Effect of Light Intensity, Carbon Dioxide Concentration, and Leaf Temperature on Gas Exchange of Spray Carnation Plants

Journal of Experimental Botany, Vol. 28, No. 102, pp. 84-95, February 1977 Effect of Light Intensity, Carbon Dioxide Concentration, and Leaf Temperature on Gas Exchange of Spray Carnation Plants H. Z. ENOCH AND R. G. HURD 1 Division of Agricultural Meteorology, Agricultural Research Organization. The Volcani Center, P.O. B.6, Bet Dagan, Israel. Received 17 April 1976 ABSTRACT The rates of CO2 assimilation by potted spray carnation plants (cv. Cerise Royalette) were determined over a wide range of light intensities (45-450 W m~ 2 PAR), CO2 concentrations (200-3100 vpm), and leaf temperatures (5-35 C). Assimilation rates varied with these factors in a way similar to the response of single leaves of other temperate crops, although the absolute values were lower. The optimal temperature for CO2 assimilation was between 5 and 10 C at 45 W m~ 2 PAR but it increased progressively with increasing light intensity and CO2 concentration up to 27 C at 450 W m~ 2 PAR and 3100 vpm CO2 as expressed by the equation 2\>pt = 6-47 + 2-336 In C + 0-031957 where C is CO 2 concentration in vpm and I is photosynthetically active radiation in W m~ 2. CO2 enrichment also increased stomatal resistance, especially at high light intensities. The influence of these results on optimalization of temperatures and CO2 concentrations for carnation crops subjected to daily light variation, and the discrepancy between optimal temperatures for growth and net photosynthesis, are discussed briefly. INTRODUCTION It is possible to effect some control over temperature and CO2 concentration in the glasshouse, with the purpose of increasing production. As a basis for setting up these conditions, it has been proposed that optimal yield will result when these parameters are continually adjusted to the intensity of the incoming light so as to provide for the maximum rate of photosynthesis (Lake, 1966). The present work presents information on the effects of temperature, CO2 concentration, and light intensity, on the net photosynthesis of whole plants so as to enable this proposal to be tested. In an earlier paper (Hurd and Enoch, 1976), some aspects of the photosynthesis and growth of spray carnation plants were considered. The spray carnation was again chosen here, both because of its commercial importance and, more importantly, also because its photosynthesis was shown to be unaffected by its previous temperature history, making the task of determining the optimal environmental factors U.K. 1 Present address: Dept. of Plant Physiology, Glasshouse Crops Res. Inst., Littlehampton, Sussex,

Enoch and Hurd Gas Exchange in Carnations 85 technically relatively simple. It also means that when adjustments to the environment are subsequently made in the glasshouse, interactions with earlier temperature regimes, as observed in other plants, will be unlikely (see Discussion). MATERIALS AND METHODS Rooted cuttings of spray carnation plants (Dianthus caryophyllus L. cv. Cerise Royalette) were planted in November in 9 cm plastic pots containing coarse sand free of organic matter and grown for 1-2 months until the one-sided leaf area was 1-2 dm 2. The plants were grown in a greenhouse at ambient atmospheric CO2 concentration and day/night temperatures of 20/ 10 C. Daily watering was with complete nutrient solution and illumination was supplemented by two 400 W HPLR Lamps (Philips, Eindhoven, The Netherlands) providing a total radiation flux density of 350-400 W m~ 2 PAR for 12 h daily. This high-light pretreatment was used to obtain plants that could give a constant net photosynthetic rate with time and little hysteresis to temperature changes during the day (see Results and Discussion). The apparatus used for measurement of net photosynthesis, dark respiration, and transpiration rate was a conventional semi-open system, similar to that described previously (Hurd and Enoch, 1976). The CO2 exchange was calculated from the rate of air through-flow and the difference in CO2 concentrations in up- and downstream air as measured in the differential mode by an URAS 2 (Hartmann and Braun, Frankfurt/Main, BRD). The CO2 concentration in the assimilation chamber was also measured by an URAS 2, used in the absolute mode. Both CO2 analyses were recalibrated each hour with a calibration gas produced by three, SA 27/3a air-mixing pumps (Wosthoff, Bochum, BRD). Transpiration was calculated from the up- and downstream dew-points as measured by a 'thermoelectric dew-point hygrometer' (MK 3, Salford Electrical Instruments Ltd., England). Illumination was from two 400 W HPLR lamps that could provide up to 450 W m~ 2 PAR; (light intensities were lowered by introducing layers of aluminum net between the lamps and the assimilation chamber). The chamber itself essentially comprised two inverted, concentric beakers between the walls of which water of known temperature could be circulated. By adjusting the water temperature over the range 1-35 C leaf temperature could be controlled between 10 and 35 C C, independent of the radiation flux density. A plant to be used for gas exchange measurements was transferred at 1700 (local time) from greenhouse to laboratory where it was watered and then drained for ] 0 min; the exposed soil surface was covered with expanded polystyrene and the pot wrapped in thermal insulation material (to minimize energy transfer and evaporation while allowing O2 and CO2 gas diffusion from the root zone). The plant was placed in the assimilation chamber and the leaf temperature adjusted to 20 C C. A CO2 concentration and light intensity appropriate to the following day's experiment was provided, as was a root temperature of 20 + 2 C, that was maintained during the measurements. Illumination only was interrupted from 1900 to 0500 of the following day. Net photosynthesis and transpiration rates were measured approximately 3 h after the lights were switched on the following morning when the rates had been constant for 20 min. The leaf temperature was then slowly changed (c. 0-3 C min -1 ) from 20 to either 10 C at high light or 5 C at low light, and gas exchange rates were measured when they had remained stable for 15 min. Measurements were repeated for each 5 C at higher leaf temperatures up to 35 C, whereupon the leaf temperature was brought back to 20 C. The gas exchange data piesented for 20 C were the means of the three values obtained. The lamps were then switched off and the dark respiration rate was determined for 1 h at a leaf temperature of 20 C C. The plant wasfinallyremoved from the chamber and its dimensions were recorded by destructive sampling, as described previously (Hurd and Enoch, 1976). Thus one plant was used for each combination of PAR and CO2 concentration and the leaf temperature of that plant was changed over the experimental ranges. While leaf temperature was changed over the range of 5-35 C or 10-35 C, the dew-points of air entering the chamber were constant at respectively 4 or 6 C, hence water vapour deficit changed between 0-3 and 4-7 kpa. In some cases, especially at low CO2 concentrations, transpiration appeared to be in a transient state although CO2 uptake was constant; in such cases the transient transpiration rate was recorded at the end of the 15 min period but the result was not used for calculation of stomatal resistance.

86 Enoch and Hurd Gas Exchange in Carnations RESULTS Preliminary experiments The plant-to-plant variation with respect to net photosynthetic rate (F) was investigated in a preliminary experiment. Nine pairs of plants were used, that had been grown outdoors at a mean photosynthetically active radiation (PAR) of about 220 W m~ 2 for 12 h per day during 1-2 months. F was measured at ambient atmospheric CO2 concentration, 20 C leaf temperature, and 450, 125, and 45 W m~ 2 PAR, respectively. The mean numerical deviation of F from F was ±0-65 mg CO2 dm~ 2 h" 1 and was smaller than the estimated accuracy of the measuring method. The mean CO2 uptake changed less than 0-1 mg dm~ 2 h" 1 per day over a period of 3 weeks. A second preliminary study was made of the effect of time of day, and of the number of hours plants were subjected to a particular environment, on the rate of net photosynthesis. At temperatures up to 25 C, any combination of light intensity and CO2 concentration gave constant net photosynthetic rates throughout an 8 h day using plants grown at high light intensities (350-400 W m~ 2 PAR). At temperatures of 30-35 C in high light and with a low CO2 concentration (that is, conditions leading to high transpiration rates), the plants tended to wilt and in this case CO2 uptake decreased after 1-2 h. Increasing the CO2 concentration tended to suppress this fall-off in CO2 uptake. The effect of leaf temperature history in the hours immediately preceding measurement was also determined. Net photosynthesis came to equilibrium rapidly without showing any hysteresis when leaf temperatures were changed at a rate of 0-3 C min" 1 or less, within the range 5-35 C. Figure 1 shows an example of measurements lasting about 11 h in which temperature was changed from 20 to 35 C, back to 13 C, and then increased to 32 C, and lowered to 24 C. The plant had a 25 c 15 O u 5 - IU 15 20 25 30 35 PIG. 1. The repeatability of CO 2 assimilation in a carnation plant submitted to a series of temperature changes over the course of 11 h. CO2 concentration, 350 vpm; light intensity, 450 W m-2; Leaf area, ~ 1 dm2.

Enoch and Hurd Gas Exchange in Carnations 87 leaf area of about 1 dm 2. Hysteresis was less than ±1 mg CO2 h~ x per plant, which was similar to the measuring accuracy of the system. At high light intensity and low CO2 concentration the best reproducibility was obtained when plants were kept only 5-10 min at each of the high temperatures (30 and 35 C). This procedure was followed subsequently. Three plants were used to investigate the influence of the previous day's irradiance on dark respiration (2?D) and the relative change in B& to a 10 C increase in temperature (Qio). A plant was placed in the assimilation chamber at 1700 (local time) at 360 vpm CO 2, 20 C leaf temperature, and 45, 125, or 450 W m" 2 PAR. The lamps were switched off between 1900 and 0400, and at 1200, giving the plants an 8 h light period. In the succeeding night, leaf temperature was decreased to 5 C and then increased to 35 C within 1 h, in which i?d was measured at 5 C intervals. Thereafter the plant was removed from the assimilation chamber and its leaf area determined by destructive sampling. At any temperature, the absolute values of BD were dependent upon the previous light level, reflecting the amounts of respirable photosynthates (Fig. 2). Log BT> was a linear function of temperature, with almost the same slope for all three prior light regimes. The Q 10 was 1-95, 1-93, and 2-12 respectively, for 45, 125, and 450 W m~ 2 PAR. For each combination of light intensity and CO2 concentration in the main experiment (see below) the mean Qio of 2-0 was used, together with the values of J?D at 20 C. 1-2 - 10-0-8 - f 0-6 - o 6' 0-4 u J 0-2 log 00 y y^ y^ O ^y ^ ' / *. / ^ y y<' -0-2 -0-4 y' ~ a _ i i 1 1 10 15 20 25 i i 30 35 FIG. 2. Dark respiration rate of carnation plants at different leaf temperatures. Values recorded during the first hour after lamps were switched off, following days with light intensity of 450 ( ), 125 ( ), or 45 W m~2 PAR ( ).

88 Enoch and Hurd Gas Exchange in Carnations to estimate R& values at the different temperatures during the preceding light period. Main experiments Net photosynthetic rate of the whole plant (F) was measured in the environmental conditions: 45, 125, 280, or 450 W m~ 2 PAR at each of the CO 2 concentrations 200, 350, 700, 1500, and 3100 vpm and, at the leaf temperatures of 10 (or 5), 15, 20, 25, 30, and 35 C. In Fig. 3A, B, C, D, and E, two curves are given for each environmental condition: the one with experimental points gives the F value obtained; the one above, with the same line designation, has the appropriate ifo value added to F. The value for F at 20 C was obtained three times (as described) for each of the 20 plants. The mean numerical deviation of F from F in the three measurements was 0-62 mg CO 2 dm-2 hr 1. 10 i; 20 25 30 35 FIGURE 3A FIG. 3. CO2 assimilation rate (F) by carnation plants in relation to light intensity, leaf temperature, and CO2 concentration. Lines with symbols represent the original F values; lines without symbols represent F + dark respiration rate RD. Arrow indicates optimal temperature for maximal F within a given light and CO2 regime. Light intensities: 450 ( ), 280 ( ), 125 ( ), 45 W m~2 PAR ( ). CO 2 concentrations (vpm): A, 200; B, 350; c, 700; r>, 1500; E, 3100. F increased with light and CO2 concentration at all temperatures to a maximum rate of 45 mg CO 2 dm" 2 h" 1 at 450 W m" 2 PAR, 3100 vpm CO 2, and at a temperature of 25 C. At the lowest light intensity, maximum net photosynthesis (F max ) was obtained at the lowest temperatures used, at all CO 2 concentrations. As light intensities and CO2 concentrations were raised, -Pmax was reached at progressively higher temperatures (Fig. 4).

Enoch and Hurd Gas Exchange in Carnations 20 5 00 8 I 8 12-4 36 32 28 24 20 16 12 or' -A- A-.. J I I I 10 15 20 25 FIGURE 3B '-a A i 30 35 ^r -4 _L ± _L 10 15 20 25 30 35 FIGURE 3O

90 Enoch and Hurd Gas Exchange in Carnations 481-44 40 36 32 28 oo 24 E o 16 12 -A-... _L _L J_ 10 15 20 25 FIGUKE 3D 30 35 In general, when allowance was made for CO2 losses by dark respiration, the sum of F and R D remained relatively constant at supraoptimal temperatures regardless of the light or CO2 conditions. This corrected F value is not the same as gross photosynthesis, since it does not include a correction for that component of respiration which takes place in light only. To give an indication of the effect of CO2 concentration on stomatal aperture, leaf resistance to water vapour transport was calculated at different substomatal CO2 concentrations at 20 C leaf temperature, from measurements of CO2 and water vapour fluxes. Data from total radiations of 148 W m~ 2 (125 W m~ 2 PAR) and 535 W m~ 2 (450 W m~ 2 PAR), that were obtained about 3 h after the lamps were lit, and where gas exchange rates had reached equilibrium, are presented in Fig. 5. Each point represents unreplicated measurements on a different plant. At a ~ 300 vpm CO2 concentration within the substomatal cavity, leaf resistance was less at the higher light intensity than at the lower one. With increasing CO2 concen-

Enoch and Hurd Gas Exchange in Carnations 91 521- E 48-44 - 40-36- 32 U 28 24 8 16 12 _ Cs ^ -, _L JL 10 15 20 25 FIGUHE 3B -D A _L 30 35 tration, leaf resistance increased at a steeper rate at the higher light intensity, giving a leaf resistance to water vapour transfer of 6-6 s cm" 1 at substomatal CO2 levels above 2400 vpm (equivalent to an external concentration of 3100 vpm). DISCUSSION The rates of carbon assimilation by carnation plants (Fig. 3) are appreciably lower than those for single leaves of other temperate crops. This is most noticeable at low light intensities, in which the rates are 2 3 mg CO2 dm" 2 h" 1 at 45 W m~ 2, compared with values of 10-20 mg CO2 dm~ 2 h -1 given by Gaastra (1959) for sugar beet and turnip. Even at higher light intensities its values are still only about half those obtained for sugar beet and turnip by Gaastra (1959), for cotton by Bierhuizen

92 Enoch and Hurd Gas Exchange in Carnations 30 r- g25.1 20 2 15 5 io I 0 100 200.100 400 500 Light intensity (W m~ 2 PAR) FIG. 4. The temperature optimum for net photosynthesis of carnation plants at different light intensities and CO2 concentrations (derived from Fig. 3). CO2 concentration (vpm): 3100 (A A), 1500 ( ), 700 ( ), 350, ( ), 200 (O O). ~ 4 0, 0 1000 2000 3000 Substomatal CO, concentration (parts I0" 6 ) FIG. 5. Leaf resistance of carnation at different substomatal CO2 concentrations and light intensities of (o O) 148 W m- 2 (125 W m-2 PAR) and ( ) 535 W m~2 (450 W rrr 2 PAR) total radiation. and Slatyer (1964), and for soybean by Brun and Cooper (1967). Photosynthesis increased substantially with CO2 enrichment. At 20 C, for example, an approximately four-fold enrichment from ambient CO2 levels increased photos3 r nthesis by about 60 per cent at three widely different light intensities (Fig. 6). The equivalent increases obtained by Gaastra (1959), Bierhuizen and Slatyer (1964), and Brun and Cooper (1967) in these conditions were even greater, i.e., about 100 per cent. The difference between these sets of results and the carnation data is presumably accounted for by the considerable respiratory load of the rest of the carnation plant. Since a component of respiration (maintenance and root respiration) is not dependent on light intensity, net CO2 uptake will be depressed progressively more at

Enoch and Htird Gas Exchange in Carnations 93 40 r- 35 30 o a U 25 20 15 10 0 ) 10 15 20 25 30 FIG. 6. The effect of CO2 enrichment on net photosynthesis of carnation at 45, 125, and 450 W m~ 2 PAR (numbers on figure) at different temperatures (derived from Fig. 3). 1500 vpm CO 2 ( ); 350 vpm CO 2 ( ). low light intensities. The carnation also has a small leaf area for a given total plant weight (for example, leaf area/total dry weight is typically only one quarter of that for similarly grown tomato plants). This will tend to emphasize the difference between net CO2 assimilation on a leaf area basis compared with a whole plant basis. The carnation is also less efficient at light interception than many other plants because of its relatively thick leaves, partly adpressed against the stem. Temperature had a marked effect on F, especially at higher rates of photosynthesis. An optimum temperature in the 15-25 C range has often been obtained for temperate crop plants (Murata and Iyama, 1963; Zelitch, 1971) and clearly the carnation comes near the bottom of this range, at ambient atmospheric CO2 concentrations. The temperature (T op t) at which maximal net photosynthesis was obtained increased with an increase in either light intensity or CO2 concentration with apparently little interaction between the two factors, suggesting they are associated with different processes (Fig. 4). A similar finding is reported by Stalfelt (1960), referring to Lundegardh's work on Solanum tuberosum. Such a response is compatible with the fact that photosynthesis depends on both a temperatureindependent light-capture process and a temperature-dependent CCVfixation process requiring no light (Heath, 1969, quoting Emerson and Arnold, 1932).

94 Enoch and Hurd Gas Exchange in Carnations Thus, at low light intensity photosynthesis is relatively independent of temperature, but as the light intensity and/or CO2 concentration increases, progressively higher temperatures are required to prevent the dark process from becoming limiting. Net photosynthesis of the whole plant is the result of gross photosynthesis less respiration during photosynthesis, i.e., light respiration, and that fraction of dark respiration which is not suppressed by light (for instance, maintenance and root respiration). Losses of CO2 in dark respiration were dependent on temperature and the previous day's light conditions. A similar correlation was found by Ludwig, Saeki, and Evans, (1965). When these repiratory CO2 losses were added to the previous day's CO2 assimilation values, the resulting level of CO2fixationwas reasonably constant above the optimal temperature for photosynthesis. This suggests that gross photosynthesis less light respiration (as opposed to dark respiration) remains almost constant in the temperature zone where it is no longer limited by the rate of the dark CO2- fixation process. Gaastra (1959) concluded that CO2 enrichment was worthwhile only at high leaf temperatures, regardless of the light intensity. The data of Fig. 6 show that, for carnation, this appears true at high light intensity only, where the rate of the dark reaction had an overriding control of photosynthesis at low temperature. When the light intensit}' was 125 W m~ 2 or less, CO2 enrichment was equally beneficial at all temperatures, at least down to 10 C. Another effect of CO2 enrichment is as an anti-transpirant. The data of Fig. 5 indicate that CO2 is more effective in this role at high light intensity. With CO2 enrichment, leaf resistance increased four-fold at high light intensity but only two-fold at the lower intensity. The original purpose of this study was to obtain the optimal combination of temperature and CO2 concentration which would maximize photosynthesis in glasshouse conditions to accord with prevailing light. This purpose was achieved. Figure 3 illustrates the relevant data while Fig. 4 gives the optimal leaf temperature for photosynthesis as varying CO2 concentrations and light intensities. As can be seen from Fig. 4, optimal temperatures (T op t) for maximal net photosynthesis (F) at given CO2 concentrations are almost linear functions of light intensity (/). Furthermore, a doubling of CO2 concentration (C) increases T O pt by a nearly equal amount at all light intensities and consequently an equation of the form T O pt = «i + «2 In C + asl represents the data well. The constants a\, «2, and a% can be determined by the leastsquares procedure. This gave the following empirical equation for the optimal temperature as a function of C and /: y opt = -6-47 + 2-336 In G + 0-03195/ where T O pt is expressed in C, C as vpm CO2, and / as W m~ 2 PAR. Caution must be exercised in applying these results for determining optimal greenhouse conditions for production of carnation cuttings or flowers. A difficulty in providing climatological guidelines rests on several observations that optimal temperatures for photosynthesis are related to prior temperatures at which the

Enoch and Hurd Gas Exchange in Carnations 95 plants were grown (Mooney and Harrison, 1970; Downton and Slatyer, 1972; Sawada and Miyachi, 1974). Such dependence was not found in the carnation (Hurd and Enoch, 1976), but even then use of our data for predicting the photosynthesis of a greenhouse carnation crop involves considerable extrapolation because of differences in plant material, age, spacing, culture, etc. Only the general trend of the experimental data is therefore relevant and for this reason we have accepted some variability arising from the lack of replication. There is another aspect to be considered before one could recommend growing plants at the temperatures optimal for photosynthesis. The low temperatures required at low light intensities and ambient atmospheric CO2 concentrations are unlikely to be optimal for growth. The fact that dark respiration increases exponentially with temperature implies that available energy for growth processes will also increase with temperature. From these considerations it appears that the original concept of controlling conditions for maximum photosynthesis of a crop of given leaf area and thus light interception, is insufficient. It must be supplemented with further information on optimal environmental conditions for growth of different plant organs so that the basis necessary for determining optimal greenhouse climates can be provided. ACKNOWLEDGMENTS We wish to thank Dr. M. Buckbinder, A.R.O., Division of Agricultural Meteorology, for his assistance with the gasometric analysis, and Mr. E. Ephrat and Dr. Y. Ben-Yaacov of the A.R.O., Division of Ornamental Crops, for providing plant material and assistance in growing it. LITERATURE CITED BIERHUIZEN, J. F., and SLATYER, R. O., 1964. Aust. J. biol. Sci. 17, 348-59. BRUN, W. A., and COOPER, R. L., 1967. Crop Sci. 7, 451-4. DOWNTON, J., and SLATYER, R. O., 1972. PL Physiol., Lancaster, 50, 518-22. GAASTRA, P., 1959. Meded. LandbHoogesch. Wageningen, 59, 1-68. HEATH, O. V. S., 1969. The physiological aspects of photosynthesis. Heinemann, London. Pp. 214 et sea. HUBD, R. G. and ENOCH, H. Z., 1976. J. exp. Bot. 27, 695-703. LAKE, J. V., 1966. Nature Lond. 209, 97-8. LUDWIO, L. J., SAEKI, T., and EVANS, L. T., 1965. Aust. J. biol. Sci. 18, 1103-18. MOONEY, H. A., and HARRISON, A. T., 1970. Proc. IBP/PP Tech. Meeting Trebon, 1969. Ed. I. Setlik. Centre Agric. Publ. and Doc, Wageningen. Pp. 411-17. MUBATA, Y., and IYAMA, J., 1963. Proc. Crop Sci. Soc. Japan, 31, 315-22. SAWADA, S., and MIYACHI, S., 1974. PL Cell Physiol., Tokyo, 15, 111-20. STALFELT, M. G., 1960. Encyl. PL Physiol. Ed. W. Ruhland. Springer Verlag. Berlin, Vol. 2, pp. 101 et seq. ZELITCH, I., 1971. Photosynthesis, photorespiration and plant productivity. Academic Press, New York. Pp. 244 et seq.