Salinity-induced Potassium Deficiency Causes Loss of Functional ...

Aust. J. Plant Physiol., 1987, 14, 351-61

Salinity-induced Potassium Deficiency Causes Loss of Functional Photosystem I1 in Leaves of the Grey Mangrove, Avicennia marina, Through Depletion of the Atrazine-binding Polypeptide Marilyn C. BallA, W. S. c h o w Band Jan M. Anderson A North Australia Research Unit 1Department of Biogeography and Geomorphology, Research School of Pacific Studies, Australian National University, G.P.O. Box 4, Canberra, A.C.T. 2601, Australia. Division of Plant Industry, CSIRO, G.P.O. Box 1600, Canberra, A.C.T. 2601, Australia.

Abstract Photosynthetic properties were studied in relation to the ionic composition of leaves of Avicennia marina grown in low and high salinity (i.e. 50 and 500 mol m - NaCl) nutrient solution containing either 0.01 o r 10 mol m - 3 K + . Leaves accumulated high concentrations of NaCl, but changes in photosynthesis were associated with changes in leaf K + concentrations. The effects occurred at two levels. (1) With decrease in leaf K + from 379 to 167 mol m-', a 21% decline in light and CO2 saturated rates of oxygen evolution per leaf area was consistent with a 24% decrease in chlorophyll content. (2) Leaves containing only 103 mol m - j K + showed drastic loss of light and COz saturated photosynthetic capacity (42%) and photochemical dysfunctioning under limiting light conditions as manifest in a 38% decrease in quantum yield. Thylakoids isolated from these low K + leaves showed n o decrease in per chlorophyll concentrations of photosystem I, cytochrome f / b complex and ATPase, but had 37% fewer atrazine-binding sites (corresponding to photosystem I1 reaction centres) than those from leaves with higher K + concentrations. The decline in atrazine-binding sites in isolated thylakoids was sufficient to account for the loss of quantum yield in intact leaves. These results identify the atrazine-binding polypeptide of photosystem I1 as one site of sensitivity to salinity-induced K + deficiency.

Introduction

Changes in the photosynthetic characteristics of halophytes with variation in salinity may be related to changes in the ionic composition of leaves and chloroplasts. Leaves of halophytes characteristically accumulate high concentrations of NaC1, which apparently function in osmoregulation of the vacuole (Munns et al. 1983). Within limits, the vacuolar concentrations of Na+ and C1- increase with increase in salinity without affecting the concentrations of these ions in other cellular compartments. For example, chloroplasts of halophytes maintain relatively constant concentrations of Na' and C1over a wide range of growth salinities and hence also leaf ion concentrations (Kaiser et al. 1983; Robinson et al. 1983; Robinson and Downton 1984, 1985). Thus, changes in the bulk leaf concentrations of Na+ and C1- are not likely to influence photosynthesis except under extreme conditions when the NaCl influx may either be insufficient to meet nutritional demands or exceed the cellular capacity for ionic compartmentation. However, variation in salinity also affects the foliar concentrations of K C, a nutrient required in relatively high concentrations for photosynthetic metabolism (Huber 1985). The concentration of K C in leaves may decline with increase in salinity, apparently because high concentrations of NaC can interfere with K C uptake by roots (Munns et al. 1983). The K + concentrations in chloroplasts of several salt-tolerant species, Suaeda maritima (Harvey et al. 1981), S. australis (Robinson and Downton 1985), and Spinacia oleracea (Kaiser et al. 1983; Robinson et al. 1983; Robinson and Downton 1984), decline with increase in the salinity in which the plants were grown. Thus, the KC 0310-7841 / 87 /030351$02.00

Marilyn C. Ball et a/.

concentrations in chloroplasts decline in parallel with salinity-induced decreases in bulk leaf K + concentrations (Robinson and Downton 1985). Hence, some of the salinityinduced changes in photosynthesis by halophytes, such as Avicennia marina, may be related t o photosynthetic requirements for K + and a decreasing capacity to satisfy those needs with increasing salinity.

Materials and Methods Plant Material Propagules of the grey mangrove, Avicennia marina (Forsk.) Vierh. var. australasica (Walp.) Moldenke, were collected from trees growing along Cullendulla Creek near Bateman's Bay, New South Wales, Australia (35"42'S., 15Oo12'E.). The propagules were placed on sand beds, subirrigated with 10 or 100% seawater, and allowed to develop under glasshouse conditions (natural sunlight, day/night air temperatures of 30120°C) until the seedlings had four leaves. The seedlings were then transferred to hydroponic cultures, and grown in aerated solutions of 10 or 100% seawater amended with 2 mol m - ) NH4N03, 0.2 mol m - j NaH2P04, and 2 mol m - 3 Fe-EDTA. The plants were grown in these two solutions for 1 week. Then the seawater solutions were replaced at a rate of 30% every 3 days with nutrient solution containing 50 or 500 mol m - 3 NaCl to approximate the salinities of 10 and 100% seawater, respectively. Other components of the nutrient solution were: 4 mol m--' Ca(NO&, 0.2 mol m - 3 NaHzP04,l mol m - 3 MgS04,2 mol m - 3 Fe-EDTA, 2 rnol m - 3 NHdNO3,0.025 mol m - 3 HsB03, 0.002 mol m - 3 MnS04, 0.002 rnol m - 3 ZnS04, 0.0005 mol m - 3 CuS04, 0.0005 mol m - 3 H2Mo04, and 0.025 mol m - 3 MgC12. The high and low potassium treatments were 10 and 0.01 mol m - 3 KCl, respectively. The high K + concentration is equivalent to that in seawater and the low K + concentration was arbitrarily chosen to produce K + deficient leaves. The plants were grown in this 2 x 2 arrangement of high/low salinity in combination with high/low K + concentrations until two pairs of leaves had been produced under the experimental conditions. All measurements were made on the youngest of these leaf pairs when they had been fully expanded for 1 week.

Photosynthetic Measurements on Leaves A leaf pair was detached and dark adapted for 20 min prior to measurements of room temperature Chl a* fluorescence characteristics in air. Excitation light from a quartz-iodide slide projector lamp was filtered through a Corning 4-72 4 mm broadband filter and adjusted to give an incident quantum irradiance of 400 pnol m - s - I. Shutter opening time was approximately 1.2 ms. Leaf fluorescence was collected with a light guide, transmitted through a Balzers B-20 685 nm interference filter and recorded with a photodiode/digital storage oscilloscope combination. Measurements were made on both leaves in the pair to check that they had similar fluorescence characteristics. One leaf was then divided into two parts, one for determination of K', Na' and C1- concentrations and the other for analysis of thylakoid composition. The second leaf was used immediately for further photosynthetic studies. The quantum yield of 0 2 evolution and the light saturated rates of photosynthesis at 25'C were measured using leaf discs (1.8 cm diameter) in air containing approximately 1% C02 in a leaf disc oxygen electrode (Hansatech, King's Lynn, U.K.) essentially as described by Delieu and Walker (1983). Oxygen exchange rates were first measured in darkness, then in a series of progressively increasing quantum irradiances, allowing at least 10 min at each irradiance to permit variables relating to gas exchange to attain steady state. Neutral density filters were used to obtain step changes in illumination provided by a quartz-iodide slide projector lamp. At the conclusion of gas exchange measurements, light absorption characteristics of the leaf disc were determined with a quantum integrating sphere. Quantum yield was calculated as the oxygen evolved per absorbed quantum over the range of light intensities at which the relationship is linear. Other details are given in Evans (1987). *Abbreviations used: CF1, coupling factor 1; Chl, chlorophyll; Cyt, cytochrome; F,, chlorophyll fluorescence level corresponding to open reaction centres; F v , variable chlorophyll fluorescence; P700, reaction centre chlorophyll of photosystem I; PS, photosystem; tricine, N-tris(hydroxymethy1)methylglycine.

Salinity-induced K C Deficiency and Photosynthesis

Measurements of Thylakoid Membrane Components Thylakoids were isolated from leaves of Avicennia marina as previously described (Ball and Anderson 1986), except that the extraction buffer contained 50 mol m - ' tricine-NaOH (pH 7.8) and the final wash buffer contained 1000 rnol m - sorbitol. The thylakoid membranes were pooled according to the salinity/KC treatment of the plants from which they had been isolated and stored at 77K until needed for assay. Chl was determined in 80% (viv) acetone (Arnon 1949). PS I1 reaction centres were assayed as atrazine-binding sites according to the general method of Tischer and Strotmann (1977), with modifications as described by Chow and Hope (1987). PS I reaction centres were assayed by the light-induced absorbance change at 703 nm, using a flash photometer constructed by Professor W. Haehnel. Details of the measurement have been reported (Chow and Hope 1987). Cyt f was estimated from hydroquinol-reduced minus ferricyanide-oxidised difference spectra (Bendall et al. 1971), using an Hitachi 557 double-beam spectrophotometer. The ~ g ~ + - s p e c i fATPase ic activity in chloroplast CFI was assayed in the presence of octyl glucoside (Pick and Bassilian 1981), with details as given in Chow and Hope (1987).

Measurements of Leaf Ionic Composition The foliar concentrations of K C , Na Farquhar 1984).

+

and C1- were determined as previously described (Ball and

Results

Ionic Composition of Leaves Avicennia marina exhibited a high degree of specificity for K + uptake under saline conditions. The K C : Na+ selectivity ratio (Pitman 1976) for leaves of plants grown under high and low K' treatments (i.e. 10 and 0.01 rnol m - 3 K', respectively) increased with increase in salinity from 50 to 500 rnol m - 3 NaCl (Table 1). The K + : Na+ selectivity ratios for leaves from the high K + treatment were similar to those previously reported for leaves of A. marina grown under similar salinity conditions (Downton 1982; Clough 1984). The extremely high selectivity ratios in leaves from the low K + treatment indicate the accumulation of high concentrations of K + in leaves relative to that in the nutrient solution. Table 1. Potassium selectivity in leaves of Avicennia marina grown under low and high salinity conditions (50 and 500 rnol m - 3 NaCl) with either 10 or 0.01 rnol m-' K + The K + : Na+ selectivity ratio (SK: N ~was ) calculated from (Kt1 Nat)/(K,/Nae) (Pitman 1976), where the subscripts t and c refer to concentrations in leaf tissue and culture solutions, respectively. Values for Kt/Nat and SK : N~ are meanfs.e.m., n = 5 Low salinity High K C Low K +

High salinity High K C Low K C

Despite the high selectivity ratios, the K f concentrations in leaves of plants grown with 10 and 0-01 rnol m - K + decreased 56 and 59%, respectively, with 10-fold increase in salinity (Table 2). Conversely, a 1000-fold decrease in the K + concentration in nutrient solutions was associated with reductions in the leaf K + concentrations of 35 and 39% in plants grown in 50 and 500 mol m - 3 NaC1, respectively. These data show that the K C concentrations in leaves of A. marina are sensitive to variations in the NaC and

Marilyn C. Ball et al.

Table 2. Ionic composition and properties of leaves of Avicennia marina in response to salinity and potassium concentration

Low and high salinities were 50 and 500 rnol m - 3 NaC1. Low and high K + concentrations were 0.01 and 10.0mol m T 3 ~ +All. values are mean?s.e.m., n=5 Low salinity High K + Low K +

High salinity High K + Low K +

Salt content (mmol k g - ' dry wt) 6 4 0 2 91 Na+ K' 1102250 C11220 2 76

1015261 6 3 9 2 40 13262 70

207 1 2 64 512271 21942 104

19352 116 320 t 40 20742 117

(mol m - leaf water) Na' 2 1 0 2 17 K 379 2 34 C1415228

3 8 8 2 12 248 2 25 511536

673 k 21 1672 24 718 2 39

728 2 25 103 t 15 747 2 24

+

K' concentrations in the culture solutions, with the accumulation of K + in leaves reduced more by increase in salinity than by decrease in the K + supply at the roots. The leaves accumulated high concentrations of Nai and C1- under all growth conditions (Table 2). Under low salinity conditions (i.e. 50 rnol m - 3 NaCl), the concentrations of Nai and Ki in leaves were inversely related such that a decrease in K' was approximately matched by an increase in Na'. This caused the Na' / K + ratio to increase from 0.60 to 1.62 while the sum of foliar Na' and K' concentrations was relatively constant and largely balanced by the C1- concentration. The same pattern of Nai and K' substitution occurred in leaves of plants grown under high salinity conditions (i.e. 500 rnol m - 3 NaCl), although these leaves contained higher total concentrations of Na' + Ki than those of plants grown under low salinity conditions. Photosynthetic Characteristics of Leaves Photosynthetic characteristics in relation to the salinity/Ki treatments and the Ki concentrations in the leaves are summarised in Table 3. Under low salinity conditions, decrease in the K' concentration of the culture solution from 10 to 0.01 rnol m - 3 had relatively little effect on photosynthetic properties. The total Chl content per unit leaf area decreased 12% from 989 to 873 pmol m - 2 with decrease in the leaf K + concentration. However, there were no measurable differences in the photosynthetic characteristics of these leaves. Under high K' conditions, increase in salinity from 50 to 500 rnol m - 3 NaCl also had relatively little effect on photosynthetic properties. The photosynthetic capacity, here considered as the light and CO2 saturated rates of 0 2 evolution, declined 21 % with increase in salinity. There was some evidence of photochemical dysfunction, as shown by the 12% decrease in quantum yield, but this was not sufficient to account for the loss in photosynthetic capacity. The Chl concentration in these leaves decreased 24% with increase in salinity, but the Chl a/b ratio was slightly higher in leaves grown under the high salinity treatment. Thus the leaves from three of the treatments, low salinity / high K' , low salinity /low K + and high salinity /high K t , had similar photosynthetic proper-

Salinity-induced K + Deficiency and Photosynthesis

355

Table 3. Chlorophyll concentrations and photosynthetic characteristics in leaf discs of Avicennia marina grown in low and high salinity (50 and 500 mol m - 3 NaCI) nutrient solutions containing either 10 or 0.01 mol m - 3 K' Values are m e a n t s.e.m., n = 5 Low salinity Low K + High K +

High salinity High K + Low K i

--

Leaf K (mol m - 3, Chl concn (pmol m - 2, Chl a / b ratio (mol mol - ') +

Photosynthetic characteristics Quantum yield (mot mol - ') Light compensation point (pmol m - 2 S-') Light and CO2 saturated rates of 0 2 evolution (pmol m-2 S-I) Dark respiration rate (02 uptake) (pmol m - s - ')

+ +

248 f 25 873f 51 2.8520.06

1 6 7 t 24 7542 56 3.11+0.02

103 t 15 387 f 29 2.8020.08

0.102 f.0.008

0.094f0.005

0.066f.0.004

19.5f.2.0

2 1 . 6 f 1.0

25.4f4.8

35.9f3.2

1 9 . 6 f 2.8

23.6f.4.3

15.4f0.74

11.3f1.7

2.1L0.1

2.2f.O.1

2.420.2

2.2f.0.1

379 34 989 37 2.95f.0.04

0.107

+ 0.005

ties, and most of the decline in photosynthetic capacity with decrease in leaf K + from 379 to 167 mol m - 3 could be associated with decline in the Chl concentration. In contrast, decrease in the K + concentration of culture solutions from 10 to 0.01 mol m - 3 under high salinity conditions induced marked changes in photosynthetic properties. Leaves from the high salinity /low K + treatment were characterised by drastic losses in Chl concentration and photosynthetic capacity, as well as photochemical dysfunction. The latter was evident in the 30% lower quantum yield in leaves from the low K + treatment than in those from the high K + treatment. It is apparent from these data that the K + concentrations in leaves from the high salinity/low K + treatment (i.e. 103 mol m-3) are below some threshold level at which metabolic dysfunction occurs in the chloroplast, and that the thylakoid membranes are sensitive to salinity-induced K' deficiency. The nature of these functional changes in photochemical activity in intact leaves was probed further with measurements of room temperature Chl a fluorescence emitted by for rise from F, to the peak, F , , increased with increase PS 11. The half time (t in salinity and decrease'in the K' concentration of the culture solution (Table 4). This increase in t 1/2 was inversely related to the leaf K + concentration. In contrast, the variable fluorescence yield (F, + F , ) I F o was insensitive to decrease in leaf K f from 379 Table 4. Room temperature Chl a fluorescence characteristics of intact leaves of Avicennia marina grown under low and high salinity conditions (50 and 500 mol m - ' N ~ C Iwith ) either 10 or 0.01 mol m - 3 K + The half time for rise from F, to the peak, F,, is indicated by t ~ m e a n t s.e.m., n= 5 Low salinity Low K + High K + Leaf K (mol m - 3, t 1/2 (ms) (Fo+Fv)IFo +

379 2 34 341 f 7 6.8f0.5

248 f 25 4 3 2 f 10 7 . 0 2 0.5

2

Values . are

High salinity High K + Low K + 167224 466 f 30 6.520.5

103215 800 f 99 2 . 9 2 0.3


to 167 rnol m - 3 , but the variable fluorescence yield of leaves with only 103 rnol m - 3 K C averaged 56% less than that in leaves with higher K + concentrations. These data show that salinity-induced K C deficiency adversely affects PS I1 functioning in vivo.

Properties of Isolated Thylakoids The composition of isolated thylakoid membranes was studied with respect to the four supramolecular complexes, i.e. Cyt f / b , CF1, P S I and PS 11, with the aim of identifying sites sensitive to salinity-induced K C deficiency (Table 5). The per Chl concentration of Cyt f / b increased with decrease in foliar K + concentration, and the CFI ATPase activity was greater in leaves containing 103 rnol m - 3 K C than in leaves with higher K C concentrations. In contrast, the per Chl concentration of PS I (assayed as P700)was insensitive to variation in leaf K C concentrations. None of these responses could have contributed to the loss of photosynthetic capacity with decreasing K C concentration. However, thylakoids isolated from leaves with only 103 rnol m - 3 K + had 37% fewer atrazine-binding sites, which represent PS I1 complexes (Steinbeck et al. 1981), than those isolated from leaves with greater K C concentrations. This depletion in atrazine-binding sites caused the ratio of functional PS I1 / P S I to be only 1 1 in thylakoids from the high salinity/low K + treatment. In contrast, the ratio of functional PS I I / P S I determined in thylakoids from all other treatments was 1.7, a value typical of healthy leaves grown under high light conditions (Anderson and Osmond 1987). These data clearly show that the atrazine-binding polypeptide of PS I1 is one site of sensitivity to salinity-induced K deficiency. +

Table 5. Composition of the thylakoids isolated from leaves of Avicennia marina grown under low and high salinities (50 and 500 mol m - 3 NaCl) with either 10 or 0 . 0 1 mol m-"+ Values are mean+s.e.m. (n), except the concentrations of atrazine-binding sites which were determined by linear regression of nine determinations (r2=O.99) in a Lineweaver-Burk type analysis (Tischer and Strotmann 1977). The P S I I I P S I ratio was calculated with the assumption that one atrazine molecule is bound per functional P S I1 Low salinity High K C Low K + Leaf K ' (mol m - -') ~~tf/b(mmolmol-'Chl) ATPase activity (mmol Pi mol - Chl s - ') P700 (mmol mol- ' Chl) Atrazine-binding sites (mmol mol - Chl) PS I1 I P S I (mol mol- ')

'

'

High salinity High K + Low K C

+

379 f.34(5) 1.80fO.15(4)

248 f 25(5) 2.05+0.05(4)

167 24(5) 2.27fO.04(4)

85.4?8.2(3) 1.57+0.03(3)

65.4f23.9(3) 1.48?0.06(3)

85.4+22.4(3) 1 1 5 . 0 2 19.8(3) 1.67f0.04(3) 1 .50(2)

2.64 1.68

2.54 1.72

2.83 1.69

103 f 15(5) 3 .24(2)

1.67 1.11

Discussion Decrease in Photosynthesis with Increase in Salinity is due to Salinity-induced K + Deficiency rather than NaCl Toxicity The present study shows that reduction in photosynthetic activity in leaves of Avicennia marina grown under high salinity conditions is due to salinity-induced K + deficiency rather than to accumulation of NaCl to toxic levels in the leaves. This conclusion is supported by three lines of evidence from the present and previous studies. Firstly, decrease in photosynthetic capacity with increase in salinity from 50 to 500 rnol m - 3 NaCl was correlated with decrease in leaf K + concentrations from 379 to 167 rnol m - 3 (Table 3). Neither the increase in salinity nor the increases in N a C and


C1- concentrations in the leaves (Table 2) were likely to have caused this decrease in photosynthetic capacity. In previous studies, the photosynthetic capacity (determined by measurement of the CO2 assimilation rate as a function of the intercellular CO2 concentration) declined 40% with increase in salinity from 50 to 500 rnol m - 3 NaCl when the leaf K f concentration decreased from 157 to 98 rnol m - 3 (Ball and Farquhar 1984), whereas photosynthesis was insensitive to this increase in salinity when the leaf K C concentrations increased from 154 to 170 rnol m - 3 (Ball 1981). Thus, decrease in photosynthetic capacity of A. marina with increase in salinity from 50 to 500 rnol m - 3 NaCl was independent of accumulation of Naf and C1- in the leaves to levels similar to those in the present study (Table 2), but was correlated with changes in the K f concentrations in the leaves (Ball 1981; Ball and Farquhar 1984). Secondly, under high salinity conditions, loss of photosynthetic capacity as well as induction of photochemical dysfunction (i.e. decrease in quantum yield) were correlated with decrease in leaf K + concentrations from 167 to 103 rnol m - 3 (Table 3). These changes in photosynthetic characteristics were not due to the concentrations of NaC and C1- per se because these concentrations were constant in the culture solutions and practically constant in leaves of both high and low K + treatments (Table 2). Finally, there is no convincing evidence of salinity-induced changes in the NaC and C1- concentrations of chloroplasts except when leaf concentrations of NaCl are very low (Kaiser et al. 1983; Robinson et al. 1983; Robinson and Downton 1984, 1985). However, decrease in the K C concentration of chloroplasts occurs in parallel with salinity-induced decrease in the K f concentration of leaves (Robinson and Downton 1985).

Biphasic Response of Photosynthesis to Salinity-induced K f DeBciency Two levels of response to leaf K + concentrations are apparent in the changes in photosynthetic characteristics of A. marina. With decrease in leaf K C from 379 to 167 rnol m - 3 , the decline in photosynthetic capacity was consistent with the decrease in Chl content per unit leaf area (Table 3). This loss in photosynthetic capacity was aparently due to decrease in the amount of photosynthetic machinery per unit leaf area. Decline in photosynthetic capacity continued with further decrease in leaf K C to 103 rnol m - 3 but, at this K + concentration, leaves showed evidence of photochemical dysfunction. The latter was apparent in the losses of quantum yield (Table 3) and variable fluorescence (Table 4). Thylakoids isolated from these low K + leaves had lower concentrations of atrazine-binding sites than those from leaves with higher K C concentrations (Table 5). These results are consistent with the sensitivity of PS I1 activity to K + deficiency in tomato leaves (Spencer and Possingham 1960). Since the polypeptide to which azido-atrazine specifically binds is the rapidly synthesised 32-34 kDa chloroplast gene product (Steinbeck et al. 1981), our results identify the atrazine-binding polypeptide of PS I1 as one site of sensitivity to salinity-induced K' deficiency. Mechanism of K C Effects on Photosynthesis Although K' is not involved specifically in photosynthetic metabolism, it is required in relatively high concentrations for other biophysical and biochemical processes which affect photosynthesis (Huber 1985). High rates of light-dependent protein synthesis by isolated pea chloroplasts require 50-70 rnol m - 3 K + in the assay medium (Fish and Jagendorf 1982). This requirement for external K' may be related to maintenance of alkaline pH in the stroma by a M~~ -activated K / H + antiporter in the chloroplast envelope (Huber and Maury 1980). There may also be a specific requirement for K C in protein synthesis in chloroplasts, as in vitro protein synthesis by other systems requires 100-150 rnol m - K and a low NaC / K + ratio; major changes in these conditions affect both the quantity and type of proteins synthesised (Lubin and Ennis 1964; +

+

+

Marilyn C . Ball et al.

Wyn Jones et al. 1979; Brady et al. 1984; Gibson et al. 1984). Such changes in the ionic composition of the cytoplasm and chloroplasts can be induced in halophytes by growth under saline conditions (Harvey et al. 1981; Kaiser et al. 1983; Robinson et al. 1983; Robinson and Downton 1984, 1985) and may well affect the synthesis and functioning of photosynthetic machinery. The atrazine-binding protein is the most likely component of the photosynthetic electron transport system to reflect failures of protein synthesis because it is synthesised at rates from 50 to 80 times greater than those of other photosynthetic membrane proteins under high irradiance (Mattoo et al. 1984). Such high rates of de novo synthesis are required to maintain the photochemical capacity of PS I1 under high light conditions because damage to the atrazine-binding protein apparently occurs as a natural consequence of its function (Arntzen et al. 1984; Kyle et al. 1984a, 1984b). If rates of protein synthesis were to decline with decrease in the K' concentration in the chloroplast and/ or with K + related changes in stromal pH, then the rates of protein synthesis might not keep pace with the rates of loss in functional atrazine-binding protein. This would cause a depletion of atrazine-binding sites in PS I1 complexes, and hence also an imbalance in the ratio of functional PS I1 /PS I as in the present study (Table 5). Implications of Depletion in the Atrazine-binding Protein for Photosynthesis by Intact Leaves The atrazine-binding protein is an integral component of PS 11, functioning in the transfer of electrons to the plastoquinone pool (see review: Kyle 1985). In PS I1 complexes which have lost atrazine-binding sites, the absorbed quanta would not be effective in driving electron transfer to the plastoquinone pool. Under light-limiting circumstances, depletion in the atrazine-binding protein would be expressed in intact leaves as a decline in quantum yield. Indeed, a 37% decrease in atrazine-binding sites was associated with a 38% decrease in quantum yield in K + deficient leaves (Table 6 ) . The decline in functional PS I1 centres was further indicated by changes in the Chl fluorescence signals (Table 6 ) . The loss of variable fluorescence might have been associated with Table 6 . Summary of relative changes in photosynthetic properties with decrease in leaf K + concentration The photosynthetic properties are expressed as a percentage of those in leaves containing the highest K concentrations, i.e. those from the low salinitylhigh K + treatment. All calculations were based on average values reported in Tables 2 and 3 +

Leaf K concentration Total Chl +

P ma, Quantum yield p700 Atrazine-binding sites PS I1 /PSI (Fo+Fv)IFo

Low salinity /low K '

High salinity / high K

Low salinity / high K +

Low salinity 1high K +

Low salinity / high K i

55 88 120 95 94 96 102 103

44 76 79 88 106 107 101 96

27 39 58 62 96 63 66 43

+

High salinity / low K '

-

increase in energy dissipation as heat in defective PS I1 centres, while the increase in t 1 , ~ could mean a slower reduction of the plastoquinone pool by the remaining functional PS I1 centres. While the results of the present study do not preclude the possibility that other electron transport components could also have been adversely affected by the growth conditions, the sensitivity of the atrazine-binding polypeptide of PS I1 to low K +

Salinity-induced K * Deficiency and Photosynthesis

concentrations is sufficient to account for most of the changes in photosynthetic characteristics of leaves grown under high salinity /low K + conditions. However, at light saturation, the loss in photosynthetic capacity in leaves from the high salinity/low K + treatment was less than might have been expected from the drastic loss in Chl concentration (Table 6). Under saturating light conditions, the rates of photosynthetic electron transfer are partly limited by electron transfer processes between the two photosystems and by the build-up of the transmembrane electrochemical gradient for protons. Hence the higher concentrations of Cyt f/b and ATPase in the K + deficient leaves (Table 5) may at least partially compensate for the loss of functional PS I1 under high irradiances. K + can be Limiting under Field Conditions The K + concentration in leaves from the high salinitylhigh K + treatment (i.e. 167 mol m-3), in which concentrations of NaCl and K + are similar to those in seawater, was similar to that found in leaves of A. marina growing naturally in tidal swamps (Popp 1984). It is apparent from the present and past studies (Ball 1981; Ball and Farquhar 1984) that such low leaf K + concentrations are at or near the lower limit that can be reached without metabolic disorders. Thus, A. marina appears to operate under natural field conditions with marginal internal K + supply despite its high affinity for K+ uptake under saline conditions (Table 1). Conclusion In summary, there are two major findings in the present study. Firstly, reduced photosynthetic activity in plants grown at high salinity may be due to salinity-induced K + deficiency rather than to NaCl toxicity in the leaves. Secondly, the atrazine-binding polypeptide is one site of sensitivity to salinity-induced K+ deficiency. These results emphasise the importance of selection for maintenance of high K + concentrations in leaves during growth under saline conditions as a basis for improvement of salinity tolerance in crop plants. Acknowledgments The authors thank Drs David Goodchild, Ta-Yan Leong and Rana Munns for critical appraisal of the manuscript, Ms Susan Allen for mangrove cultivation, Mr Jim Caldwell for K+ and Na+ determinations, and Ms Janet Sincock for secretarial assistance. This research was supported in part by a grant from the Rural Credits Development Fund of the Reserve Bank to M.C.B. References Anderson, J. M., and Osmond, C. B. (1987). Shade-sun responses: compromises between acclimation and photoinhibition. Top Photosynth. 9, in press. Arnon, D. I. (1949). Copper enzymes in isolated chloroplasts. Polyphenol oxidase in Beta vulgaris. Plant Physiol. 24, 1-15. Arntzen, C. J., Kyle, D. J., Wettern, M., and Ohad, I. (1984). Photoinhibition: a consequence of the accelerated breakdown of the apoprotein of the secondary acceptor of photosystem 11. In 'Biosynthesis of the Photosynthetic Apparatus: Molecular Biology, Development and Regulation'. (Eds R. Hallick, L. A. Staehelin and J. P . Thornber.) U.C.L.A. Symposium Series No. 14, pp. 313-24. (A. R. Liss Inc.: New York.) Ball, M. C. (1981). Physiology of photosynthesis in two mangrove species: responses to salinity and other environmental factors. Ph.D. Thesis, Australian National University. Ball, M. C., and Anderson, J. M. (1986). Sensitivity of photosystem I1 to NaCl in relation to salinity tolerance. Comparative studies with thylakoids of the salt-tolerant mangrove, Avicennia marina, and the salt-sensitive pea, Pisum sativum. Aust. J. Plant Physiol. 13, 689-98.


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