Dirunal leaf movement, chlorophyll fluorescence and carbon assimilation in soybean grown under different nitrogen and water availabilities

Diurnal heliotropic leaf movements, photosynthetic gas exchange, and the ratio of variable fluorescence to maximum fluorescence (Fv/Fm) of unrestrained and horizontally restrained leaves from soybean (Glycine max cv. Cumberland) plants grown in two different water and two different nitrogen treatments were measured. Leaves of plants grown in low water or low nitrogen availability treatments displayed more pronounced diaheliotropism (solar tracking) in the afternoon and a longer period of paraheliotropism (light avoiding) at midday relative to those of well-watered, high-nitrogen-grown plants. Photosaturated photosynthetic rates and the photon flux required to saturate photosynthesis were reduced by water stress and nitrogen deficiency. Compared to horizontal leaves, irradiance on orienting leaves was nearer to the breakpoint of the photosynthetic light response curve, where photosynthesis is co-limited by ribulose biphosphate regeneration and carboxylation. This would increase the carbon return on investments of nitrogen into photosynthesis. A positive linear relationship between Fv/Fm and quantum yield of photosynthesis was measured. Leaves of low-nitrogen-grown plants had earlier and more prolonged reductions in Fv/Fm at midday compared to leaves of high nitrogen grown plants of the same water treatment. Within the same water and nitrogen treatment, horizontally restrained leaves had lower midday Fv/Fm in relation to orienting leaves. Nitrogen deficiency and water stress enhanced this difference such that horizontally restrained leaves of low water and low nitrogen grown plants had earlier and longer midday depressions in Fv/Fm.     

Physiological behaviour, oxidative damage and antioxidative protection of olive trees grown under different irrigation regimes

Irrigation effects were investigated on an 8-year-old olive (Olea europaea L., cv. Cobrançosa) commercial orchard located in northeast Portugal. Trees were subjected to a rainfed control (T0) and three treatments (T1, T2, T3) that received a seasonal water amount equivalent to 30%, 60% and 100% of the estimated local evaporative demand by a drip irrigation system. Irrigation increases the photosynthetic activity of olive trees, in association with increases in water status, and reduces the midday and afternoon depression in gas exchange. The closely association between photosynthetic rate (A) and stomatal conductance (g _s) revealed that the decline in net photosynthesis over the course of the day was largely a consequence of stomatal limitation. However, the ratio of intercellular to atmospheric CO₂ concentration increased markedly from morning to midday in non-irrigated plants, in spite of lower g _s, suggesting that non-stomatal limitations of photosynthesis also occur when environmental conditions become more stressful. The occurrence of perturbations at chloroplastic level in rainfed plants was demonstrated by a lower maximum photochemical efficiency of photosystem II during the afternoon. Chlorophyll fluorescence measurements also revealed the occurrence of a dynamic photoinhibition in irrigated trees, mainly in T2 and T3, which seemed to be effective in protecting the photosynthetic apparatus from photodamage. Irrigation enhances antioxidant protection and decreases the oxidative damage at leaf level. Leaves grown under rainfed conditions revealed symptoms of oxidative stress, like the reduction (14%) in chlorophyll concentration and the increased levels (57%) of lipid peroxidation. We also found that the scavenging function of superoxide dismutase was impaired in rainfed plants. In contrast, the low thiobarbituric acid reactive substances concentration in T3 indicates that irrigation enhances the repairing mechanisms and decreases the oxidative damage by lipid peroxidation. Accordingly, leaves in T3 treatment had high levels of –SH compounds and the highest antioxidant potential. Meanwhile, the finding that guaiacol peroxidase activity increased in rainfed plants, associated with the appearance of oxidative damage, suggests that this enzyme has no major antioxidative function in olive.     

Afternoon Depression In Photosynthesis in Grapevine Leaves--Evidence for a High Light Stress Effect

A midday depression in net photosynthesis and in stomatal conductancewas observed when leaves of well-watered Vitis vinifera plantswere subjected to a diurnal pattern of variation in leaf temperatureand leaf-to-air water vapour pressure difference similar toa summer day, while photon flux density was kept constant at1450 µmolm^–2 s^–1,. When leaves were kept atconstant leaf temperature (22.5°C) and leaf-to-air watervapour presure difference (8.5 Pa kPa^–1) at the same lightintensity, stomata opened with the onset of illumination andmaximal conductance and photosynthesis values were observedabout 1 h later. Subsequently, conductance and photosynthesisdecreased gradually. Leaf water potential never dropped below{macron}0.3 MPa. Leaves kept under constant environmental conditionsshowed an afternoon decline in photosynthesis at high internalCO₂, in carboxylation efficiency and in maximum conductanceas well as an increase in stomatal sensitivity to CO₂. Whenthe photon flux density during the day was reduced to 750 µmolm^–2 s^–1, the afternoon depression in gas exchangerates was attenuated. To evaluate the possible effects of highlight stress on changes in chloroplastic behaviour we comparedlight response curves of photosynthesis determined with an oxygenelectrode, in the morning and in the afternoon after the plantswere exposed to either high or moderate photon flux densities.A significant depression in photosynthetic capacity was foundby this method in high light treated leaves, but not in leavespreviously exposed to moderate photon flux density. Apparentquantum yield decreased in the afternoon, particularly afterexposure to high light. Maximum chlorophyll a fluorescence at22°C was reduced and the quenching of fluorescence afterreaching the peak was slower in the afternoon than in the morning,especially in high light-treated leaves. Changes in the patternsof chlorophyll fluorescence kinetics were observed after lighttreatment, i.e. in the afternoon, with oscillations either absent(after high light) or significantly reduced (after moderatelight) in comparison to the morning. The significance of theseresults is discussed and it is suggested that a direct inhibitoryeffect of high light at the chloroplast level provides the bestinterpretation for the observed afternoon decline in photosyntheticrate. Key words: Carboxylation efficiency, chlorophyll fluorescence, photosynthetic capacity, quantum yield, stomatal conductance, Vitis vinifera L.     

Photosynthetic performance and light response of two olive cultivars under different water and light regimes

The olive tree (Olea europaea L.) is commonly grown in the Mediterranean area, where it is adapted to resist periods characterized by severe drought and high irradiance levels. Photosynthetic efficiency (in terms of F_v/F_m and Φ_PSII), photochemical (q_P) and nonphotochemical quenching (NPQ) were determined in two-year-old olive plants (cultivars Coratina and Biancolilla) grown under two different light levels (exposed plants, EP, and shaded plants, SP) during a 21-day controlled water deficit. After reaching the maximum level of drought stress, plants were rewatered for 23 days. During the experimental period, measurements of gas exchange and chlorophyll (Chl) fluorescence were carried out to study the photosynthetic performance of olive plants. The synergical effect of drought stress and high irradiance levels caused a reduction of gas exchange and photosynthetic efficiency and these decreases were more marked in EP. EP showed a higher degree of photoinhibition, a higher NPQ and a lower q_P if compared to SP. Coratina was more sensitive to high light and drought stress but also showed a slower recovery during rewatering, whereas Biancolilla showed a less marked photosynthesis depression during drought and a considerable resilience during rewatering. The results confirm that photoinhibition due to high light intensity and water deficit can be an important factor that affects photosynthetic productivity in this species.     

Epicuticular Phenolics Over Guard Cells: Exploitation for in situ Stomatal Counting by Fluorescence Microscopy and Combined Image Analysis

Guard cells emit an alkali-induced, blue fluorescence upon excitationby ultraviolet radiation (emission maximum energy at 365 nm).Fluorescence emission of guard cells was brighter than thatof the neighbouring epidermal cells in a number of wild andcultivated plants including conifers, but the relative fluorescenceintensity and quality was species-dependent. Three representativeplants possessing stomatal complexes which differed morphologicallywere studied: Olea europaea, Vicia faba and Triticum aestivum.Immersing leaves of these plants in chloroform for 30 s (therebyremoving epicuticular waxes) significantly reduced the intensityof the fluorescence emitted by guard cells. This indicates thatguard cell fluorescence could be due to either an increasedconcentration of fluorescing compounds (probably wax-bound phenolics),or a thicker cuticular layer covering the guard cells. Giventhat the alkali-induced blue fluorescence of the guard cellsis a common characteristic of all plants examined, it couldbe used as a rapid and convenient method for in situ measurementsof the number, distribution and size of stomatal complexes.The proposed experimental procedure includes a single coatingof the leaf surface by, or immersion of the whole leaf in, a10% solution of KOH for 2 min, washing with distilled water,and direct observation of the leaf surface under the fluorescencemicroscope. Fluorescence images were suitable for digital imageanalysis and methodology was developed for stomatal countingusing Olea europaea as a model species. Copyright 2001 Annalsof Botany Company Cuticle, epicuticular waxes, fluorescence microscopy, image analysis, phenolics, stomata     

Physiological and anatomical changes induced by drought in two olive cultivars (cv Zalmati and Chemlali)

Photosynthetic gas exchange, vegetative growth, water relations and fluorescence parameters as well as leaf anatomical characteristics were investigated on young plants of two Olea europaea L. cultivars (Chemlali and Zalmati), submitted to contrasting water availability regimes. Two-year-old olive trees, grown in pots in greenhouse, were not watered for 2 months. Relative growth rate (RGR), leaf water potential (Ψ_LW) and the leaf relative water content (LWC) of the two cultivars decreased with increasing water stress. Zalmati showed higher values of RGR and LWC and lower decreased values of Ψ_LW than Chemlali, in response to water deficit, particularly during severe drought stress. Water stress also caused a marked decline on photosynthetic capacity and chlorophyll fluorescence. The net photosynthetic rate, stomatal conductance, transpiration rate, the maximal photochemical efficiency of PSII (F _v/F _m) and the intrinsic efficiency of open PSII reaction centres (F′ _v/F′ _m) decreased as drought stress developed. In addition, drought conditions, reduced leaf chlorophyll and carotenoids contents especially at severe water stress. However, Zalmati plants were the less affected when compared with Chemlali. In both cultivars, stomatal control was the major factor affecting photosynthesis under moderate drought stress. At severe drought-stress levels, the non-stomatal component of photosynthesis is inhibited and inactivation of the photosystem II occurs. Leaf anatomical parameters show that drought stress resulted in an increase of the upper epidermis and palisade mesophyll thickness as well as an increase of the stomata and trichomes density. These changes were more characteristic in cv. ‘Zalmati’. Zalmati leaves also revealed lower specific leaf area and had higher density of foliar tissue. From the behaviour of Zalmati plants, with a smaller reduction in relative growth rate, net assimilation rate and chlorophyll fluorescence parameters, and with a thicker palisade parenchyma, and a higher stomatal and trichome density, we consider this cultivar more drought-tolerant than cv. Chemlali and therefore, very promising for cultivation in arid areas.     

Scanning Electron Microscopy Analysis of Exine Patterns in Cultivars of Olive (Olea europaea L.)

Scanning electron microscope analysis of exine patterns in thepollen of nine cultivars of the olive tree (Olea europaea L.)has revealed specific differences between them. The differencesinclude variation in size, form and profile of the meshes andin the thickness of the muri. Every cultivar possesses its ownmorphological characteristics which are constant and are foundin successive years. Olea europaea L, olive, pollen, exine patterns     

On the mechanism of salt tolerance in olive (Olea europaea L.) under low- or high-Ca2+ supply

The effect of changes in Ca²⁺/Na⁺ ratios at the root zone has been reported in Olea europaea, a species mostly cultivated in calcareous soils. Plants were exposed to low (2.0 mM, low-Ca) or high-Ca²⁺ supply (9.0 mM, high-Ca) and supplied with 0 or 200 mM NaCl. Measurements were performed on water relations, gas exchange and photosynthetic performances, ion fluxes at whole-plant and leaf level, Na⁺ allocation at organismal level, the elemental and soluble carbohydrate concentration in the leaf. Most parameters were also measured during a period of relief from salinity stress, as Olea europaea suffers from fluctuating root zone NaCl concentrations over the whole growing season. High-Ca²⁺ supply decreased stomatal conductance, especially during the first two weeks of treatment. In response to salinity stress (i) leaf turgor potential was more severely depressed in high-Ca than in low-Ca plants, whereas net CO₂ assimilation rate and relative growth rate were unaffected by root zone Ca²⁺ concentrations (ii) high-Ca plants had a markedly superior ability to both exclude Na⁺ from the shoot and to selectively transport K⁺ over Na⁺ than low-Ca plants; (iii) both CO₂ carboxylation efficiency and maximal efficiency of PSII photochemistry (F_v/F_m) were significantly smaller in low-Ca than in high-Ca plants, likely as a result of a greater accumulation of toxic ions. Consistently, when osmotic stress was relieved by supplying plants with good quality water (relief period), both photosynthetic (+44%) and growth rates (+65%) recovered to a markedly superior degree in high-Ca than in low-Ca plants which had been previously treated with 200 mM NaCl. We conclude that (1) high-Ca²⁺ supply expose olive leaves to a more severe dehydration, but allowed to restrict both the entry and the allocation of potentially toxic ions to sensitive shoot organs; (2) a transient restriction of water-mass flow to the shoot during salinization may be of relatively minor significance in Olea europaea, which is very tolerant to drought; (3) overall salt tolerance in Olea europaea, as in most evergreen sclerophylls inhabiting Mediterranean areas, tightly depends upon the ability to reduce water uptake and transpiration during the dry/warm period and to recover photosynthetic and growth rates when low-salinity flood water is available. Therefore, data from the present experiment allow conclude that an increase in root zone Ca²⁺ concentration enhances tolerance to salinity stress in olive plants.     

Physiological responses of different olive genotypes to drought conditions

Gas exchange rates, chlorophyll fluorescence, pressure–volume relationships, photosynthetic pigments, total soluble sugars, starch, soluble proteins and proline concentrations were investigated in five Olea europaea L. cultivars with different geographical origins (Arbequina, Blanqueta, Cobran?osa, Manzanilla and Negrinha) grown under Mediterranean field conditions. We found considerable genotypic differences among the cultivars. Comparing the diurnal gas exchange rates, we observed that Cobran?osa, Manzanilla and Negrinha had high photosynthetic rate than Arbequina and Blanqueta. The first group reveals to be better acclimated to drought conditions, and appears to employ a prodigal water-use strategy, whereas Blanqueta and Arbequina, with high water-use efficiency, appear to employ a conservative water-use strategy. The degree of midday depression in photosynthesis was genotype dependent, with a maximum in Arbequina and a minimum in Negrinha. The reductions in the photosynthetic rate were dependent from both stomatal and non-stomatal limitations. Elastic adjustment plays an important role as drought tolerance mechanism. The group of cultivars that employ a prodigal water-use strategy revealed high tissue elasticity, whereas Arbequina and Blanqueta revealed high tissue rigidity. We also identified the existence of drought tolerance mechanisms associated with soluble proteins accumulation in the foliage. The high levels of soluble proteins in Arbequina may represent an increased activity of oxidative stress defence enzymes and may also represent a reserve for post stress recovery. In all cultivars, especially in Manzanilla, free proline was accumulated in the foliage. The discussed aspects of drought stress metabolism may have an adaptative meaning, supporting the hypothesis that olive cultivars native to dry regions, such as Cobran?osa, Manzanilla and Negrinha, have more capability to acclimate to drought conditions than cultivars originated in regions with a more temperate climate, like Arbequina and Blanqueta.     

Leaf gas exchange,water relations,nutrient content and growth in citrus and olive seedlings under salinity

The effects of salinity on growth, leaf nutrient content, water relations, gas exchange parameters and chlorophyll fluorescence were studied in six-month-old seedlings of citrus (Citrus limonia Osbeck) and rooted cuttings of olive (Olea europaea L. cv. Arbequina). Citrus and olive were grown in a greenhouse and watered with half strength Hoagland’s solution plus 0 or 50 mM NaCl for citrus, or plus 0 or 100 mM NaCl for olive. Salinity increased Cl⁻ and Na⁺ content in leaves and roots in both species and reduced total plant dry mass, net photosynthetic rate and stomatal conductance. Decreased growth and gas exchange was apparently due to a toxic effect of Cl⁻ and/or Na⁺ and not due to osmotic stress since both species were able to osmotically adjust to maintain pressure potential higher than in non-salinized leaves. Internal CO₂ concentration in the mesophyll was not reduced in either species. Salinity decreased leaf chlorophyll a content only in citrus.     

Limitations to photosynthesis of lettuce grown under tropical conditions: alleviation by root-zone cooling.

Aerial parts of lettuce plants were grown under natural tropical fluctuating ambient temperatures, but with their roots exposed to two different root-zone temperatures (RZTs): a constant 20 degrees C-RZT and a fluctuating ambient (A-) RZT from 23-40 degrees C. Plants grown at A-RZT showed lower photosynthetic CO2 assimilation (A), stomatal conductance (gs), midday leaf relative water content (RWC), and chlorophyll fluorescence ratio Fv/Fm than 20 degrees C-RZT plants on both sunny and cloudy days. Substantial midday depression of A and g(s) occurred on both sunny and cloudy days in both RZT treatments, although Fv/Fm did not vary diurnally on cloudy days. Reciprocal temperature transfer experiments investigated the occurrence and possible causes of stomatal and non-stomatal limitations of photosynthesis. For both temperature transfers, light-saturated stomatal conductance (gs sat) and photosynthetic CO2 assimilation (A(sat)) were highly correlated with each other and with midday RWC, suggesting that A was limited by water stress-mediated stomatal closure. However, prolonged growth at A-RZT reduced light- and CO2-saturated photosynthetic O2 evolution (Pmax), indicating non-stomatal limitation of photosynthesis. Tight temporal coupling of leaf nitrogen content and P(max) during both temperature transfers suggested that decreased nutrient status caused this non-stomatal limitation of photosynthesis.     

Photoinhibition of Photosynthesis Without Net Loss of D1 Protein in Wheat Leaves Under Field Conditions

To determine whether the net loss of D1 protein is the main cause of photoinhibition of photosynthesis in wheat leaves under field conditions in the absence of any environmental stress other than strong sunlight, the D1 protein content, photosynthetic evolution of oxygen and chlorophyll a fluorescence parameters were measured in field grown wheat leaves. After exposure to midday strong light for about 3 h, apparent photosynthetic quantum efficiency (Φ), Fv/Fm and Fo in wheat leaves declined, and these parameters recovered almost completely 1 h after transfer to the weak light of 30~40 ttmol photons · m-2 · s-1. No evident change in the D1 protein content was observed in the leaves after exposure to midday strong light for 3 h. After 3 hours exposure to strong light, the slow-relaxed fluorescence quenching in the leaves treated with streptomycin (SM) increased much more than that in the control leaves, but there was no effect SM on the recovery of Fv/Fm and F0; dithiothretol (DTT) treatment enhanced photoinhibition of photosynthesis and reduced the D1 protein content in the leaves after exposure to midday strong light. These results indicated that under field conditions with no environmental stress other than strong sunlight, photoinhibition of photosynthesis in wheat leaves was not due to the net loss of D1 protein, and it could be attributed mainly by the increased nonradiative energy dissipation.     

Tropic leaf movements, photosynthetic gas exchange, leaf δ13C and chlorophyll a fluorescence of three soybean species in response to water availability

Midday leaf angle, photosynthetic gas exchange, stable carbon isotope ratio ( δ ¹³C), and chlorophyll a fluorescence among three wild soybeans, Glycine soja, G . tomentella and G . tabacina, from habitats with different water availability were examined. Plants grown under low water availability had reduced leaf area, photosynthetic and electron transport rates, more positive δ ¹³C values, and more vertical midday leaf angles. The three species differed in midday leaflet orientation, leaf size, photosynthesis and fluorescence responses to water availability. The species differences were consistent with the water availability of their habitat. G . soja , which grows in the wettest habitats, was shown to be the most photosynthetically susceptible to low water treatment and tended to have the most vertical midday leaflet angles. In contrast, G . tabacina , distributed in the driest habitats, had the least vertical midday leaflet angles and the lowest photosynthetic sensitivity to low water availability. G . tomentella , inhabiting an intermediate habitat, had intermediate midday leaf angles and photosynthetic responses. Our results support the hypothesis that paraheliotropic leaf movements respond in concert with photosynthetic characteristics in soybean leaves such that water use efficiency is enhanced and the risk of photoinhibition under water stress conditions is reduced.     

Limitations due to water stress on leaf net photosynthesis of Quercus coccifera in the Portuguese evergreen scrub

Summary Gas exchange characteristics in leaves of the sclerophyll shrub Quercus coccifera were studied in the natural habitat in Portugal during spring and during the summer dry period. Compared to other sclerophyll species growing at the same site, photosynthesis in leaves of Quercus coccifera was less affected by water stress. Moderate water stress after six weeks of drought led to large decreases in stomatal conductance but no change in mesophyll photosynthetic capacity as compared to late spring. Leaf internal CO₂ pressure remained near 220 bar during diurnal courses in the spring. On midsummer days, leaf internal CO₂ decreased from a late morning value of 200 bar to a late afternoon value of approximately 150 bar. In contrast to Quercus suber (Tenhunen et al. 1984), restriction of CO₂ supply due to stomatal closure reduced net CO₂ uptake at midday and in the afternoon during midsummer. A decrease in leaf carboxylation efficiency and an increase in CO₂ compensation point at midday also played an important role in determining the diurnal course of net photosynthesis. During the late stages of drought in September, severe water stress led to reduction in mesophyll photosynthetic capacity and further reduction in leaf conductance. The observed decrease in mesophyll photosynthetic capacity was correlated with decrease in the daily minimum leaf water potential to greater negative values than-30 bar. At this time, CO₂ saturated photosynthetic rates decreased as much as 50% over the course of a day when measured at constant saturating light, 32° C leaf temperature, and a water vapor mole fraction difference between leaf and air of 30 mbar bar^-1.     

Effects of drought on photosynthesis in Mediterranean plants grown under enhanced UV-B radiation

The effects of drought on the photosynthetic characteristics of three Mediterranean plants (olive, Olea europea L.; rosemary, Rosmarinus officinalis L.; lavender, Lavandula stoechas L.) exposed to elevated UV-B irradiation in a glasshouse were investigated over a period of weeks. Drought conditions were imposed on 2-year-old plants by withholding water. During the onset of water stress, analyses of the response of net carbon assimilation of leaves to their intercellular CO2 concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bisphosphate on photosynthesis. Measurements of chlorophyll fluorescence were used to determine changes in the efficiency of light utilization for electron transport, the occurrence of photoinhibition of photosystem II photochemistry and the possibility of stomatal patchiness across leaves. The first stages of water stress produced decreases in the light-saturated rate of CO2 assimilation which were accompanied by decreases in the maximum carboxylation velocity and the capacity for regeneration of ribulose 1,5-bisphosphate in the absence of any significant photodamage to photosystem II. Leaves of rosemary and lavender were more sensitive than those of olive during the first stages of the drought treatment and also exhibited increases in stomatal limitation. With increasing water stress, significant decreases in the maximum quantum efficiency of photosystem II photochemistry occurred in lavender and rosemary, and stomatal limitation was increased in olive. No indication of any heterogeneity of photosynthesis was found in any leaves. Drought treatment significantly decreased leaf area in all species, an important factor in drought-induced decreases in photosynthetic productivity. Exposure of plants to elevated UV-B radiation (0.47 W m(-2)) prior to and during the drought treatment had no significant effects on the growth or photosynthetic activities of the plants. Consequently, it is predicted that increasing UV-B due to future stratospheric ozone depletion is unlikely to have any significant impact on the photosynthetic productivity of olive, lavender and rosemary in the field.     

Irrigation effects on daily and seasonal variations of trunk sap flow and leaf water relations in olive trees

Irrigation effects on whole-plant sap flow and leaf-level water relations were characterised throughout a growing season in an experimental olive (Olea europaea L.) orchard. Atmospheric evaporative demand and soil moisture conditions for irrigated and non-irrigated olive trees were also monitored. Whole-plant water use in field-grown irrigated and rain fed olive trees was determined using a xylem sap flow method (compensation heat-pulse velocity). Foliage gas exchange and water potentials were determined throughout the experimental period. Physiological parameters responded diurnally and seasonally to variations in tree water status, soil moisture conditions and atmospheric evaporative demand. There was a considerable degree of agreement between daily transpiration deduced from heat-pulse velocity and that determined by calibration using the Penman–Monteith equation in the field. Summer drought caused decreasing leaf gas exchange and water potentials, and a progressive increase in hydraulic conductance (stronger in non-irrigated than irrigated trees), probably attributable to modifications in hydraulic properties at the soil-root interface. Negligible hysteresis, attributable to low plant capacitance, was observed in the relationship between leaf water potential and sap flow. A proportional decrease in maximum daily leaf conductance with increasing vapour pressure deficit was observed, while mean daytime canopy stomatal conductance decreased with the season. As a result, plant water use was limited and excessive drought stress prevented. Non-irrigated olive trees recovered after the summer drought, showing a physiological behaviour similar to that of irrigated trees. In addition to physiological and environmental factors, there are endogenous keys (chemical signals) influencing leaf level parameters. Olive trees are confirmed to be economical and sparing users of soil water, with an efficient xylem sap transport, maintenance of significant gas exchange and transpiration, even during drought stress.     

干旱胁迫下油橄榄品种光合特性研究

为揭示油橄榄(Olea europaea L.)耐旱性与光合特性之间的关系,以筛选出的适宜于半干旱川西南地区种植的7个引进油橄榄品种为供试材料,采用盆栽模拟干旱胁迫的方法,研究持续干旱胁迫对其光合特性的影响。结果表明:(1)随着干旱胁迫程度加剧,7个油橄榄品种叶片相对含水量均显著降低,至干旱胁迫后期(25d),各品种叶片均出现大幅失水,其中品种‘科拉蒂’失水率最高(45.79%),而品种‘小苹果’失水率最低(25.52%),说明‘小苹果’叶片在干旱胁迫下较其他油橄榄品种具有更高保水能力。(2)随着干旱胁迫程度加剧,7个油橄榄品种叶片光合色素含量均不同程度降低,表明光合色素分解量大于合成量;干旱胁迫持续25d时,品种‘豆果’的叶绿素a和叶绿素b含量下降幅度最大(P0.05),品种‘皮削利’类胡萝卜素含量下降幅度最大(P0.05),而品种‘小苹果’叶绿素a含量下降幅度最小。(3)随着干旱胁迫的持续进行,各油橄榄品种叶片净光合速率(Pn)、蒸腾速率(Tr)、气孔导度(Gs)和胞间CO2浓度(Ci)均不同程度降低,而水分利用效率(WUE)则呈上升趋势;干旱胁迫期间,品种‘佛奥’的Pn、Tr和Ci以及‘皮削利’的Gs降幅均高于其他品种,而‘小苹果’的Pn、Gs和Ci降幅均为最小且WUE上升幅度最大。研究发现,在持续干旱胁迫条件下,油橄榄幼苗叶片均大幅失水,光合色素结构被破坏、色素分解、含量降低,同时气孔关闭蒸发减少,光合作用减弱,而供试油橄榄品种中‘小苹果’对干旱胁迫的适应性最强,适宜于在半干旱的川西地区种植。     

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

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

The Intracellular Location of Catechol Oxidase: in the Olive Fruit

Catechol oxidase in young olive fruits was located on the innerface of chloroplast thylakoids and in the mitochondria. Thiswas demonstrated by observing the oxidation products of 3,4-dihydroxyphenyl-alanine(DOPA) with the electron microscope in fruit segments prefixedwith glutaraldehyde. Diethyl dithiocarbamate (DIECA), an inhibitorof the enzyme, prevented the accumulation of oxidation productsof DOPA. Olea europaea L, olive fruits, catechol oxidase, ultrastructure     

Summer survival of leaves in a soft-leaved shrub (Phlomis fruticosa L., Labiatae) under Mediterranean field conditions: avoidance of photoinhibitory damage through decreased chlorophyll contents

Photosynthetic pigments and relative water content of youngleaves of P. fruticosa decreased considerably with the onsetof the summer dry period and stabilized at low values for thelast two summer months, while leaf growth was arrested. Correspondingdecreases in photochemical efficiency of photosystem II, asjudged by chlorophyll fluorescence measurements of predarkenedleaves, were, however, negligible. Following the first autumnheavy rains, growth was restored and photosynthetic pigmentsand relative water content increased to the pre-drought values.The results indicate that the reduction of chlorophylls doesnot result from severe photoinhibitory damage but, instead,it may be an adaptive response against the adverse conditionsof the Mediterranean summer. Some photosynthetic and photoprotective characteristics of P.fruticosa leaves at two stages of their development, i.e. atthe severely dehydrated state with arrested growth during latesummer and after their revival following the first heavy autumnrains were compared. Apart from the chlorophyll loss, the photonyield of O₂ evolution and the photosynthetic capacity at saturatedCO₂ were considerably suppressed during the summer, indicatingthat the extremely low net photosynthetic rates observed inthe field were the combined result of stomatal and mesophylllimitations. Epoxidation state was low at midday during thesummer, indicating an active, photodissipative xanthophyll cycle.Although zeaxanthin content did not increase at midday afterthe rains, the potential of the cycle was maintained in therevived leaves, as judged by the high concentrations of thecycle components. After the rains, the activities of the anti-oxidantenzymes (superoxide dismutase, ascorbate peroxidase) remainedrelatively unchanged on a chlorophyll basis, but increased whenexpressed on a leaf surface area or protein basis. It may beconcluded that P. fruticosa leaves avoid severe photoinhibitoryand oxidative damage during the long, warm, dry and sunny Mediterraneansummer by reducing light harvesting and electron flow capacity,whilst maintaining an adequate photoprotective ability. Thepreservation of a remarkable photodissipative and anti-oxidativepotential after the rains may be related to the low predictabilityof precipitation even during the rainy winter. Key words: Chl loss, anti-oxidant enzymes, xanthophyll cycle, photosynthesis, water stress