90, had little or nowax deposits on their cuticles. The two maize cultivars, NB611and N7A, were non-glossy with dense wax covering; no trichomeswere observed until the 5 to 7 leaf stage. Thus, the glossycharacter was correlated with the reduction or absence of waxdeposits on the leaf surfaces, while hairiness might occur ineither glossy or non-glossy genotypes. Unlike sorghum and maize,in which all leaves after the fifth or seventh were glossy,pearl millet showed no glossiness through the ninth leaf. Measurementsshowed that cuticular transpiration of glossy leaves was oftenmore than double that of non-glossy leaves. Comparisons amongsorghums showed that non-glossy lines had higher stomatal densitiesthan glossy lines. Epicuticular wax, trichome, glossy mutant, stomata, cuticular transpiration, Sorghum bicolor, (L.) Moench, Zea mays L., Pennisetum americanum, (L.) Leeke     

Evidence for facultative deciduousness in Colophospermum mopane in semi‐arid African savannas

Leaf phenology dictates the time available for carbon assimilation, transpiration and nutrient uptake in plants. Understanding the environmental cues that control phenology is therefore vital for predicting climate‐related changes to plant and ecosystem function. In contrast to temperate systems, and to a lesser degree, tropical forest systems, the cues initiating leaf drop in tropical savannas are poorly studied. We investigated the cues for leaf fall in a tropical monodominant arid savanna species, Colophospermum mopane, using an irrigation experiment. We tracked soil moisture, solar radiation, air temperature, leaf water status, leaf health and leaf carbon balance through the dry season in both irrigated and control plants. Water was the primary cue driving leaf loss of C. mopane rather than temperature or light. Trees watered throughout the dry season retained their canopies. These leaves remained functional and continued photosynthesis throughout the dry season. Leaf carbon acquisition rates did not decline with leaf age but were affected by soil moisture availability and temperature. Leaf loss did not occur when leaf carbon gain was zero, or when a particular leaf carbon threshold was reached. Colophospermum mopane is facultatively deciduous as water availability determines leaf drop in this widespread arid savanna species. Obligate deciduosity is not the only successful strategy in climates with a long dry season.     

Leaf breakdown patterns in a NW Italian stream: Effect of leaf type, environmental conditions and patch size

We studied the decomposition process and macroinvertebrate colonisation of leaf packs to determine to what extent leaf consumption and invertebrate abundance depend on the pollution level, season, leaf type and patch size. We exposed 400 leaf packs made of two leaf types, alder and chestnut, at two sites of the Erro River (NW Italy) with different environmental alteration levels. Leaf packs were set out as three patch sizes (alone, or in groups of 6 or 12). A first experiment was carried out in winter and a second in summer. Leaf packs were retrieved after 15, 30, 45 and 60 days of submersion to determine the leaf mass loss and to quantify the associated macroinvertebrates. Natural riverbed invertebrates were collected in the same areas. Patch size, season, leaf type and pollution level significantly affected mass loss. The breakdown process was faster for alder leaves, during summer, at the unpolluted site, and in smaller patches. Leaf type and patch size did not affect macroinvertebrate density and richness, but the highest taxon richness was found in winter and at the unpolluted site. There were more shredders and predators than in the natural riverbed. Our study supports two recent ideas regarding leaf processing in streams: that patch size influences the leaf breakdown rate and that the breakdown rate can be used to evaluate water quality and environmental health.     

Chloroplast response to low leaf water potentials: I. Role of turgor

The effect of decreases in turgor on chloroplast activity was studied by measuring the photochemical activity of intact sunflower (Helianthus annuus L. cv. Russian Mammoth) leaves having low water potentials. Leaf turgor, calculated from leaf water potential and osmotic potential, was found to be affected by the dilution of cell contents by water in the cell walls, when osmotic potentials were measured with a thermocouple psychrometer. After the correction of measurements of leaf osmotic potential, both the thermocouple psychrometer and a pressure chamber indicated that turgor became zero in sunflower leaves at leaf water potentials of −10 bars. Since most of the loss in photochemical activity occurred at water potentials below −10 bars, it was concluded that turgor had little effect on the photochemical activity of the leaves.