Midday depression in root respiration of <Emphasis Type="Italic">Quercus crispula</Emphasis> and <Emphasis Type="Italic">Chamaecyparis obtusa</Emphasis>: its implication for estimating carbon cycling in forest ecosystems

We observed the phenomenon of midday depression in the rate of tree root respiration. Diurnal changes in the root respiration rate of Quercus crispula and Chamaecyparis obtusa were measured under intact conditions using a closed chamber method and a soil respiration measurement system (LI-6400 with a root respiration chamber) in a forest in the foothills of Mt. Fuji. After the measurement of intact root respiration in the field, the root was excised and taken to the laboratory, and the temperature dependence on the respiration rate of the detached root was measured using an open-flow gas exchange system with an infrared gas analyzer (LI-6252). The measurement was conducted in September 2003, August and November 2005, and June 2006. Whereas the root respiration rate of both species under intact conditions increased with increasing soil and root temperatures from dawn to early morning, the respiration rate decreased around midday from 10:00 to 15:00 despite an increment of soil and root temperatures. There was no clear relationship between the intact root respiration rate and root temperature in either species, although the detached root respiration rate of both increased exponentially with the temperature. The amount of the CO₂ efflux estimated using the temperature dependence of detached root respiration tended to underestimate the actual measurement value (intact respiration rate) by 20–50% in both species. These results indicate that evaluating midday depression in root respiration would be important for a more accurate estimation of the carbon cycle or net ecosystem production in forests.     

Changes in photosynthetic capacity,carboxylation efficiency,and CO2 compensation point associated with midday stomatal closure and midday depression of net CO2 exchange of leaves of Quercus suber

The carbon-dioxide response of photosynthesis of leaves of Quercus suber, a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a standard time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO₂ pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO₂ uptake. Midday depression of CO₂ uptake is the result of i) a decrease in CO₂-saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO₂ response curve (carboxylation efficiency), and iii) a substantial increase in the CO₂ compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO₂ pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO₂ may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.Abbreviations and symbols CE carboxylation efficiency - NP net photosynthesis rate - PAR photosynthetically active radiation - P_i leaf internal CO₂ partial pressure - W water vapor mole fraction difference between leaf and air - T CO₂ compensation pressure Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday     

Midday depression of tree root respiration in relation to leaf transpiration

The root respiration rate often shows an exponential or a linear relationship with temperature under laboratory conditions. However, under intact conditions in the field, the root respiration rates of some tree species decreased around midday despite an increment of the root temperature (Bekku et al. 2009). To clarify the cause of midday depression, we examined the relationships between the intact root respiration and parameters of leaf gas exchange through the simultaneous field measurement of the gas exchange in the leaf and root of Quercus crispula and Chamaecyparis obtusa, which are canopy trees. There were no significant relationships between the root respiration rates (R _r) and the parameters of leaf gas exchange in the field. However, in C. obtusa, the relationships between R _r and the transpiration rates (E) at 1 h before the measurement of R _r were fitted by logarithmic function with a determination coefficient of 0.60–0.89. In the light-manipulation experiments using saplings, R _r had significant positive correlations with E at 20 min before the measurement of R _r, root temperature (T _r), and the photosynthesis (P _n) at 20 min before the measurement of R _r. We examined which factor, P _n or E, affects the root respiration rate through a manipulation experiment using a growth chamber regulating the ambient CO₂ concentration and relative humidity independently under constant air temperature and photosynthetic photon flux density. As a result, the root respiration rates changed corresponding to E and not P _n. These results suggest that the root respiration rate of trees changes significantly in the daytime and is affected by the leaf transpiration rate as well as the temperature.     

Use of Azolla as a test organism in a growth chamber of simple design

Summary The construction and application of a new type of growth chamber, in which different growth conditionsi.e.: temperature, humidity, pH, light intensity, light colour, change in nutrient composition and gas exchange can easily be controlled, are presented. The method has previously been applied to twoAzolla speciesviz. Azolla filiculoides, which is cold tolerant andAzolla pinnata (distinguished in Vietnam as the form Xanh), which is heat tolerant. In the growth chamber natural growth conditions of the Azolla —Anabaena azollae symbiotic association were imitated as much as possible. For testing the system, methods discussed earlier^8,14 and some previously presented data, concerning photosynthetic activities, such as oxygen evolution and nitrogen fixation (acetylene reduction) of twoAzolla species³⁹, were partially used. Biomass ofA. filiculoides was measured and reactions to its environment at conditions when grown in the field and in the growth chamber, were studied. Growth and photosynthesis measurements were performed under special light conditions and with whole plants grown under laboratory conditions. Anthocyanin synthesis was studied in relation with humidity. Anthocyanin spectra were analyzed by means of a spectrum-deconvolution method. On leave from the Department of Plant Physiology of the University of Hanoi, Vietnam.     

The midday depression of CO2 assimilation in leaves of Arbutus unedo L.: diurnal changes in photosynthetic capacity related to changes in temperature and humidity

Parts of attached leaves of the sclerophyllous shrub Arbutus unedo were subjected to simulated mediterranean days. Gas exchange was recorded in order to recognize the causes of the midday depression in CO₂ assimilation. Depressions could be induced in part of a leaf: they were local responses. The CO₂-saturation curves of photosynthesis, determined during the morning and afternoon maxima of CO₂ assimilation and during the minimum at midday, established that depressions in CO₂ assimilation were in one-half of the investigated cases totally caused by reversible reductions in the photosynthetic capacity of the leaves, and in the other half almost totally caused by such reductions. An analysis of 37 daily courses showed that morning reductions and afternoon recoveries of stomatal conductance and rate of photosynthesis occurred simultaneously and in proportion to each other, with the result that the partial pressure of CO₂ in the intercellular spaces remained more or less constant. Midday depressions occurred also in detached leaves standing in water. The initiation of a midday depression was not caused by a circadian rhythm, nor was high quantum flux or high temperature a requirement. There was no correlation between the rate of water loss from the leaves, or the amount of water lost, with the degree of reduction of the photosynthetic capacity. However, depressions occurred if an apparent threshold in the water-vapor pressure difference between leaf and air was exceeded. This critical value varied between about 20 and 30 mbar, depending on the leaf investigated. The dominating role of humidity in the induction of the midday depression was further demonstrated when leaf temperature was held constant and the vapor-pressure difference was made to follow the pattern of the mediterranean day: depressions occurred. Depressions however were hardly noticeable when the water-vapor pressure difference was held constant and leaf temperature was allowed to vary. In another set of experiments, leaves were subjected to variations in temperature and humidity independent of the time of the day, under otherwise constant conditions. Photosynthetic capacity and stomatal conductance proved to be almost insensitive to changes in temperature (in a range extending from 20 to 37° C) as long as the water vapor-pressure difference was held constant. If it was not, the rate of photosynthesis began to decline with increasing temperature after a threshold water-vapor pressure difference was exceeded. The position of the resulting apparent temperature optimum of photosynthesis depended on the humidity of the air. We suggest that the ability of A. unedo to respond to a dry atmosphere with a reversible reduction of its photosynthetic capacity (by a still unknown mechanism) is the result of a co-evolution with the development of a strong stomatal sensitivity to changes in humidity.     

The effect of CO2 enrichment on leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie

Open-top chambers were used to study the effects of CO₂ enrichment on leaf-level photosynthetic rates of the C₄ grass Andropogon gerardii in the native tallgrass prairie ecosystem near Manhattan, Kansas. Measurements were made during a year with abundant rainfall (1993) and a year with below-normal rainfall (1994). Treatments included: No chamber, ambient CO₂ (A); chamber with ambient CO₂ (CA); and chamber with twice-ambient CO₂ (CE). Measurements of photosynthesis were made at 2-hour intervals, or at midday, on cloudless days throughout the growing season using an open-flow gas-exchange system. No significant differences in midday rates of photosynthesis or in daily carbon accumulation as a result of CO₂ enrichment were found in the year with abundant precipitation. In the dry year, midday rates of photosynthesis were significantly higher in the CE treatment than in the CA or A treatments throughout the season. Estimates of daily carbon accumulation also indicated that CO₂ enrichment allowed plants to maximize carbon acquisition on a diurnal basis. The increased carbon accumulation was accounted for by greater rates of photosynthesis in the CE plots during midday. During the wet year, CO₂ enrichment decreased stomatal conductance, which allowed plants to decrease transpiration while still photosynthesizing at rates similar to plants in ambient conditions. During the dry year, CO₂ enrichment allowed plants to maintain photosynthetic rates even though stomatal conductance and transpiration had been reduced in all treatments due to stress. Estimates of instantaneous water-use efficiency were reduced under CO₂ enrichment for both years. This revised version was published online in June 2006 with corrections to the Cover Date.     

Gas exchange pattern in the two‐spot ladybird beetle,Adalia bipunctata,differs in dry vs. moist air

Gas exchange patterns in the ladybird beetle, Adalia bipunctata (L.) (Coleoptera: Coccinellidae), were investigated using an infrared gaseous analyser (IRGA) and a coulometric O₂ respirometer (manometric–volumetric system). Before testing, the beetles were kept either in dry (dehydrated) or moist (hydrated) conditions for 1 day. Their subsequent gas exchange patterns did not depend on their state of humidity but rather were controlled by the humidity of the insect chamber during gas exchange measurement. If this chamber contained dry air, the beetles exhibited CO₂ release by burst, which we interpreted as cyclic gas exchange (CGE) with inter‐burst periods, but if the chamber was switched to contain moist air, then cyclic CO₂ release was soon abandoned and a pattern of continuous gas exchange appeared. Measurements with the coulometric respirometer in moist air showed that continuous gas exchange was often associated with weak abdominal pulsations, which we interpreted as active ventilation. Their metabolic rate was lower during gas exchange cycles than during continuous gas exchange. We revealed that in the ladybird beetle metabolic rate increased in moist air when the gas exchange pattern transitioned from cyclic to continuous.     

Stomatal closure induced by high vapor pressure deficit limited midday photosynthesis at the canopy top of <Emphasis Type="Italic">Fagus crenata</Emphasis> Blume on Naeba mountain in Japan

Diurnal changes in gas exchange, chlorophyll fluorescence and leaf water potential (_leaf) were measured to determine the environmental and physiological factors that limit carbon gain in the horizontal leaves of Fagus crenata Blume at the canopy top. Although midday depression of the net CO₂ assimilation rate (A_n) and stomatal conductance (gH₂O) were clearly evident on a fine day, the potential quantum yield of PS II (F_v/F_m) was fairly constant around 0.83 throughout the day. This result indicates that the leaves at the canopy top do not suffer from chronic photoinhibition, and the excess energy is dissipated safely. Large reversible increases in non-photochemical quenching (NPQ) were evident on fine days. Therefore, the non-radiative energy dissipation of excess light energy contributed to avoid chronic photoinhibition. The electron transfer rate (ETR) reached maximum during the midday depression, and thus there was no positive relation between ETR and A_n under high light conditions, indicating a high rate of photorespiration and the absence of non-stomatal effect during midday. The protective mechanisms such as non-radiative energy dissipation and photorespiration play an important role in preventing photoinhibitory damage, and stomatal limitation is the main factor of midday depression of A_n. To separate the effect of air to leaf vapor pressure deficit (ALVPD) and leaf temperature (T_leaf) on gas exchange, the dependencies of A_n and gH₂O on ALVPD and T_leaf were measured using detached branches under controlled conditions. A_n and gH₂O were insensitive to an increase in T_leaf. With the increase in ALVPD, A_n and gH₂O exhibited more than a 50% decrease even though water supply was optimum, suggesting the dominant role of high ALVPD in the midday depression of gH₂O. We conclude that midday depression of A_n results from the midday stomatal closure caused by high ALVPD.     

Comparison of photosynthetic activity of <Emphasis Type="Italic">Orychophragmus violaceus</Emphasis> and oil-seed rape

Orychophragmus violaceus, Brassica campestris cv. Chuanyou No.8, and Brassica juncea cv. Luzhousileng diurnal changes of net photosynthetic rate (P _N) and activities of carbonic anhydrase (CA) of leaves were studied. One uni-modal curve occurred at the diurnal changes of P _N in O. violaceus, but bimodal curves were found in B. campestris and B. juncea. Thus photosynthetic midday depression was not found in O. violaceus but in both Brassica species. Midday depression of P _N in O. violaceus was not related to high temperature or low humidity at midday but to the activity of CA.     

Modelling Diurnal Courses of Photosynthesis and Transpiration of Leaves on the Basis of Stomatal and Non-Stomatal Responses,Including Photoinhibition

A mathematical model for photoinhibition of leaf photosynthesis was developed by formalising the assumptions that (1) the rate of photoinhibition is proportional to irradiance; and (2) the rate of recovery, derived from the formulae for a pseudo first-order process, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is integrated into a photosynthesis-stomatal conductance (g _s) model that combines net photosynthetic rate (P _N), transpiration rate (E), and g _s, and also the leaf energy balance. The model was run to simulate the diurnal courses of P _N, E, g _s, photochemical efficiency, i.e., ratio of intercellular CO₂ concentration and CO₂ concentration over leaf surface (C _i/C _s), and leaf temperature (T ₁) under different irradiances, air temperature, and humidity separately with fixed time courses of others. When midday depression occurred under high temperature, g _s decreased the most and E the least. The duration of midday depression of g _s was the longest and that in E the shortest. E increased with increasing vapour pressure deficit (VPD) initially, but when VPD exceeded a certain value, it decreased with increasing VPD; this was caused by a rapid decrease in g _s. When air temperature exceeded a certain value, an increase in solar irradiance raised T ₁ and the degree of midday depression. High solar radiation caused large decrease in initial photon efficiency (). P _N, E, and g _s showed reasonable decreases under conditions causing photoinhibition compared with non-photoinhibition condition under high irradiance. The T ₁ under photoinhibition was higher than that under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in C _i/C _s at midday implies that stomatal closure is a factor causing midday depression of photosynthesis.     

Effect of nitrogen-deficiency on midday photoinhibition in flag leaves of different rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars

Effects of nitrogen (N)-deficiency on midday photoinhibition in flag leaves were compared between two contrastive Japanese rice cultivars, a traditional japonica cultivar with low yield, cv. Shirobeniya (SRB), and a japonica-indica intermediate type with high yield, cv. Akenohoshi (AKN). Both cultivars were grown under high-N and low-N conditions. At midday, low-N supply resulted in more intensive reductions in net photosynthetic rate, stomatal conductance, maximal quantum yield of photosystem II (PSII) and quantum yield of PSII electron transport in SRB than in AKN, indicating that SRB was more strongly photoinhibited than AKN under low-N condition. At midday, the low-N plants of two cultivars showed higher superoxide dismutase (SOD) activities than the high-N plants. However, ascorbate peroxidase (APX) activity was maintained in AKN but significantly decreased in SRB under low-N condition (N-deficiency). In contrast, hydrogen peroxide (H₂O₂) content in SRB significantly increased under low-N condition, indicating that the susceptibility to midday photoinhibition in the low-N plants of SRB is related to the increased H₂O₂ accumulation. It is suggested that the midday depression in photosynthesis may be a result of oxidative stress occurring in the low-N plants in which antioxidant capacity is not enough to cope with the generation of H₂O₂. Therefore, H₂O₂-scavenging capacity could be an important factor in determining the cultivar difference of midday photoinhibition in flag leaves of rice under low-N condition.     

海岸不同坡向滨麦光合特性与风速异质环境的关系

在自然条件下,测定分析了夏季和秋季静风天及秋季大风天,海岸沙丘不同坡向环境因子及滨麦株高、叶片叶绿素含量、土壤含水量和光合日变化特性,以期明晰滨麦响应不同风速环境的生理调控机理。结果表明,不同坡向环境异质性明显,滨麦形态可塑性强。海岸迎风坡风大、温度低、湿度大,滨麦植株低矮、叶绿素含量较高;背风坡温度高、土壤干旱、空气流动差,滨麦植株高大,叶绿素含量低。在夏秋季静风天,迎风坡和背风坡滨麦叶片Pn出现光合"午休"现象,但迎风坡滨麦日均Gs、Tr、Pn均显著高于背风坡;而在秋季大风天,迎风坡和背风坡滨麦叶片Pn"午休"现象消失,并且背风坡滨麦日均Pn、Tr、Gs均显著高于迎风坡滨麦。同坡向相比,秋季大风天迎风坡滨麦日均Pn、Tr、Gs明显较秋季静风天低,而背风坡滨麦日均叶片Pn、Tr、Gs却较静风天分别增高126%、66.3%、134%。强海风吹袭引发迎风坡温度降低、滨麦叶片摇摆、气孔导度降低导致Pn下降,而强海风使背风坡空气流动加快、温度降低、气孔导度增大、"午休"消失使Pn增高。不同坡向滨麦对不同海风风速的适应表现出明显的光合生理可塑性,它在滨麦适应不同风力、提高其光合速率和增加物质积累上具有重要作用。而滨麦的形态和光合生理可塑性可能是其在不同海风强度下生存、生长、实现种群扩张的重要生理调控机理,这一特性在未来作物、牧草和树木抗风、抗盐育种中具有重要应用价值。     

Spatio‐temporal water dynamics in mature Banksia menziesii trees during drought

Southwest Australian Banksia woodlands are highly diverse plant communities that are threatened by drought‐ or temperature‐induced mortality due to the region's changing climate. We examined water relations in dominant Banksia menziesii R. Br. trees using magnetic leaf patch clamp pressure (ZIM‐) probes that allow continuous, real‐time monitoring of leaf water status. Multiple ZIM‐probes across the crown were complemented by traditional ecophysiological measurements. During summer, early stomatal downregulation of transpiration prevented midday balancing pressures from exceeding 2.5 MPa. Diurnal patterns of ZIM‐probe and pressure chamber readings agreed reasonably well, however, ZIM‐probes recorded short‐term dynamics, which are impossible to capture using a pressure chamber. Simultaneous recordings of three ZIM‐probes evenly spaced along leaf laminas revealed intrafoliar turgor gradients, which, however, did not develop in a strictly basi‐ or acropetal fashion and varied with cardinal direction. Drought stress manifested as increasing daily signal amplitude (low leaf water status) and occasionally as rising baseline at night (delayed rehydration). These symptoms occurred more often locally than across the entire crown. Microclimate effects on leaf water status were strongest in crown regions experiencing peak morning radiation (East and North). Extreme spring temperatures preceded the sudden death of B. menziesii trees, suggesting a temperature‐ or humidity‐related tipping point causing rapid hydraulic failure as evidenced by collapsing ZIM‐probe readings from an affected tree. In a warmer and drier future, increased frequency of B. menziesii mortality will result in significantly altered community structure and ecosystem function.     

Cultivation of Escherichia coli to high cell densities in a dialysis reactor

High cell density cultivation of Escherichia coli on a glycerol-based mineral medium was studied. The cultivation was done in a dialysis reactor composed of two chambers. The inner chamber is formed and separated from an outer chamber by a membrane. Fresh medium was continuously exchanged with medium in the outer chamber so that both glycerol and other components of the medium were supplied to the inner chamber through the membrane. Inhibitory substances diffused from the inner to the outer chamber and were subsequently removed with effluent from the outer chamber. Initially, mathematical models were used to describe the process. The optimal cultivation parameters, such as the initial glycerol concentrations in the two chambers, the desired transport rate across the membrane, glycerol concentration in the feed/dialysing medium, and the time to start the medium exchange, were determined from preliminary experiments and calculations. The actual cultivation results agreed very well with the model predictions. A very high cell concentration of 174 g dry weight/1 was obtained. This cell concentration is within the range of the maximum theoretical concentration of E. coli in culture broth (160–200 g/l).Dedicated to the 60th birthday of Prof. Dr. D. Vortmeyer Correspondence to: H. Märkl     

Abiotic factors affecting Diachasmimorpha longicaudata (Hymenoptera: Braconidae) activity as a natural enemy of Ceratitis capitata (Diptera: Tephritidae) under semi‐natural conditions in the Mediterranean region

The larval–pupal endoparasitoid Diachasmimorpha longicaudata (Hymenoptera: Braconidae) is currently the most commonly employed biological control agent against Tephritid fruit flies in the Americas. However, this parasitoid remains largely ignored and is not used in many regions, including the Mediterranean Basin. In this study, the potential of D. longicaudata as a biocontrol agent against the Mediterranean fruit fly (medfly) Ceratitis capitata (Diptera: Tephritidae) was addressed in an area of eastern Spain (the Valencian community). The parasitic activity of parasitoids and the effects of climatic conditions were evaluated throughout a 1‐year period in field‐cage experiments in which parasitoids were confined with apples artificially infested with medfly larvae. The following parameters were calculated and related statistically to several environmental conditions: the parasitism rate, the induced mortality and progeny sex ratio. The results show that D. longicaudata is able to parasitize medfly larvae throughout the year under semi‐natural conditions. Important fluctuations in the parasitism rate (from almost zero to 42%) and the induced mortality (from 6% to 80%) were partially influenced by climatic conditions. The parasitism rate increased with mean temperature and decreased with mean relative humidity, while the induced mortality decreased with minimum relative humidity. The optimal climatic conditions for the activity of the parasitoid were a mean temperature of 16–24°C combined with a relative humidity of 45%–60%. Overall, these results suggest that reduction in the medfly population due to D. longicaudata activity is feasible and provide information about the optimal time period for parasitoid release in the field. In conclusion, D. longicaudata has a significant potential to control C. capitata in the Mediterranean region.     

Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO(2) Exchange in Leaves of Welwitschia mirabilis

In well-watered plants of Welwitschia mirabilis, grown in the glass-house under high irradiance conditions, net CO₂ assimilation was almost exclusively observed during the daytime. The plants exhibited a very low potential for Crassulacean acid metabolism, which usually resulted in reduced rates of net CO₂ loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats, CO₂ assimilation rates in the light were markedly depressed under conditions resembling those occurring during midday, when leaf temperatures and the leaf-air vapor pressure differences were high (36°C and 50 millibars bar⁻¹, respectively). Studies on the relationship between CO₂ assimilation rate and intercellular CO₂ partial pressure at various temperatures and humidities showed that this decrease in CO₂ assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO₂ diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher ¹³C content of Welwitschia compared to the majority of C₃ species.     

Photosynthetic responses of Populus przewalski subjected to drought stress

Cuttings of P. przewalski were exposed to two different watering regimes which were watered to 100 and 25 % of field capacity (WW and WS, respectively). Drought stress not only significantly decreased net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), efficiency of photosystem 2 (PS2) (F_v/F_m and yield), and increased intrinsic water use efficiency (WUE_i) under controlled optimal conditions, but also altered the diurnal changes of gas exchange, chlorophyll fluorescence, and WUE_i. On the other hand, WS also affected the P _N-photosynthetically active radiation (PAR) response curve. Under drought stress, P _N peak appeared earlier (at about 10:30 of local time) than under WW condition (at about 12:30). At midday, there was a depression in P _N for WS plants, but not for WW plants, and it could be caused by the whole microclimate, especially high temperature, low relative humidity, and high PAR. There were stomatal and non-stomatal limitations to photosynthesis. Stomatal limitation dominated in the morning, and low P _N at midday was caused by both stomatal and non-stomatal limitations, whereas non-stomatal limitation dominated in the afternoon. In addition, drought stress also increased compensation irradiance and dark respiration rate, and decreased saturation irradiance and maximum net photosynthetic rate. Thus drought stress decreased plant assimilation and increased dissimilation through affected gas exchange, the diurnal pattern of gas exchange, and photosynthesis-PAR response curve, thereby reducing plant growth and productivity.     

Patterns of airborne conidia of Stemphylium vesicarium, the causal agent of brown spot disease of pears, in relation to weather conditions

Seven-day volumetric spore samplers were installed in pear orchards of northern Italy, in the years between 1993 and 2002, and operated continuously during the development of brown spot epidemics (mid-April–mid-August), caused by Stemphylium vesicarium. Aerial concentration of conidia was recorded at 2 h intervals to study their diurnal and seasonal patterns and the influence of weather conditions. The diurnal periodicity of aerial conidia showed a peak around midday and low counts in the dark. The increase in spore concentration was significantly correlated with the reduction of relative humidity and wetness in early morning, and the increase of wind in late morning and afternoon. Conidia of S. vesicarium became easily airborne to form a regular component of the air-spora in pear orchards, while ascospores were caught only sporadically. Differences between years concerned total spore counts and numbers of peaks (defined as days with more than 30 conidia/m³ air per day). Periods with highest spore counts occurred in late-May to early-June (in 2 years), mid to end of June (5 years), or after mid-July (3 years). There was a significant correlation between spore peaks and days with favourable weather conditions, defined as days with air temperature between 15 and 25°C and high humidity, particularly a wet period longer than 10 h. Occurrence of one or more consecutive days with favourable weather conditions determined an increase in the airborne concentration of conidia, which usually lasted some days and then decreased.     

Day-night changes in leaf water relations associated with the rhythm of crassulacean acid metabolism in Kalanchoë daigremontiana

A study was made of the day-night changes under controlled environmental conditions in the bulk-leaf water relations of Kalanchoë daigremontiana, a plant showing Crassulacean acid metabolism. In addition to nocturnal stomatal opening and net CO₂ uptake, the leaves of well-watered plants showed high rates of gas exchange during the whole of the second part of the light period. Measurements with the pressure chamber showed that xylem tension increased during the night and then decreased towards a minimum at about midday; a significant increase in xylem tension was also seen in the late afternoon. Cell-sap osmotic pressure paralleled leaf malate content and was maximum at dawn and minimum at dusk. The relationship between these two variables indicated that the nocturnally synthesized malate was apparently behaving as an ideal osmoticum. To estimate bulk-leaf turgor pressure, values for water potential were derived by correcting the pressurechamber readings for the osmotic pressure of the xylem sap. This itself was found to depend on the malate content of the leaves. Bulk-leaf turgor pressure changed rhythmically during the day-night cycle; turgor was low during the late afternoon and for most of the night, but increased quickly to a maximum of 0.20 MPa around midday. In water-stressed plants, where net CO₂ uptake was restricted to the dark period, there was also an increase in bulk-leaf turgor pressure at the start of the light period, but of reduced magnitude. Such changes in turgor pressure are likely to be of considerable ecological importance for the water economy of crassulacean-acid-metabolism plants growing in their natural habitats.Abbreviation and symbols CAM Crassulacean acid metabolism - P turgor pressure - osmotic pressure - water potential Dedicated to Professor Dr. H. Ziegler on the occasion of his 60th birthday