Environmentaland ExperimentalBotany, Vol. 34, No. 2, pp. 141 151, 1994
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THE I M P A C T OF CANOPY G R O W T H A N D T E M P O R A L C H A N G E S IN R A D I A T I O N ON THE D Y N A M I C S OF CANOPY CARBON A S S I M I L A T I O N F O R K I W I F R U I T (ACTINIDIADELICIOSA) VINES D U R I N G SPRING j. G. BUWALDA The Horticulture and Food Research Institute of New Zealand Ltd, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand
(Received 17 August 1993; accepted in revisedform 12 November 1993)
BUWALDAJ. G. The impact of canopy growth and temporal changes in radiation on the dynamics of canopy carbon assimilation for kiwifruit (Actinidia deliciosa) vines during spring. ENVIRONMENTALAND EXPERIMENTALBOTANY34, 141--151, 1994. --Diel trends of canopy CO2 assimilation (A) for four kiwifruit (Actinidia deliciosa) vines during spring were measured during a 78-day period from 12 days after bud burst until the summer solstice. The canopy of each vine was enclosed in an opensystem gas exchange cuvette, and a computer-controlled sampling and logging system enabled multiplexed analysis of gas exchange rates for each vine. The canopy leaf area, estimated on five occasions, increased on average from 0.25 m 2 m -2 at 15 days after bud burst to 2.39 m 2 m -2 at 85 days after bud burst. Inter-vine differences in leaf area were consistent throughout the measurement period. Diurnal integrals of quantum flux density (Q), measured above the canopy within the cuvette, varied from 9 to 56 #mol m -2 day-1, and did not show a clear tendency to increase with increasing daylength towards the summer solstice. Mean daily air temperatures within the cuvettes tended to increase during the measurment period, from 12-14°C soon after bud burst to 15-21°C near the summer solstice. Diel integrals of canopy A tended to increase during the measurment period, from ca - 0 . 7 #mol CO2 m 2 day l soon after bud burst to as high as 1.4 #mol CO2 m 2 day-1 near the summer solstice. Inter-vine differences at any stage of the season were related to differences in leaf area. The temporal trend for increasing diel integrals of canopy A during spring followed the increasing leaf area, although large day-to-day differences at any stage of the season could be associated with concomitantly varying diurnal integrals of O. The temporal trend of increasing canopy A was almost entirely due to increasing day-time A, as night-time A (negative) changed relatively little during the measurement period. Asymptotic exponential curves were used to describe the relationship between instantaneous Q and canopy A at three stages of the season. These relationships indicated that the initial response of canopy A to increasing incident Q (the apparent quantum yield) was 0.03, 0.07 and 0.08 mol CO2 m o l - ~ Q at ca 35, 55 and 85 days after bud burst, respectively. The quantum saturated rate of canopy A at any stage, however, responded significantly to inter-vine differences in leaf area, ranging from 9.6 to 11.9 #mol CO~ m -2 s -~ at ca 35 days after bud burst, 25.7-29.7 #mol CO2 m -~ s ~ at ca 55 days after bud burst, and 29.0-37.8 #mol CO 2 m 2 s-I at 85 days after bud burst. The temporal changes and inter-vine differences at any stage in the quantum saturated rate of canopy A could be related to canopy leaf area.
Key words: Actinidia deliciosa, canopy growth, carbon assimilation, leaf area, quantum flux density. 141
142
J. G. BUWALDA INTRODUCTION
THE carbon economy of a growing plant, represented by the balance of carbon acquisition due to photosynthesis and carbon loss due to respiration, determines the carbon available for synthesis in new plant tissue. For annual plants, this balance and hence growth are closely dependent on current photosynthesis, as stored carbon pools (e.g. in the seed) are small relative to the absolute growth rate. The temporal dynamics of carbon acquisition and allocation for growth are less directly related in perennial deciduous plants. For such plants, hydrolysis of carbon stored in perennial structures over winter represents the primary carbon source for maintenance and growth processes during spring when leaf area and hence radiation interception and photosynthesis are relatively low. Indeed, simulation modelling ~2) and analysis of the seasonal dynamics of starch pools within the kiwifruit vine/~9) have indicated that carbon reserves may be depleted to support growth and maintenance for up to 100 days after bud burst. Carbon budgets for whole-plants can be estimated by extrapolating from defined rates of photosynthesis and respiration for component tissues of the plant, often using simulation modelling to account for the many potential interactions involved. (2'12'16) However, spatial and temporal heterogeneity of many of the variables controlling carbon acquisition and utilization limit the value of such extrapolations. The development of opensystem gas exchange measurement systems for entire plant canopies now enables direct measurement of the temporal dynamics of whole-canopy carbon assimilation. (t°'tT'IS'21) Such systems have also been developed for perennial fruiting plants, (23) and are being used increasingly to describe responses to environmental variables including radiation and temperature. (3'11'22) In this paper, we investigate limitations to whole canopy CO 2 assimilation for kiwifruit vines during spring, to characterize the importance of inter-plant differences in leaf growth and temporal difference in diel integrals of quantum flux density (Q). Whilst hydrolysis of carbon reserves at this time is clearly important, critical plant development stages such as flowering may be particularly sensitive to perturbations in the supply
or demand for carbon within the whole plant. !9i An improved understanding of underlying mechanisms is important for identifying opportunities to overcome limitations to carbon assimilation during this critical stage of the season. MATERIALS AND METHODS
Plants
Measurements were made during spring 1992, using kiwifruit vines (Actinidia deliciosa var. deliciosa, cv Hayward) growing in an orchard near Hamilton, New Zealand (latitude 38.2°S). The vines were planted in 1984, spaced 2.5 m apart in rows 4.5 m apart, with canopies trained on a T-bar trellis, iSi The trellis rows ran approximately north-south, with the inclined faces of the T-bar trellis facing east and west, respectively. The vines were fertilized before bud burst, with 180 kg N ha i, 160 kg K ha i, and 60 kg P ha -1. The vines were irrigated with mini-sprinklers, at rates approximately equal to the evapotranspiration lOSS. Four vines were selected in late winter for uniformity, and dormant-pruned to a cordon length of 2.4 m and a lateral span of 4.2 m (2.1 m either side of the cordon), giving a canopy area of 10 m 2. Interplant differences in canopy carbon assimilation were therefore expected to reflect subsequent differences in leaf growth and assimilation rates. Vegetative growth commenced with bud burst on year day 265. Flowering occurred during year days 318-324. Flowers were pollinated by hand, and fruit numbers reduced to ca 300 per vine within seven days of flowering. Gas exchange measurement system
The measurement system has been described in detail elsewhere. (4/Each of the open-system gas exchange cuvettes comprised a frame constructed of 200 mm galvanized iron pipe clad on the walls and roof with ethyl vinyl acetate (EVA) film of 200 ~m thickness. The cuvette, with a basal floor area o f ca 10 m 2 and a volume o f ca 26 m 3, was positioned on a timber floor, elevated 0.2 m above the ground. Air was pumped continuously through each cuvette, at a flow rate o f ca 0.5-1.5 m 3 s 1. Air speed (m s -x) through the flow delivery tube for each cuvette was measured continuously using cup anemometers ("Maximum
DYNAMICS OF CANOPY CARBON ASSIMILATION Halt Effect" wind-speed generator, M a x i m u m Inc., New Bedford, Massachusetts, U.S.A.). Flow rates were estimated from the measured wind speeds, after the wind speed sensors had been calibrated according to a measured elevation of C O 2 partial pressure within the cuvette while introducing a known flux of CO2. Air entering the cuvette was dispersed through a perforated E V A tube (2.4 m x 0.5 m diameter), with the perforations positioned to direct the air upwards and outwards, enhancing mixing of air within the cuvette. Flow rates equivalent to a complete change of the cuvette air volume every 20 50 s ensured high b o u n d a r y layer conductance and a small difference between ambient air temperature and that within the cuvette. T h e maintenance of a small positive air pressure, relative to ambient, within the cuvette, ensured that leaks from the cuvette were primarily outwards. Q u a n tum flux density (Q, #mol m -2 s 1) above the plant within each cuvette was measured using LI 190 q u a n t u m sensors (Li-Cor, Lincoln, Nebraska, U.S.A.) located immediately below the cladding at the roof apex. Air temperature within the cuvette was measured with thermistors (Grant CS, G r a n t Instruments, Cambridge, U.K.). Four cuvettes were multiplexed to a single set of analtyical instruments, using a personal computer (PC) for control of the multiplexer and recording of data. The canopy net carbon assimilation rate (A, #tool CO2 m -2 s-1) was calculated from the difference in CO2 partial pressures between air entering the cuvette and air within the cuvette. I n d e p e n d e n t air lines conducted reference and measurement samples to the analytical instruments. Reference air for this differential analysis was sampled from the tube delivering air to the cuvette. Measurement air was sampled from four positions within the cuvette. T h e reference and measurement air streams were each d r a w n to the analytical instruments through tubes of equal volume (1.5 1), at flow rates o f ca 0.2 1 s i. Multiplexed sampling of reference and measurement air streams for the four cuvettes was facilitated by a purpose-built solenoid switching unit. O p e n i n g and closing of solenoids at the end of each of the reference and measurement air lines was regulated from the PC, enabling reference and measurement gas to be sampled from one cuvette at a time. T h e sampled
143
reference and measurement air lines were divided, with sub-samples directed through a differential CO2 infra red gas analyser ( I R G A , A D C M K I I I , Analytical Development C o m p a n y , Hoddesdon, U.K.) and over dew-point hygrometers (MTS M K I, Walz Mel3- und Regeltechnik, Effeltrich, Germany). A sub-sample of the reference air stream was also passed through an absolute CO2 I R G A (ADC M K II). Instantaneous measurements of differential CO2 and H 2 0 partial pressures and absolute CO2 partial pressure in the reference air stream were typically made every 4 s, and averaged every 60 s. Sequential sampling from the four cuvettes involved an equilibration interval of 120 s followed by a recording interval of 300 s (i.e. 420 s per cuvette). T h e zero-setting of the differential I R G A was automatically checked every 2 hr, during an interval of 480 s when reference air from one of the cuvettes was directed through both cells of the differential I R G A and over both dew point hygrometers. Thus, the measurement system enabled canopy A to be estimated two times each hour for each cuvette, using the mean of 75 instantaneous readings during a 300 s interval for each estimate. Earlier work indicated that such equilibration and measurement times were sufficient for estimating instantaneous rates of canopy A including responses to short-term changes in environmental variables such as incident radiation./3'4~ Nocturnal (i.e. night-time) estimates of the rate of c a n o p y A were often sensitive to the rate of change of ambient CO2, although estimates of the nocturnal integral of canopy A were usually considered valid. (3/The I R G A s were calibrated manually for zero and span, usually twice weekly, using CO2-free "synthetic air" and a span gas with 395 /~mol CO2 mol 1 (balanced with nitrogen), respectively. Measurements All measurements were made during spring 1992. The leaf area index of each of the four vines was estimated on five occasions; viz., year days 280, 300, 320, 337 and 350. Contacts with leaves as a needle was passed through the canopy were counted at 200 positions (on a grid, with centres at 0.1 m x 0.4 m) on each vine, at an angle normal to the trellis surface at each position. T h e leaf area index at each position, expressed as the leaf
144
J. G. BUWALDA
area per unit trellis surface area, was estimated according to the leaf layers so counted, with a cosine correction for a typical distribution of leaf inclination angles/7/ and the inclination relative to the trellis surface at each measurement point. The total vine leaf area was estimated as the spatial integral of these point estimates of leaf area index. This total was expressed per unit trellis surface area for the whole vine, which coincidentally equalled the ground area occupied by the vine; i.e. the basal floor area of the cuvette. Measurements of canopy A, Q and cuvette air temperature commenced on year day 277 (ca 12 days after bud burst), and continued until year day 354 (the summer solstice). For kiwifruit vines, a large proportion of canopy growth is typically completed during this time./5/ For selected periods, measured A was related to Q above the vine, using an asymptotic exponential function: (6)
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Seasonal dynamics of radiation and canopy gas exchange T h e spring period used for these measurements was characterized by highly variable radiation conditions. Diurnal (i.e. day-time) integrals of Q (Qd) during this period (Fig. 3a) ranged from 9 m o l m 2 d a y - I (year day 298) to 56 mol m 2 d a y - l (year day 328). The large day-to-day variation in Q largely masked the tendency for
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FIG. 3. Temporal trend of (a) diel integral of quantum flux density (Qd), (b) mean daily temperature, and (c) diel integral of canopy CO2 assimilation (Ad) for each of tour vines. Data are presented only for those 24-hr periods in which data collection was uninterrupted. increasing Od with increasing day length. M e a n daily temperature tended to increase during the season, from 12-14°C soon after bud burst to 1521°C near the summer solstice (Fig. 3b). At any stage of the growing season, the mean daily temperature tended to be lower on days with low Qa than on days with high Qa. Measurements of c a n o p y A were frequently interrupted, as instrument calibrations were checked, access to the vine within the cuvette was required (e.g. for estimation of leaf area index), and during power or instrument failures. Accordingly, complete diel (i.e. 24 hr) data sets were obtained for 47 days only, although data for more than 95% of the day were available for 67 days. For each day with uninterrupted recording of canopy A, the diel integral of c a n o p y A (Aa) was estimated from the mean rate measured two times each hour (Fig. 3c). At the start of the season, Aa was consistently negative (i.e. c a n o p y respiration exceeded canopy photosynthesis), and as low as - 0 . 7 mol CO2 m ~ day -1. After year day 300, Aa was consistently positive, and thereafter tended to increase with time, ranging from 0.7 to 1.4 tool C O 2 m 2 d a y - l during the last three days of measurement. Daily variations in A d were how-
145
ever very large. In particular, Ad fell to relatively low levels between year days 333 and 348, when diurnal integrals of Q were also relatively low. T h e inter-vine difference in A d ranged considerably, from less than 0.10 mol CO2 m 2 d a y - l on year days 298 and 326, to more than 0.40 mol CO2 m ~ day i on year days 347 and 349. For almost every day during the measurement period, Aa was lower for vine 3 than for the other vines. For much of the measurement period, Aa was higher for vine 2 than for the other vines. Nocturnal rates of canopy A (i.e. respiration) ranged typically from - 4 to - 7/~mol CO2 m - 2 s-1 during the first 20 days after measurements began, and from - 6 to - 8 #mol CO2 m -2 s ~during the last 20 days of the measurement period. Nocturnal integrals of canopy A accordingly ranged from - 0 . 2 4 to - 0 . 3 5 mol CO2 m -2 d a y - ~early in the season, to - 0.17 to - 0.24 mol C O 2 m -2 day ~ at the end of the measurement period. T h e tendency for less negative nocturnal integrals of canopy A as the season progressed was associated mainly with nocturnal periods of shorter duration rather than less negative rates of canopy A during the nocturnal period. Early in the season, A d was lower (i.e. more negative) than the nocturnal integral, for all vines, due to negative rates of canopy A for much or all of the diurnal period then. By the end of the measurement period, the nocturnal integral of canopy A typically represented 12-40% of the diurnal integral of canopy A. This range reflected day-today differences in Qd, rather than vine-to-vine differences in Ad.
Relationship between quantum flux density and canopy carbon assimilation Diurnal variations in Q and A were compared, to examine the dependence of A on Q at different stages during canopy development, using pairs of days with low and high Qa. Such a comparison is shown in Fig. 4, for the time about 30~1,0 days after bud burst when the leaf area index of the vines was ca 0.6-0.8 m 2 m 2. Data from year days 298 and 304 were used to represent cloudy and sunny days with low and high diurnal integrals of Q, respectively. Air temperatures within the cuvettes were similar for these days, averaging 13.5°C on the cloudy day and 14.8°C on the sunny day. For each day, the values of Q mea-
146
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sured within all cuvettes are presented together (Figs 4a a n d 4b), while c a n o p y A measured in each of the cuvettes is shown s e p a r a t e l y (Figs 4 c 4j). Q was always less than 500 # m o l m 2 s - i d u r i n g the cloudy day, b u t exceeded 1900 #mol m -2 s i d u r i n g the sunny day. F o r both days, c a n o p y A increased and decreased with r a p i d l y c h a n g i n g Q at the start and end of the day, respectively, and r e m a i n e d relatively constant for a long period d u r i n g the day. F o r all vines, c a n o p y A was considerably higher on the sunny d a y comp a r e d to the cloudy day, but inter-vine differences were also a p p a r e n t . At any time a n d on both days, vines 1 a n d 3 typically showed lowest c a n o p y A (ca3 # m o l C O 2 m - 2 s l a n d 11 # m o l C O 2 • 2 s - l on the cloudy a n d sunny days, respectively) a n d vine 2 showed the highest c a n o p y A (ca 6 #mol CO~ m -2 s -1 and 15 # m o l CO2 m -2 s -1 on the cloudy and sunny days, respectively). At subsequent m e a s u r e m e n t times, c a n o p y A at a n y r a d i a t i o n level tended to increase, b u t continued to be higher on sunny days c o m p a r e d to cloudy days. N e a r the s u m m e r solstice (e.g. y e a r
days 347, 353), c a n o p y A was up to 40 # m o l CO2 m -2 s -1, b u t showed large responses to withinand b e t w e e n - d a y variations in Q (Fig. 5). A t this time, c a n o p y A was consistently highest for vine 2 a n d lowest for vine 3. Fluctuations in o v e r h e a d Q of shorter d u r a t i o n t h a n a complete s a m p l i n g cycle (1680 s) did not lead to measured responses in c a n o p y A within all cuvettes. F o r example, the d i u r n a l trends on y e a r d a y 298 (Fig. 4) i n d i c a t e d short periods of increased Q at a b o u t 13.40 hr and 15.15 hr. During the first of these events, a clear increase in c a n o p y A was noted for cuvettes 3 a n d 4, while c a n o p y A in cuvette 1 a p p e a r e d to be unaffected. Conversely, c a n o p y A d u r i n g the second of these events responded most noticeably in cuvette 1, while c a n o p y A in cuvettes 3 a n d 4 was h a r d l y affected. Similarly on y e a r d a y 304 (Fig. 4), canopy A in each of the cuvettes showed a p p a r e n t l y variable responses to the fluctuations in Q between 12.00 a n d 15.00 hr. I n the multiplexed analysis of gas exchange used here, with a m e a s u r e m e n t time for each cuvette of 300 s within
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DYNAMICS OF CANOPY CARBON A S S I M I L A T I O N a total m e a s u r e m e n t cycle time (for all four cuvettes) of 1680 s, short fluctuations in Q did not necessarily coincide with a m e a s u r e m e n t interval for a cuvette. Considering c a n o p y A within a n y cuvette against Q d u r i n g the s a m p l i n g time only for that cuvette removes this a p p a r e n t difference in response. T h e relationship of c a n o p y A to Q ( E q u a t i o n 1) between y e a r days 292 a n d 304 is shown in Fig. 6, with d a t a a n d fitted asymptotic exponential curves shown s e p a r a t e l y for each vine. T h e values of the p a r a m e t e r s fl a n d p for curves fitted indep e n d e n t l y to d a t a for the tbur cuvettes did not differ significantly• H e n c e the regression procedure was modified, to estimate first c o m m o n values of the p a r a m e t e r s / ~ and p, using d a t a for all vines, a n d then vine-specific values for the p a r a m e t e r A~, only (Fig. 6). This relationship, a c c o u n t i n g for 72% of the v a r i a t i o n in measured c a n o p y A, i n d i c a t e d that q~ at this early stage of the season was 0.032 mol CO~ t o o l - ~ Q and that A~a~ranged from 9.64 # m o l CO~ m - 2 s z for vine 3 to 11.93 # m o l CO~ m -~ s 1for vine 2. Estimates of ( ~ for the four vines at this stage of the season
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r a n g e d only from 1060-1130 # m o l m - " s 1, averaging 1090 # m o l m 2 s - I . A s y m p t o t i c exponential relationships with c o m m o n values offl a n d p a n d vine specific values of Asat were similarly used to describe the relationship of c a n o p y A to Q for each of the vines, for the periods from y e a r days 316 to 327 (Fig. 7) and y e a r days 344 to 354 (Fig. 8). These relationships explained 92 and 91°/o of the v a r i a t i o n in measured c a n o p y A d u r i n g these periods, respectively. F o r the period from y e a r days 316 to 327, estim a t e d q~ was 0.067 mol CO2 m o l - 1 Q, while estim a t e d A ~ r a n g e d from 25.7 /tmol CO2 m 2 s - i for v i n e 3 to 2 9 . 7 / ~ m o l C O 2 m - 2 s i for v i n e s 2 and 4. Estimates of ~ for vines r a n g e d only from 1370 to 1440 #tool m 2 s-~, averaging 1400 # m o l m -2 s ]. For the period from y e a r days 344-354, estimated ~ was 0.080 mol CO2 m o l l Q, while estimated A ~ r a n g e d from 29.0 /~mol CO2 m -2 s ~forvine3to37.8#molCO~m ~s-~forvine 2. E s t i m a t e d ( ~ at this time r a n g e d only t)om 1340 to 1460 # m o l m ~ s -~, averaging 1400 # m o l m - 2 s 1. T h e ontogenetic trend in the response of can-
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Fie.. 6. Relationship of canopy CO2 assimilation (A) t o quantum flux density (Q) between year days 292 and 304. Each data point is the mean of 75 instantaneous assessments at 4 s intervals during a 300 s period. The lines are fitted asymptotic exponential regressions; (a) A = 10.30-11.52" 0.9972 Q, (b) A = 11.93-11.52" 0.9972 Q,(c) A = 9 . 6 4 11.52"0.9972 Q,(d) A = 11.5111.52" 0.9972 Q.
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gested that A~a, should maximize at a leaf area of 2.53 m 2 m -2, thereafter declining. Inter-vine differences in estimated ~b were not significant. However, the ratio of this parameter to leaf area at year days 300 and 320 was almost identical (ca 0.046 m 2 leaf area), although this ratio declined by year day 350 (ca 0.033 m 2 leaf area). Intervine differences in estimated O~a~were also very small, and ontogenetic changes were proportionally much smaller than changes in leaf area.
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FIG. 8. Relationship of canopy CO2 assimilation (A) to quantum flux density (Q) between year days 344 and 354. Each data point is the mean of 75 instantaneous assessments at 4 s intervals during a 300 s period. The lines are fitted asymptotic exponential regressions; (a) A = 34.02-36.31" 0.9978 Q, (b) A = 37.81 36.31" 0.9978 Q, (c) A = 29.01-36.81" 0.9978 Q, (d) A = 31.77-36.31" 0.9978 ~. opy A to Q can be related to ontogenetic changes in leaf area for each of the vines. A quadratic regression indicated that inter-vine differences and ontogenetic changes in leaf area accounted for 95% of the variation in the vine-specific estimates of A~at (Fig. 9). This regression further sug40 'tO
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Canopy carbon balance The diel carbon balances (Ad), estimated as the temporal integral of instantaneous rates of A during a 24-hr period, changed from negative to positive at ca 35 days after bud burst (Fig. 3). This change was largely a consequence of increasing integrals of diurnal photosynthesis (positive A), as integrals of nocturnal respiration (negative A) remained relatively stable throughout the measurement period. During the first 15-20 days after bud burst, however, the diurnal integral of A was only slightly higher (i.e. less negative) than the nocturnal integral of A. While diurnal A was obviously limited by low leaf area and hence radiation interception at this time, the contribution of photosynthesis m a y have been masked by increased respiration at higher temperatures ~L'22/ during the day. Nocturnal respiration rates measured here in spring were similar, while diurnal photosynthesis rates were up to 3 0 - 5 0 % higher compared to those reported earlier for a single kiwifruit vine in late summer. (3/ T h e vines used here developed leaf areas up to two times larger than those for the vines used in the earlier work./3~ Hence, the strong relationship between canopy A and leaf area (Fig. 9) can account for the higher diurnal photosynthesis rates recorded here. T h e low fruit biomass during the current measurements would have limited the nocturnal respiration rates compared to measurements made in late summer when the fruit biomass would be much larger, thus offsetting at least some of the expected impact of the higher leaf biomass on nocturnal respiration. The Aa rates reported here appear similar in
DYNAMICS OF CANOPY CARBON ASSIMILATION magnitude to those for other fruiting plants. For an apple tree, mean net carbon assimilation rates ranging from 0.04 mol CO2 m 2 d a y - 1 when the leaf area was 0.1 m 2 m - 2 , t o 0.21 mol CO2 m 2 day-~ when the leaf area was 1.2 m 2 m 2, have been reported. (23/ While Ad measured here with kiwifruit vines clearly varied with Qd, average rates during the season increased from ca - 0 . 1 0 mol CO2 m -2 day -I when the leaf area was 0.25 m 2 m 2, to ca 0.75 mol CO2 m -2 day -I when the leaf area was 2.0 m 2 m 2 (Figs 1 and 3). Interplant differences in canopy carbon balance were clearly associated with differences in leaf growth (Figs 1 and 2). The transition from negative to positive Aa and the peak Aa values were broadly proportional to the leaf area. The carbon balance as measured here is distinct from the balance between carbon supply and demand. In addition to carbon utilization for respiration within the canopy, the balance between carbon supply and demand also included carbon utilization for growth of tissues within the canopy and for growth and maintenance of tissues within the root systems, Nevertheless, negative values of Ad, recorded as late as 33 days after bud burst in the present study, highlight the duration of deficits between carbon supply and demand for perennial fruiting plants. Such deficits, identified directly with 14C experimentation, (14)or indirectly with simulation modelling ~2) and tissue starch analysis, Cl9/highlight two critical periods coinciding with the very early stages of shoot growth and fruit growth, respectively. Indeed, Ad values estimated here did not reflect the deficit between carbon supply and demand typically associated with the onset of fruit growth (about year day 300 in our study), probably because a large fraction of the carbon allocated to the fruit at this time is incorporated in synthesized tissue rather than lost in respiration./2°! Quantum response of net canopy carbon assimilation The temporal dynamics of net canopy carbon assimilation for deciduous fruiting plants depend strongly on the rate of canopy growth in spring, and hence the dynamics of radiation interception. However, the large day-to-day variations in Ad at any stage of the measurement period, associated with variations in Qd, highlight the value of describing canopy photosynthesis in terms of the
149
relationship between A and Q. The asymptotic exponential curves used here to describe this relationship include parameters that provide convenient estimates of the apparent quantum yield of canopy A (~b), the quantum saturated rate of A (Asat) , and the Q required to saturate A (Qs,,). Estimates ofq~ should ideally be based on intercepted Q, rather than Q incident on a horizontal surface above the canopy as in this study. Where radiation incident on the inclined faces of the Tbar canopy is higher than that on a horizontal surface above the canopy (e.g. at early and late times during the day), 4) may be over-estimated. Nevertheless, temporal changes in estimated q5 reflect changes in the relationship between incident radiation and canopy photosynthesis coincidental with the growing leaf canopy. Increasing Q~at during the season can be attributed to increasing leaf area and hence a requirement for increasing Q above the canopy to enable quantum saturation of A at leaves in shaded positions within the canopy. The relative stability in estimates of Q~at (ca 1400/~mol m 2 s-l) beyond year day 320 is therefore surprising, as the leaf area of the vines was certainly increasing beyond that time. However, the estimate of Q~atcannot be isolated from estimates ofq~ within the asymptotic exponential equations fitted here. Hence, a tendency to over-estimate q~, as discussed above, would lead to under-estimates of Qsat" Nevertheless, canopy carbon assimilation was frequently limited by low O. While Q was less than Qsat for 30-50% of the diurnal period on days without cloud (Fig. 4b), Q almost never exceeded O~at on the many days with partial or complete cloud cover. A~at for a canopy should increase with increasing leaf area and hence radiation interception, at least until an optimum leaf area above which the respiratory carbon costs associated with synthesis and maintenance of marginal leaf area exceed the photosynthetic carbon gains. Temporal changes in leaf photosynthetic capacity will accordingly cause temporal changes in A~atfor the canopy. A~a~ for single leaves on kiwifruit vines can increase from ca 9 #mol CO2 m 2 s-~ at about 30 days after leaf emergence to ca 16-20 #mol CO2 m 2 s -I at 90-150 days after leaf emergence. (6/ Such a change can explain, at least partly, the temporal increase of canopy A s a t measured here.
150
J. G. BUWALDA
The " o p t i m u m " leaf area associated with the peak for A~at predicted by the regression describing the change in A~at with increasing leaf area (2.53 m 2 m 2 Fig. 9) coincides temporally with the approximate canopy leaf area at the end of the measurement period used in this study. It is therefore not possible to confirm the predicted decline in Asat at higher leaf areas using the data presented here. Net carbon assimilation for the canopy, and hence estimated Asat, will be sensitive to factors influencing photosynthetic and respiratory components of all canopy constituents including leaves, fruit and shoots. In particular, the carbon costs associated with fruit growth are likely to have affected strongly the temporal changes in Asat. As leaves and fruit continued to grow, cumulative respiration from the canopy would increase, thus reducing the response of canopy A to increasing leaf area, even where canopy photosynthesis is still increasing. Indeed, simulation modelling has indicated that, from about 90 days after bud burst (ca year day 355), the carbon allocated for maintenance within the entire vine exceeds that allocated for growth./2) Spatial heterogeneity of leaf area (Fig. 2) and the orientation of leaf surfaces within the canopy could influence the response of canopy photosynthesis to leaf area. The relatively high leaf area near the central cordon will cause rapid attenuation of O within that region, while Qd on the surface of the inclined portions of the kiwifruit canopy is typically as much as 50% lower than that for the central horizontal section./7) Accordingly, the " o p t i m u m " vine leaf area m a y increase with increasing uniformity of leaf area distribution within the canopy. Sink regulation of photosynthesis m a y also contribute to the temporal changes of As~t. During late summer, when the canopy leaf area is temporally stable, midday declines in canopy photosynthesis have been associated with enhanced photosynthesis during the early morning when ambient CO2 levels can be as high as 700 p p m and exceed 400 p p m for up to 4 hr after d a w n ] 4) Such declines are therefore consistent with feedback inhibition, or sink regulation, of photosynthesis. (15) Similar regulation has been noted in association with the presence or absence of fruit. (~3) Decreases in whole-canopy photosynthesis (50-60%) and respiration (33 50%) for
apple trees associated with the removal of fruit have also been reported recently. (23i In that study, leaf area was not significantly affected by the presence or absence of fruit so the decreased photosynthesis was consistent with sink regulation of source activity. The decreased respiration can be explained only partly by the removal of the carbon sink for fruit growth and maintenance, indicating that fruiting may also influence respiratory carbon consumption elsewhere in the plant. However, the impact on source activity of ontogenetic changes in sink demand, such as those due to changing fruit growth rates during the season, is not clear.
Acknowledgements I am grateful to John Meekings for technical support, and to Isabelle Gravett for curve fitting. This work was funded by the New Zealand Foundation for Research, Science and Technology and the New Zealand Kiwifruit Marketing Board.
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