Planta

Planta (1988) 174:159-165

9 Springer-Verlag 1988

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Recovery and its dependence on temperature D.H. Greer and W.A. Laing Plant Physiology Division, DSIR, Private Bag, Palmerston North, New Zealand

Abstract. Recovery of photoinhibition in intact leaves of shade-grown kiwifruit was followed at temperatures between 10 ~ and 35 ~ C. Photoinhibition was initially induced by exposing the leaves for 240 min to a photon flux density (PFD) of 1 500 ~tmol. m - 2. S - 1 at 20 ~ C. In additional experiments to determine the effect of extent of photoinhibition on recovery, this period of exposure was varied between 90 and 400 rain. The kinetics of recovery were followed by chlorophyll fluorescence at 77K. Recovery was rapid at temperatures of 25-35 ~ and slow or negligible below 20 ~ C. The results reinforce those from earlier studies that indicate chilling-sensitive species are particularly susceptible to photoinhibition at low temperatures because of the low rates of recovery. At all temperatures above 15 ~ C, recovery followed pseudo firstorder kinetics. The extent of photoinhibition affected the rate constant for recovery which declined in a linear fashion at all temperatures with increased photoinhibition. However, the extent of photoinhibition had little effect on the temperature-dependency of recovery. An analysis of the fluorescence characteristics indicated that a reduction in non-radiative energy dissipation and repair of damaged reaction centres contributed about equally to the apparent recovery though biochemical studies are needed to confirm this. From an interpretation of the kinetics of photoinhibition, we suggest that recovery occurring during photoinhibition is limited by factors different from those that affect post-photoinhibition recovery. Abbreviations and symbols: Fo, F,., F~= instantaneous, maximum, variable fluorescence; KD, Kr, Ke, K r = r a t e constants for non-radiative energy dissipation, fluorescence, photochemistry, transfer to photosystem I; k(PI), k(R)=rate constants for photoinhibition and recovery; PFD = p h o t o n flux density; PSI, II = photosystem I, II; ~bi = photon yield of photosynthesis (incident light)

Key words: Actinidia - Chlorophyll fluorescence - Photoinhibition of photosynthesis - Temperature and recovery from photoinhibition.

Introduction Photoinhibition of photosynthesis is a reversible process that affects the photon yield and light-saturated rates of photosynthesis (see Powles 1984 for review). Until recently, the ability of higher plants to recover from photoinhibition was poorly understood. To redress this, Greer et al. (1986) studied the effects of temperature and photon flux density (PFD) on recovery from photoinhibition in Phaseolus vulgar&. Their results showed that recovery was temperature-dependent with little or no recovery occurring below 15 ~ C and maximum recovery at 30 ~ C. They showed that recovery occurred concomitantly with photoinhibition and suggested that the temperature-dependency of photoinhibition was, in part, determined by the effect of temperature on recovery. Thus net photoinhibition was the difference between high-light damage (gross photoinhibition) and recovery. A similar conclusion was reached by Samuelsson et al. (1985) for the cyanobacterium Anacystis nidulans. From this, it has been proposed that the inhibition of recovery by low temperatures may be the reason that some species are particularly susceptible to photoinhibition at low temperatures (Greer et al. 1986; Oquist et al. 1987). The exposure of shade-grown leaves to, high light results in a decrease in the maximum chlorophyll fluorescence (F,,) at 77K and 692 nm and a concurrent increase in the instantaneous fluorescence (Fo) (Powles and Bj6rkman 1982; Greer et al. 1986). These changes in fluorescence reflect chan-

160

D.H, Greer and W.A. Laing: Recovery from photoinhibition in kiwifruit

ges in non-radiative energy dissipation and in the photochemical activity of photosystem II (PSII) (0gren and 0quist 1984; Bj6rkman 1987a; Demmig and Bj6rkman 1987). In previous studies of recovery, as measured by chlorophyll fluorescence, it has been assumed that the repair of damaged reaction centres was the sole contributor to that recovery. However, it is now apparent that recovery probably includes a reduction in non-radiative energy dissipation as well as repair of damaged reaction centres. In studies where non-radiative energy dissipation increased without concomitant reaction-centre damage, it has been shown that this energy dissipation was readily and rapidly reversed (Demmig et al. 1987). It remains to be seen if this holds where an increase in non-radiative energy dissipation accompanies the inactivation of the primary photochemistry of PSII. We have established that shade-grown kiwifruit leaves are susceptible to photoinhibition, particularly at low temperatures where severe inactivation of PSII reaction centres occurs (Greer et al. 1988). The objective of this study was to measure recovery from photoinhibition in kiwifruit leaves with particular emphasis on the effect of temperature. Changes in the rate constants for energy transfer within PSII during recovery were also determined to ascertain the contribution of reaction-centre repair to the apparent recovery. Using the data from this study along with that for photoinhibiti0n in our previous study, the relationship between recovery and photoinhibition was investigated.

Table 1. Changes in the chlorophyll fluorescence characteristics at 77K in kiwifruit leaves treated to a P F D of 1 500 gmol. m - 2 s - 1 for 240 rain at 20 ~ C Characteristic

Fo F,, F~

F~/Fm

Before exposure After exposure

Change

(mean-+SE)

(mean-+SE)

(%)

12.2 4-0.5 100.0 87.9 -t-0.5 0.879 _+0.05

J4.9 32.1 17.3 0.529

+22.0 -67.9 -80.3 - 39.8

-t-0.6 _+1.5 +1.2 _+0.02

Each value is the mean of duplicate measurements from 12 leaves

phyll fluorescence emitted from PSII at 77K, respectively, and Fv is the variable fluorescence, where F~ = F,,-Fo.

Data analysis. Data were fitted to an equation described in the text using non-linear regression analysis.

Results

Changes in 77K chlorophyll fluorescence during photoinhibition and recovery. The fluorescence characteristics of the intact kiwifruit leaves before and after treatment with a photoinhibitory PFD of 1 5 0 0 g m o l - m - Z . s - 1 at 20 ~ for 240min are shown in Table 1. These constitute the control values a n d those values for leaves after the standard photoinhibition treatment. An example of the time course of recovery in Fo, Fv, and Fv/F,, that occurred subsequently once the light was reduced to 20 g m o l . m - 2 - s - 1 is shown in Fig. 1. The data were fitted to the following equation for a pseudo first-order process.

Material and methods

Ft -=Fo~-(Foo-Fi) e -k'

Plant material. Rooted cuttings o f Actinidia deliciosa (A. Chev.)

where F~ is the variable fluorescence at any time, t. Fi and k are fitted parameters where Fi is the initial level of F~ during recovery and k is the observed first-order rate constant. Foo was a constant determined from the pre-photoinhibition control value. The lines on Fig. 1 are the best fit to eq. 1. Fo declined over the 400-rain recovery period to approach the pre-photoinhibition level in this time. Over the same period, both F~ and F~/Fm increased but did not reach full recovery; F~ had recovered to about 50% in 420 min whereas FdFm had recovered to about 90%. Only F~ data are subsequently presented particularly since Greer et al. (1988) have established that changes in Fv were linearly correlated with the changes in photon yield of 02 evolution in kiwifruit leaves. However, it should be noted that qualitatively similar results of recovery were obtained using Fo/Fm to those using F~.

C.F. Liang et A.R. Ferguson were grown in 9 1 pots in a 40: 30:10 (by vol.) Opiki loam: sand :coarse pumice growing medium with incorporated fertilizer. Further details of the cultural and growth conditions were described by Greet et al. (1987).

Recovery. Photoinhibition was induced by exposing a fully mature leaf for 240 min at 20 ~ C to a P F D of 1500 I~mol"m - 2. s as described by Greer et al. (1987). Recovery was followed subsequently at a P F D of 20 gmol. m - z. s - 1, provided by a 40-W incandescent lamp. Leaf temperature was held constant, between 10 ~ and 35 ~ C, during the recovery period. To examine the effect of the extent of photoinhibition on recovery, the duration of the photoinhibitory high-light exposure was varied between 90 and 400 min. Recovery assay. Duplicate leaf samples (10 mm diameter) were punched from random locations in the leaf at intervals throughout a recovery period. These samples were held in the dark on moistened filter paper for at least 30 rain, then chlorophyll fluorescence at 77K was measured as described by Greer et al. (1988). Fo and Fm are the instantaneous and maximum chloro-

(1)

D.H. Greer and W.A. Laing: Recovery from photoinhibition in kiwifruit 1O0

=

i

i

i

161

i

70

Fv/F,

60

80

50 •

9 40

60 i.

30 2O 10 20

Fo G - c c o

o

,",

16o

^ ~

0

0

0

0

0

10"C

6 5'0

260 Time (rain)

Fig. l. The time course of the changes in the instantaneous fluorescence, (o--o), variable fluorescence, (e e), and the fluorescence ratio at 77K, (m--m) during recovery at 25~ and a PFD of 20 lamol-m -a.s -1 for an intact leaf of kiwifruit following photoinhibition at 20~ C and 1500 I~mol-m 2. s- 1 for 240 rain. The fluorescence data is normalised to a pre-photoinhibition Fm value of 100. The lines are best fit to eqn. 1

Effects of temperature on the time-course of recovery. T h e time courses o f changes in F~ o f leaves recovering f r o m p h o t o i n h i b i t i o n at t e m p e r a t u r e s o f 10 ~ 20 ~ a n d 30 ~ C are s h o w n in Fig. 2A. N o r e c o v e r y was a p p a r e n t in 400 rain at I 0 ~ while at 20 ~ C, only 2 5 % r e c o v e r y o f F~ h a d o c c u r r e d in this time. A t 30 ~ C F, h a d r e c o v e r e d a b o u t 9 0 % in 400 rain. This shows t h a t r e c o v e r y was t e m p e r a t u r e - d e p e n d e n t . A l t h o u g h r e c o v e r y was slow at 20 ~ C, Fig. 2 B shows t h a t full r e c o v e r y c o u l d be achieved in longer times. Similar l o n g - t e r m results were o b t a i n e d (not shown) for o t h e r t e m p e r a t u r e s between 15 ~ a n d 35 ~ C.

Effect of extent of photoinhibition on recovery. W h e n the extent o f p h o t o i n h i b i t i o n was varied by v a r y i n g the d u r a t i o n o f the high-light t r e a t m e n t , the kinetics o f r e c o v e r y were changed. This is s h o w n in Fig. 3 for leaves recovering after e x p o sure to high light for 90 a n d 300 rain. T h e 90-rain e x p o s u r e resulted in 4 8 % inhibition o f Fv a n d the s u b s e q u e n t r e c o v e r y was r a p i d a n d c o m p l e t e d within 360 rain. F o r leaves p r e t r e a t e d for 300 rain, F~ was 8 7 % inhibited a n d r e c o v e r y was slow, a n d only 5 1 % c o m p l e t e d within 420 rain. T h u s recovery was d e p e n d e n t on the extent o f p h o t o i n h i b i tion. A similar effect o f extent o f p h o t o i n h i b i t i o n o f s u b s e q u e n t r e c o v e r y was also n o t e d at o t h e r t e m p e r a t u r e s (not shown). T o q u a n t i f y the effect o f the extent o f d a m a g e on recovery, the first-order rate c o n s t a n t for recovery [k(F~)], was d e t e r m i n e d (eq. 1). T h e extent o f p h o t o i n h i b i t i o n was d e t e r m i n e d f r o m the following e q u a t i o n

1. o 260 2. o 360 3&o 460Time (rain)

1 O0 80

B

60 40 20 0

(~

5E)O 10'00 15'00 20'00 25'00

--

(rain)

Time

Fig. 2A, B. The time course of recovery of Fv of an intact kiwifruit leaf at A 10~ 20~ 30~ C and B 20~ C from photoinhibition following a 240-rain exposure to a PFD of 1500 gmol.m-2s- 1 at 20~ C Extent o f p h o t o i n h i b i t i o n

=

1-Fi/Fc

where Fi is the value o f Fv at the start o f the recovery p h a s e a n d Fc, the c o n t r o l value p r i o r to p h o toinhibition. T h e r e l a t i o n s h i p between k(Fv) a n d the extent o f p h o t o i n h i b i t i o n at t e m p e r a t u r e s o f 20 ~ a n d 30~ is s h o w n in Fig. 4. A t b o t h t e m p e r a t u r e s , the rate c o n s t a n t increased in a linear fashion as d a m a g e decreased. T h e intercepts o f the regressions were n o t significantly different b u t the slope o f the relationship at 3 0 ~ was threefold higher t h a n t h a t at 20 ~ C.

Effect of temperature on the rate constant for recovery. T h e effect o f t e m p e r a t u r e on the i n t e r p o l a t e d rate c o n s t a n t for recovery, [k(Fv)], for a c o n s t a n t extent o f 7 5 % p h o t o i n h i b i t i o n (equivalent to a b o u t a 200-min e x p o s u r e to the high light) is s h o w n in Fig. 5. T h e rate c o n s t a n t increased as a n a p p a r e n t l y linear function o f t e m p e r a t u r e f r o m 0 . 8 - 1 0 - 3 . m i n - 1 at 1 5 ~ (the lowest t e m p e r a t u r e at which r e c o v e r y could be dete~ted) to

162

D . H . G r e e r a n d W . A . L a i n g : R e c o v e r y f r o m p h o t o i n h i b i t i o n in kiwifruit

100 8Oso~

i

n

,.-, 4 T c-

PI

am

E z

O2 x I

40

9 9

la_ ~e 0

o

1;o

4;o

5;o

1'o

Time (rain)

n

- -

2'o

A 02)

n

u

i

8

3'5

Fig. 5. T h e rate c o n s t a n t for recovery, [k(F~)], for intact kiwifruit leaves as a f u n c t i o n o f t e m p e r a t u r e at a c o n s t a n t level of 75% photoinhibition

i

1

i

Kp

7O "'7 t-

2'5

Temperature

Fig. 3. T h e time course o f recovery o f F~ o f a n intact leaf o f kiwifruit at 30 ~ C a n d a P F D o f 20 g m o l . m - 2 - s 1 following a 90- or a 3 0 0 - m i n e x p o s u r e to a P F D o f 1 500 g m o l . m - 2. s at 20 ~ C

u

r

i

9 9

..~ so

o~

E6 ~Q I

o 9-v

x

ou 40 4

u~2

20

--Sg 10 0

;

1;o

3;o

4;o

"l'Tme (rain)

Extent of Photoinhlbition Fig. 4. The rate constant for the recovery of Fv, [k(Fv)], at t e m p e r a t u r e s o f 20 ( o - - o ) a n d 30 ~ C ( e - - e ) in intact kiwifruit leaves as a f u n c t i o n o f the extent o f p h o t o i n h i b i t i o n d a m a g e i n d u c e d by e x p o s u r e to a P F D o f 1 500 g m o l . m - 2. s - 1 at 20 ~ C for d u r a t i o n s r a n g i n g b e t w e e n 90 a n d 400 m i n . T h e extent o f p h o t o i n h i b i t i o n w a s m e a s u r e d as m i n u s t h e ratio o f Fv at the e n d o f the p h o t o i n h i b i t i o n t r e a t m e n t to t h a t o f the c o n t r o l before p h o t o i n h i b i t i o n

Fig. 6. T h e time course o f t h e c h a n g e s in the calculated rate

4.1.10- 3.min- 1 at 35 ~ C. This temperature-dependency of recovery was, however, apparently not further dependent on the extent of photoinhibition.

stant for non-radiative energy dissipation and transfer to PSI, KDr, calculated from the changes in F,,, declined with similar kinetics to also approach the pre-inhibition level. At other temperatures (not shown) the changes in Ke and KDr followed a similar time course to that shown in Fig. 6.

Changes in the estimated energy-transfer rate constants during recovery. During recovery, the rate constant for PSII photochemical activity, Kp as calculated from the changes in Fo, increased at an initially rapid rate that subsequently declined to approach the pre-inhibition level in about 400 min (Fig. 6). In the same time, the combined rate con-

c o n s t a n t for p h o t o c h e m i s t r y , Kp (e . ) a n d the c o m b i n e d rate c o n s t a n t for n o n - r a d i a t i v e energy dissipation a n d transfer to PSI, KDr (o o) d u r i n g t h e recovery f r o m p h o t o i n h i b i t i o n o f intact kiwifruit leaves at 30 ~ C a n d a P F D o f 20 g m o l . m - 2. s -

Discussion

Leaves of shade-grown kiwifruit recovered from photoinhibition once the high-light stress was removed. However, the recovery that occurred sub-

D,H. Greer and W.A. Laing: Recovery from photoinhibition in kiwifruit

sequently was dependent on temperature with maximum rates of recovery at temperatures of 25-35 ~ C, slow rates below 20 ~ C and no apparent recovery at 10 ~ C. Thus, the temperature-dependence of recovery in kiwifruit leaves is similar to that in Phaseolus vulgaris although maximal rates of recovery in Phaseolus are higher (Greet et al. 1986). This supports the conclusion that kiwifruit leaves are more susceptible to photoinhibition than Phaseolus leaves (Greer et al. 1988) in that the low rates of recovery would exacerbate photoinhibition. As shown in this study, there was no other intrinsic limitation to complete recovery at temperatures where recovery was slow (i.e. 15-20 ~ C). Recovery from photoinhibition in kiwifruit leaves was also directly dependent on the extent of the initial photoinhibitory damage. Similar effects of the extent of photoinhibition on the subsequent recovery have been observed by Nilsen et al. (1984) and 0gren et al. (1984) in Lemna gibba, by Krause et al. (1985) in Spinacea oleracea and by Demmig and Bj6rkman (1987) in Hedera canariensis and Monstera deliciosa. However, apparently only in kiwifruit has the interactive effects of temperature and the extent of photoinhibition been defined. There was a threefold higher rate of decrease in the rate constant for recovery, k(F~) at 30 ~ C compared to that at 20 ~ C, as the extent of photoinhibition increased (Fig. 4). Recovery at high temperatures was thus relatively more dependent on the extent of photoinhibition than it was at low temperatures. However, the relative effect of temperature on the rate constant for recovery was apparently independent of the extent of photoinhibition. One explanation for the effect of the extent of photoinhibition on recovery is that as photoinhibition increased in severity, secondary damage occurred other than in the reaction centres. The additional damage could lead to a more general deterioration of the biosynthetic machinery facilitating the low rates of recovery. There is some evidence that tong-term photoinhibition treatments on isolated chloroplasts result in the loss of polypeptides at or close to the reaction centres (Dos Santos and Hall 1982). When the effect of the extent of photoinhibition on recovery in kiwifruit leaves was taken into account, the rate constant for recovery increased linearly with temperature from 10 ~ to 35 ~ C. The temperature-dependency of recovery in kiwifruit therefore differed slightly from that in Phaseolus vulgaris which was curvilinearly dependent on temperature (Greer et al. 1986). However, over all temperatures, the rate constant for recovery in kiwifruit was lower than that in Phaseolus. In both

163

species, the high rate constants for recovery at high temperatures are consistent with the lower extents of photoinhibit~on at these temperatures (Greer et al. 1988). Similarly, the susceptibility of leaves to photoinhibition at low temperatures could be attributed, in part, to the low rates of recovery at these temperatures. Changes in the fluorescence characteristics during exposure to high light reflect changes in the rate constant for non-radiative energy dissipation, KD, and PSII photochemistry, Kp. Increases in KD confer some protection on the reaction centres from excessive excitation energy while decreases in Kp result from damage at or near the reaction centres (Ogren and Oquist 1984; Bj6rkman 1987 a; Demmig and Bj6rkman 1987). The reversal of these changes in fluorescence subsequently in weak light should, therefore, be indicative of a relaxation in non-radiative energy dissipation and repair of the photochemical damage. In photoinhibited leaves of Monstera deliciosa, the reversal of Fo and F,, was very slow and incomplete after 200 rain (Demmig and Bj6rkman 1987). In kiwifruit leaves, the recovery of Fo and Fm was relatively faster and complete within 400 min under comparable ,conditions. These data indicate that the reversal of the changes in both KD and Kp followed similar time courses (Fig. 6) and for both species, relaxation of KD and repair of Kv was relatively slow. Thus recovery was apparently contributed to jointly by repair of damaged reaction centres and the reduction in non-radiative energy dissipation. Recovery of photochemical damage has been shown in Anacystis nidulans (Samuelsson et al. 1985), Chlamydomonas reinhardii (Ohad etal. 1984) and Phaseolus vulgaris (Greer et al. 1986) to require chloroplast-encoded protein synthesis. The conversion of the carotenoid pigment violaxanthin to zeaxanthin has been proposed by Demmig et al. (1987) to facilitate the increased dissipation of excess excitation energy during exposure to high light. The reversal of this conversion was also shown to occur when Kz~ was initially induced to increase by treating leaves to low 02 and zero CO2 in non-photoinhibitory light. However, recovery from high-light exposures which have resulted in reaction-centre damage, has not yet apparently been shown to parallel a decrease in the amount of zeaxanthin. Similarly, no time course of protein synthesis has been shown to occur cor~comitant with recovery. Therefore, further studies are needed to establish if the biochemical mechanisms involved in the recovery process are as slow as the data here indicate. Furthermore, temPerature effects on these mechanisms are required to deter-

164

D.H. Greet and W.A. Laing: Recovery from photoinhibition in kiwifruit

--"'-,12

Tr

El0 V

O

8

k(PI) 6 Q_ .~ 4

.i-o

o e- 2 0 o,, 0

1'0

1'5

2'0

2'5

Temperature ('C)

Fig. 7. The calculated rate constants for photoinhibition, k(PI) (e--o) and recovery during photoinhibition, k(R) (o--o) as a function of temperature for intact kiwifruit leaves mine w h y recovery at low temperatures, in particular, does not occur. It has been p r o p o s e d that p h o t o i n h i b i t i o n induced in leaves exposed to high light is the net difference between the rate o f d a m a g e and the rate o f c o n c o m i t a n t repair (Samuelsson e t a l . 1985; G r e e r et al. 1986). A simple model based on this p r o p o s a l has been developed here (see Appendix) assuming first-order kinetics for b o t h the p h o t o i n hibition and the recovery occurring during photoinhibition. Because net p h o t o i n h i b i t i o n followed first-order kinetics ( G r e e r et al. 1988), recovery during p h o t o i n h i b i t i o n has been assumed to be ind e p e n d e n t o f the extent o f p h o t o i n h i b i t i o n . T h e rate constants o f the m o d e l for net photoinhibition, k(PI), a n d recovery d u r i n g p h o t o i n h i b i t i o n , k(R), as a function o f t e m p e r a t u r e were calculated using the d a t a o f G r e e r et al. (1988) and are shown in Fig. 7. The estimated rate constant, k(R) apparently increased with increasing t e m p e r a t u r e in a linear fashion whereas k(PI) a p p e a r e d to decline only slightly with increasing temperature. H o w ever, k(R) always a p p e a r e d to be m u c h less than k(PI) such that where recovery was maximal (30-35 ~ C), k(PI) would be three- to f o u r f o l d higher t h a n k(R). This is consistent with the high propensity o f kiwifruit leaves to be photoinhibited, even at the higher temperatures. This interpretation implies that the rates o f rec o v e r y in weak light, that is following p h o t o i n h i b i tion, are higher t h a n the rates o f recovery in high light, that is, during p h o t o i n h i b i t i o n . The rate constants for recovery after p h o t o i n h i b i t i o n were f r o m 1.5- to 1.8-fold higher t h a n the estimated rate constants for recovery during p h o t o i n h i b i t i o n and this difference might be expected to increase with lower extents o f p h o t o i n h i b i t i o n (see Fig. 4). Thus, re-

covery after p h o t o i n h i b i t i o n would a p p e a r to be at least quantitatively different f r o m recovery during photoinhibition. Nevertheless, recovery during p h o t o i n h i b i t i o n appears to be as temperature-dep e n d e n t as the observed recovery after photoinhibition. This at least affirms the conclusion that increased susceptibility to p h o t o i n h i b i t i o n at low temperatures results, in part, f r o m the lack o f recovery at these temperatures. F u r t h e r studies are needed to resolve why postp h o t o i n h i b i t i o n recovery depends on the extent o f damage. Similarly, there is a need to resolve whether recovery during and after p h o t o i n h i b i t i o n involves different mechanisms or at least is limited by different factors. It is, however, a p p a r e n t f r o m this and o u r earlier studies (Greer et al. 1986, 1987) that t e m p e r a t u r e has i m p o r t a n t effects on b o t h p h o t o i n h i b i t i o n and recovery and these effects must be a c c o u n t e d for when determining the imp o r t a n c e o f p h o t o i n h i b i t i o n on crop g r o w t h and productivity. Appendix." A model f o r photoinhibition and concomitant recovery Making the assumption that recovery occurs concomitantly with photoinhibition and that the kinetics of recovery and photoinhibition can be described adequately by a pseudo first-order process then the following model can be used to derive kinetic equations xk(~PI)x* K(R)

(1A)

where X is the concentration of undamaged reaction centres

and X* the concentration of photoinhibited reaction centres. Variable fluorescence measurements are assumed to measure X. k(PI) and k(R) are the photoinhibition and concomitant recovery rate constants respectively. The recovery process modelled here assumes the rate constant for recovery during photoinhibition, k(R) is independent of the extent of damage. Assuming that X+ X* is a constant (i.e. no destruction of chlorophyll occurs) it can be shown that during photoinhibition Ft = Fo~- (F~-Fi)e-k~

(2A)

where/Co= k(PI) + k(R),

(3A)

F~ is the initial variable fluorescence and F~ is the steady-state variable fluorescence under photoinhibitory conditions. Equation 2A closely describes the photoinhibition data and is the same as eq. 1 in Greer et al. (1988). Under steady-state photoinhibition conditions (i.e. t= oo) k(PI)F~ = k(R)(Fc-Fo~) From eqns. 3A and 4A k(PI) can be calculated

(4A)

k(PI) = [ko(F,-F~o)]/F, while k(R) can be calculated from eqn. 3A.

(5A)

References Bj6rkman, O. (1987a) High irradiance stress in higher plants and interaction with other stress factors. Progr. Photosynth. Res. 4, 11 18

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