Photosynthetic Responses of a Temperate Liana to Xylella fastidiosa Infection and Water Stress

Xylella fastidiosa is a xylem‐limited bacterial plant pathogen that causes bacterial leaf scorch in its hosts. Our previous work showed that water stress enhances leaf scorch symptom severity and progression along the stem of a liana, Parthenocissus quinquefolia, infected by X. fastidiosa. This paper explores the photosynthetic gas exchange responses of P. quinquefolia, with the aim to elucidate mechanisms behind disease expression and its interaction with water stress. We used a 2 × 2‐complete factorial design, repeated over two growing seasons, with high and low soil moisture levels and infected and non‐infected plants. In both years, low soil moisture levels reduced leaf water potentials, net photosynthesis and stomatal conductance at all leaf positions, while X. fastidiosa‐infection reduced these parameters at basally located leaves only. Intercellular CO₂ concentrations were reduced in apical leaves, but increased at the most basal leaf location, implicating a non‐stomatal reduction of photosynthesis in leaves showing the greatest disease development. This result was supported by measured reductions in photosynthetic rates of basal leaves at high CO₂ concentrations, where stomatal limitation was eliminated. Repeated measurements over the summer of 2000 showed that the effects of water stress and infection were progressive over time, reaching their greatest extent in September. By reducing stomatal conductances at moderate levels of water stress, P. quinquefolia maintained relatively high leaf water potentials and delayed the onset of photosynthetic damage due to pathogen and drought‐induced water stress. In addition, chlorophyll fluorescence measurements showed that P. quinquefolia has an efficient means of dissipating excess light energy that protects the photosynthetic machinery of leaves from irreversible photoinhibitory damage that may occur during stress‐induced stomatal limitation of photosynthesis. However, severe stress induced by disease and drought eventually led to non‐stomatal decreases in photosynthesis associated with leaf senescence.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Differences in physiological adaptation of <Emphasis Type="Italic">Haberlea rhodopensis</Emphasis> Friv. leaves and roots during dehydration–rehydration cycle

The ecophysiological responses of the homoiochlorophyllous desiccation-tolerant (HDT) plant Haberlea rhodopensis showed that this plant could tolerate water deficit and both leaves and roots had high ability to survive severe desiccation. The changes and correlation between CO₂ assimilation, stomatal conductance, contents of photosynthetic pigments, root respiration and specific leaf area during dehydration–rehydration cycle were investigated. The physiological activity of leaves and roots were examined in fully hydrated (control) plants and during 72 h of dehydration, as well as following 96 h of rehydration every 6 and 24 h. After 6 h of dehydration, the stomatal conductance declined and the intercellular CO₂ concentration increased. The reduction in CO₂ assimilation rate was observed after 54 h of dehydration. There was a good correlation between the root respiration and water content. Our results showed that the plasticity of adaptation in leaves and roots were different during extreme water conditions. Roots were more sensitive and reacted faster to water stress than leaves, but their activity rapidly recovered due to immediate and efficient utilization of periodic water supply.     

Production of paramylon,a β‐1,3‐glucan,by heterotrophic cultivation of Euglena gracilis on potato liquor

Euglena gracilis is shown to be able to grow on potato liquor as the main medium component leading to an interesting biotechnological product represented by paramylon – a β‐1,3‐glucan – and, at the same time, revaluating an otherwise annoying waste stream of the potato‐starch industry. Paramylon mass fractions of about 75% are obtained for biomass concentrations of 15 g/L during simple batch cultivation under heterotrophic conditions. Supplementation of the growth medium with glucose and the vitamins B₁ and B₁₂ are shown to improve growth rate as well as paramylon content. E. gracilis grows best at about 27.5°C without requiring pH control.     

Host specific variation in photosynthesis of an obligate xylem‐tapping mistletoe Dendrophthoe curvata in a Bornean heath forest

Most mistletoe–host ecophysiological studies have paid attention to the influence of parasites on host performance. This paper explored the impact of varying hosts on the photosynthesis of a single mistletoe species. Here, we studied an obligate xylem‐tapping tropical mistletoe (Dendrophthoe curvata (Blume) Miquel) parasitizing four different hosts (Acacia auriculiformis A. Cunn. Ex Benth, Andira inermis (W. Wright) DC., Mangifera indica L. and Vitex pinnata L.) in a homo geneous tropical heath forest patch in Brunei Darussalam. We compared photosynthetic capacity and photosynthesis‐related characteristics of the mistletoe on four different hosts to evaluate the overall impact of hosts on the parasite. Results showed that the mistletoe–host patterns of CO₂ assimilation rates, transpiration rates and water use efficiency varied significantly based on the host. In the D. curvata–Vitex pinnata association, the mistletoe exhibited significantly lower CO₂ assimilation rates but showed no significant variations in transpiration rates and water use efficiency when compared to the host. In D. curvata–Andira inermis and D. curvata–Mangifera indica associations, the mistletoe showed significantly higher photosynthetic rates than the hosts, whereas in the D. curvata–Acacia auriculiformis association, there was no significant difference in photosynthetic rates between the counterparts. Host specificity also significantly influenced some mistletoe photosynthetic parameters such as light saturated photosynthesis, specific leaf area, leaf chlorophyll content, CO₂ assimilation rates, stomatal conductance, transpiration rates and water use efficiency. Different tree hosts intrinsically offer different resources to their obligate mistletoe parasites based on their physiology and environmental parameters. We argue that host‐specific responses have driven these intra‐specific variations in mistletoe physiology. This study provides background for future investigation on potential host‐regulated mechanisms that drive functional changes in host‐dependent mistletoes.     

Herbivore perception decreases photosynthetic carbon assimilation and reduces stomatal conductance by engaging 12‐oxo‐phytodienoic acid,mitogen‐activated protein kinase 4 and cytokinin perception

Herbivory‐induced changes in photosynthesis have been documented in many plant species; however, the complexity of photosynthetic regulation and analysis has thwarted progress in understanding the mechanism involved, particularly those elicited by herbivore‐specific elicitors. Here, we analysed the early photosynthetic gas exchange responses in Nicotiana attenuata plants after wounding and elicitation with Manduca sexta oral secretions and the pathways regulating these responses. Elicitation with M. sexta oral secretions rapidly decreased photosynthetic carbon assimilation (A_C) in treated and systemic (untreated, vascularly connected) leaves, which were associated with changes in stomatal conductance, rather than with changes in Rubisco activity and 1‐5 ribulose‐1,5‐bisphosphate turnover. Phytohormone profiling and gas exchange analysis of oral secretion‐elicited transgenic plants altered in phytohormone regulation, biosynthesis and perception, combined with micrografting techniques, revealed that the local photosynthetic responses were mediated by 12‐oxo‐phytodienoic acid, while the systemic responses involved interactions among jasmonates, cytokinins and abscisic acid signalling mediated by mitogen‐activated protein kinase 4. The analysis also revealed a role for cytokinins interacting with mitogen‐activated protein kinase 4 in CO₂‐mediated stomatal regulation. Hence, oral secretions, while eliciting jasmonic acid‐mediated defence responses, also elicit 12‐oxo‐phytodienoic acid‐mediated changes in stomatal conductance and A_C, an observation illustrating the complexity and economy of the signalling that regulates defence and carbon assimilation pathways in response to herbivore attack.     

Carbon dioxide enrichment alleviates heat stress by improving cellular redox homeostasis through an ABA‐independent process in tomato plants

Plant responses to elevated CO₂ and high temperature are critically regulated through a complex network of phytohormones and redox homeostasis. However, the involvement of abscisic acid (ABA) in plant adaptation to heat stress under elevated CO₂ conditions has not been thoroughly studied. This study investigated the interactive effects of elevated CO₂ (800 μmol·mol^?1) and heat stress (42 °C for 24 h) on the endogenous level of ABA and the cellular redox state of two genotypes of tomato with different ABA biosynthesis capacities. Heat stress significantly decreased maximum photochemical efficiency of PSII (Fv/Fm) and leaf water potential, but also increased levels of malondialdehyde (MDA) and electrolyte leakage (EL) in both genotypes. Heat‐induced damage was more severe in the ABA‐deficient mutant notabilis (not) than in its parental cultivar Ailsa Craig (Ailsa), suggesting that a certain level of endogenous ABA is required to minimise the heat‐induced oxidative damage to the photosynthetic apparatus. Irrespective of genotype, the enrichment of CO₂ remarkably stimulated Fv/Fm, MDA and EL in heat‐stressed plants towards enhanced tolerance. In addition, elevated CO₂ significantly strengthened the antioxidant capacity of heat‐stressed tomato seedlings towards a reduced cellular redox state for a prolonged period, thereby mitigating oxidative stress. However, elevated CO₂ and heat stress did not alter the endogenous level of ABA or the expression of its biosynthetic gene NCED2 in either genotype, indicating that ABA is not involved in elevated CO₂‐induced heat stress alleviation. The results of this study suggest that elevated CO₂ alleviated heat stress through efficient regulation of the cellular redox poise in an ABA‐independent manner in tomato plants.     

Bundle sheath extensions affect leaf structural and physiological plasticity in response to irradiance

Coordination between structural and physiological traits is key to plants' responses to environmental fluctuations. In heterobaric leaves, bundle sheath extensions (BSEs) increase photosynthetic performance (light‐saturated rates of photosynthesis, A_max) and water transport capacity (leaf hydraulic conductance, K_leaf). However, it is not clear how BSEs affect these and other leaf developmental and physiological parameters in response to environmental conditions. The obscuravenosa (obv) mutation, found in many commercial tomato varieties, leads to absence of BSEs. We examined structural and physiological traits of tomato heterobaric and homobaric (obv) near‐isogenic lines grown at two different irradiance levels. K_leaf, minor vein density, and stomatal pore area index decreased with shading in heterobaric but not in homobaric leaves, which show similarly lower values in both conditions. Homobaric plants, on the other hand, showed increased A_max, leaf intercellular air spaces, and mesophyll surface area exposed to intercellular airspace (S_mes) in comparison with heterobaric plants when both were grown in the shade. BSEs further affected carbon isotope discrimination, a proxy for long‐term water‐use efficiency. BSEs confer plasticity in traits related to leaf structure and function in response to irradiance levels and might act as a hub integrating leaf structure, photosynthetic function, and water supply and demand.     

Guard cell photosynthesis is critical for stomatal turgor production,yet does not directly mediate CO2‐ and ABA‐induced stomatal closing

Stomata mediate gas exchange between the inter‐cellular spaces of leaves and the atmosphere. CO₂ levels in leaves (Ci) are determined by respiration, photosynthesis, stomatal conductance and atmospheric [CO₂]. [CO₂] in leaves mediates stomatal movements. The role of guard cell photosynthesis in stomatal conductance responses is a matter of debate, and genetic approaches are needed. We have generated transgenic Arabidopsis plants that are chlorophyll‐deficient in guard cells only, expressing a constitutively active chlorophyllase in a guard cell specific enhancer trap line. Our data show that more than 90% of guard cells were chlorophyll‐deficient. Interestingly, approximately 45% of stomata had an unusual, previously not‐described, morphology of thin‐shaped chlorophyll‐less stomata. Nevertheless, stomatal size, stomatal index, plant morphology, and whole‐leaf photosynthetic parameters (PSII, qP, qN, F_V′/F_M′) were comparable with wild‐type plants. Time‐resolved intact leaf gas‐exchange analyses showed a reduction in stomatal conductance and CO₂‐assimilation rates of the transgenic plants. Normalization of CO₂ responses showed that stomata of transgenic plants respond to [CO₂] shifts. Detailed stomatal aperture measurements of normal kidney‐shaped stomata, which lack chlorophyll, showed stomatal closing responses to [CO₂] elevation and abscisic acid (ABA), while thin‐shaped stomata were continuously closed. Our present findings show that stomatal movement responses to [CO₂] and ABA are functional in guard cells that lack chlorophyll. These data suggest that guard cell CO₂ and ABA signal transduction are not directly modulated by guard cell photosynthesis/electron transport. Moreover, the finding that chlorophyll‐less stomata cause a ‘deflated’ thin‐shaped phenotype, suggests that photosynthesis in guard cells is critical for energization and guard cell turgor production.     

Elevated atmospheric CO2 alters stomatal responses to variable sunlight in a C4 grass

Native tallgrass prairie in NE Kansas was exposed to elevated (twice ambient) or ambient atmospheric CO₂ levels in open-top chambers. Within chambers or in adjacent unchambered plots, the dominant C₄ grass, Andropogon gerardii, was subjected to fluctuations in sunlight similar to that produced by clouds or within canopy shading (full sun > 1500 μmol m⁻² s⁻¹ versus 350 μmol m⁻² s⁻¹ shade) and responses in gas exchange were measured. These field experiments demonstrated that stomatal conductance in A. gerardii achieved new steady state levels more rapidly after abrupt changes in sunlight at elevated CO₂ when compared to plants at ambient CO₂. This was due primarily to the 50% reduction in stomatal conductance at elevated CO₂, but was also a result of more rapid stomatal responses. Time constants describing stomatal responses were significantly reduced (29–33%) at elevated CO₂. As a result, water loss was decreased by as much as 57% (6.5% due to more rapid stomatal responses). Concurrent increases in leaf xylem pressure potential during periods of sunlight variability provided additional evidence that more rapid stomatal responses at elevated CO₂ enhanced plant water status. CO₂-induced alterations in the kinetics of stomatal responses to variable sunlight will likely enhance direct effects of elevated CO₂ on plant water relations in all ecosystems.     

Potassium mediates coordination of leaf photosynthesis and hydraulic conductance by modifications of leaf anatomy

Typical symptoms of potassium deficiency, characterized as chlorosis or withered necrosis, occur concomitantly with downregulated photosynthesis and impaired leaf water transport. However, the prominent limitations and mechanisms underlying the concerted decreases of leaf photosynthesis and hydraulic conductance are poorly understood. Monocots and dicots were investigated based on responses of photosynthesis and hydraulic conductance and their components and the correlated anatomical determinants to potassium deficiency. We found a conserved pattern in which leaf photosynthesis and hydraulic conductance concurrently decreased under potassium starvation. However, monocots and dicots showed two different hydraulic‐redesign strategies: Dicots tended to show a decreased minor vein density, whereas monocots reduced the size of the bundle sheath and its extensions, rather than the minor vein density; both of these strategies may restrain xylem and outside‐xylem hydraulic conductance. Additionally, potassium‐deprived leaves developed with fewer mesophyll cell‐to‐cell connections, leading to a reduced area being available for liquid‐phase flow. Further quantitative analysis revealed that mesophyll conductance to CO₂ and outside‐xylem hydraulic resistance were the major contributors to photosynthetic limitation and increased hydraulic resistance, at more than 50% and 60%, respectively. These results emphasize the importance of potassium in the coordinated regulation of leaf photosynthesis and hydraulic conductance through modifications of leaf anatomy.     

Elevated [CO2] alleviates the impacts of water deficit on xylem anatomy and hydraulic properties of maize stems

Plants can modify xylem anatomy and hydraulic properties to adjust to water status. Elevated [CO₂] can increase plant water potential via reduced stomatal conductance and water loss. This raises the question of whether elevated [CO₂], which thus improves plant water status, will reduce the impacts of soil water deficit on xylem anatomy and hydraulic properties of plants. To analyse the impacts of water and [CO₂] on maize stem xylem anatomy and hydraulic properties, we exposed potted maize plants to varying [CO₂] levels (400, 700, 900, and 1,200 ppm) and water levels (full irrigation and deficit irrigation). Results showed that at current [CO₂], vessel diameter, vessel roundness, stem cross-section area, specific hydraulic conductivity, and vulnerability to embolism decreased under deficit irrigation; yet, these impacts of deficit irrigation were reduced at elevated [CO₂]. Across all treatments, midday stem water potential was tightly correlated with xylem traits and displayed similar responses. A distinct trade-off between efficiency and safety in stem xylem water transportation in response to water deficit was observed at current [CO₂] but not observed at elevated [CO₂]. The results of this study enhance our knowledge of plant hydraulic acclimation under future climate environments and provide insights into trade-offs in xylem structure and function.     

Neither xylem collapse,cavitation, or changing leaf conductance drive stomatal closure in wheat

Identifying the drivers of stomatal closure and leaf damage during stress in grasses is a critical prerequisite for understanding crop resilience. Here, we investigated whether changes in stomatal conductance (g_s) during dehydration were associated with changes in leaf hydraulic conductance (K_leaf), xylem cavitation, xylem collapse, and leaf cell turgor in wheat (Triticum aestivum). During soil dehydration, the decline of g_s was concomitant with declining K_leaf under mild water stress. This early decline of leaf hydraulic conductance was not driven by cavitation, as the first cavitation events in leaf and stem were detected well after K_leaf had declined. Xylem vessel deformation could only account for <5% of the observed decline in leaf hydraulic conductance during dehydration. Thus, we concluded that changes in the hydraulic conductance of tissues outside the xylem were responsible for the majority of K_leaf decline during leaf dehydration in wheat. However, the contribution of leaf resistance to whole plant resistance was less than other tissues (<35% of whole plant resistance), and this proportion remained constant as plants dehydrated, indicating that K_leaf decline during water stress was not a major driver of stomatal closure.     

Differences in gas exchange contribute to habitat differentiation in Iberian columbines from contrasting light and water environments

During photosynthesis, respiration and transpiration, gas exchange occurs via the stomata and so plants face a trade‐off between maximising photosynthesis while minimising transpiration (expressed as water use efficiency, WUE). The ability to cope with this trade‐off and regulate photosynthetic rate and stomatal conductance may be related to niche differentiation between closely related species. The present study explored this as a possible mechanism for habitat differentiation in Iberian columbines. The roles of irradiance and water stress were assessed to determine niche differentiation among Iberian columbines via distinct gas exchange processes. Photosynthesis–irradiance curves (P–I curves) were obtained for four taxa, and common garden experiments were conducted to examine plant responses to water and irradiance stress, by measuring instantaneous gas exchange and plant performance. Gas exchange was also measured in ten individuals using two to four field populations per taxon. The taxa had different P–I curves and gas exchange in the field. At the species level, water stress and irradiance explained habitat differentiation. Within each species, a combination of irradiance and water stress explained the between‐subspecies habitat differentiation. Despite differences in stomatal conductance and CO₂ assimilation, taxa did not have different WUE under field conditions, which suggests that the environment equally modifies photosynthesis and transpiration. The P–I curves, gas exchange in the field and plant responses to experimental water and irradiance stresses support the hypothesis that habitat differentiation is associated with differences among taxa in tolerance to abiotic stress mediated by distinct gas exchange responses.     

The influence of CO2 enrichment, phosphorus deficiency and water stress on the growth, conductance and water use of Pinus radiata D. Don

Abstract. Seedlings of Pinus radiata D. Don were grown in growth chambers for 22 weeks with two levels of phosphorus, under either well-watered or water-stressed conditions at CO₂ concentrations of either 330 or 660mm³ dm^?3. Plant growth, water use efficiency and conductance were measured and the relationship between these and needle photosynthetic capacity, water use efficiency and conductance was determined by gas exchange at week 22. Phosphorus deficiency decreased growth and foliar surface area at both CO₂concentrations; however, it only reduced the maximum photosynthetic rates of the needles at 660 mm³ CO₂ dm^?3 (plants grown and measured at the same CO₂ concentration). Water stress reduced growth and foliar surface area at both CO₂ concentrations. Increases in needle photosynthetic rates appeared to be partly responsible for the increased growth at high CO₂ where phosphorus was adequate. This effect was amplified by accompanying increases in needle production. Phosphorus deficiency inhibited these responses because it severely impaired needle photosynthetic function. The relative increase in growth in response to high CO₂ was higher in the periodically water-stressed plants. This was not due to the maintenance of cell volume during drought. Plant water use efficiency was increased by CO₂ enrichment due to an increase in dry weight rather than a decrease in shoot conductance and, therefore, transpirational water loss. Changes in needle conductance and water use efficiency in response to high CO₂ were generally in the same direction as those at the whole plant level. If the atmospheric CO₂ level reaches the predicted concentration of 660 mm³ dm^?3 by the end of next Century, then the growth of P. radiata will only be increased in areas where phosphorus nutrition is adequate. Growth will be increased in drought-affected regions but total water use is unlikely to be reduced.     

Contemporary evolution of an invasive grass in response to elevated atmospheric CO2 at a Mojave Desert FACE site

Elevated atmospheric CO₂ has been shown to rapidly alter plant physiology and ecosystem productivity, but contemporary evolutionary responses to increased CO₂ have yet to be demonstrated in the field. At a Mojave Desert FACE (free‐air CO₂ enrichment) facility, we tested whether an annual grass weed (Bromus madritensis ssp. rubens) has evolved in response to elevated atmospheric CO₂. Within 7 years, field populations exposed to elevated CO₂ evolved lower rates of leaf stomatal conductance; a physiological adaptation known to conserve water in other desert or water‐limited ecosystems. Evolution of lower conductance was accompanied by reduced plasticity in upregulating conductance when CO₂ was more limiting; this reduction in conductance plasticity suggests that genetic assimilation may be ongoing. Reproductive fitness costs associated with this reduction in phenotypic plasticity were demonstrated under ambient levels of CO₂. Our findings suggest that contemporary evolution may facilitate this invasive species' spread in this desert ecosystem.     

Stomatal function,density and pattern,and CO2 assimilation in Arabidopsis thaliana tmm1 and sdd1‐1 mutants

• Stomata modulate the exchange of water and CO₂ between plant and atmosphere. Although stomatal density is known to affect CO₂ diffusion into the leaf and thus photosynthetic rate, the effect of stomatal density and patterning on CO₂ assimilation is not fully understood.
• We used wild types Col‐0 and C24 and stomatal mutants sdd1‐1 and tmm1 of Arabidopsis thaliana, differing in stomatal density and pattern, to study the effects of these variations on both stomatal and mesophyll conductance and CO₂ assimilation rate. Anatomical parameters of stomata, leaf temperature and carbon isotope discrimination were also assessed.
• Our results indicate that increased stomatal density enhanced stomatal conductance in sdd1‐1 plants, with no effect on photosynthesis, due to both unchanged photosynthetic capacity and decreased mesophyll conductance. Clustering (abnormal patterning formed by clusters of two or more stomata) and a highly unequal distribution of stomata between the adaxial and abaxial leaf sides in tmm1 mutants also had no effect on photosynthesis.
• Except at very high stomatal densities, stomatal conductance and water loss were proportional to stomatal density. Stomatal formation in clusters reduced stomatal dynamics and their operational range as well as the efficiency of CO₂ transport.

     

Rapid hydraulic recovery in Eucalyptus pauciflora after drought: linkages between stem hydraulics and leaf gas exchange

In woody plants, photosynthetic capacity is closely linked to rates at which the plant hydraulic system can supply water to the leaf surface. Drought‐induced embolism can cause sharp declines in xylem hydraulic conductivity that coincide with stomatal closure and reduced photosynthesis. Recovery of photosynthetic capacity after drought is dependent on restored xylem function, although few data exist to elucidate this coordination. We examined the dynamics of leaf gas exchange and xylem function in Eucalyptus pauciflora seedlings exposed to a cycle of severe water stress and recovery after re‐watering. Stomatal closure and leaf turgor loss occurred at water potentials that delayed the extensive spread of embolism through the stem xylem. Stem hydraulic conductance recovered to control levels within 6 h after re‐watering despite a severe drought treatment, suggesting an active mechanism embolism repair. However, stomatal conductance did not recover after 10 d of re‐watering, effecting tighter control of transpiration post drought. The dynamics of recovery suggest that a combination of hydraulic and non‐hydraulic factors influenced stomatal behaviour post drought.     

Stress resistance induced by paramylon treatment in Artemia sp.

A study was undertaken to investigate the effectiveness of paramylon extracted from the non-photosynthetic WZSL mutant of Euglena gracilis in potentiating the resistance of the brine shrimp Artemia sp. to stress conditions resulting from poor growth medium quality and daily handling. The experimental design incorporated four different treatments: I) paramylon addition/no growth medium replacement; II) no paramylon addition/no growth medium replacement; III) paramylon addition/growth medium replacement; IV) noparamylon addition/growth medium replacement. As shown by the survival curves of Artemia sp., the addition of paramylon significantly reduced susceptibility of animals to the stressors. Paramylon effectively increased also the ability of offspring to withstand poor water quality and handling damages. Without paramylon almost all offspring died before adulthood, whereas addition of paramylon allowed the animals to moult to the adult stage. Moreover, reproductive success (higher number of living offspring) was enhanced considerably in animals treated with paramylon treated under both stress conditions. These results show that paramylon extracted from Euglena represents a promising biologically active compound for aquaculture purposes. It could be used as a purified product or as component of whole cells, since the Euglena mutant, because of the high intracellular amount of paramylon it can accumulate, could be added to the feed or to water in tanks and ponds without prior processing. This revised version was published online in August 2006 with corrections to the Cover Date.     

Elevated [CO2] does not ameliorate the negative effects of elevated temperature on drought‐induced mortality in Eucalyptus radiata seedlings

It has been reported that elevated temperature accelerates the time‐to‐mortality in plants exposed to prolonged drought, while elevated [CO₂] acts as a mitigating factor because it can reduce stomatal conductance and thereby reduce water loss. We examined the interactive effects of elevated [CO₂] and temperature on the inter‐dependent carbon and hydraulic characteristics associated with drought‐induced mortality in Eucalyptus radiata seedlings grown in two [CO₂] (400 and 640 μL L^?1) and two temperature (ambient and ambient +4 °C) treatments. Seedlings were exposed to two controlled drying and rewatering cycles, and then water was withheld until plants died. The extent of xylem cavitation was assessed as loss of stem hydraulic conductivity. Elevated temperature triggered more rapid mortality than ambient temperature through hydraulic failure, and was associated with larger water use, increased drought sensitivities of gas exchange traits and earlier occurrence of xylem cavitation. Elevated [CO₂] had a negligible effect on seedling response to drought, and did not ameliorate the negative effects of elevated temperature on drought. Our findings suggest that elevated temperature and consequent higher vapour pressure deficit, but not elevated [CO₂], may be the primary contributors to drought‐induced seedling mortality under future climates.