Photosynthetic responses to light variation in rainforest species

Oecologia 9 Springer-Verlag1986 Oecologia (Berlin) (1986) 69:517-523 Photosynthetic responses to light variation in rainforest species I. Induction under constant and fluctuating light conditions* Robin L. Chazdon** and Robert W. Pearcy Department of Botany, University of California, Davis, CA 95616, USA Summary. Photosynthetic induction under constant and fluctuating light conditions was investigated in intact leaves of Alocasia macrorrhiza and Toona australis, two species native to Australian rainforests. When leaves were exposed to saturating light following a long period at low light intensity, an induction period of 25-40 rain was required before steady-state photosynthesis was achieved. A long induction period was required regardless of plant growth conditions (high vs. low light) and ambient CO2 concentrations during mesurement. In low-light grown A. macrorrhiza, the initial slope of the relationship between assimilation and internal CO2 pressure increased 7-fold from 30 s following illumination to the end of the induction period. Both stomatal and carboxylation limitations play a role in photosynthetic induction, but carboxylation limitations predominate during the first 6-10 min. In both species, leaf induction state increased 2 to 3-fold during a sequence of five 30- or 60-s lightflecks separated by 2 min of low light. Rates of induction during 60-s lightflecks and during constant illumination were similar. Induction loss in low-light grown leaves of Alocasia macrorrhiza required more than 60 rain of continuous exposure to low light conditions. These results suggest that, under forest understory conditions, leaves are at intermediate induction states for most of the day. The ability to utilize sunflecks may therefore be strongly influenced by the ability of leaves to maintain relatively high states of induction during long periods of low light. Light conditions in forest understory habitats are highly variable. The low levels of diffuse light incident during most of the day can be punctuated by sunflecks that may increase photon flux density (PFD) by over two orders of magnitude within a few seconds. Although they may occur at any one spot for only a small fraction of the day, sunflecks can contribute 50 to 80% of total daily PFD in evergreen tropical forest understory habitats (Bj6rkman and Ludlow 1972; Pearcy 1983; Chazdon and Fetcher 1984). Measurements of photosynthesis in these environments similarly show that a large fraction of the total daily carbon gain can be attributed to sunfleck periods (Bj6rkman et al. 1972; Pearcy * Supported by National Science Foundation Grant BSR 8217071 and USDA Grant 85-CRCR-l-1620 ** Current address and offprint requests to." Department of Botany, University of California, Berkeley, CA 94720, USA and Calkin 1983; Chazdon 1986). Although steady-state photosynthetic light responses have been reported for a number of tropical species, little attention has focused on photosynthetic responses to the rapidly fluctuating light conditions typical of sunfleck periods. Under these conditions, analyses of transient photosynthetic responses are required to investigate the determinants of leaf carbon gain (Gross and Chabot 1979; Gross 1982; Pearcy et al. 1985). It has long been known that, when a leaf is exposed to saturating light intensities following a long period of darkness or low light, maximum photosynthetic rates are achieved only after an induction period ranging from minutes to several hours (Osterhout and Haas 1917; Rabinowitch 1956). The induction requirement for maximum photosynthetic rates is an intrinsic feature of photosynthesis and has been widely demonstrated in intact leaves, algae, protoplasts, and isolated chloroplasts (Edwards and Walker 1983). Although the processes that occur during induction are poorly understood, there is good evidence that induction is primarily associated with reactions of the photosynthetic carbon reduction cycle, rather than with photochemical reactions (Edwards and Walker 1983). However, beyond this fundamental observation, it is difficult to separate the relative importance of stomatal limitations, substrate concentrations, and enzyme activation during induction in intact leaves. Moreover, little is known about the potential significance of the induction requirement to light utilization during sunfleck periods. In this study, we have determined the time-course of induction in two contrasting species with the objective of investigating how induction influences the utilization of sunflecks; Alocasia macrorrhiza (L.) G. Don (Araceae) is an understory herb, and Toona australis (F. Muell.) Harms (Meliaceae) is a canopy tree. Both species commonly occur in humid tropical and subtropical forests of Queensland, Australia. Field measurements with A. maerorrhiza in a shaded understory have shown that about 40% of the daily CO2 uptake occurred during a few brief sunflecks (Bj6rkman et al. 1972). In contrast, T. australis is a fast-growing gap species that regenerates under open conditions and hence may depend less on sunflecks for carbon gain (G. Stocker, pers. comm). The time-course of photosynthetic induction was studied under conditions of constant light and fluctuating light. We also describe the time-course of induction loss in A. macrorrhiza and examine the role of stomatal limitations during induction in A. macrorrhiza. Carbon gain and photosynthetic efficiency during simulated 518 sunflecks as influenced by induction are discussed elsewhere (Chazdon and Pearcy 1986). M a t e r i a l s and methods Seeds of A. macrorrhiza and T. australis were collected in a rainforest area near Atherton, Queensland (Lat. 16 ~ S, alt. 760 m). After germination, the plants were grown in a glasshouse under conditions described elsewhere (Pearcy et al. 1982). Low light growth conditions were created with a shadecloth enclosure within the glasshouse (8% transmittance). At least 6 months following growth under these conditions, measurements of COz exchange were made on single attached leaves in an open-circuit gas exchange apparatus described elsewhere (Pearcy et al. 1985). All measurements were made on young, fully-expanded leaves and were replicated 3-6 times with different leaves. A. macrorrhiza were grown under both high and low light conditions, whereas T. australis were grown only under high light in the glasshouse. Induction responses were measured as described by Pearcy et al. (1985). In the experiments described here, the shutter was controlled directly from a Commodore model 8032 microcomputer, which was also connected to a Hewlett Packard model 3497 data acquisition system. A 15 W incandescent bulb located above the leaf chamber provided background low light throughout all induction and lightfleck experiments. Raw data were stored on floppy disks during data acquisition and rates of COz and water vapor exchange were computed later. During measurements under constant light, the signals from the COz analyzer, humidity sensor, and leaf thermocouples were logged at 5-s intervals for the first 10 to 20 min and subsequently at 5- to 30-s intervals. Except as noted below, all measurements were made using a reference CO2 pressure of 360 gbar. Leaves used in induction measurements were not exposed to PFD above 10 g m o l m -2 s -1 for at least 2 h previous to measurement. During measurements of induction under fluctuating light, signals from the CO2 analyzer, humidity sensor, and leaf thermocouple were recorded every 1.3 s. Leaves were exposed to a series of 5 lightfiecks separated by 2 rain of low light. Within an experiment, lightflecks were either 30 or 60 s long. Leaf induction state after each lightfleck was determined as follows: Induction state ( % ) - PLF-- - PL • l o o Pn-- PL (1) where Per is the measured CO2 assimilation rate at the end of the lightfleck, and PL and Pn are the steady-state CO2 assimilation rates at low and high light levels, respectively. Background low and lightfleck PFD were 10 and 500 I.tmol m -2 s -~ and 15-20 and 1,200 lamol m -z s -~, for A. macrorrhiza and T. australis, respectively. Lightfleck P F D were above light saturation as determined from steady-state light response curves for both species, but did not result in photoinhibition, even during exposures of 1-2 h. Low-light P F D were just above the light compensation points of both species. Induction measurements during fluctuating light were made at a flow rate through the chamber and analyzer of 3.5-4.0 1 min -~ to minimize lags due to the analyzer response. To further minimize instrumentation lags, all electronic filtering in the analyzer circuits was switched off. Under these conditions, the total response time of the analyzer and tubing was reduced to 4.3 s. Rates of CO2 uptake were corrected for instrumentation lags due to volume of the chamber as described by Pearcy et al. (1985). To determine the relative importance of stomatal and non stomatal limitations during induction in low-light grown A. macrorrhiza, the time-course of photosynthesis and stomatal conductance in one leaf were determined under 6 different ambient CO2 pressures ranging from 130 to 1,284 gbar. Measurements were made at flow rates of 0.6-0.7 1 min -~. The induction response followed a light increase from 12 to 400 lamol m -2 s -1. Based on data derived from these time-courses, the relationship between CO2 assimilation (A) and intercellular CO2 pressure (Ci) was determined at various times during the induction. These "transient" A/Ci curves were then used to calculate stomatal limitation as described by Farquhar and Sharkey (1982). Non-stomatal carboxylation limitations were calculated at the same elapsed times as follows: 9 ~ . Carboxylation limitation- P(Ci)eq-P(fl)t .... x 1 ,00 (2) P(Ci)eq w h e r e Ci is the intercellular CO2 pressure at each time period, and e(ci)eq and P(Ci)t .... are the CO2 assimilation rates at this Ci during equilibrium and transient conditions, respectively. This limitation is a measure of the relative difference between transient assimilation rates and steady-state assimilation rates at the same Ci, and therefore represents limitations due to reactions of the photosynthetic carbon reduction cycle rather than those that directly affect diffusion of CO2. It should be noted that stomatal limitations are calculated on an absolute basis (infinite stomatal conductance), while carboxylation limitations are calculated relative to a minimal steady-state value. Furthermore, these limitations are not additive because of the non-linearity of photosynthetic responses to CO2 (Sharkey 1985). The time-course of induction loss in A. macrorrhiza was determined by exposing fully-induced leaves to periods of low light ( 1 0 g m o l m -z s -1) ranging from 2 to 60rain. After exposures to low light, leaves were given a 30-s exposure to high light (500 gmol m 2 s- 1). Leaf induction state following low light periods was determined using equation 1, based on the CO2 assimilation rate reached after 30 s of illumination. Results Time-course of induction under constant light When leaves were exposed to saturating light following a long period at low light, an induction period of 26 to 48 min was required before steady-state photosynthesis rates were reached. The time-course of induction did not differ significantly between high- and low-light grown A. macrorrhiza, or between A. macrorrhiza and T. australis (Table 1). Eight to 12 min were required to reach 50% of the steady-state rates (Table 1). In low-light grown A. macrorrhiza leaves, induction appeared to occur in two distinct phases, an initial rapid phase followed by a slow, gradual rise to the steady-state rate (Fig. 1 A). In contrast, T. australis did not exhibit an initial rapid phase (Fig. 2A). In both species, increases in stomatal conductance often lagged behind increases in CO2 assimilation, but once stomatal conductance began to increase, the time-course of photosynthesis and 519 Table 1. Time (min) required to reach 50%, 90% and 100% of steady-state assimilation rates (~tmol CO2 m -2 s -1) of leaves of A. macrorrhiza and T. australis. Measurements were made at ambient CO2 pressures of 355-360 pbar. Values are means+ 1 standard deviation Species/ Trt n 50% 90% 100% (min) (min) (min) A ~E 12 Steady-state rate (~tmol COz m 2 s- 1) I 0 I I I 200 A. macrorrhiza Low-light 4 High-light 4 8.9_+1.7 21.8+1.5 10.1___2.5 27.4_+5.0 34.7_+2.8 4.8_+1.4 42.0+6.0 16.7_+1.7 150 E -~ 7". australis High-light 6 11.8+3.9 21.8_+4.8 29.2-+5.7 12.3+2.8 E 100 50 ~1 0 7r 6 A ~176176176176176176 ~~176 ~176176 7 E 3 4 i i .I 250 -_ 150 O 2 4 I I I I 75 E E 50 400 B lOO E 100 50 ] 125 ? i ..t: 200 o 7r 300 Ij,,f,f"" E .~ 350 ./f" ~176176176176176176176176176 ~176 ~176176176176 i I 800 i 1200 1gO0 2000 Time, seconds Fig. 2 A-C. Time course of A assimilation, B stomatal conductance (g), and C intercellular CO2 pressure (Ci) during photosynthetic induction of a high-light grown leaf of Toona australis. The arrow indicates when light was increased from 20 to 1,200 ~tmol m - 2 s- 1 25 i 0 I I 350 300 mg~ i Table 2. Time (min) required to reach 50%, 90% and 100% of steady-state photosynthesis rates (~tmol CO2 m -2 s-x) during inC duction of a leaf of Alocasia macrorrhiza at different ambient CO2 pressures ~ -~176176176176176176176176176176 "*~176 ~176176 "~176 I I i 250 200 O- tjIl ~ ~ ~ B,e 50% 90% 100% Steady-state Rate pressure (~tbar) (rain) (min) (min) (~tmolCO2 m-2 s-1) 130 200 640 844 1,284 1.1 5.3 4.6 5.9 2.0 19.0 23.2 14.7 13.3 11.7 43.5 45.2 28.6 34.3 26.3 1.1 2.0 6.0 5.8 7.5 Ambient 150 CO 2 100 50 0 i 400 I 800 12100 16i00 2000 Time, seconds Fig. 1 A-C. Time course of A assimilation, B stomatal conductance (g), and C intercellular CO2 pressure (Ci) during photosynthetic induction of a low-light grown leaf of Alocasia macrorrhiza. The arrow indicates when light was increased from 10 to 400 ktmol m - 2 S-1 conductance a p p e a r e d nearly coincident (Figs. 1 A, B and 2A, B). Intercellular CO2 pressure showed an initial d r o p upon illumination and then remained relatively stable during the induction period (Fig. I C and 2C). In general, Ci remained above 200 labar t h r o u g h o u t the entire period. Stomatal limitation during induction In A. macrorrhiza, a long induction period was required at ambient CO2 pressures as high as 1,284 p b a r (Table 2). The long induction requirement at these high CO2 pressures cannot be explained by diffusion limitations, and must therefore reflect decreases in carboxylation limitations during the induction period. A t ambient CO2 pressures below 200 labar, CO2 assimilation increased slowly and continuously with time. However, at ambient CO2 pressures above 200 ~tbar, the increase in assimilation rate with time appeared biphasic, with an early rapid phase followed by a slow, sigmoidal phase (Fig. 1 A), The biphasic nature o f the induction became more p r o n o u n c e d at the highest CO2 pressures. The relationship between CO2 assimilation and Ci at different time periods following illumination showed an increase in assimilation rate at a given C~ over time (Fig. 3). 520 10 9 T E 8 ? E o E steady state -----e~' 960 s /e~e~e....~'* 6 o 4 "~ s .E = ~120 s a o60s v 0 I I I I 250 500 750 1000 , , , ~ ? E o Intercellular C02 pressure, p,bor Fig. 3. The relationship between assimilation and intercellular C O 2 pressure (C~) at five time periods during photosynthetic induction and at steady-state of a low-light grown leaf of Alocasia macrorrhiza. At each time period, points represent assimilation and C~ values measured at different ambient CO2 pressures ) . ? E E E g J~ 4 ,3 -1 + .o[ ,I +oL . I 2 o --9 1 ~ o .9 ~" ,f "~+~ t 9 , ." : O0 9 i l i..~.:.~ .... ~,.a; ~.'. ~,H~. ~~,-,~'" 9 1' T P I 0 , l , , I I , 200 400 I I 400 500 9 200 1O0 0 i 0 Time, I 600 , I 200 I 400 Time, 10 JD :r . I 600 I 800 1000 seconds Fig. 5A-C. Time course of A assimilation, B stomatal conductance (g), and C intercellular CO2 pressure (C+) during a series of five 60-s lightflecks of a high-light grown leaf of Toona australis. The arrows indicate times of light increases and decreases. Two min of low light separate each lightfleck 40 20 . t-[ 20o._t-T, = : I ? E E . . 50 30 . 100 ' E '' 400 5 o E ::::L "" :. . 0o- 7ca I T.,~ 1oo[ 2 0 ! l o 240 s e//~, o E E ago s / ' 5 -6 I 800 i 1000 seconds Fig, 4A-C. Time course of A assimilation, B stomatal conductance (g), and C intercellular CO2 pressure (C+) during a series of five 60-s lightflecks of a low-light grown leaf of AIocasia macrorrhiza. The arrows indicate times of light increases and decreases. Two min of low light separate each lightfleck Because of irregularities in the biphasic nature of the responses, individual data points exhibited some scatter but the time dependent increase in the initial slopes of the relationship between CO2 assimilation rate and Ci is clearly evident. The initial slope increased approximately 7-fold from 30 s following the light increase to the end of the induction period. At 30 s, CO2 uptake at a Ci of 300 Ixbar was 0.8 ~tmol CO2 m -2 s -1 compared to 4.2 lamol CO2 m - 2 s-1 at equilibrium (Fig. 3). These curves suggest that most of the initial lag in photosynthesis can be explained by carboxylation limitations. Analyses of stomatal and carboxylation limitations of photosynthesis indicated that both components are involved in the induction process, but carboxylation limitations predominate for the first 6-10 min following illumination. Stomatal limitations increased later during induction, but still represented only a small part of the overall limitation on photosynthetic rate. At an ambient CO2 pressure of 360 ktbar, stomatal limitation was only 8% at 30 s, whereas carboxylation limitations were 82% (relative to 0 at equilibrium). Stomatal limitation increased to 22% 1 min after illumination and remained between 23-32% during the entire induction period, whereas carboxylation limitations decreased throughout the induction period. Induction under fluctuating light During a sequence of lightflecks, rates of CO2 assimilation and stomatal conductance increased in both species (Figs. 4A, B and 5A, B). Following a sequence of five 60-s lightflecks (total elapsed time of 13 min), leaves of low-light grown A. macrorrhiza had reached 75% of their steady-state CO2 assimilation rate, whereas leaves of T. austral& had reached 52% of their steady-state assimilation rate (Table 3). The time-course of induction during 30-s lightflecks 521 1 O0 Table 3. Relative induction state (% full induction) of leaves of A. macrorrhiza and T. australis during a sequence of five 30- and \ 60-s light flecks at saturating light intensity. Flecks were separated by 2 min of low light. Values are means_+SD for 3-4 leaves. Fleck length (s) Fleck # 60 Induction State (%) A. macrorrhiza T. australis Elapsed time (min) 1 2 3 4 5 39.5__+23.0 49.1_+28.5 57.5_+29.1 65.1_+30.5 74.8_+27.2 14.3__+ 7.4 23.9__+10.2 34.8__+11.5 44.0_+13.7 52.3_+14.4 1 4 7 t0 13 1 2 3 4 5 23.7+ 6.2 31.2_+ 5.4 36.2+ 5.4 41.5_____ 4.1 48.3-+ 3.0 10.3_+ 9.8 15.9+13.9 21.0__+15.3 25.8_+15.3 31.0_+15.1 0.5 3.0 5.5 8.0 10.5 Q. 0 g 30 was relatively slower in both species; after 5 lightflecks (total elapsed time of 10.5 min), leaves of A. macrorrhiza and T. australis reached 48 and 31% of their respective steadystate rates (Table 3). At any given time period during the lightfleck sequence, the degree of induction was greater in leaves exposed to 60-s lightflecks (Table 3). Within both species, leaves varied widely in their responses to the first lightleck, and therefore to subsequent lightflecks. Coefficients of variation for percent full induction during the first 60-s lightfleck were 58% and 52% in A. macrorrhiza and T. australis, respectively. Although the rate of induction did not differ significantly between species (Student's t-test on arcsine-transformed data; P > 0.05), these data suggest that leaves of A. macrorrhiza reached higher levels of induction during the first lightfleck, and maintained these higher levels throughout the sequence (Table 3). Stomatal responses to lightflecks were consistently more rapid in T. australis than in A. macrorrhiza (Figs. 4 B and 5 B). However, CO2 assimilation rates were also higher in T. australis, and the resulting Ci during lightfleck sequences were similar in the two species (Figs. 4 C and 5 C). During lightfleck sequences, Ci remained above 200 labar, as during induction under constant light (Figs. 1 C, 2C, 3 C, and 4C). In T. australis, there appeared to be two components to the stomatal response: a rapid one that corresponded to the light increases and decreases during individual lightflecks, and a slow one that corresponded to the apparent increase in induction state. Within each species, the rate of induction during the 60-s lightfleck sequences was not substantially different from the rate of induction during constant illumination (Figs. 1 A and 2A). Because different sets of leaves were used and sample sizes were small, it is difficult to make meaningful quantitative comparisons between these two experiments. Nevertheless, the data clearly indicate that sunflecks, if they are not too brief and infrequent, may be as effective as constant illumination in promoting photosynthetic induction. Time-course o f induction loss & low light In leaves of low-light grown A. macrorrhiza, complete induction loss required over 60 min of exposure to low-light "0 C 40 20 0 210 r 0 610 ' ' I 180 Low-light period, minutes Fig. 6. The time-course of induction loss of low-light grown leaves of Alocasia macrorrhiza following different periods of exposure to low light conditions (Fig. 6). The loss of induction in low light followed a negative exponential function; the half-time for induction loss was approximately 25 min (Fig. 6). After 60 min at low light, leaves dropped to 43 % relative induction. When these leaves were initially exposed to a 30-s lightfleck following more than 3 h of exposure to low light, the CO2 assimilation rate was 20% of the difference between the steady-state low-light and high-light rates (Fig. 6, Table 3). Although the full time course of induction loss in T. austral& was not investigated, after 2 min of low light, relative induction dropped to 89,5%, and responses to subsequent lightflecks further decreased (Chazdon and Pearcy 1986). Based on these observations, it appears that induction loss in high-light grown T. austral& occurs more rapidly than in low-light grown A. macrorrhiza. Discussion Photosynthetic induction in intact leaves is a complex phenomenon potentially involving both stomatal and carboxylation limitations to photosynthetic rate. The data presented here suggest that, during induction, increases in carboxylation capacity rather than stomatal conductance are primarily responsible for the increasing photosynthetic rate. Similar results were obtained with leaves of two Hawaiian understory species (Pearcy et al. 1985). One possible problem with this interpretation, however, is that a cuticular conductance to water vapor but not to CO2 could result in an overestimation of the intercellular CO2 pressures and hence an underestimation of the stomatal limitations. The effect of a cuticular conductance would be greatest at the low stomatal conductances present early in induction. The mechanistic basis of the induction requirement of photosynthesis is poorly understood, but it probably involves requirements for autocatalytic buildup of sufficient pools of carbon reduction cycle metabolites and/or light activation of photosynthetic enzymes (Walker 1980). Several photosynthetic enzymes, including ribulose-l,5-bisphosphate carboxylase (RUBISCO) are known to be light activated. Moreover, under steady-state conditions there is a strong linear relationship between RUBISCO activity and the initial slope of the CO2 dependence curve of assimilation (von Caemmerer and Farquhar 1981). The mechanistic 522 basis for the increasing slope of the transient A/Ci curves during induction may be the light activation requirement of RUBISCO (Fig. 3). However, a role for activation of other enzymes, or for metabolite pool size changes cannot be ruled out. The apparently nearly constant intercellular COz pressures after the initial drop during induction suggests that stomatal opening may somehow be closely coupled to the increasing photosynthetic capacity during induction. Further evidence of the close correlation between stomatal conductance and photosynthesis under transient conditions is shown by the relative constancy of the calculated stomatal limitation throughout the induction period. A close correlation between stomatal conductance and steady-state photosynthetic rate has been observed for a variety of plants (Wong et al. 1979, Korner et al. 1979, Schulze and Hall 1982), but little attention has been given to correlated responses under transient conditions. Stomatal opening and induction in a C4 tree species was much faster than in a C3 tree species, even though carboxylation limitations predominated during induction in both species (Pearcy et al. 1985). Slow stomatal responses to sunflecks of some tree species have been suggested as a possible basis for species differences in shade tolerance (Woods and Turner 1971). However, measurements of A. macrorrhiza in its native habitat indicate that stomatal conductances are probably sufficiently high that they would impose only a small limitation on photosynthesis during sunflecks (Bj6rkman et al. 1972). The increase in COz assimilation rate during fluctuating light conditions clearly shows that constant light is not required for induction (Figs. 4 and 5), which had been previously demonstrated for leaves of other forest species (Pearcy et al. 1985). Thus, the occurrence of a lightfleck appears to prime the leaf, through its effect on induction, such that the leaf is better able to utilize subsequent lightflecks. The greater rate of induction during 60-s lightflecks compared to 30-s lightflecks suggests that induction is sensitive to lightfleck length (Table 3) with the total exposure time, rather than the total number of photons received, probably being the significant factor. The time-course of induction in wheat leaves was relatively independent of light intensity (McAlister 1937). The loss of induction in low-light grown leaves of A. macrorrhiza was considerably slower than the rate of induction under both constant and fluctuating light. Deactivation of RUBISCO in wheat seedlings was also found to be considerably slower than activation (Perchorowicz et al. 1982). Also, changes in ribulose-l,5-bisphosphate (RuBP) pool sizes in Xanthium strumarium leaves following a decrease in light were consistent with a relatively slow deactivation of RUBISCO (Mott et al. 1984). Leaves of A. macrorrhiza consistently exhibited a faster initial rate of induction than leaves of T. australis under both constant and fluctuating light conditions (Figs. 1A, 2A, 4A, and 5A). This difference may reflect species-specific differences in requirements for activation of photosynthetic enzymes, or in metabolite pool sizes. The slower rate of induction loss in A. macrorrhiza also suggests that this species is able to maintain RUBISCO (or other carbon reduction cycle enzymes) in a more activated state and/or maintain larger pool sizes of carbon reduction cycle intermediates under low-light conditions. These differences in induction responses should result in more efficient utiliza- tion of sunflecks by A. macrorrhiza than by T. australis (Chazdon and Pearcy 1986). In a Hawaiian forest understory, sunflecks are clustered in their temporal distribution with periods of relatively frequent sunflecks interspersed with periods with few or no sunflecks. Most sunflecks in this forest occurred within 2 min of another sunfleck (Pearcy and Calkin 1983). Similar patterns have been observed in other temperate and tropical forests, including those where A. macrorrhiza is common (Bj6rkman et al. 1972) Thus, the occurrence of a sunfleck is a good predictor that another will occur within a short time. A rapid induction response coupled with a slow rate of deactivation may serve to insure that sunflecks are efficiently utilized. The data presented here, along with measurements of light in tropical and subtropical forests (Bj6rkman and Ludlow 1972, Pearcy and Calkin 1983, Chazdon 1986), suggest that, in the understory, leaf induction state may significantly limit the capacity of a leaf to utilize a sunfleck. Exposure to several, short sunflecks is sufficient to stimulate partial induction following a long period of darkness or low light. Yet leaves are rarely exposed to sufficiently long periods of high light to maintain a fully-induced photosynthetic apparatus for very long. Consequently, steady-state photosynthesis rates may poorly reflect actual rates of photosynthesis during sunfleck periods. Studies of leaf carbon gain in understory species will therefore need to incorporate transient photosynthetic responses at intermediate induction states. References Bj6rkman O, Ludlow MM (1972) Characterization of the light climate on the floor of a Queensland rainforest. 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