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Published in Crop Sci. 44:806-811 (2004).
© 2004 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

CROP PHYSIOLOGY & METABOLISM

Sorghum and Salinity

II. Gas Exchange and Chlorophyll Fluorescence of Sorghum under Salt Stress

Godfrey Wafula Netondoa, John Collins Onyangoa and Erwin Beck*,b

a Department of Botany, Faculty of Science, Maseno University, P.O. Private Bag, Maseno, Kenya
b Department of Plant Physiology, University of Bayreuth, Universitätstraße 30, 95440, Bayreuth, Germany

* Corresponding author (erwin.beck{at}uni-bayreuth.de).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Photosynthetic activity decreases when plants are grown under saline conditions leading to reduced growth and productivity. Leaf growth, gas exchange, and chlorophyll fluorescence of two sorghum [Sorghum bicolor (L.) Moench] varieties, Serena and Seredo, were measured in response to increasing NaCl concentration. Sorghum plants were grown in sand culture under controlled greenhouse conditions. The NaCl concentrations in complete nutrient solution were 0 (control), 50, 100, 150, 200, and 250 mM. Salinity significantly (P ≤ 0.01) reduced leaf area by about 86% for both varieties of sorghum. Chlorophyll a and b, net CO2 assimilation, stomatal conductance, and transpiration rate decreased significantly (P ≤ 0.01) with the increase in salinity, and these decreases were similar for the two sorghum varieties. Salt induced decreases for these physiological traits ranged from 75 to 94%. Photochemical efficiency of PSII (Fv/Fm) and photochemical quenching coefficient (qP) decreased by about 9 and 10%, respectively, for both varieties, and electron transport rate (ETR) decreased by 20 and 25% for Serena and Seredo. In contrast, non-photochemical quenching (NPQ) significantly (P ≤ 0.01) increased by 44 and 50% for Serena and Seredo. The results indicate that salinity affected photosynthesis per unit leaf area indirectly through stomatal closure, and to a smaller extent through direct interference with the photosynthetic apparatus. In addition, salinity decreases whole plant photosynthesis by restricting leaf area expansion. This effect starts from low levels of salinity, in contrast to that of net photosynthesis per unit leaf area, which occurs at higher levels of NaCl concentration.

Abbreviations: Chl, Chlorophyll • DASA, days after start of salt application


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SALINITY IS ONE of the most important environmental factors limiting crop production of marginal agricultural soils in many parts of the world. Salinity effects on plants include ion toxicity, osmotic stress, mineral deficiencies, physiological and biochemical perturbations, and combinations of these stresses (Munns, 1993, 2002; Neumann, 1997; Yeo, 1998; Hasegawa et al., 2000).

Sorghum is moderately tolerant to salinity (Maas et al., 1986) and is widely grown in semiarid areas of East Africa on soils prone to salinity. Substantial genotypic differences exist among sorghum cultivars in response to salinity (Weimberg et al., 1982). Classical methods of screening for salt tolerance are based on yield response and are time consuming and expensive. Measurements of major physiological traits, including photosynthesis, can be used to monitor noninstantaneous plant responses to salt stress (Belkhodja et al., 1994). Salt stress decreases photosynthesis through stomatal and nonstomatal factors (Yeo et al., 1985; Sharma and Hall, 1991; Dionisio-Sese and Tobita, 2000). The latter are not yet fully understood. Measurements of chlorophyll a fluorescence provides quantitative information about photosynthesis through noninvasive means (van Kooten and Snel, 1990). Salinity responses of chlorophyll a fluorescence have been studied in barley (Hordeum vulgare L., Larcher et al., 1990; Belkhodja et al., 1994), rice (Oryza sativa L., Lutts et al., 1996), and sorghum (Sharma and Hall 1991, 1992; Lu and Zhang, 1998). Detached leaves, leaf discs, cell cultures, or isolated chloroplasts have been used to quantify chlorophyll a fluorescence but investigations of intact leaves under field conditions are scarce. Previous reports of NaCl effects on chlorophyll a fluorescence of diverse plant organ, tissue, and cell preparation have been inconsistent. For instance, Larcher et al. (1990), Brugnoli and Lauteri (1991), Mishra et al. (1991), Belkhodja et al. (1994)(1999) and Jimenez et al. (1997) reported no significant change in the photosynthetic quantum yield (Fv/Fm) in response to NaCl treatments. These authors concluded that Fv/Fm was not a useful indicator of salt stress. In contrast, Smillie and Nott (1982), Bongi and Loreto (1989) and Misra et al. (2001) suggested Fv/Fm was an early indicator of salt stress. Yet, Fv/Fm could have been due to stressors associated with increasing NaCl concentration (Everard et al., 1994; Lutts et al., 1996). Photosynthetic performance is usually enhanced by additional environmental stresses, for example high light irradiance (Sharma and Hall, 1991, 1992, Jimenez et al., 1997), and low or high temperatures (Larcher et al., 1990; Lu and Zhang, 1998) can be associated with NaCl stress and contribute to Fv/Fm response.

There are only limited data available concerning salinity response of variables other than the Fv/Fm ratio. It is still an open question as to whether salinity directly affects functioning of PSII or whether stomatal closure is the main factor inhibiting photosynthesis and biomass production under NaCl stress. The present study was conducted to quantify and evaluate the responses of total leaf area and physiological traits of photosynthesis to increasing NaCl concentrations in the rooting medium of two drought resistant varieties of Kenyan sorghum (Serena and Seredo). Varietal differences could be used as a source of variation for future plant breeding programs. These measurements of variables related to photosynthetic performance will compliment measurements of growth, water relations, and mineral ion distribution in tissues to salt stress (Netondo et al., 2004).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Growth and Salt Treatment
Two Kenyan varieties of sorghum, Serena and Seredo, were grown as detailed previously (Netondo et al., 2004). In brief, single plants were grown in 1.5-L pots in a greenhouse at the University of Bayreuth, Germany, from October 1997 to January 1998. Day and night temperatures were 34 and 24°C, respectively, with natural dawn and dusk transition of photoperiod. Average photon flux density from 350 to 400 µmol quanta m–2 s–1 (maximum 600 µmol quanta m–2 s–1 at 1200 h) comprised natural illumination fortified by light from tungsten lamps for 12 h. Recorded relative humidity values ranged from 32% during day to 80% at night. Plants were grown in sand irrigated with a nutrient solution (pH of 6–6.5) prepared according to Maas et al. (1986). The conditions are similar to those in Kenya, except light intensities, which are higher, ranging from about 600 to 2000 µmol quanta m–2 s–1 (Kinyamario and Imbamba, 1992).

Salt treatment was started 14 d after planting. Sodium chloride was added to nutrient solution to provide final concentrations of 0 (control), 50, 100, 150, 200, and 250 mM. The respective concentrations corresponded to electrical conductivities of 0.01, 3.42, 6.74, 9.66, 12.40, and 15.01 dS m–1. The NaCl concentration greater than 50 mM were imposed incrementally by 50-mM steps every 2 d until final concentrations were reached. Irrigation was applied twice per day.

Leaf Area Measurements
Total leaf area per plant was measured at harvest with a leaf area meter (Model 3000, LI–COR Inc.; Lincoln, NE) 25 d after start of salt application (DASA).

Gas Exchange Measurements
A calibrated portable photosynthesis system (LI-6400; LI-COR, Inc.; Lincoln, NE) was used to measure net CO2 assimilation rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration 25 DASA. Each measurement of fully expanded leaves was repeated with leaves of similar age (leaf number 8 from below) of three plants, between 0900 and 1500 h. Carbon dioxide was provided from a steady external source (LI-COR Inc.) and maintained at 400 µL L–1 within the leaf chamber throughout the measurement. Photosynthetically active radiation (PAR) of 1200 µmol m–2 s–1 was supplied by the 6400-02 LED (LI-COR) light source in the leaf chamber. Calculations were made according to von Caemmerer and Farquhar (1981).

Chlorophyll Measurement
Chlorophyll concentration was determined with four replicate plants, from fully expanded leaves (leaf number 8). A leaf sample of 0.1 g was ground and extracted with 5 mL of 80% (v/v) acetone in the dark. The slurry was filtered and absorbancies were determined at 645 and 663 nm. Concentration of chlorophyll a (Chl a), and chlorophyll b (Chl b) were estimated by the equations of Arnon (1949).

Chlorophyll Fluorescence Measurements
Chlorophyll fluorescence was determined with intact plants in the greenhouse with a Pulse-amplitude-modulated chlorophyll fluorometer (PAM: H. Walz GmbH, Effeltrich, Germany). Leaves previously selected for measurement of gas exchange were used for fluorescence measurements. Measurements were made on upper (adaxial) surface of leaves, which had been predarkened for at least 30 min. Measurements included: basic fluorescence upon exposure to weak light after dark adaptation (Fo), basic fluorescence after light adaptation (F'o), maximum fluorescence (Fm), maximum fluorescence yield of a light adapted leaf exposed to a pulse of saturating light (F'm), and steady state fluorescence of a light adapted leaf (Fs) (van Kooten and Snel, 1990). These measurements were used to determine quenching and electron transport rate. The potential maximal efficiency of PSII (Fv/Fm) of dark adapted leaves was calculated as Fv/Fm = (Fm – Fo)/Fm. Photochemical quenching (qP) was calculated as qP = /. Nonphotochemical quenching (NPQ), which refers to the nonradiative dissipation of energy, was calculated as NPQ = /F'm. Electron transport rate (ETR) was calculated as ETR = {Delta}F/Fm x PF; where PF stands for photon flux (µmol m–2 s–1) and {Delta}F represents F'm – Fs. Fluorescence nomenclature is according to van Kooten and Snel (1990).

Statistical Analysis
The experiment was a complete randomized design consisting of six salinity treatments, two varieties and three replications. Analysis of variance (ANOVA) was performed with the statistical program Minitab (Minitab Inc.; College Park, PA), involving two levels of classification (salinity and variety) with interactions. Means were separated using the least significant difference (LSD) test at 5% level. LSD (0.05) values were compounded from ANOVA computations as opposed to paired comparisons (Sokal and Rohlf, 1995). For statistical comparisons the LSD (0.05) value can be used to compare the difference between any combination of two means within a table or figure. The variety by salinity level interactions were significant at P ≤ 0.05 for all the ANOVA analyses where LSD (0.05) value has been presented.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Effect of Salinity on Leaf Growth
Increasing NaCl concentrations in the irrigation solution significantly (P ≤ 0.01) decreased total leaf area of both sorghum varieties (Fig. 1) . At 250 mM NaCl total leaf area was reduced by 80% compared with the controls. The leaf area of both varieties was nearly equal at all NaCl concentrations, except 100 mM, suggesting no difference in varietal response to NaCl.



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  		Fig. 1. Effects of increasing NaCl concentration in the irrigation medium on total leaf area per plant of the two sorghum varieties Serena and Seredo. Means of 4 replicates ±SD.

 
Effect of Salinity on Leaf Chlorophyll Concentration
Chl a and Chl b declined significantly (P ≤ 0.01) with increasing salinity (Fig. 2) . The 250 mM NaCl treatment induced 70 and 58% decrease in Chl a concentration in Serena and Seredo, respectively. Similarly, Chl b concentration decreased with 68 and 69% for Serena and Seredo. Serena had significantly more Chl a and Chl b than Seredo at NaCl concentrations of 0 and 50 mM. The strongest effect of salt occurred with Serena between 50 and 100 mM NaCl. Chlorophyll a/b ratio was not affected significantly (P > 0.05) by increasing salinity.



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  		Fig. 2. Effects of increasing NaCl concentration in the irrigation medium on chlorophyll a (A), chlorophyll b (B), and chlorophyll a/b ratio (C) in the two sorghum varieties Serena and Seredo. Means of 4 replicates ±SD.

 
Effect of Salinity on Gas Exchange
Net CO2 uptake rates (Fig. 3A) of both varieties decreased by about 25% with increasing external NaCl concentration up to 200 mM. At 250 mM external NaCl concentration, however, net CO2 uptake was almost completely inhibited, having decreased by more than 90%. The impact of salinity on stomatal conductance (Fig. 3B) and transpiration (data not shown) was similar to that on net CO2 uptake. There was no significant (P ≥ 0.05) effect of salinity on Ci (Fig. 3C). Varietal differences were not significant, although at 250 mM NaCl Serena had rather low Ci while Seredo exhibited extremely high values.



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  		Fig. 3. Effects of increasing NaCl concentration in the irrigation medium on CO2 assimilation rate (A), stomatal conductance (B), and intercellular CO2 concentration (C) of the two sorghum varieties Serena and Seredo. Means of 3 replicates ±SD.

 
A positive correlation (r2 = 0.83 and 0.91 for Serena and Seredo, respectively) between CO2 net uptake and stomatal conductance was observed for both varieties (Fig. 4A and B) .



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  		Fig. 4. Relationship between CO2 assimilation rate and stomatal conductance for NaCl stressed sorghum varieties Serena (A) and Seredo (B). Data points represent individual measurements.

 
Effect of Salinity on Chlorophyll Fluorescence
Up to an external NaCl concentration of 200 mM, quantum yield, as indicated by Fv/Fm, was in the range of 0.77 to 0.79 for Serena and Seredo, respectively (Fig. 5) . A significant, but not dramatic decline of about 9% occurred for plants irrigated with 250 mM NaCl in the nutrient solution. Concomitantly with this decrease the rate of electron transport dropped by about 20% for Serena, and 25% for Seredo. This decrease was accompanied by a more than twofold increase of NPQ, in both varieties (50% for Serena and 44% for Seredo). Photochemical fluorescence quenching, on the other hand was not dramatically impaired, as it declined in both varieties by not more than 10% between controls and 250 mM NaCl (data not shown).



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  		Fig. 5. Effects of increasing NaCl concentration in the irrigation medium on Fv/Fm ratio (A), electron transport rate (B), and non-photochemical quenching (C) of the two sorghum varieties Serena and Seredo. Means of 3 replicates ±SD.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Total leaf area of sorghum plants was markedly reduced by salinity (Fig. 1). A decrease in leaf area may be attributed to early senescence and death, reduced growth rate or delayed emergence (Bernstein et al., 1993). In the experiments reported, folded upper parts of the leaf, rapid drying of tips of lamina, or both were indicative of abnormal leaf development. Unfolded older leaves tended to trap younger leaves whose margins finally showed several fissures. This abnormal leaf development has been termed "developmentally perturbed leaf syndrome" (Amzallag et al., 1993) and has been associated with deficiency of calcium (Maas and Grieve, 1987; Grieve and Maas, 1988). Indeed, in the two sorghum varieties investigated in this work, a reduced Ca2+ concentration of leaf blades was measured under salt-stress (Netondo et al., 2004). The negative impact of NaCl on leaf area development was evident at the lowest salt concentration (50 mM) and increased linearly with increasing salinity. This is considered as one of the major reasons for a lowered carbon gain and growth under salt stress.

There was little impact of salt on CO2 assimilation rates at moderate NaCl concentrations, as has been reported also for other varieties of sorghum (Masojidek and Hall, 1992; Nagy et al., 1995; Sharma and Hall, 1991). The sorghum response to NaCl is similar to that of Phaseolus vulgaris L. (Seemann and Critchley, 1985; Brugnoli and Lauteri, 1991), Triticum aestivum L., and Hordeum vulgare L. (Rawson, 1986; Sharma and Hall, 1991). The reduction of CO2 assimilation rate at the highest salt concentration, especially with variety Seredo, could be attributed to NaCl effects on stomatal closure (Fig. 3 and 4). The small and statistically nonsignificant decrease in Ci at salt concentrations up to 200 mM indicates photosynthesis was not substantially affected. Closure of the stomata could reduce internal CO2 concentration and CO2 assimilation rate (Dionisio-Sese and Tobita, 2000). However, a positive correlation between stomatal conductance and CO2 assimilation rate suggests stomatal conductance as the primary factor limiting photosynthesis under salt stress.

Stomatal factors are generally more significant at medium salinities and nonstomatal limitations are more relevant at high salinity (Everard et al., 1994). Direct effect of salinity on photosynthetic CO2 net uptake may reflect several impacts, two of which could be substantiated in our study: (i) a reduced chlorophyll concentration of the leaves of plants grown at NaCl concentrations higher than 100 mM and (ii) decrease of the concentrations of essential ions such as Ca2+ and Mg2+ in the mesophyll cells (Netondo et al., 2004). As discussed by Reddy and Vora (1986), salinity could affect chlorophyll concentration of leaves through inhibition of synthesis of chlorophyll or an acceleration of its degradation. Impairment of the carboxylation capacity, which in turn inhibits electron transport, is indicated by the measurements of chlorophyll a fluorescence.

A significant decrease of photosynthetic quantum yield was observed with both sorghum varieties at high salinity, which underlines the drop of photosynthetic net CO2 uptake. In contrast to slight decrease of the CO2 assimilation rates caused by stomatal closure at external NaCl concentrations up to 200 mM, photosynthetic quantum yield, electron transport and photochemical fluorescence quenching were unaffected. Nonphotochemical quenching increased slightly in plants grown at 200 and 250 mM NaCl. These findings contrast with those reported by Sharma and Hall (1991)(1992) on other varieties of sorghum, who observed a reduction of the quantum yield by 14 to 17% between 0 and 200 mM NaCl. Their results were, however, produced with isolated chloroplasts and not with intact leaves. Data from intact leaves of sorghum (Lu and Zhang, 1998) and other plant species (Larcher et al., 1990; Belkhodja et al., 1994; Mishra et al., 1991) were interpreted to indicate that salinity does not affect the Fv/Fm ratio. Contradictory results have been reported for rice, mungbean [Vigna radiata (L.) R. Wilczek], and Brassica seedlings (Lutts et al., 1996; Dionisio-Sese and Tobita, 2000; Misra et al., 2001). These results imply that effects of salt on potential photochemical efficiency of PSII may be species specific.

A reduced quantum yield of about 0.72 as obtained in our experiments (Fig. 5A), may result from a structural impact on PS II (Everard et al., 1994; Lutts et al., 1996; El-Shintinawy, 2000) although Lu and Zhang (1998) found PSII to be highly resistant to salinity stress. Salinity has been concluded to affect reaction centers of PSII either directly (Masojidek and Hall, 1992) or via an accelerated senescence (Hasson and Poljakoff-Mayber, 1981; Kura-Hotta et al., 1987). A structural change of PS II, its immediate surrounding or both is suggested by the increase of NPQ in plants grown at a salt concentration higher than 150 mM (Fig. 5C). The rise in NPQ may also reflect the fact that reduced CO2 assimilation decreases demand for products of electron transport, and thus increases thermal dissipation of light energy.

There was a decline in electron transport rate by approximately 20 to 25% for both varieties between controls and 250 mM NaCl (Fig. 5B). This effect appears mild and indicates that electron flow was not severely affected. High external salt concentrations could affect thylakoid membranes by disrupting lipid bilayer or lipid-protein associations and thus impair electron transport activity (Reddy and Vora, 1986). Previous measurement of linear electron transport (Mishra et al., 1991) revealed that salinity does not affect electron transport in wheat. Effects of salinity on rate of electron transport could, however, be species specific (Mishra et al., 1991).

In conclusion, results presented here show that salinity reduces leaf growth and gas exchange variables. The remarkable reduction of total plant leaf area is likely to affect whole plant photosynthesis, contributing to the low biomass production. Effects of external NaCl on CO2 assimilation and other gas exchange variables started at relatively low concentrations. However, a drastic impact on the primary photosynthetic processes was observed only at high NaCl concentrations beyond 200 mM. Chlorophyll a fluorescence appears to be a useful indicator of salt stress at high NaCl concentrations.


   ACKNOWLEDGMENTS
 
We thank the Deutsche Akademische Austauschdienst (DAAD) for financial support to GWN, Dr. Jens Hansen for technical assistance with the chlorophyll fluorescence and gas exchange measurements and Prof. Dr. E. Steudle for providing research facilities.

Received for publication December 2, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 


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