Photosynthesis

Increased atmospheric [CO2] reduces photorespiration of C3 plants by promoting carboxylation and diminishing oxygenation of the photosynthetic enzyme Rubisco (Sage, 1994). This change, together with the increased CO2 diffusion gradient into the leaves, leads to enhanced photosynthetic rates. However, in some cases this effect appeared to be not maintained in the long term. Leaves of C3 plants that were exposed over prolonged periods of time to enhanced [CO2] have sometimes shown decreased photosynthetic capacity. This phenomenon, called acclimation or down-regulation of photosynthetic capacity, has been the subject of several investigations (Long and Drake, 1992; Sage, 1994), but many of the results remain contradictory. This issue is of interest as it affects the prediction of the likely consequences of rising [CO2] on carbon uptake and storage by crops. In the OTC experiments conducted in the Netherlands (A.H.C.M. Schapendonk, personal communication, 1995), signs of acclimation or down-regulation of photosynthesis were observed as the initially large CO2 stimulation of photosynthesis decreased during the period of active tuber filling.

To understand and analyse the potential consequences of such acclimation for growth and yield, a mechanistic simulation model of potato growth (Spitters et al., 1986; Schapendonk et al., 1995) was used. Leaf gross CO2 assimilation was simulated according to the biochemical model described by

Farquhar et al. (1980) and von Caemmerer and Farquhar (1981), which is based on Michaelis—Menten kinetics and simulates the kinetics of electron flow and carboxylation rates. From these relationships, the effects of temperature, photosynthetic photon flux density (PPFD) and [CO2] on gross leaf photosynthesis were calculated. Gross leaf photosynthesis is known to be determined by two rate-limiting processes: the production of reducing equivalents in the electron-transport chain and the rate of carbon fixation in the Calvin cycle. At low PPFD, the electron-transport rate, equivalent to energy delivery, is limiting. Elevated [CO2] has a positive effect due to more efficient energy utilization by suppression of photorespiration. At high PPFD the positive effect is enhanced because the carboxylation rate, linked with the availability of CO2, is rate-limiting. In fact, in the OTC experiment, doubling [CO2] resulted in 24-40% increases in tuber yields. Simulations showed that the yield effect of increased [CO2] would have been almost two times as large, if acclimation had not down-regulated the photosynthetic rates. This shows that acclimation may be responsible for a major reduction of the CO2 effect on biomass and tuber production (Fig. 9.4).

Acclimation was also observed in the FACE experiment, but this was caused by earlier senescence of the plants exposed to elevated [CO2].

Fig. 9.4. Simulated (dotted line) and observed total biomass (minus roots) dry matter production for ambient (open symbols) and doubled (closed symbols) [CO2] treatments measured in the open-top chambers (OTC) experiment in Wageningen in 1995 in two different maturity class of potatoes: (a) early variety and (b) late variety.

Fig. 9.4. Simulated (dotted line) and observed total biomass (minus roots) dry matter production for ambient (open symbols) and doubled (closed symbols) [CO2] treatments measured in the open-top chambers (OTC) experiment in Wageningen in 1995 in two different maturity class of potatoes: (a) early variety and (b) late variety.

Photosynthetic acclimation in the FACE experiment was investigated by gas exchange. The leaves of potato plants grown in the field at ambient, 460, 560 and 660 |mmol mol-1 were periodically collected to determine rates of assimilation (A) versus intercellular [CO2] (C). Ribulose 1,5-bisphosphate (RuBP) saturated (Vcmax, maximum carboxylation rate) and RuBP regeneration limited (Jmax, electron transport-mediated regeneration capacity) carboxylation rates were calculated for each A/Ci curve by fitting a deterministic photosynthesis model (Farquhar and von Cammerer, 1982) to the data. The measurements were made during the period between flowering and maximum leaf area expansion and later when the tubers were growing.

Leaf senescence was also investigated at the time of the second round of photosynthesis measurements. It was assumed that as broad leaves senesce, their reflectance increases in the green wavelength region, peaking at 550 nm because of chlorophyll degradation (Knipling, 1970). Hence, leaf reflectance was assessed on upper canopy leaves by means of a laboratory spectro-radiometer (Benincasa et al., 1988; Malthus and Madeira, 1993).

Measurements in the first part of the growing season showed that the response of intercellular [CO2] of potato leaves was not affected by long-term elevated [CO2] exposure in the FACE (Fig. 9.5). Subsequent measurements made later in the season showed that leaf photosynthesis decreased progressively in the higher [CO2] environments (data not shown). This decrease in photosynthetic capacity was due to senescence, which was shown by leaf reflectance in leaves grown in high [CO2] (Fig. 9.6). The reflectance measured at 550 nm was well correlated (the regression equation is y = 0.0248 + 7.788x, with r = 0.99) with the loss of photosynthetic capacity.

Leaf N concentration decreased faster in the leaves grown under high [CO2] than in those grown under ambient [CO2], thus further supporting the conclusion that leaf senescence was accelerated in plants grown in high [CO2]. Such loss of leaf N led to a reduction in leaf photosynthetic capacity. Data from this experiment clearly indicated that there was no down-regulation of photosynthetic capacity in potato leaves exposed to increasing [CO2], at least during the main part of the growing season. When tubers are actively growing, they provide a large sink for carbon fixed by the leaves. This likely prevents negative feedback effects of carbohydrate accumulation in leaves that otherwise might lead to a permanent loss of photosynthetic capacity.

Plants grown under elevated [CO2] may continuously fix a larger amount of carbon than those grown at ambient concentration and, not surprisingly, increasing [CO2] has large and progressive positive effects on tuber yields (see section 9.2.3). However, accelerated senescence of the leaves in the three elevated [CO2] FACE plots was a likely consequence of the higher canopy temperatures during daytime hours with elevated [CO2], or a result at the end of the growing season of the carbohydrate storage capacity of the tubers being more rapidly exhausted in the plants grown in high [CO2] than in those grown in ambient [CO2], or both. Whatever the case, tuber growth under elevated [CO2] had indirect consequences on the photosynthetic properties of the leaves, mediated by this acceleration of leaf senescence. Overall, the results obtained in these experiments with potato suggest that photosynthetic

Fig. 9.5. Relationship between carbon assimilation (A) and internal leaf carbon dioxide concentration (Ci) measured in July on potato leaves growing in different CO2 concentrations. Error bars indicate standard deviation of assimilation (A) and CO2 intercellular concentrations (Ci). (a) Leaves grown at ambient [CO2]; (b) Leaves grown at 460 |mmol mol-1; (c) Leaves grown at 560 |mmol mol-1; (d) Leaves grown at 660 mmol mol-1.

Fig. 9.5. Relationship between carbon assimilation (A) and internal leaf carbon dioxide concentration (Ci) measured in July on potato leaves growing in different CO2 concentrations. Error bars indicate standard deviation of assimilation (A) and CO2 intercellular concentrations (Ci). (a) Leaves grown at ambient [CO2]; (b) Leaves grown at 460 |mmol mol-1; (c) Leaves grown at 560 |mmol mol-1; (d) Leaves grown at 660 mmol mol-1.

Ambient 460 560 660

C02 treatment

Fig. 9.6. Leaf reflectance (%) of potato leaves grown at different [CO2] treatments measured at 550 nm in late August (significant at P< 0.01). Ambient [CO2] was approximately 360 |mmol mol-1.

acclimation is a complex process involving interactive effects of elevated [CO2] on source/sink relations in the plants (Ziska et al., 1995).

Photosynthetic carbon supply may interact with the availability of inorganic nutrients and with the canopy energy balance as reflected by canopy temperature. It remains doubtful if the decreased leaf N concentrations that have been observed in leaves grown at elevated [CO2], are per se a sign of decreased photosynthetic capacity. Tuber and root crops do not make an exception to this observation, as decreased leaf N was also observed in plants grown under elevated [CO2] on kohlrabi (Sritharan et al., 1992), radish (McKeehen et al., 1995) and potato (Miglietta et al., 1998).

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