Journal of Experimental Botany, Vol. 51, No. 348, pp. 1309-1317,
July 2000
© 2000 Oxford University Press
Original Papers |
Effects of drought on photosynthesis in Mediterranean plants grown under enhanced UV-B radiation
Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK
Received 13 October 1999; Accepted 12 March 2000
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Abstract |
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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.
Key words: Drought, Lavandula stoechas, Mediterranean vegetation, Olea europea, photosynthesis, Rosmarinus officinalis, ultraviolet-B, water stress.
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Introduction |
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During the summer, Mediterranean vegetation is often subjected to periods of severe drought and leaves exhibit large reductions in relative water content and water potential (Kyparissis et al., 1995
; Scarascia-Mugnozza et al., 1996). Frequently, large decreases in photosynthetic activity are associated with such changes in water status (Angelopoulos et al., 1996; Munné-Bosch et al., 1999). However, the mechanistic bases of this inhibition of photosynthesis are not well understood.
During periods of drought large depressions in photosynthesis of Mediterranean plants during midday are often observed and maximum rates of CO2 assimilation occur only during early morning and late afternoon when temperatures are relatively low (Schulze and Hall, 1982; Tenhunen et al., 1987; Chaves, 1991; Pereira and Chaves, 1993). Such midday depressions in CO2 assimilation are associated with stomatal closure, which simultaneously reduces water loss and daily carbon assimilation (Pereira and Chaves, 1993). In C3 plants when stomata close in response to drought and CO2 assimilation is reduced, the photosynthetic reduction of O2 via photorespiration increases and serves as a sink for excess excitation energy in the photosynthetic apparatus (Cornic and Briantais, 1991). However, increases in the rate of photorespiratory reduction of O2 are not sufficient to dissipate the excess excitation energy in PSII antennae and, consequently, increased dissipation of this energy as heat occurs in order to minimize photodamage to PSII reaction centres (Baker, 1993). Midday depression of PSII photochemical efficiency in Mediterranean shrubs has been reported previously (Demmig-Adams et al., 1989). Under severe water stress, electron transport to O2 and increased quenching of excitation energy in the PSII antennae may be unable to dissipate the excess excitation energy in the PSII antennae and photodamage of PSII will result, with a possible net loss of D1 protein of PSII reaction centres (Baker, 1993; Cornic, 1994). Such effects can have significant consequences for the photosynthetic productivity of plants (Long et al., 1994).
Drought-induced midday depression of photosynthesis in Mediterranean plants may also involve heterogeneity of leaf photosynthesis. Such heterogeneity can be the consequence of patchy stomatal closure and/or collapse of parts of the mesophyll due to loss of turgor, associated with a low lateral CO2 diffusion capacity (Cornic and Massacci, 1996), and result in decreases in the photosynthetic efficiency and capacity of leaves.
During periods of depression of photosynthesis in droughted Mediterranean plants in the field, leaves will be exposed to high fluxes of photosynthetically-active and UV-B radiation, both of which are potentially damaging for the photosynthetic apparatus. Exposure of leaves to high doses of UV-B can result in loss of Rubisco and photochemically competent PSII complexes and induce stomatal closure, although under moderate stresses and levels of UV-B radiation associated with the predicted future atmospheric ozone depletion no significant decreases in photosynthetic performance of leaves have been observed (reviewed in Allen et al., 1999). However, in the severe Mediterranean environment, the interactive effects of drought and high light stress during periods of exposure to increased UV-B irradiance might have serious consequences for photosynthetic productivity and for the distribution of vegetation in the region (Peñuelas et al., 1998).
The aim of this study was to determine the effects of drought on the photosynthetic characteristics of the leaves of three native Mediterranean plants (lavender, olive and rosemary) when irradiated with a high flux of UV-B radiation. Analyses of the response of net CO2 assimilation to intercellular CO2 concentration and chlorophyll fluorescence measurements allowed evaluation of the relative limitations to leaf photosynthesis imposed by changes in stomatal conductance, carboxylation efficiency, capacity for regeneration of ribulose 1,5-bisphosphate (RuBP) and PSII electron transport efficiency.
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Materials and methods |
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Growth and treatment conditions
Two-year-old plants of lavender (Lavandula stoechas L.), olive (Olea europea L.) and rosemary (Rosmarinus officinalis L.) were obtained from a local nursery (Scarletts, Essex, UK) and grown in 1.5 dm3 pots (depth of 33.5 cm) in a glasshouse at the University of Essex as described previously (Nogués et al., 1998
). Minimum PPFD during a 16 h photoperiod was maintained at approximately 500 µmol m-2 s-1 by supplementary lighting. Mean temperature and vapour pressure deficit were maintained at approximately 23/19 °C and 1.7/1.3 kPa day/night, respectively. Plants were watered to saturation on alternate days with Hoagland's solution.
Plants were placed in a transparent UV-exposure cabinet within the glasshouse, which has been previously described previously (Allen et al., 1997), and irradiated with UV-B for 14 h each day (from 06.00 h until 20.00 h) during the photoperiod for 8 weeks. The UV spectrum at the top of the plants was measured with a scanning spectroradiometer (SR 991-PC, Macam Photometrics, Livingston, UK). Glasshouse and cabinet transmission of UV-A radiation, supplemented by the UV fluorescent lamps, ensured that UV-A exposure was maintained for photorepair and for flavonoid biosynthesis (Teramura and Ziska, 1996). The biologically weighted UV-B dosages according to the generalized plant action spectrum (normalized to 300 nm; Caldwell, 1971) for the UV-B and control treatments were 0.47 W m-2 (24 kJ m-2 d-1) and 0.001 W m-2, respectively. The UV-exposure cabinet was divided into two independent sections: one without UV-B and one with UV-B radiation. The sections were regularly exchanged to minimize any between-section differences other than in UV-B treatment.
Eight weeks after placing the plants in the UV-exposure cabinet half the plants were subjected to progressive drought by withholding water. Well-watered and droughted plants were divided equally between the two sections in a split-plot design. Consequently, plants were subjected to one of four treatments: (i) without UV-B radiation and well-watered, (ii) UV-B radiation and well-watered, (iii) without UV-B radiation and droughted, and (iv) UV-B radiation and droughted. Four plants of each species were used in each treatment. Lavender plants were subjected to these treatments for 2 weeks, rosemary plants for 3 weeks and olive plants for 4 weeks. Leaf water potential (
w) was measured twice a week at 08.00 h using a pressure chamber (PMS Instrument Co., Corvallis, Oregon, USA), with damp paper at the bottom of the chamber to avoid excessive evaporation during the measurements. Relative water content (RWC) of the leaves was determined as (FW–DW)/(TW–DW)x100, where FW is the fresh weight, DW is the dry weight and TW is the turgid weight of the leaf after equilibration in distilled water for 24 h. Daily evaporation rate (E) of the whole plant was calculated from the change in weight of the pots (water loss by the soil was minimized by using cellulose film sealed on top of the pots) as described previously (Nogués et al., 1998).
Leaf gas exchange and fluorescence analysis
An infrared gas analyser (CIRAS-1, PP Systems, Hitchin, UK) was used, as previously described (Farage et al., 1991; Nogués et al., 1998) to estimate net CO2 assimilation rates (A) and intercellular CO2 concentrations (ci) at a PPFD of 1600 µmol m-2 s-1 using equations developed by von Caemmerer and Farquhar (von Caemmerer and Farquhar, 1981). Measurements were made on attached single olive leaves using a plant leaf chamber type N (PLCN; PP Systems). A plant leaf chamber type C (PLCC; PP Systems) was used to measure attached apical non-woody rosemary and lavender shoots. Stomatal limitation (l), which is the proportionate decrease in light-saturated net CO2 assimilation attributable to stomata, was calculated by the method of Farquhar and Sharkey (Farquhar and Sharkey, 1982). Estimations of the maximum carboxylation velocity of Rubisco (Vc,max) and the maximum electron transport rate contributing to RuBP regeneration (Jmax) were made by fitting a maximum likelihood regression below and above the inflexion of the A/ci response using the method of McMurtrie and Wang (McMurtrie and Wang, 1993).
Steady-state modulated chlorophyll fluorescence of the adaxial surface of attached leaves was measured using a fluorimeter (PAM-2000, H Walz GmbH, Effeltrich, Germany) during the gas exchange measurements. Calculations were made from fluorescence parameters of the maximum quantum efficiency of PSII photochemistry (given by Fv/Fm) and the relative quantum efficiency of PSII electron transport (
PSII; estimated from (
–
)/; Genty et al., 1989), the efficiency of excitation energy capture by open PSII reaction centres (
/) and photochemical quenching (qp) as described previously (Andrews et al., 1993). Measurements of Fv/Fm were made after dark adaptation for 15 min and PSII, / and qp were measured at a PPFD of 500 µmol m-2 s-1, which was similar to the minimum mean growth PPFD.
All gas exchange and fluorescence measurements were made each day between 10.00 and 16.00 h.
Images of the relative quantum efficiency of PSII electron transport (PSII) of mesophyll cells across leaves were produced from high-resolution fluorescence imaging as previously described (Oxborough and Baker, 1997).
The abaxial leaf conductance of attached olive leaves was measured in situ at midday using an automatic transit-time porometer (AP4, Delta-T Devices, Cambridge, UK), taking measurements from at least six leaves per treatment according to Nogués et al. (Nogués et al., 1998).
Pigment analysis
Water-soluble pigments (flavonoids and anthocyanins) were extracted from leaves at the end of the drought treatment using the method of Jordan et al. (Jordan et al., 1994). Leaves were ground to a powder in liquid nitrogen before extraction in 10 cm3 of acidified methanol (HCl:methanol, 1:99, v/v). Absorption spectra of the extracts were determined using a Cary 210 spectrophotometer (Varian, Palo Alto, CA, USA), and the flavonoid and anthocyanin contents were estimated from absorbances at 300 and 530 nm, respectively.
Plant biomass analysis
At the end of the drought treatment plants were harvested and oven dried at 80 °C for 2 d, and analyses of biomass of shoots and roots were carried out. Total plant leaf area was estimated prior to drying using a flat-bed scanner (Hewlett-Packard ScanJet model IIcx, San Diego, USA) and analysed with an image processing program (Nogués et al., 1998).
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Results |
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The changes in plant water status during 2, 3 and 4 weeks of drought in lavender, rosemary and olive plants, respectively, are shown in Fig. 1
. The plants were grown for 8 weeks without UV-B radiation or with 0.47 W m-2 UV-B. Well-watered leaves had an RWC of c. 87% and w of c. -1.2 MPa throughout the measurement period for the three species (data not shown). No major changes in the RWC of olive leaves with (UVB) or without UVB (non-UVB) treatment were observed throughout the 4 week drought (Fig. 1a). After 4 weeks of drought, w of olive leaves had decreased from c. -1.3 to -4.8 MPa in both non-UVB and UVB treatments (Fig. 1b). In rosemary leaves, RWC and w decreased rapidly to c. 40% and c. -4.0 MPa, respectively, over 3 weeks of drought in both non-UV-B and UV-B treatments (Fig. 1d, e). RWC and w also decreased rapidly (from c. 80% to 25% and from c. -1.3 to -4.0 MPa, respectively, over 2 weeks of drought) in both non-UV-B and UV-B-treated lavender plants (Fig. 1g, h). The daily plant evaporation rate (E) increased with plant growth in all species in both non-UVB and UVB treatment throughout the experiment (Fig. 1c, f, i).
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) or with 0.47 W m-2 UV-B (•). The UV-B treatment was also maintained during the drought period. Leaves of well-watered plants had an RWC of c. 87% and Analyses of A/ci curves throughout the treatments allowed determination of the changes in the light saturated rate of CO2 assimilation (Asat), maximum carboxylation velocity of Rubisco (Vc,max), maximum potential rate of electron transport contributing to RuBP regeneration (Jmax) and stomatal limitation to Asat (l) for olive leaves or apical non-woody rosemary and lavender shoots (Fig. 2
). Four weeks of drought markedly decreased Asat, Vc,max and Jmax and increased l (from c. 20 to 40%) in olive leaves (Fig. 2a–d). After one week of severe water stress, Asat, Vc,max and Jmax had substantially decreased, and l increased, in both rosemary (Fig. 2e–h) and lavender (Fig. 2i–l) leaves. Further decreases in Asat, Vc,max and Jmax and increase in l were observed in lavender leaves after the 2 weeks of drought (Fig. 2i–l). UV-B treatment did not induce any significant changes in any of these photosynthetic parameters of the three species studied throughout the measurement period (Fig. 2). Four weeks of drought decreased abaxial stomatal conductance (gs) of olive leaves, measured in situ using an AP4 porometer, from c. 102.3±11.6 to c. 14.6±3.9 mmol m-2 s-1 (data not shown).
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The changes in the maximum efficiency of PSII photochemistry after 15 min dark-adaptation (Fv/Fm), quantum yield of PSII electron transport (
PSII), efficiency of energy capture by open PSII reaction centres (/) and photochemical quenching (qp) at the growth PPFD of 500 µmol m-2 s-1 during 2, 3 and 4 weeks of drought in lavender, rosemary and olive plants, respectively, are shown in Fig. 3. No significant changes in the dark-adapted Fv/Fm of olive leaves in either non-UVB or UV-B treatment were observed throughout the 4 week drought (Fig. 3a). After 4 weeks of drought, PSII of olive leaves had decreased from c. 0.6 to 0.4 in both non-UVB and UV-B treatments (Fig. 3b). Decreases in / accounted for the changes in PSII (Fig. 3c). No changes were observed in qp throughout the 4 week of drought in non-UV-B or UV-B-treated olive plants (Fig. 3d). In rosemary leaves, after the first 2 weeks of drought no change was observed in Fv/Fm (Fig. 3e), whereas PSII, / and qp showed a slight decrease (Fig. 3, f–h). Further decreases in all the parameters were observed after the third week of the drought in non-UV-B and UV-B treated rosemary leaves (Fig. 3, e–h). In lavender leaves, a similar pattern of changes in Fv/Fm, PSII, / and qp were observed over the 2 weeks of drought. No changes were observed in Fv/Fm after the first week (Fig. 3i), but there were slight decreases in PSII, / and qp (Fig. 3j–l). Further decreases in all the parameters were observed after the second week of drought in non-UV-B and UV-B treated lavender leaves (Fig. 3i–l).
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Imaging of chlorophyll a fluorescence from leaves was carried out in order to determine whether any heterogeneity of
PSII existed between mesophyll (palisade) cells (data not shown). Although imaging confirmed that PSII decreased in UV-B irradiated and 4 week droughted olive, 3 week droughted rosemary and 2 week droughted lavender compared to control leaves, no significant level of heterogeneity in PSII was observed in any leaves.
Water-soluble pigments that absorb UV-B radiation (flavonoids and anthocyanins) are considered to play a major role in protecting plants from UV-B damage. The effects of increased UV-B exposure for 8 weeks and after 2, 3 and 4 weeks of drought on the pigment contents in leaves of lavender, rosemary and olive plants are shown in Table 1. Drought treatments had no significant effect on the pigment contents of leaves of the three species (data not shown). Flavonoid and anthocyanin concentrations were significantly increased by c. 22% and 37%, respectively, in UV-B treated olive plants (Table 1). However, neither flavonoid nor anthocyanin concentrations were significantly changed by UV-B irradiance in rosemary or lavender leaves (Table 1). Differences in the pigment concentrations with UV-B radiation treatment were similar whether expressed on the basis of leaf area (data not shown) or fresh weight.
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The effects of 4, 3 and 2 weeks of drought on several plant growth characteristics in olive, rosemary and lavender plants grown in UV-B, respectively, are shown in Table 2
. The UV-B treatment had no significant effect on any of the parameters studied for any of the species (data not shown). Drought significantly reduced plant height, leaf area, number of leaves per plant, total dry weight, leaf dry weight, and shoot dry weight in olive plants; plant height, leaf area, number of leaves per plant, specific leaf area, and leaf area ratio in rosemary plants; and leaf area, number of leaves per plant, specific leaf area, leaf weight ratio, leaf area ratio, and leaf dry weight in lavender plants (Table 2). Plant and soil water contents were significantly reduced by drought in the three species studied (Table 2).
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Discussion |
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The drought treatment resulted in large decreases in
w in all three species after 1 week. However, RWC decreased markedly only for lavender and rosemary (Fig. 1
), with decreases in olive leaves being small even after 4 weeks of drought (Fig. 1). Accompanying these changes in water status in all species were large decreases in the photosynthetic gas exchange parameters Asat, Vc,max and Jmax (Fig. 2). Consequently, drought-induced decreases in photosynthetic capacity of the leaves are accompanied by reductions in carboxylation efficiency and the ability to regenerate RuBP. RuBP regeneration could be limited either by an inability to supply reductants and ATP from electron transport or an inactivation or loss of Calvin cycle enzymes other than Rubisco (Baker et al., 1997). The large depressions in Jmax occurring after 1 week (Fig. 2) were not accompanied by such large changes in the relative quantum efficiency of electron flux through PSII at the growth PPFD (PSII; Fig. 3). This suggests that, initially, during the development of water stress decreases in the ability to regenerate RuBP cannot be attributed to a reduction in non-cyclic electron transport and the ability to produce ATP and reductants, unlike the situation in sunflower where inhibition of RuBP regeneration induced by water stress has been attributed to decreases in ATP supply resulting from a loss of ATP synthase (Tezara et al., 1999). Allen et al. have shown that decreases in Jmax induced by UV-B irradiation are associated with decreases in sedoheptulose 1,7 bisphosphatase, a key regulatory enzyme in the Calvin cycle (Allen et al., 1998). Decreases in Vc,max (Fig. 2) are likely to result from loss or inactivation of Rubisco (Allen et al., 1997). Consequently, an attractive unifying hypothesis to explain the simultaneous decreases in Vc,max and Jmax during drought is that inactivation or loss of both Rubisco and other key Calvin cycle enzymes is occurring and may result in a decrease in carboxylation efficiency and account for decreases in Vc,max, whilst decreases in other Calvin enzyme activities may result in a decrease in the rate of regeneration of RuBP and a decrease in Jmax (Baker et al., 1997).
The drought-induced decreases in PSII in rosemary and lavender were accompanied by similar decreases in / and qP, whereas in olive only / decreased and qP remained relatively constant (Fig. 3). This highlights an interesting difference in the response of the photosynthetic apparatus to drought between olive and the other two species. The magnitude of PSII is determined by the product of the efficiency of excitation energy capture by ‘open’ PSII reaction centres, which is estimated by /, and the proportion of PSII reaction centres that are open, which is estimated by qP (Genty et al., 1989). Decreases in / are associated with increases in excitation energy quenching in the PSII antennae and are generally considered indicative of ‘down-regulation’ of electron transport (Horton et al., 1996). Consequently, the decreases in / exhibited during drought in all the species can be taken as indicative of a physiological regulation of electron transport by increasing excitation energy quenching processes in the PSII antennae. Decreases in qP are attributable to either decreases in the rate of consumption of reductants and ATP produced from non-cyclic electron transport relative to the rate of excitation of open PSII reaction centres or damage to PSII reaction centres. The large drought-induced decreases in qP in rosemary and lavender could to be due to a combination of both of these factors. During the first week of the drought treatment of these two species no significant change in dark-adapted Fv/Fm was observed (Fig. 3) indicating that no photodamage to PSII reaction centres or development of slowly relaxing excitation energy quenching had been induced by the drought, i.e. quenching that did not relax during the 15 min dark-adaptation period. The very large decreases in the gas exchange parameters Asat, Vc,max and Jmax that occur in rosemary and lavender after the first week of drought (Fig. 2) when only relatively small decreases in Fv/Fm and / are found (Fig. 3) suggests that demand for reductants and ATP has decreased dramatically and this is a major factor in the closure of PSII reaction centres. However, after 2 and 3 weeks of drought large decreases in Fv/Fm of lavender and rosemary leaves, respectively, were observed (Fig. 3), indicating that either PSII reaction centres had been damaged, or slowly relaxing quenching had been induced. Clearly, negligible photodamage to PSII occurs during drought in olive leaves since no significant changes are found in Fv/Fm over a 4 week period (Fig. 3) during which very large decreases in Asat, Vc,max and Jmax occur (Fig. 2). Consequently, the drought-induced decreases in PSII that occur in olive are attributable to ‘down-regulation’ of electron transport. This study supports the contention that photodamage to PSII reaction centres is not a primary factor in the depression of CO2 assimilation of the leaves induced by water stress. However, photoinhibitory damage to PSII may be a secondary effect of drought in rosemary and lavender.
No heterogeneity of PSII at the mesophyll (palisade) cellular level was observed in any of the water-stressed plants (data not shown). Consequently ‘patchiness’ of photosynthesis is not a factor in limiting photosynthetic activity in these species during periods of drought stress. It is possible that such ‘patchiness’ occurs only when dehydration is very rapid and thus it may not occur in the field (Cornic and Massacci, 1996).
In leaves of all three species the capacity for CO2 assimilation decreases to almost zero (Fig. 2). However, in olive after 4 weeks of drought treatment when there is negligible CO2 assimilatory capacity PSII remains at approximately 50% of the non-stressed leaves (Fig. 3). This suggests that a considerably greater rate of non-cyclic electron transport is occurring than is required to maintain CO2 assimilation. An alternative sink to CO2 assimilation for electrons would be oxygen reduction by photorespiration and/or a Mehler reaction, although in droughted bean leaves it has been shown that photorespiration does not act to protect the photosynthetic apparatus from photodamage (Brestic et al., 1995). In the droughted bean leaves increases in zeaxanthin content of the thylakoids were correlated with increased antennae quenching and this appeared to be a major mechanism for protecting the photosynthetic apparatus from photodamage (Brestic et al., 1995). A similar situation is likely to be occurring in the droughted olive leaves.
Drought produced large increases in stomatal limitation in the three species (Fig. 2). In rosemary and lavender the large increases in stomatal limitation accompanied the decreases in photosynthesis parameters (Fig. 2) and, consequently, stomatal closure would appear to be an important factor contributing to the depressed CO2 assimilation. However, during the initial stages of the development of water stress in olive leaves, only small increases in stomatal limitation were observed when very large depressions in the photosynthetic parameters were occurring (Fig. 2). With time drought treatment produced a large increase in stomatal limitation in olive leaves, similar to that observed earlier in rosemary and lavender (Fig. 2).
The photosynthetic productivity of a plant is determined by the quantity of photosynthetically active radiation intercepted and the efficiency with which this intercepted radiation is utilized for net dry matter production. Factors associated with the drought-induced decreases in the efficiency of light utilization have been addressed above and are clearly important in reducing photosynthetic productivity. However, the large drought-induced decreases in leaf area occurring in all three species studied (Table 2) will be associated with large decreases in the ability of the plants to capture photosynthetically active radiation and be a major factor in determining the drought-induced depressions in photosynthetic productivity of these species.
Exposure of lavender, olive and rosemary leaves to 0.47 W m-2 UV-B during severe drought treatments had no significant effects on leaf water relations and photosynthetic activities or on growth characteristics. This was a somewhat surprising finding since it is well established that high fluxes of UV-B result in closure of stomata and depressions in photosynthetic activities in many other species under non-drought conditions (Teramura and Sullivan, 1994; Teramura and Ziska, 1996) and recent studies on pea demonstrated a significant interaction between UV-B radiation and drought treatments (Nogués et al., 1998). Also previous studies have suggested that high levels of UV-B inhibit photosynthetic performance of Mediterranean vegetation (Grammatikopoulus et al., 1994; Petropoulou et al., 1995; Nikolopoulus et al., 1995; Drilias et al., 1997). In the present study olive leaves responded to the UV-B treatment by increasing significantly their flavonoid and anthocyanin contents (Table 1), which presumably offers protection from the high UV-B level. Interestingly, the flavonoid and anthocyanin contents of the lavender and rosemary leaves did not change significantly with exposure to high UV-B (Table 1). However, the pigment contents of control leaves of these species are considerably higher than those found in the olive leaves (Table 1) thus suggesting that lavender and rosemary leaves are well protected from exposure to high UV-B and do not need to synthesize additional pigments to effect protection from the UV-B. The UV-B treatment of 0.47 W m-2 used in this study is equivalent to a daily dosage of 24 kJ m-2 d-1, which approximates to a level that is c. four times the level in midsummer in the Mediterranean. This UV-B level is considerably in excess of the predicted increases in UV-B reaching the earth's surface due to depletion of stratospheric ozone (Allen et al., 1998). Consequently, these data would suggest that increases in UV-B radiation are unlikely to pose a threat to the photosynthetic productivity in the field of lavender, olive or rosemary in the foreseeable future.
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Acknowledgments |
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This research was supported by a research grant to NRB from the UK Natural Environment Research Council and to SN from Generalitat de Catalunya/MEC. We thank Kevin Oxborough for assistance with the fluorescence imaging system.
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Notes |
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1 Present address: Departament de Biologia Vegetal, Facultat de Biologia, Universitat de Barcelona, Av. Diagonal 645, E-08028 Barcelona, Spain.
2 To whom correspondence should be addressed. Fax: +44 1206 873416. E-mail: baken{at}essex.ac.uk
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Abbreviations |
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Asat, light-saturated net CO2 assimilation rate; ci, intercellular CO2 concentration; E, daily plant evaporation rate;
, maximal and variable fluorescence yields in a light-adapted state; gs, stomatal conductance; Fv/Fm, ratio of variable to maximal fluorescence yield in dark-adapted leaves; Jmax, maximum potential rate of electron transport contributing to RuBP regeneration; l, stomatal limitation to Asat; RuBP, ribulose 1,5-bisphosphate; RWC, relative leaf water content; Vc,max, maximum carboxylation velocity of Rubisco; PSII, relative quantum efficiency of photosystem II photochemistry; w, leaf water potential. |
References |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Allen DJ, McKee IF, Farage PK, Baker NR.1997. Analysis of the limitation to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B. Plant, Cell and Environment 20, 633–640.
Allen DJ, Nogués S, Baker NR.1998. Ozone depletion and increased UV-B radiation: is there a real threat to photosynthesis?. Journal of Experimental Botany 49, 1775–1788.
Allen DJ, Nogués S, Morison JIL, Greenslade PD, Mcleod AR, Baker NR. 1999. A thirty percent increase in UV-B has no impact on photosynthesis in well-watered and droughted pea plants in the field. Global Change Biology 5, 213–222.
Andrews JR, Bredenkamp GJ, Baker NR.1993. Evaluation of the role of state transitions in determining the efficiency of light utilization for CO2 assimilation in leaves. Photosynthesis Research 38, 15–26.
Angelopoulos K, Dichio B, Xiloyannis C.1996. Inhibition of photosynthesis in olive trees (Olea europea L.) during water stress and rewatering. Journal of Experimental Botany 47, 1093–1100.
Baker NR.1993. Light-use efficiency and photoinhibition of photosynthesis in plants under environmental stress. In: Smith JAC, Griffiths H, eds. Water deficits: plant responses from cell to community. Oxford: ßios Scientific Publishers, 221–235.
Baker NR, Nogués S, Allen DJ.1997. Photosynthesis and photoinhibition. In: Lumsden PJ, ed. Plants and UV-B: responses to environmental change. Cambridge: Cambridge University Press, 95–111.
Brestic M, Cornic G, Fryer MJ, Baker NR.1995. Does photorespiration protect the photosynthetic apparatus in French bean leaves from photoinhibition during drought stress? Planta 196, 450–457.
Caldwell MM.1971. Solar UV irradiation and the growth and development of higher plants. In: Giese AC, ed. Photophysiology, Vol. 6. New York: Academic Press, 131–177.
Chaves MM.1991. Effects of water deficits on carbon assimilation. Journal of Experimental Botany 42, 1–16.
Cornic G.1994. Drought stress and high light effects on leaf photosynthesis. In: Baker NR, Boyer JR, ed. Photoinhibition of photosynthesis from molecular mechanisms to the field. Oxford: ßios Scientific Publishers, 297–313.
Cornic G, Briantais J-M.1991. Partitioning of photosynthetic electron flow between CO2 and O2 reduction in a C3 leaf (Phaseolus vulgaris L.) at different CO2 concentrations and during drought stress. Planta183, 178–184.
Cornic G, Massacci A.1996. Leaf photosynthesis under drought stress. In: Baker NR, ed. Photosynthesis and the environment. The Netherlands: Kluwer Academic Publishers, 347–366.
Demmig-Adams B, Adams III WW, Winter K, Meyer A, Schreiber U, Pereira JS, Kruger A, Czygan FC, Lange OL.1989. Photochemical efficiency of photosystem II, photon yield of O2 evolution, photosynthetic capacity, and carotenoid composition during the midday depression of net CO2 uptake in Arbutus unedo growing in Portugal. Planta 177, 377–387.
Drilias P, Karabourniotis G, Levizou E, Nikolopoulos D, Petropoulou Y, Manetas Y.1997. The effects of enhanced UV-B radiation on the Mediterranean evergreen sclerophyll Nerium oleander depend on the extent of summer precipitation. Australian Journal of Plant Physiology 24, 301–306.
Farage PK, Long SP, Lechner EG, Baker NR.1991. The sequence of change within the photosynthetic apparatus of wheat following short-term exposure to ozone. Plant Physiology 95, 529–535.
Farquhar GD, Sharkey TD.1982. Stomatal conductance and photosynthesis. Annual Review Plant Physiology 33, 317–345.
Genty B, Briantais JM, Baker NR.1989. The relationship between the quantum yield of photosynthetic electron transport and quenching of chlorophyll fluorescence. Biochimica et Biophysica Acta 990, 87–92.
Grammatikopoulus G, Karabourniotis G, Kyparissis A, Petropoulou Y, Manetas Y.1994. Leaf hairs of olive (Olea europea) prevent stomatal closure by ultraviolet-B radiation. Australian Journal of Plant Physiology 21, 293–301.
Horton P, Ruban AV, Walters RG.1996. Regulation of light harvesting in green plants. Annual Review of Plant Physiology and Plant Molecular Biology 4, 655–684.
Jordan BR, James PE, Strid A, Anthony RG.1994. The effect of ultraviolet-B radiation on gene expression and pigment composition in etiolated and green pea leaf tissue: UV-B induced changes are gene-specific and dependent upon the development stage. Plant, Cell and Environment 17, 45–54.
Kyparissis A, Petropoulou Y, Manetas Y.1995. 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. Journal of Experimental Botany 46, 1825–1831.
Long SP, Humphries S, Falkowski PG.1994. Photoinhibition of photosynthesis in nature. Annual Review of Plant Physiology and Plant Molecular Biology 45, 633–662.[Web of Science]
McMurtrie RE, Wang Y-P.1993. Mathematical models of the photosynthetic responses of tree stands to rising CO2 concentrations and temperatures. Plant, Cell and Environment 16, 1–13.
Munné-Bosch S, Nogués S, Alegre L.1999. Diurnal variations of photosynthesis and dew absorption by leaves in two evergreen shrubs growing in Mediterranean field conditions. New Phytologist 144, 109–119.
Nikolopoulus D, Petropoulou Y, Kyparisis A, Manetas Y.1995. Effects of enhanced UV-B radiation on the drought semi-decidous Mediterranean shrub Phlomis fruticosa under field conditions are season-specific. Australian Journal of Plant Physiology 22, 737–745.
Nogués S, Allen DJ, Morison JIL, Baker NR.1998. Ultraviolet-B radiation effects on water relations, leaf development, and photosynthesis in droughted pea plants. Plant Physiology 117, 173–181.
Oxborough K, Baker NR.1997. An instrument capable of imaging chlorophyll a fluorescence from intact leaves at very low irradiance and at the cellular and sub-cellular levels of organization. Plant, Cell and Environment 20, 1473–1483.
Peñuelas J, Filella I, Llusià J, Siscart D, Piñol J.1998. Comparative field study of spring and summer leaf gas exchange and photobiology of the Mediterranean trees Quercus ilex and Phyllyrea latifolia. Journal of Experimental Botany 49, 229–238.
Pereira JS, Chaves MM.1993. Plant water deficits in Mediterranean ecosystems. In: Smith JAC, Griffiths H, eds. Water deficits. Plant responses from cell to community. Oxford: ßios Scientific Publishers, 237–251.
Petropoulou Y, Kyparissis A, Kikolopoulos D, Manetas Y.1995. Enhanced UV-B radiation alleviates the adverse effects of summer drought in two Mediterranean pines under field conditions. Physiologia Plantarum 94, 37–44.
Scarascia-Mugnozza G, de Angelis P, Matteucci G, Valentini R.1996. Long-term exposure to elevated [CO2] in a natural Quercus ilex L. community: net photosynthesis and photochemical efficiency of PSII at different levels of water stress. Plant, Cell and Environment 19, 643–654.
Schulze ED, Hall AE.1982. Stomatal responses, water loss and CO2 assimilation rates of plants in contrasting environments. In: Lange OL, Nobel PS, Osmond CB, Ziegler H, eds. Physiological plant ecology. II. Water relations and carbon assimilation. Berlin: Springer, 615–676.
Tenhunen JD, Catarino FM, Lange OL, Oechel WC (eds).1987. Plant responses to stress. Functional analysis in Mediterranean ecosystems. NATO ASI Series. Ecological Sciences, Vol. 15. Berlin: Springer-Verlag.
Teramura AH, Sullivan JH.1994. Effects of UV-B radiation on photosynthesis and growth of terrestrial plants. Photosynthesis Research 39, 463–473.
Teramura AH, Ziska LH.1996. Ultraviolet-B radiation and photosynthesis. In: Baker NR, ed. Photosynthesis and the environment. The Netherlands: Kluwer Academic Publishers, 435–450.
Tezara W, Mitchell VJ, Driscoll SD, Lawlor DW.1999. Water stress inhibits plant photosynthesis by decreasing coupling factor and ATP. Nature 401, 914–917.
von Caemmerer S, Farquhar GD.1981. Some relations between the biochemistry of photosynthesis ant the gas exchange of leaves. Planta 153, 376–387.[Web of Science]
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