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Keywords:

  • Olea europea;
  • chloroplast CO2 concentration;
  • mesophyll and stomatal conductances;
  • photosynthesis;
  • salt-stress

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Olive (Olea europea L) is one of the most valuable and widespread fruit trees in the Mediterranean area. To breed olive for resistance to salinity, an environmental constraint typical of the Mediterranean, is an important goal. The photosynthetic limitations associated with salt stress caused by irrigation with saline (200 mm) water were assessed with simultaneous gas-exchange and fluorescence field measurements in six olive cultivars. Cultivars were found to possess inherently different photosynthesis when non-stressed. When exposed to salt stress, cultivars with inherently high photosynthesis showed the highest photosynthetic reductions. There was no relationship between salt accumulation and photosynthesis reduction in either young or old leaves. Thus photosynthetic sensitivity to salt did not depend on salt exclusion or compartmentalization in the old leaves of the olive cultivars investigated. Salt reduced the photochemical efficiency, but this reduction was also not associated with photosynthesis reduction. Salt caused a reduction of stomatal and mesophyll conductance, especially in cultivars with inherently high photosynthesis. Mesophyll conductance was generally strongly associated with photosynthesis, but not in salt-stressed leaves with a mesophyll conductance higher than 50 mmol m−2 s−1. The combined reduction of stomatal and mesophyll conductances in salt-stressed leaves increased the CO2 draw-down between ambient air and the chloroplasts. The CO2 draw-down was strongly associated with photosynthesis reduction of salt-stressed leaves but also with the variable photosynthesis of controls. The relationship between photosynthesis and CO2 draw-down remained unchanged in most of the cultivars, suggesting no or small changes in Rubisco activity of salt-stressed leaves. The present results indicate that the low chloroplast CO2 concentration set by both low stomatal and mesophyll conductances were the main limitations of photosynthesis in salt-stressed olive as well as in cultivars with inherently low photosynthesis. It is consequently suggested that, independently of the apparent sensitivity of photosynthesis to salt, this effect may be relieved if conductances to CO2 diffusion are restored.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

The cultivation of olives, one of the most economically valuable trees in the Mediterranean countries, is highly encouraged because of its limited water requirement in areas which are subjected to prolonged summer drought (Larcher, De Morales & Bauer 1981; Chartzoulakis, Michelakis & Tsompanakis 1992). Olives are often irrigated with saline water, especially in coastal areas. Salt reduces photosynthesis and impairs growth (Munns 1993). The tolerance of olives to salt is believed to be intermediate (Rugini & Fedeli 1990) but, since the genetic diversity is high in olives, there are large differences in salt tolerance among genotypes and cultivated varieties (Tattini 1994).

In herbaceous species, such as spinach, the photosynthesis of salt-stressed plants is initially constrained by resistance to CO2 diffusion (Delfine et al. 1998, 1999). When leaf salt (Na) concentration is moderate, a remarkable increase of mesophyll resistance, coupled with an increase of stomatal resistance, lowers the chloroplast CO2 concentration and limits photosynthesis. Photosynthesis limitation is partially reversible if salt accumulation inside the leaves is reduced by irrigation with salt-free water (Delfine et al. 1999). An irreversible impairment of the photosynthetic apparatus, associated with a reduction of ribulose-1,5-bisposphate carboxylase/oxygenase (Rubisco) activity and content, occurs when the stress is prolonged and salt continues to accumulate in the leaves (Delfine et al. 1999).

Sclerophyllous olive leaves have a constitutively packed mesophyll which may limit CO2 entry in the chloroplasts and photosynthesis (Bongi & Loreto 1989). Under salt-stress conditions, olive leaves become thicker and more succulent (Bongi & Loreto 1989). Increasing leaf thickness may further reduce the mesophyll conductance by extending and making more tortuous the CO2 pathway toward the chloroplasts (Longstreth & Nobel 1979; Evans et al. 1994; Syvertsen et al. 1995). Bongi & Loreto (1989) also reported a reduction of mesophyll conductance associated with increasing olive-leaf thickness, but this reduction was surprisingly moderate and the overall mesophyll conductance was comparable with that subsequently estimated in herbaceous leaves (Loreto et al. 1992). Harley et al. (1992) reviewed possible source of errors in the estimation of mesophyll conductance obtained by using different methods. The method used by Bongi & Loreto (1989), in particular, may not have correctly estimated the electron transport rate of olive leaves and, consequently, may have underestimated the substantial reduction of mesophyll conductance that probably occurs in salt-stressed leaves. A much more precise estimation of the electron transport rate has been obtained by using chlorophyll fluorescence (Genty, Briantais & Baker 1989). By simultaneously using chlorophyll fluorescence and gas-exchange it has been possible to estimate the electron transport rate using two independent methods. Discrepancies between the electron transport measured through fluorescence and that calculated through gas-exchange indicate the occurrence of finite mesophyll resistances reducing the chloroplast CO2 concentration and which may be easily calculated (Loreto et al. 1992). A first objective of our study was to understand whether mesophyll conductance is always involved in limiting photosynthesis of leaves exposed to a moderate salt stress, or if, contrary to the case of herbaceous plants, tree species with inherently low mesophyll conductance do not experience further restrictions to CO2 entry.

Our second objective was to identify the possible causes of the different sensitivity of photosynthesis to salt in olive cultivars. In Crete (Greece) there are several olive cultivars with interesting commercial characteristics, but the fresh water available for irrigation is limited. It is important to select cultivars that may give reliable performances when irrigated with saline water. We compared the photosynthetic performances of six olive cultivars irrigated with saline water. By measuring the gas-exchange and chlorophyll fluorescence simultaneously, we estimated in vivo and in situ the different components that may affect photosynthesis under salt stress, including photochemical efficiency, Rubisco activity and mesophyll and stomatal conductances.

MATERIALS AND METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Plant material, experimental conditions and statistical treatment

One-year-old-plants of Greek olive cultivars (Adramitini, Agouromanaki, Chalkidikis, Kerkiras, Throubolia and Valanolia) were grown in the greenhouse of the Subtropical Plants and Olive Tree Institute in Chania, Crete, Greece. Plants were grown in 8.5 × 10−3 m3 pots containing a sand : perlite mixture (1 : 3). The total number of plants used within this experiment was 48 (eight plants per cultivar, four of which were maintained in control conditions and the other four were salt-stressed). The plants were grown following a completely randomized design. Control plants were irrigated twice daily for 30 s through a closed recycling system with a 50% strength Hoagland solution. In salt-stressed plants, 200 mm NaCl was added to the irrigation water, but this was done by starting with a solution 25 mm, and incrementing this concentration by 25 mm per day to avoid salt shock. This treatment resulted in a water electrical conductivity of 24.6 dS m−1. The salt treatment was initiated on 22 May 2000, when the plants had developed shoots 15–20 cm long.

The experimental measurements were carried out between 40 and 45 d after beginning the treatment, when plant growth was still not impaired by the stress, and no visible sign of toxicity (leaf chlorophyll chlorosis, burning of leaf edges or leaf dropping) was observed. The measurements were carried out during the morning (1000–1200 h), when the air temperature was 28–30 °C, vapour pressure difference between leaf and air (VPD) was lower than 2 kPa, and light intensity was 800–1000 µmol photons m−2 s−1 because of the greenhouse shading with black nets. No measurements were carried out during the afternoon because of the increasing air temperature which increased the VPD to more than 2 kPa, making it difficult to compare the photosynthetic rates with those obtained in the morning.

Gas-exchange and fluorescence measurements were repeated on four different, randomly chosen plants per treatment. Two leaves were sampled on each plant: the last leaf fully expanded before the imposition of salt stress (old) and the last fully expanded leaf which developed after the onset of salt stress (young). Means (n = 4) ± standard deviations are reported when old and young leaves were treated separately (Figs 1 & 2). Mean differences were statistically separated by Tukey test. When old and young leaves were not treated separately, eight single measurements for each cultivar are shown (Figs 3–6). Regression coefficients were calculated with SigmaPlot version 3.1 (SPSS Science, Chicago, IL, USA). Water deficit was assessed on three leaves per cultivar. Treatments (control and salt stress) were compared by pooling together measurements on different cultivars. Means ± standard deviations (n = 18) are shown in Fig. 3 and statistical differences were assessed by Tukey test.

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Figure 1. Photosynthesis of control (black bars) and salt-stressed (white bars) leaves of six different olive cultivars. Measurements were carried out in the last leaf developed before the onset of salt stress (old, a) and the last fully expanded leaf which developed after the onset of salt stress (young, b). Cultivars are ranked according to the photosynthesis of controls in old leaves. Means ± standard deviations (n = 4) are reported. Statistical significance of difference between control and salt-stressed leaves of each cultivar was assessed by Tukey test. * represents significance at 5% level of confidence.

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Figure 2. Relationship between photosynthesis reduction (% of controls) and Na accumulation in old (black) and young (white) leaves of olive cultivars. Symbols represent the different cultivars as shown in the legend. Means ± standard deviations (n = 4) are reported.

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Figure 3. Leaf Na accumulation (left axis), water potential (Ψ, left axis) and relative water content (RWC, right axis) of control (black bars) and salt-stressed (grey bars) olive leaves. All values are presented as mean + standard deviation (n = 24 for Na accumulation, n = 18 for water deficit measurements). Statistical significance of difference between control and salt-stressed leaves was assessed by Tukey test. * represents significance at 5% level of confidence.

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Figure 4. Relationship between photosynthesis and photochemical efficiency estimated by the quantum yield of PSII in dark adapted leaves (Fv/Fm) in control (black) and salt-stressed leaves (white). The different symbols indicate single measurements on leaves of the six different olive cultivars as shown in the legend.

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Figure 5. Relationship between photosynthesis and stomatal (a) or mesophyll conductance (b) in control (black) and salt-stressed (white) leaves of different olive cultivars. Each symbol represent an olive cultivar as shown in Fig. 4.

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Figure 6. Relationship between photosynthesis and CO2 draw-down from ambient (Ca) concentration to the intercellular (Ci) concentration (a), or from Ca to the chloroplast (Cc) concentration (b). Symbol legend as in Fig. 4.

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Gas-exchange and fluorescence measurements

Measurements of gas exchange were made in situ by using a portable infrared analyser (Licor 6400; Li-Cor Inc., Lincoln, NE, USA) with no temperature and light control system. Thus every measurement was carried out under ambient temperature and light intensity, as specified previously. The gas exchange cuvette window was modified to accommodate the fluorescence probe (MiniPAM; Walz, Effeltrich, Germany). The tip of the optic fibre of the MiniPAM was inserted in one of the window extremities at an angle of 45°. With this setting the optic fibre was placed at about 1 cm from the leaf without shading it. A leaf was clamped in the cuvette and exposed to the ambient light intensity until photosynthesis was steady. Then, photosynthesis, stomatal conductance and chlorophyll fluorescence yield were simultaneously measured. The fluorescence yield (i.e. the quantum yield of PSII in the light, ΔF/Fm) was measured by using a saturating pulse (10 000 µmol  m−2 s−1) of white light (Genty et al. 1989). The electron transport rate was measured by fluorescence as indicated by (Genty et al. 1989). This required also a measurement of electron transport under low (2%) O2 to calibrate the system under non-photorespiratory conditions. The electron transport rates measured under low O2 by fluorescence and gas-exchange were not statistically different. Once the electron transport rate is known, the mesophyll conductance is the only unknown variable in the analytical equation calculating the electron transport proposed by Farqhuar, von Caemmerer & Berry (1980), and can be easily calculated as explained in details by Harley et al. (1992) and Loreto et al. (1992).

The photochemical efficiency was assessed in the same leaves that were used for gas exchange by measuring the quantum yield of PSII (the ratio between variable and maximal fluorescence Fv/Fm) after 12 h of dark adaptation. Details about fluorescence nomenclature and measurements are reported by Van Kooten & Snel (1990). The 12 h dark-adapted leaves were also used to detect dark respiration by conventional gas-exchange.

Na accumulation and water content measurements

Leaves used for gas exchange measurements were harvested, dried in an oven at 70 °C for 48 h and then ground to a fine powder. One-half gram of the ground leaf material was transferred to a 250 mL beaker with 100 mL dilute nitric acid for 24 h. Sodium concentration on the extracts was determined using a flame photometer (PEP-7; Jenway, Dunmow, UK).

Pre-dawn water potential and relative water content were measured at the time at which the leaves were harvested for Na determination. Leaves were collected at 0600 h. Water potential was measured with a PMS (Corvallis, OR, USA) pressure bomb.

RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

The cultivars were significantly different for their inherent photosynthesis, that is, for the photosynthetic rate in non-stressed (control) conditions (Fig. 1). Photosynthesis was inhibited when a moderate accumulation of salt in the leaves occurred. The inhibition was observed in both old and young leaves (Fig. 1a & b). Photosynthesis was generally more inhibited (relative to controls) in cultivars with inherently high rates than in those with inherently low rates. Throubolia and Valanolia were the cultivars most sensitive to salt with photosynthesis reduction averaging 90% in both old and young leaves. Kerkiras photosynthesis was also reduced in salt-stressed leaves but remained higher than in all other salt-stressed cultivars. On the other hand, salt-dependent photosynthesis inhibition was lower, and even not statistically significant, in cultivars with inherently low photosynthesis such as Adramitini and Chalkidikis. Changes of photosynthesis were not associated with changes of dark respiration. Respiration was 0.9 ± 0.6 µmol CO2 m−2 s−1 (mean ± standard deviation) and was not statistically different among cultivars and between treatments.

Photosynthesis inhibition was not associated with Na accumulation inside the leaves (Fig. 2). Na accumulation was not significantly different in old and young leaves. The only exception was found in the old leaves of Throubolia in which Na was more than 0.5% of the leaf dry weight and photosynthesis was severely inhibited. Since we did not observe a significantly different sensitivity of photosynthesis to salt in young and old leaves, the results concerning these two samples will be presented and discussed together, starting with the following figure. Na accumulation caused a reduction of leaf pre-dawn water potential and relative water content (Fig. 3). As in the case of Na, there was no clear association between the increased water deficit and photosynthesis inhibition at cultivar and leaf age level.

The photochemical efficiency, as estimated by the ratio between variable and maximal fluorescence, was above 0.700 in most of the control leaves and was slightly associated with the photosynthetic rate of the single leaves (Fig. 4, r2 of the linear regression = 0.57). In salt-stressed leaves, however, a photochemical efficiency ranging between 0.800 and 0.500 was found, independently of the low photosynthetic rates.

There was a good linear correlation (r2 = 0.70) between photosynthesis and stomatal conductance when controls and salt-stressed leaves were pooled together (Fig. 5a). An even better correlation (r2 = 0.80) was found between photosynthesis and mesophyll conductance. However, photosynthesis was apparently not influenced by mesophyll conductances higher than 50 mmol m−2 s−1 in salt-stressed leaves (Fig. 5b).

Stomatal and mesophyll resistances progressively reduce the CO2 concentration reaching the chloroplasts. The inverse correlation between CO2 draw-down caused by low stomatal conductance (the difference between ambient and intercellular CO2 concentration, Ca − Ci) and photosynthesis was, however, absent both in controls and salt-stressed leaves (r2 = 0.05 for the pooled data). In particular, different photosynthesis corresponded to similar CO2 draw-down in control leaves, and the opposite was true in salt-stressed leaves (Fig. 6a). When the CO2 draw-down caused by both low stomatal and mesophyll conductances (the difference between ambient and chloroplast CO2 concentration, Ca − Cc) was compared with the corresponding photosynthesis, a significant inverse correlation (r2 = 0.70) was obtained (Fig. 6b).

DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Photosynthesis was inhibited by salt stress but to a variable extent in the six cultivars tested. In general, the highest inhibition was observed in the cultivars which had inherently high photosynthesis (Fig. 1) and stomatal conductance (Fig. 5). A higher uptake and accumulation of salt in the leaves of cultivars with higher stomatal conductance could explain their stress sensitivity. However, salt accumulated to the same extent in leaves of all cultivars with only one exception (Fig. 2). Thus, as already shown for herbaceous species such as rice (Yeo et al. 1990), the selection of genotypes for salt resistance on the basis of salt accumulation in leaves may often be unsuccessful. Photosynthesis was found to be independent of Na concentration also in olive recovering from salt stress (Tattini et al. 1995). Perhaps olive photosynthesis was indirectly limited by the lower water uptake in moderately salt-stressed plants, whereas the toxic effect caused by salt accumulation would become visible at higher Na concentration, as already proposed for the limitation of salt-stressed plant growth (Munns 1993; Neumann 1997). We show that the accumulation of Na in fact significantly reduced both water status parameters (Fig. 3) although no clear association between water deficit and photosynthesis reduction was found (data not shown). In general, photosynthesis was sensitive to very low Na accumulation in the leaves, if compared with results obtained in spinach on the same leaf dry weight basis (Delfine et al. 1999). This may be due to the elevated dry weight of sclerophyllous olive leaves. However, a similar result is obtained if Na accumulation is calculated as tissue water concentration rather than as a percentage of leaf dry weight (data not shown).

It has been reported that salt accumulates prevalently in old olive leaves that eventually drop (Bongi & Loreto 1989). This would allow young leaves to be relatively unaffected by salt and to efficiently photosynthesize, and is similar to a mechanism of salt exclusion (Munns 1993). In our experiment a high salt accumulation was evident only in old Throubolia leaves, but photosynthesis was equally inhibited in both young and old leaves (Fig. 2). Furthermore, no leaf shedding was observed in the salt-tolerant Frantoio olive cultivar (Tattini et al. 1995). Evidently this mechanism of resistance to salt is not a common trait in olive cultivars.

In order to explain the causes of the different photosynthesis inhibition of our olive cultivars, we tested the functionality of the photochemical apparatus, and the substrate availability for photosynthesis which may in turn cause biochemical limitations. The ratio between variable and maximal fluorescence in dark-adapted leaves (Fv/Fm) was not constant, but was very often within a physiological range (> 0.700) (Björkman & Demmig 1987) in control leaves of all cultivars. Variations of this fluorescence parameter were slightly associated with variations of photosynthesis (Fig. 4). This indicates that, when light is the limiting factor, the photochemical efficiency may effectively contribute to set aphotosynthesis ceiling for non-stressed olive leaves. In salt-stressed leaves, the variability of photochemical efficiency was much higher than in controls and Fv/Fm values as low as 0.500 were found. This span of Fv/Fm values, however, was not associated with photosynthesis changes. We conclude that in salt-stressed leaves, and particularly in those of cultivars Chalkidikis and Valanolia, the photochemical efficiency was strongly impaired, although this should not have been directly responsible for the observed photosynthetic limitation observed in our experimental conditions (i.e. at high light intensity). This strong impairment of photochemical reaction was not observed in salt-stressed herbaceous plants (Brugnoli & Lauteri 1991; Delfine et al. 1999). The absence of photochemical and biochemical impairments allowed photosynthesis recovery in herbaceous plants when salt-stress was relieved (Delfine et al. 1999). We speculate that in olive, on the other hand, photosynthesis of recovering plants might be limited or delayed by the photochemical impairment. Dark adaptation allowed us also to measure respiration. We found no clear effect of salt stress on dark respiration and interpret this as a further indication that the stress was mild and did not activate repair mechanisms that could be assessed from an increase of the respiratory cost. It should be mentioned that the limited amount of leaf material introduced in our gas-exchange cuvette made the respiratory signal very small and rather inaccurate. Our measurements of dark respiration are however, indirectly confirmed by the estimation of Rd on the basis of CO2 response as shown in our companion article (Centritto, Loreto & Chartzoulakis 2003).

Photosynthesis at low internal CO2 concentration (assuming the intercellular CO2 equal to the chloroplastic CO2) estimates the Rubisco activity (Farquhar & Sharkey 1982). Photosynthesis of control and salt-stressed plants was similar in most of the cultivars under low internal CO2, as shown by the response of photosynthesis to mesophyll conductance (Fig. 5). In Fig. 6, the CO2 draw-down was calculated from Fig. 5 data to emphasize possible changes in photosynthesis at low CO2. This elaboration confirmed that salt did not cause an evident shift of the photosynthetic rate when the CO2 draw-down was heavy and the internal CO2 was low. Therefore, we think that Rubisco activity was not generally impaired in olive salt-stressed plants. This is also confirmed by laboratory experiments (photosynthesis response to CO2 and calculation of Vcmax) made on plants with contrasting salt sensitivity (cv. Kerkiras and Chalkidikis), as shown in our companion paper (Centritto et al. 2003). A similar result was also shown for herbaceous species exposed to moderate salt-stress but accumulating more salt than in our experiment (Delfine et al. 1998, 1999). It should be pointed out, however, that salt-stressed plants of cv. Agouromanaki and Valanolia do show a different slope of the relationship between photosynthesis and CO2 draw-down with respect to the other plants. Although more replications would be needed to ascertain the significance of this shift, this result indicates that salt may affect the biochemical reactions of some olive cultivars.

It has been reported that a moderate salt accumulation results in a partially reversible reduction of stomatal and mesophyll conductances in spinach leaves. This, in turn, inhibits photosynthesis (Delfine et al. 1999). Bongi & Loreto (1989) also attributed the inhibition of photosynthesis in salt-stressed olive leaves to a reduced mesophyll conductance. However, the mesophyll conductance reduction was too small in their salt-stressed leaves to explain the rather strong photosynthesis inhibition at high VPD. At low VPD, on the other hand, the mesophyll conductance was surprisingly high for a sclerophyllous plant (Lloyd et al. 1992; Loreto et al. 1992). In our experiment, which was conducted at a VPD that was lower than 2 kPa, olive mesophyll conductance was always below 250 mmol m−2 s−1 and often below 100 mmol m−2 s−1. The technique used in our study to estimate mesophyll conductance is very precise within this range of values (Harley et al. 1992). Therefore it is likely that in olive leaves the chloroplast CO2 concentration is lower than previously reported by using an iterative method for its calculation (Bongi & Loreto 1989). Photosynthesis of control leaves was associated with stomatal and mesophyll conductance whereas in stressed leaves mesophyll conductances higher than 50 mmol m−2 s−1 did not result in increasing photosynthesis (Fig. 5b). Apparently there are factors other than mesophyll conductance limiting photosynthesis in salt-stressed olive leaves that maintain photosynthesis higher than 5 µmol m−2 s−1, such as in cultivar Kerkiras. However, this limitation does not operate when mesophyll conductance is strongly reduced by the stress and the CO2 concentration at the chloroplast is low, such as in all other cultivars.

Photosynthesis of control and stressed plants was not strongly associated with the draw-down of CO2 caused by stomatal resistances (the difference between ambient and intercellular CO2 concentration, Ca − Ci) (Fig. 6a). However, a strong inverse correlation between photosynthesis and the CO2 draw-down was found when taking into account not only stomatal but also mesophyll conductance (Ca − Cc) (Fig. 6b). This correlation holds also for those stressed plants on which photosynthesis was not limited by high mesophyll conductance (Fig. 5). Interestingly, control leaves of cultivars with inherently low photosynthesis also fit this relationship as, for example, in the case of cultivar Chalkidikis. When leaves of this cultivar were salt-stressed, no further significant reduction of the CO2 draw-down (and of photosynthesis, see Fig. 1) was found. This is a clear indication that the chloroplast CO2 concentration, as set by the combination of stomatal and mesophyll resistances to CO2 diffusion, is the main limitation of photosynthesis for cultivars characterized by low photosynthesis as well as for cultivars sensitive to, and strongly affected by salt-stress.

In conclusion we found that the sensitivity to salt of olive photosynthesis is not dependent on Na accumulation, at least when salt accumulation is moderate. The sensitivity to salt is higher in cultivars with inherently high photosynthesis (e.g. Kerkiras) than in those with low photosynthesis (e.g. Chalkidikis). This is neither fully explained by the observed reduction of photochemical efficiency nor by possible changes in Rubisco properties of salt-stressed leaves. The very good association found between low photosynthesis and low mesophyll conductance, or high CO2 draw-down, both in cultivars with inherently low photosynthesis and in salt-stressed plants of all cultivars, indicates that the main limitation of photosynthesis in olive is the low chloroplastic CO2 concentration. This suggests that the stress effect may be reversed if the conductance to CO2 diffusion is restored. It also indicates that olive cultivars with inherently low stomatal and mesophyll conductance probably will be little affected by salt-stress. If these traits are associated with a sustained productivity, these cultivars may be of interest in the future for the exploitation of saline soils and for the use of saline water.

ACKNOWLEDGMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

This work has been carried out within the frame of the Greek-Italian Bilateral Cooperation for Research & Technology 1999–2001 (Contract no. GSRT-18345). The assistance of Mrs Maria Moutsopoulou during gas-exchange measurements is gratefully acknowledged.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES
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Received 8 October 2002; received in revised form 14 October 2002; accepted for publication 14 October 2002