13.5.2  Leaf gas exchange

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Generalising from extensive data on container-grown tropical species, light-saturated photosynthesis is increased (on average by 50%) and gs is decreased (on average by 50%) in response to a doubling of ambient CO2 concentration provided pot size is sufficiently large. An increase in assimilation coupled to a decrease in gs may appear counter-intuitive. However, in a CO2-enriched environment, CO2 concentration within a leaf is still larger (often double) than that in a leaf in ambient air, and thus assimilation rate is still enhanced.

These results are consistent with a range of results observed for temperate and boreal tree responses (Eamus and Jarvis 1989). Increased assimilation responses may be attributed to increased substrate concentration (CO2) and decreased photorespiration. Few measurements of the activation state of Rubisco have been made for tropical species, but declines in carboxylation efficiency as inferred from A:pi curves have been observed for Eucalyptus tetrodonta and other tropical species (Eamus et al. 1995; Ziska et al. 1991). A decline in the amount and activity of Rubisco in response to CO2 enrichment has been observed in a range of annual and tree species, and probably represents an optimisation response, whereby nitrogen is relocated from non-limiting steps in photosynthesis (carbon fixation) to those steps limiting the rate of photosynthesis. In a CO2-enriched environment carbon supply to Rubisco is non-limiting but electron transport or Calvin cycle turnover may have become limiting.

Some of the increase in assimilation rate, expressed as per unit leaf area, is attributable to an increase in the thickness of photosynthetic tissue (mesophyll), resulting in a decreased specific leaf area (area of leaf per unit dry mass of leaf). Typically specific leaf area decreases by approximately 10–40%. Although some of this decrease may result from increased leaf density, reflecting increased starch storage in leaves growing with CO2 enrichment, much of the decrease in specific leaf area is the result of thicker leaves, with either more layers of photosynthetic cells or thicker cells. Consequently the mass of photosynthetic tissue per unit leaf area increases, and assimilation rate per unit leaf area increases. Nevertheless, when assimilation rates are expressed as per unit dry mass, they are still significantly increased by CO2 enrichment because of an increase in substrate supply and decreased photorespiration.

A loss of photosynthetic potential has been observed in some tropical tree species, including Eucalyptus tetrodonta, but not in others (e.g. Mangifera indica, mango). This loss of potential can be shown by comparing the assimilation rate at a common pi of trees grown in ambient conditions with trees grown in enriched conditions. By using a common pi, any variation in assimilation rate due to differences in substrate concentration is removed. The loss of photosynthetic potential is often due to the loss of activity or a decline in the amount of Rubisco. Foliar nitrogen content frequently declines in CO2-enriched trees, reflecting a change in allocation of plant nitrogen.

Instantaneous rates of assimilation for trees grown in ambient but measured in enriched conditions are often larger than those of trees grown in enriched but measured in ambient conditions (Figure 13.3). This results not only from down-regulation but also a decline in stomatal density (Table 13.1), which restricts entry of CO2. Stomatal aperture is important in plants grown with CO2 enrichment but measured in ambient conditions because of the decrease in CO2 concentration upon transfer to ambient conditions. When measured in a CO2-enriched environment, the larger gradient in CO2 concentration between ambient air and leaf airspaces compensates for reduced gs, but when measured in ambient conditions a lower stomatal density results in a lower pi and hence a lower assimilation rate than is measured for ambient-grown trees measured in ambient conditions.

Decreased gs is due to reduced stomatal density (Table 13.1) as well as reduced aperture. Given an increase in assimilation but a decline in gs, instantaneous water use efficiency (ratio of assimilation to transpiration) also increases. Such increases may delay onset of drought or reduce the severity of a drought, but a more likely scenario in nature is that the increase in leaf area per plant, plus the increase in leaf temperature often associated with a decreased leaf transpiration rate, will offset increased water use efficiency.