SCIENTIA HORTICULTURR Scientia Horticulturae 63 (1995) 83-100 zyxwvutsrqponmlkjihgfedcbaZYXWVU Net photosynthesis response to light and air CO2 concentration of Begonia X hiemalis: whole plant measurements and modelling Panagiotis Giaglaras a, Maria Baille b, Alain Baille a,* “Institut National de la Recherche Agronomique, Station de Bioclimatologie, 84143 Montfavet Ceakx, France YYorniti National Interprofessionel de l’Horticulture, Dipartement Technique, 94152 Rungis Cedex, France Accepted 28 November 1994 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR Abstract Response of net photosynthesis (Pn) of Begonia plants to photosynthetic photon flux density (I,) and carbon dioxide concentration (C) was measured and modelled. Measurements were performed on whole plants at different growth stages in either controlled (assimilation chambers) or in situ conditions (glasshouse). A rectangular hyperbola was chosen as the response function of net photosynthesis to I,, C and leaf area (A). Parameterisation and validation of the model was carried out using Pn measurements on Begonia plants, cv. “Line”. The model was then tested for its extrapolative ability with two other data sets: one obtained in assimilation chambers with different cuhivars (cv. “Heidi”, and shoot cuttings of the Rosalie group) and the other from in situ measurements in glasshouse with the cv. “Line”, using a portable assimilation chamber. Prediction of Pn was satisfactory under normal conditions of air temperature (T,), vapour pressure deficit (VPD) and I, in glasshouse production. Discrepancies between measured and estimated rates of Pn occurred under (over25”C) andVPD (morethan 1.5kPa),and high I, (more than 700 pm01 photon m -2s-1),T, can possibly be attributed to effects of stomata1 closure on the net photosynthetic rate. Keywords: Ornament&; Pot plants; Assimilation chambers; Effective leaf area * Corresponding author. Abbreviations: PPFD, photosynthetic photon flux density (pm01 photon me2 s-l); Pn, whole plant net photosynthesis rate (pg CO* per plant s-l); I,, incident PPFD at the top of the canopy (pm01 photon me2 s-l); T., air temperature (“C) ; C, air carbon dioxide concentration (~1 l- ’ ) ; VPD, vapour pressure saturation deficit of the air (kPa) ; A, leaf area ( m* per plant) ; Ab, basal area ( m zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ - * per plant) ; A,, effective leaf anza ( m* per plant) ; gc,iand g,j conductance to CO2 and to water vapour (where the. index i may be b (boundary), (mesophyll), t (total)), (molm-*s-‘);r,,,cropsurfaceresistance (sm-‘). 0304-4238/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO304-4238(94)00755-l s (stomatal), m 84 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83400 zyxwvutsrqponmlkjihgfedcbaZYX 1. Introduction Models for crop growth simulation are expected to be useful tools in greenhouse horticulture, for planning purposes and climate optimisation (Liebig, 1989). They can give valuable information on the duration of the cultivation period or the dry matter production under different climate conditions, therefore contributing to improved decision-making (Buchwald, 1987; Mann and Krug, 1989; Leutscher, 1990) on both strategic and tactical levels (Challa and Van Straten, 1991). However, development of decision support systems, including physical, physiological and economic models, requires a good knowledge of longand short-term plant responses to climate conditions (Challa and Schapendonk, 1986). Concerning Begonia X El&or cultivars, which are commonly used in greenhouse pot plant production, few data are available in the literature. The role of carbon dioxide enrichment and artificial lighting to shorten the duration of the cultivation period and to improve plant quality has been demonstrated by several workers (Mathijssen, 1985; Bierman, 1988; Hendriks, 1988; Verberkt, 1990). Experimental data on development and flowering showed that a shortage of assimilates for plant growth occurs at low light intensities (Powell and Bunt, 1985; Simmonds and Nelson, 1988). Literature on photosynthetic response is very scarce. As far as we know, only one work deals with photosynthesis of Begonia (Bierhuizen et al., 1984). The aims of this paper are to present ( 1) experimental data on the response of Begonia net photosynthesis to PPFD and CO* concentration, and (2) a model for predicting the net photosynthetic rate which could be used in algorithms for short-term greenhouse climate optimisation or as the photosynthesis module in a general crop growth model for Begonia. 2. Materials and methods 2.1. Plant material Six groups of plants (Cl-C6) were used in the present study. Plant material and climate conditions were different for each group: ( 1) for the C l-C3 groups, leaf cuttings of Begonia X hiemalis cv. “Heidi” were grown in a glasshouse at INRA Montfavet (southern France) from February to May 1992 (Cl ), Five weeks after planting, 100 plants from the greenhouse were placed in a growth chamber with constant air temperature (20 f 2°C) and relative air humidity (70 f 10%) and divided in two groups, C2 and C3, differing in the level of incident PPFD: 200 pm01 photon m-’ s- ’ and 400 pmol photon m -’ s-’ for C2 and C3, respectively. (2) For the C4 and C5 groups, shoot cuttings of Begonia X hiemulis cv. “Line” were grown in the same glasshouse during two periods: October 1992-February 1993 (i.e. a winter culture; C4) and March-June 1993 (i.e. a spring culture; C5). (3) For the C6 group, shoot cuttings of Rosalie group cultivars were grown during winter 1992-1993, in a production glasshouse, at CNIH (Comite National Interprofessionel de l’Horticulture), Angers (western France). P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83-100 85 2.2. Cultivation practice Shoot and leaf cuttings were grown in plastic pots ( 14 cm diameter, 1 1 volume) with a mixture of 20% perlite and 80% peat, placed on metallic benches inside a 60 m* heated glasshouse. One hundred and fifty plants was selected at the same developmental stage (five apparent leaves and no secondary shoots) and used for Groups Cl-C5. During the cultivation period, plant density was progressively decreased, from 45 to 8 plants m-* of bench, to avoid light and space competition between plants. The plants were fertilised with a commercial mixture ( l-0.3-1.44) plus micro-elements. Electric conductivity and pH of the irrigation water were 1.4 (mS cm-‘) and 6.3, respectively. Before each fertigation, the pots were drained with water for 5 min to avoid accumulation of salts in the substrate. Hence, the substrate conductivity was maintained between 1 and 1.5 mS cm-‘. 2.3. Glasshouse equipment and climate The heating setpoint was 20°C. Air ventilation (roof opening) operated when temperature was higher than 25°C. Three tungsten lamps (60 W Osram) per bench, located 1.5 m above the plant level along the middle axis of each bench, were used to ensure a 16 h photoperiod during the vegetative stage of the plants. PPF’D (I,, 400-700 nm), dry bulb (T,) and wet bulb (T,,,) air temperature were measured at the top of the canopy, and water vapour pressure of the air (e,) was estimated from the T, and T,,, measurements. Hourly mean values of Z,, T,, and e, were recorded by a data logger (Campbell Scientific, Logan, UT, USA; 21X). Mean saturation vapour pressure (e,) and vapour pressure deficit (e, - e, = VPD) were calculated from T, and e,. 2.4. Carbon dioxide assimilation rate measurements Carbon dioxide exchange rates of whole plants were measured as follows: ( 1) in assimilation chambers (semi-open system), for plants from all groups (Cl-C6). The Pn data measured on plants of the C4 and C5 cultivations (cv. “Line”) were separated in two data sets; one set was used for identification of the model parameters and the second set for their validation. The data obtained on plants of Cl, C2 and C3 groups (cv. “Heidi”), and C6 group (Rosalie) were used to test the extrapolative capacity of the model (different cultivars and greater leaf areas). (2) In in situ (greenhouse) conditions, with a portable (U-6200, Li-Cor, Lincoln, NB) photosynthesis meter, only for plants of the C5 cultivation (cv. “Line”). Those data were used to test the model for its extrapolative capacity (different environment). 2.4.1. Assimilation chambers The two chambers, control system and operating process have been described by Longuenesse et al. ( 1982) and Gary ( 1988a). The measurements of Pn were conducted on plants with different growth stages, taken in the glasshouse. Throughout the cultivation period, two pot plants per week were taken and placed in the assimilation chambers for 4 days. 86 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA P. Giaglaras et al. /Scientia Horticuhrae 63 (1995) 83- 100 zyxwvutsrqponmlkjihgfedcbaZY START 0 1 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO 2 3 END Photoperiod Assimilation chamber Tie (2h) PPFD 6 levels co2 3 levels Fig. 1. Schematic representationof the PPFDand COzsequencesappliedduring measurementsof the wholeplant netphotosynthesis rate in the assimilation chambers. Nutrient solution was supplied by capillarity from a stirred water tank located inside each chamber. Air temperature inside the chambers was maintained constant at 20 f 2°C day and night, and relative air humidity at 75 f 10%. PPFD was measured at the top of the plant by means of quantum sensors (Li-Cor) in the same way as in the glasshouse. The CO, exchange rate of the plants was obtained from the air flux measurements (mass flowmeters, Brooks, Inc.) and from the measurements of CO, concentrations (IRGA analyser, ADC) at the input and output of the chambers. The influence of water concentration changes and inertia of the system were taken into account, using the approach of Gary ( 1988b). Pn was recorded continuously during periods while PPFD and CO* concentration followed predefined sequences (Fig. 1) . CO1 concentration varied from one photoperiod to the other: 900 ~1 l- ‘, 400 ~1 l- ’ and 600 ~1 1-l on the first, second and third day, respectively. During a photoperiod, PPFD changed every 2 h from high (700 pm01 m-* s- ‘) to zero levels with five intermediate levels (approximately 50, 100,250,400 and 500 pm01 m-* SK’) . At night, CO2 concentration was maintained at 400 pull-‘. At the end of the third day, the plant shoot was cut and the respiration of substrate plus root was measured both in darkness and in light (700 pm01 m-* s-l). In the following, Pn rates refer to the calculated mean steady-state values corrected for the substrate CO2 efflux. 2.4.2. In situ measurements Net photosynthesis of whole plant (cv. “Line”) was also measured in the glasshouse under natural climatic conditions. For this purpose, the portable photosynthesis system was modified in the following way: the leaf chamber was replaced by a hemispheric glass chamber enclosing the plant in a volume of 21.1 1 (30 cm diameter, 35 cm height). A stainless steel base supported the glass cover and the sensors; a small fan mixed the internal atmosphere. Tests of the system tightness gave very satisfactory results, even in conditions of high CO2 concentration gradients. P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83-100 87 PPFD was measured at plant level by a quantum sensor. For Pn measurements, the same CO* concentrations as in the assimilation chambers were applied. At the end of the Pn measurements, plant leaf area (A) and shoot dry matter (DM,) were obtained by conventional destructive analysis. 3. The model At whole plant level, net photosynthesis rate (Pn) can be considered as the difference between gross photosynthesis (Pg) and dark respiration (Rd). Assuming that respiration at light is equal to Rd, we have Pn=Pg-Rd (1) (where Pn, Pg and Rd are in pg COP per plant s- ’ ) . Pg integrates leaf gross photosynthesis (Pgl, pg CO2 m-* s-‘) over the leaf area. A rectangular hyperbola (Thornley, 1976) was adopted for expressing the dependence of Pg, vs. 1, (pm01 photon m-* s-l): pgr=aI,W%m~ + Pgm, (2) where Pgm, is the maximum value of Pgl at light saturation level and a! is the incident PPFD use efficiency ( pg CO, per pm01 photon). As suggested by Acock ( 1991), the effect of CO2 concentration on the leaf gross photosynthesis, in the range of CO2 concentrations used, can be expressed as Pgm, = rC (3) where r is the total (stomata plus mesophyll) leaf CO, conductance zyxwvutsrqponmlkjihgfedcbaZYXWVU ( pg CO2 m-* s- ’ 1 PI-‘) and C the CO, concentration (~11~r ). The measurements at whole plant level showed that, for a given light (I,) and CO2 level (C), Pg increases with leaf area (A, m* per plant) until it reaches a plateau. The selfshading effect of A was described in the following way: we assumed that only part of the whole plant leaf area responds to the incident PPFD, the so-called “effective leaf area” (A,, m* per plant). A, was related to leaf area using the following formula: A,=A(-) a+A where u (m* per plant) is a statistical parameter called “effective leaf area coefficient”. The effects of leaf area distribution, orientation and age on Pg are included in this parameter. The dependence of A, on A for several values of cr is shown in Fig. 2. The basal area of the plants, Ab (i.e. the horizontal projection on the ground of the plant canopy; m2 per plant) is a function of the whole plant leaf area. The following mathematical expression was used for fitting: Ab =plA +pd2 +p+i3 +p4A4 +p# (5) 88 P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83-100 V I 0.05 I I I 0.15 I 0.25 Leaf area, A (In2 plant- I) Fig. 2. Shape of the relation between the effective (A,) and the total leaf area (A) for various values of the effective leaf area coefficient CT(0.05,0.10,0.15 and 0.20 m2 per plant). withp, =0.91,p,= -9.16,p3=56.3,p,= - 1.54X lO’andps= 1.58X 102.Theestimated relation is given by the continuous line in Fig. 3 (Z?‘=O.85, RSE= 0.00655 me2 per plant). Eq. (5) was used to estimate the leaf area index (L = A/Ah, m2 mm2), a common variable in light interception laws applied in crop growth models. The dark respiration (Rd) , under conditions of constant air temperature and growth rate, can be considered to be mainly dependent on the amount of shoot dry matter (DM,, g per plant) : Rd = pDMs (6) where p is the specific dark respiration ( pg CO g- ’ s-i) of the plant. Substituting Eqs. (2)-(4) and (6) in Eq. ( 1) leads to CYZTC Pn=&-f--$(-& + - PDM, (7) which describes the net photosynthesis of the whole plant in relation to Z,, C, A and DM,, with four parameters ((Y,7, u and /3) to be identified. In the following, the values of these parameters were derived by means of non-linear least-squares fitting of the experimental data gathered from the assimilation chambers, using the S-PLUS statistical routine (Statistical Sciences Inc., 1991). The goodness of fit between the estimated and measured values was evaluated using the relation %timated = a + bPnmeasured (8) The determination coefficient ( R2) and residual standard error (RSE, g CO2 s- ’ per plant) were calculated. The residuals were plotted versus the estimated values and the independent variables, C, IO,A and DM,. P. Giaglaras et al. /Scientia Horticulturae 63 (199s) 83- 100 89 0.10 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA _ 0.08 ‘r B -z 0.06 3 9 0.04 -: 0.02 0.0 0.05 0.10 0. is A (m2 0.20 fitted 0.25 0.35 area (A), for plants of Groups Cl- Assimilation groups h yB 0.30 plant-l) Fig. 3. Relationship between the basal plant area (A*) and the whole plant leaf C6: measured values ( X ) and fitted curve (Eq. (5)). 100 curve 80 “0. -20 0 chamber measurements et C5 (cv. “Line”) / second data set 0% 0 C4 20 Pnmeasured 40 60 80 (Pg Co2 S-’ Pkmt-‘1 Fig. 4. Estimated vs. measured Pn values: comparison of the modelestimations (Eq. (7) ) with Pn values obtained in the assimilation chambers using plants from Groups C4 and C5 (cv. “Line”). 90 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83- 100 zyxwvutsrqponmlkjihgfedcb 4. Results 4.1. Modeljitting and parameter estimation The range of measured dependent and independent variables used for the parameter identification and the values of the estimated parameters are given in Table 1. Comparison of the measured and estimated rates of Pn (Fig. 4) indicates a good agreement: a = 0.79 L-0.40 pg CO* SK’ per plant; b = 0.95 f 0.02, R2 = 0.94 and RSE = 4.9 pg CO1 s- ’ per plant. The residuals followed a normal distribution and did not show any correlation with the independent variables used in the model nor with the model estimations. The confidence interval of the model predictions, at the 95% confidence level, is * 8.04 j_kgCO2 s- ’ per plant. 4.2. Model validation Comparison of measured and estimated values, using the second set of Pn data obtained in the assimilation chambers on plants of C4 and C5 cultivations, showed a good agreement (Fig. 5): a= -0.20f0.33 pg CO2 s-r per plant; b = 1.04 f 0.01 and R2 = 0.96. Table 1 Ranges of the independent variables used from the model parameterisation and estimated values of the parameters using data obtained in assimilation chambers for BegoniaX hiemalis (cv. “Line”) plants, with constant air temperature (20 rt 2°C) and air relative humidity (75 f 10%) Units Measured range I.cg COP s-r per plant - 13-+72 Dependent variable Pn, whole plant net photosynthesis Independent variables I,, incident PPFD pm01 photon m-* s-’ /.&11-I C, air carbon dioxide concentration A, total leaf area DM ,, shoot dry matter O-630 300-1000 m* per plant g per plant 0.029-0.216 1.5-18.3 Parameters Units Estimated values Standard error a, incident PPFD use efficiency 7, conductance to carbon dioxide o, effective leaf area coefficient p, specific respiration pg CO* per pmol photon 2.9 0.17 pg COz m -zs-‘lILl-’ 2.85 0.21 m2 per plant 0.15 0.02 /.&gco* g-1 s-’ 0.45 0.05 P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83- 100 Assimilation 91 chamber measurements groups C4 et C5 (cv. “Line”) first data set ‘I -20 -20 n /./// I I I I I I 0 20 40 60 80 too Pflmeasured (Pg co2 s-I plant-‘) Fig. 5. Estimated vs. measured Pn values in the validation test: comparison of estimated values (E!q. (7)) with the second data set of Pn (plants cv. “Line” from Groups C4 and C5 measured in the assimilation chambers). Assimilation chamber measurements groups C / I, C2, C3 (cv. “Heidi”) and C6 (cv. “Rosalie”) / I I I I I J -20 0 20 40 60 80 IO0 L Pnmeasured (Pg WI s-’ pkint-‘1 Fig. 6. Estimated vs. measured Pn values in the extrapolation test: model ability to predict the Pn values measured in the assimilation chambers with plants from Groups Cl-C3 (cv. “Heidi”) and C6 (Rosalie). 92 P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83400 100 Rosalie group plant S = 0.44 m2 plant-t 80 k&30 ‘In 0” 40 U on z $ 20 measuredat : * !mplI-’ 0 6oopIl-I 0 4ooplI-1 0 I -20 0 IO0 200 3w 400 500 600 IO (pm01 mm2 s-l) Fig. 7. Measured and estimated response of Pn to PPFD and CO* concentration for a plant from Group C6 (Rosalie) having leaf area (0.44 m* per plant) twice that of the largest plant used for the identification of model parameters. 0, Measurements at 400 ~1 I-‘; 0, measurements at 600 /.rll-‘; A, measurements at 900 ~1 I-‘. Continuous lines represent model estimations for the same PPFD and COa levels. “in- situ” measurements group C5 (cv. “Line”) 0 20 40 60 80 100 f’nmeasured (Pg CO:! s-’ Plant-‘) Fig. 8. Estimated vs. measured values for Pn measurements obtained in situ (cv. “Line”). 93 zyxwvutsrq P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83- 100 4.3. Extrapolative tests First, the predictive value of the model was checked against measurements obtained in assimilation chambers with the C 1 to C3 (cv. “Heidi’ ’ ) plants. The results (Fig. 6) showed that the agreement is satisfactory, although we note a slight overestimation for high levels of CO* and Z, (a = 1.94 + 0.30 pg CO% s- ’ per plant; b = 1.24 + 0.01 and R* = 0.95). 60 measurements group C5 (cv. “Line”) + (a) ++ “in- silt/ ++ + + + + + P + + + _. + + 20 I I I I 25 30 35 40 Ta W) “in-situ”measurements + group C5 (cv. “Line”) + + + + t + ++ + + + k ++ + + + + (b) + + + ++ + ._. + + 0 I I I I 2 3 I zyxwvutsrqponmlkjihgfedcbaZ 4 VfD (kPa) Fig. 9. Difference ( APn) between estimated and measured values of Pn (for in situ measurements, cv. “Line”) vs. (a) air temperature ( T,) ; (b) vapour pressure saturation deficit zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ (VPD) . Horizontal dashed lines represent the 95% confidence interval of the model predictions. 94 P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83- 100 The prediction reliability of the model for plants with larger leaf area was tested using data obtained on plants of the Rosalie group (C6) having a leaf area of 0.44 m* per plant, e.g. twice the leaf area (0.22 m* per plant) of the largest plant used for the model parameter identification. The model simulated fairly well the measured values until Z, reaches about 250 pmol photon m -2 s-’ . For higher values of Z,, the model significantly overestimates the measured Pn rates, especially at the 600 and 900 Z~ll-’ levels (Fig. 7). Finally, the model was checked against data obtained in situ (Fig. 8). In this case, the predictions were less satisfactory, with a significant overestimation of Pn by the model: u= 10.43& 1.40 Z.&gco, s-l per plant; b = 1.26 f 0.09 and R* = 0.40. Analysis of the residuals showed that the deviations (A Pn = Pnestimted - Pnmeasured)are important under conditions of high temperature (Fig. 9), which were associated with both elevated PPFD (more than 700 pm01 m-* s-’ ) and high VPD (over 1.5 kPa) levels. zyxwvutsrqponmlkjihgfedcbaZY 5. Discussion Data on net photosynthesis usually found in the scientific literature refer to the leaf or crop level, but studies at the whole plant level are less frequent. Measurements at leaf scale are currently available, as they are much easier to obtain than measurements at the whole plant level. Extrapolation from leaf level to whole plant level needs the estimation of the plant leaf area distribution and light extinction, sometimes combined with a directional treatment of incident light. Such an approach does not significantly improve the predictions at the whole plant level if not supported by a detailed description of the leaf photosynthetic characteristics and by the characterisation of the microclimate inside the plant, i.e. temperature and CO2 distribution (Boote and Loomis, 1991) . Obtaining these inputs is not straightforward. In contrast, the PPFD level above the plant canopy is commonly available in climate-controlled greenhouses, and total leaf area and shoot dry matter can be predicted from simple growth models. These reasons plead for the development of a photosynthesis model at the whole plant level, applicable and usable in production conditions. In fact, the model presented contains some elements relative to the leaf level (i.e. Pgl estimation), but was parameterised and validated with measurements performed at the whole plant level. This approach results in the identification of parameters that are analogous to those at the leaf level. 5.1. Light use eflciency , a The parameter cr, calculated on the basis of incident radiation, was found equal to 2.9 pg CO2 per pmol photon (Table zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH 1 ), or 0.066 mol CO2 per mol photon. This parameter is analogous to the quantum yield (although based on incident rather than absorbed radiation), for which Ehleringer and Bjijrkman (1977) gave a nearly constant average value for C3 species of 0.073 mol CO2 per mol photon. If we assume that Begonia leaf absorptance is 0.85, the quantum yield for Begonia would be equal to 0.066 mol CO, per mol photon i 0.85, i.e. 0.078 mol CO2 per mol photon, which is very near the above-mentioned average value for C3 plants. P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83-100 95 Table 2 Estimated light use efficiency values for COz concentration level Mean COz concentration (CL1 I-‘) U (kg CO2 per wool photon) Standarderror Residualstandard error ( P g CO* pm01 per photon) (pgCO,s-‘perplant) Degrees of freedom 380 620 900 2.80 2.91 2.61 0.06 0.05 0.04 3.62 4.14 5.11 216 137 146 Bierhuizen et al. ( 1984) reported lower values for Begonia X hiemalis zyxwvutsrqponmlkjihgfedcbaZY cv. “Tiara” plants: ’ and 8.0 pg CO 2 J- ’ for 21% and 1% O,, which correspond respectively to 0.029 and 0.04 mol CO, per mol photon and 0.04 mol COZ per mol photon (assuming that 1 Joule me2 s-l is equal to 4.54 pmol photon m P-2s- ‘, Varlet-Grancher et al., 1989). To verify if (Yis CO,-dependent, the measured Pn data were separated according to the COZ concentration level (400,600 and 900 ~11~‘). The model was fitted to each set of data, with r, p and u constant (Table 1). The light-use efficiency changes slightly with CO, concentration in the range 300-900 /..~ll- ’ (Table 2). The a value obtained at 900 ~1 1-l is smaller than that obtained at 600 ~1 l- ‘: 0.066 ( ~0.001) mol CO* per mol photon and 0.059 ( &0.0009) mol CO1 per mol photon, respectively. This result could indicate that CO2 saturation at low light levels was already achieved at the 600 ~1 1-l level for Begonia plants and that higher CO* concentrations have a negative effect. 5.8 pg CO 2 J- 5.2. Carbon dioxide conductance, T The parameter 7 of the model (2.9 pg CO2 s - ’ m - ’ l/.~l- ’ , Table 1) is analogous to the total CO, conductance usually referred to as g,, (mol mm2 s-‘) in the literature (Ball, 1988). The value of T can be converted to mol m-* s- ’ by dividing it by 44 (e.g. r= 0.068 molm-*s- ‘). This value is significantly higher than the CO2 conductance that can be deduced from the data of Bierhuizen et al. ( 1984). Replacing the Pgm, value reported by Bierhuizen et al. in Eq. (3) with C= 300 ~11~‘, results in rvalues of 0.0097 mol m-* s-’ and 0.0164 mol m-* SK’ for the 21% and 1% O2 levels, respectively. On the other hand, Acock et al. ( 1978), using the same formulation for leaf net photosynthesis of chrysanthemum plants, found r= 0.12( 10.049) molm-*s-l, i.e. nearly twice the estimated value of T. Assuming that mesophyll conductance to CO2 (g,,,) equals 2.5-4 times the total conductance to CO2 (g,,) and that the stomatal (g,,,) and the boundary layer conductances to water (g,,,+) , are respectively 1.56 and 1.37 times the stomata1 (g,,,) and the boundary layer conductance to CO, (gc,b), the estimated r value corresponds to g,,r ranging from 0.14 (g,,,,=4g,,) to 0.17 (gc,nr=2.5gc,,) mol m-* s-‘, respectively. Values of g,,, and g,,, are expected to vary with light, CO*, VPD and wind velocity near the leaf boundary layer. Baille et al. ( 1994) reported surface resistances to water vapour transfer ( rw,J of a greenhouse Begonia X hiemalis crop increasing from 270 to 900 s m- ’ with VPD increasing from 0.2 to 1.5 kPa, which correspond to a decrease from 0.165 to 0.05 mol m-* se1 in terms of conductance. For the mean VPD value in the assimilation chambers (0.55 zyxwvutsrqponml kPa) , 96 P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83- 100 their formula will give a surface conductance g’,,c= 0.124 molm-*s-i. This value is smaller compared with the range of gw,rvalues given above, and this difference can be explained by the higher boundary conductance inside the assimilation chamber. The estimated value for r did not change significantly when the model was fitted using the CO*dependent (Yvalues given in Table 2. In Eq. (3) the CO2 compensation point was omitted. This simplification was adopted because Pn data with saturating light and low CO2 concentration values needed for a precise estimation of this parameter were not available. The effect of a constant compensation point value (50 ppm) on the estimation of T was not very significant. 5.3. “Effective leaf area’ ‘, A, The effective leaf area approach was able to simulate in a simple manner the dependence of the whole plant gross photosynthesis on light extinction, leaf position and orientation relative to incident light and leaf age. The results of the model at the whole plant level using this approach were satisfactory. The value found for u was 0.15 m* per plant (Table 1). With this value, Eq. (4) gives A, = 0.74A when A = 0.05 m* per plant, and A, = 0.37A when A =0.25 m* per plant. 5.4. Total specijc dark respiration coefJicient, /3 An attempt was also made to compare our value of the specific respiration coefficient (/3=0.45 pg CO2 g-‘s-l) with values given in the literature. An estimate of Begonia dark respiration per unit leaf area was derived from the net photosynthesis model proposed by Bierhuizen et al. ( 1984), by extrapolating the proposed equation for PPFD = 0. Multiplying this dark respiration value by the mean leaf area of the plants used in their experiments (0.019 m* per plant) and dividing by the shoot dry matter (0.75 g per plant), resulted in p = 0.98 pg CO,g- ‘s- ‘. This value is high compared with our value (0.45 pg CO2 g-i s - ‘) . To explain this difference, we analysed dark respiration using McCree’s model (McCree, 1970) : Rd = gGR + mDM , (9) where g is the growth respiration coefficient ( pg CO2 pug- ‘)zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPO , GR the growth rate (hg s- ’ ) and m the specific maintenance respiration rate (pg CO2 g - ’ s- ‘). Equating Eq. (6) to Eq. (9) and solving for the p coefficient gives p=gGRIDM ,+m=gRGR+m (10) where RGR is the relative growth rate ( pg g-i s-r ). Eq. ( 10) indicates that p includes both the growth and maintenance respiration, and corresponds in fact to the specific respiration coefficient r, as defined by Thomley ( 1970). From Eq. ( lo), the value of /3 is expected to vary proportionally to RGR during plant growth. The RGR of the C4 and C5 (cv. “Line”) plants is high at early growth stages (approximately 0.58 pg g - ’ s- ’ ) and tends to a constant value as the plants become larger (approximately 0.23 pg gg ’ s- ‘). Taking into account that Bierhuizen et al. ( 1984) measured Pn on young Begonia plants (A = 0.019 m* per plant), which are expected to P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83-100 91 have high RGR values, it can be concluded that the observed difference between the estimated value of p (0.45 pg CO* g - ’ s - ’ ) and that estimated from the Bierhuizen et al. (1984) model (0.98 pg CO* g-’ s-l), is due to growth stage and RGR differences. The estimated value of p is a mean value for the overall production period of Begonia plants. 5.5. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Extrapolation tests The 24% overestimation of the net photosynthesis found with the first extrapolation test (Fig. 6) corresponds to data obtained with the cultivar “Heidi”. The fitting of the model to those data suggests that the plants of this cultivar have lower light use efficiency ((Y= 1.64 f0.07 pg CO* per pmol photon), lower conductance to CO2 (T= 1.9 f 0.08 pg CO2 s-’ m-’ 1 ~1~‘), greater effective leaf area coefficient (a= 0.205 m2 per plant) and lower specific dark respiration (p = 0.305 f 0.025 /.Lg co* g-1 s-l>, compared with the values of the cultivar “Line” (Table 1). Additional data from appropriate experiments are needed to verify the hypothesis that the parameters of the model are subjected to variations owing to genetic variability of Begonia X hiemalis plants. The validation test vs. plant leaf area showed that we cannot use the same value of u (0.145 m* per plant) for leaf areas beyond 0.25 m* per plant. Curve fitting to the data shown in Fig. 7, corresponding to plants with large leaf area (0.44 m2 per plant), would give a=0.117 m* per plant. The comparison between the predictions of the model with the in situ (glasshouse) measurements (cv. “Line”) shows that air temperatures above 25°C and VPD values zyxwvutsrqpo measured - -- 0 100 nyxk.1 estimations Eivxhuizcn et al. (1984) 200 300 400 cl 0 Pn v alues plant I plant 2 500 600 lo (lmol mm2 s-l) Fig. 10. Comparison of the predictions from Bierhuizen model (dashed line) and present model (continuous line) with Pn data obtained in the assimilation chambers in the C range 35OAOO$l-’ for two Begonia X hienrnlis plants (0, Plant 1 with A = 0.025 m* per plant and D&f,= 1.37 g per plant; 0, Plant 2 with A = 0.216 m* per plant and DM,= 17.94 g per plant). 98 P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83400 greater than 1.5 kPa limit the assimilation rates of Begonia X hiemulis plants (Fig. 9). Stomatal and non-stomatal effects (photorespiration, photochemistry) could explain this observation (Comic et al. 1992). Recently, Baille et al. ( 1994) found that the transpiration rate of a Begonia X hierndis crop decreases when VPD rises above 1.2 kPa. Thus, it could be proposed that stomata1 closure was the main cause of the Pn overestimation by the model. The available transpiration rate measurements at the whole plant level make the subject of a study for the investigation of the above hypothesis and for the quantification of its effects on Pn. Neglecting the stomata1 response to VPD and the effect of CO? on cr (Acock, 199 1) in the present net assimilation model could affect the model predictions if the application ranges of temperature and VPD are large. However, if the model is used under the controlled climate conditions of production greenhouses (i.e. similar to our assimilation chamber conditions), the predictive value of the model can be considered as satisfactory. The model of whole plant net photosynthesis presented here was also compared with the model of Bierhuizen et al. ( 1984). The predictions of the two models are shown in Fig. 10, for two plants (A, =0.025 m* per plant, DM ,, = 1.37 g per plant and A2= 0.216 m* per plant, DM s2 = 17.94 g per plant) and for a CO2 concentration of 370 ~11~‘. As PPFD rises, the predictions of the presented model follow the measured values better than the Bierhuizen model, which predicts light saturation at a lower value of PPPD. This could be a specific characteristic of the “Tiara” cultivar, or the result of low light acclimation of the plants studied by Bierhuizen et al. ( 1984). 6. Conclusions The presented model for predicting Begonia net photosynthesis using four explanatory variables (incident PPFD, CO2 concentration, leaf area and shoot dry matter) gives satisfactory results throughout the entire cultivation period and for a wide range of cultivars (“Line”, “Heidi” and Rosalie group cultivars). Predictions of Begonia plants net photosynthesis are reliable in the ranges of climatic variables similar to those under which the model was calibrated: T,E [ 18, 25]“C, Z,,E [0, 7001 pm01 photon m-* s-i, VPDE [0, 1.51 kPa and CE [ 350,950] ~11~‘. The parameter values estimated from model fitting on Pn measurements from assimilation chambers were similar to those reported in the literature for C3 plants. Crop history (climate conditions, cultural practice, origin, etc.) did not seem to influence the net photosynthesis rate at the whole plant level. Plant structure and leaf area effects were significant and the “effective leaf area” approach was found convenient to account for these effects. Model fitting on a wider range of leaf area should improve the estimation of the effective leaf area and the influence of the “self shading effect” on the light-Pn conversion. Cultivar and growth stage differences are probably at the origin of the observed discrepancy with the previously reported Pn model for Begonia X hiemalis plants of Bierhuizen and al. ( 1984). In conclusion, the present model can be used for the estimation of the carbon balance of whole Begonia plants throughout their cultivation period in production greenhouses. It can provide a basis for comparison of the photosynthetic characteristics of different BegoniaX hierndis cultivars at the leaf and/or at the whole plant scale for plant breeding P. Giaglaras et al. /Scientia Horticulturae 63 (1995) 83400 99 purposes. 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