Effect of [CO2 on photosynthesis

Elevated [CO2] increases the rate of net photosynthesis (Pn) and decreases the rate of photorespiration (Pr) in C3 plants such as wheat. The causes are well understood: the enzyme responsible for the reaction of CO2 with ribulose bisphosphate (RuBP) is ribulose bisphosphate carboxylase-oxygenase (Rubisco). The reaction is not saturated at current [CO2]; also oxygen reacts with RuBP, producing phosphoglycolate which is metabolized, ultimately releasing CO2 in Pr. Doubling current [CO2] saturates Rubisco and greatly decreases Pr, thus increasing Pn by 30-40% (in bright light and at 25-30°C) and the rate of carbon assimilates supply (sugars and starch) to the plant (Long, 1991). Although elevated [CO2] also decreases the stomatal conductance (gs), with potentially large reductions in water loss, the increased atmospheric [CO2] is more than sufficient to compensate so that the internal CO2 concentration, [CO2]i, is greatly increased, hence the increased Pn. The response of leaf Pn to

[CO2]i for different light and temperature values is well predicted by the standard model of the process (Farquhar and von Caemmerer, 1982).

Wheat exhibits these typical responses and there is little varietal variation. Often the initial stimulation of Pn in C3 plants at elevated [CO2] decreases in plants grown at elevated [CO2]. This is associated with a general loss of photosynthetic capacity of plants grown in elevated [CO2], i.e. Pn measured at any given [CO2] is smaller than that of plants grown in ambient [CO2] (Sage,

1994). This is usually termed acclimation, but as increases also occur it is better to refer to it as negative or positive acclimation. The possible mechanisms of such acclimatory processes have been discussed by Lawlor and Keys (1993) and Drake et al. (1997). There are two explanations: (i) loss of photosynthetic components; or (ii) decreased activation of components ('down-regulation'). Both affect Rubisco. 'Up-regulation' is less frequently observed.

In studies on wheat grown under elevated [CO2], a range of different responses of photosynthetic capacity have been observed. Individual plants, grown under controlled environmental conditions, lost photosynthetic capacity with reduced amounts and activation states of Rubisco (McKee and Woodward, 1994a,b). In the later FACE studies, there was no acclimation due to growth under elevated [CO2] so that photosynthesis was stimulated when measured under elevated [CO2] (Garcia et al., 1998). Earlier studies indicated substantial stimulation of Pn of leaves (31%) in well-watered and fertilized crops by 550 cf. 380 |mmol CO2 mol-1 (Nie et al., 1995a). For leaves just emerged, elevated [CO2] during growth had no effect on photosynthetic capacity or on amounts of any of the major proteins in any leaf examined. For flag leaves during grain-fill, there was a substantial effect on photosynthetic capacity. Rubisco declined by 45% in control leaves but by 60% in elevated [CO2] (Nie et al., 1995a,b). However, care must be exercised in interpreting these studies, due to the cooler control compared with elevated [CO2] treatments caused by omission of blowers (Garcia et al., 1998). With correct controls, the effect of CO2 was only seen during late grain-filling and particularly with N deficiency. Also, carboxylation efficiency was decreased, but by variable amounts in different leaves (Brooks et al., 1996; Adam et al., 1997; Wall et al., 1997). Similar results were obtained in OTC experiments carried out in several European countries on the spring wheat cv. Minaret (Mitchell et al., 1999). Here, there was also no effect of elevated [CO2] on photosynthetic capacity before anthesis, but there was a decrease in flag leaves photosynthesis during grain-fill, especially in crops with less applied N. Delgado et al. (1994) did not observe acclimation. However, in a subsequent, similar study on the same variety (Lawlor et al., 1995 and unpublished) there was a progressive decrease in photosynthetic capacity in leaves of later insertion, and loss of Rubisco, demonstrating that subtle changes in environment can affect photosynthetic capacity responses to elevated [CO2].

The supply of N is critical in determining the response of photosynthetic capacity. Increased photosynthetic capacity and Rubisco content (positive acclimation) in response to elevated [CO2] have been observed in young plants grown with warm temperatures and unlimited N supply (e.g. Habash et al.,

1995). Possibly, such conditions stimulate extra root growth and N uptake.

Growth at elevated [CO2] had no effect on photosynthetic capacity in young leaves of wheat with varying N supply but decreased it as leaves aged, and this effect was greater at lower N supplies (Theobald et al., 1998). This work also showed that the critical factor is whether the N treatment results in a difference in leaf N content. This explains why elevated [CO2] does not always result in decreased capacity even at low N supply (e.g. Delgado et al., 1994).

There have been studies to separate the two explanations for the effects of [CO2] on photosynthetic capacity. There is little evidence of down-regulation of photosynthetic capacity at high light (Delgado et al., 1994; Lawlor et al., 1995; Nie et al., 1995a,b; Theobald et al., 1998). McKee and Woodward (1994a) interpreted the down-regulation of Rubisco to occur when it is in excess compared with some other limitation on Pn. Changes in amounts of photo-synthetic components, and Rubisco in particular, are therefore responsible for loss of photosynthetic capacity.

From all the above evidence, many of the observed effects of elevated [CO2] on photosynthetic capacity in wheat can be explained in terms of changes in the sink-source balance, i.e. the result of more C assimilate (Morison and Lawlor, 1999), echoing the conclusions of Rogers et al. (1996). For example, if N supply is fixed, and canopy growth and leaf area are stimulated at elevated [CO2], leaf N content and photosynthetic capacity are inevitably decreased. If elevated [CO2] has stimulated ear and grain formation, they provide a greater sink for remobilization of N from photosynthetic tissue during grain-fill; thus, green area senescence may be accelerated. There is also the confounding effect of reduced stomatal conductance at elevated [CO2] (see below), which may increase leaf temperature and accelerate senescence. When N is supplied, such that additional root growth at elevated [CO2] can capture more N (and also use more assimilates), very different results are obtained (Habash et al., 1995), or the effect of elevated [CO2] on N content of leaves is not decreased (Rogers et al., 1996). Therefore, the response of photosynthetic capacity to elevated [CO2] supports the hypothesis that a general consequence of elevated [CO2] is altered sink-source relations. The relationships between Rubisco content, thylakoid ATP-synthase and leaf N were unaltered in wheat leaves grown at elevated [CO2]. A CO2-specific response would be expected to decrease Rubisco preferentially (Theobald etal., 1998).

The mechanism by which the changes in leaf N and photosynthetic capacity occur are still uncertain. Part of it possibly occurs as a consequence of the feedback regulation exerted by increased carbohydrates (hexoses or, specifically, glucose) on gene expression (van Oosten and Besford, 1996). However, in a FACE experiment, mRNA transcripts for a number of photosynthetic components, including the large and small subunits of Rubisco, were poorly correlated with the concentrations of carbohydrates (e.g. glucose-6-phosphate, sucrose and starch), which were different between the CO2 treatments (Nie et al., 1995b). Also, in wheat in a controlled-environment experiment, changes in amounts of Rubisco were not correlated with the concentrations of carbohydrates in leaves or stems (Lawlor et al., 1995 and unpublished). Thus, the sugar repression model of photosynthetic acclimation

(Sheen, 1994) is not operating in a simple way in wheat grown in elevated [CO2]. The complex effects observed are likely the result of several dynamic, interacting mechanisms operating to balance changing demand and supplies for C and N (Morison and Lawlor, 1999).

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