2.2.5 - Environmental physiology of C₃ versus C₄ photosynthesis

Rubisco is characterised by its low affinity for its productive substrate, CO₂ and slow catalytic turnover rate (i.e., 1-3 cycles per sec). Importantly, Rubisco reacts with O₂ (photorespiration), and this culminates in loss of CO₂ and energy. In C₃ plants, photorespiration can drain more than 25% of fixed CO₂ under non-stressful conditions. The ratio of photorespiration to photosynthesis increases with increasing temperature and decreasing intercellular CO₂ such as occurs when stomatal conductance is reduced under water stress. C₃ plants compensate for Rubisco’s inefficiencies by (i) opening their stomata to increase CO₂ diffusion into chloroplasts, which increases water loss and lowers leaf-level water use efficiency, WUE; and (ii) investing up to 50% of leaf nitrogen in Rubisco, which lowers their leaf-level nitrogen use efficiency, NUE.

The C₄ pathway supercharges photosynthesis and suppresses photorespiration by operating a CO₂ concentrating mechanism which elevates CO₂ around Rubisco. Although C₄ photosynthesis incurs additional energy, the energy cost of photorespiration exceeds that of the CO₂ concentrating mechanism above 25^oC. Hence, higher radiation use efficiencies (i.e., efficiency of converting absorbed radiation into biomass) have been recorded for C₄ than C₃ crops. High bundle sheath CO₂ concentration saturates C₄ photosynthesis at relatively low intercellular CO₂, allowing C₄ plants to operate with lower stomatal conductance. Thus, leaf-level WUE is usually higher in C₄ than C₃ plants. Relative to C₃ plants, Rubisco of C₄ plants is faster (higher turnover rate) and operates under saturating CO₂. Thus, C₄ plants typically achieve higher photosynthetic rates with about 50% less Rubisco and less leaf nitrogen. Hence, photosynthetic NUE is higher in C₄ than C₃ plants. Accordingly, C₄ plants are advantaged relative to C₃ plants in hot and nitrogen-poor environments with short growing seasons, hence their great abundance in wet/dry tropics such as Northern Territory savannas.

As mentioned earlier, more than 50% of C₄ plants are grasses. C₄ grasses are confined to low latitudes and altitudes, whereas C₃ species dominate at higher latitudes and altitudes. Generally, C₄ species frequently occur in regions of strong irradiance. Ehleringer and colleagues (Ehleringer et al. 1997) proposed that these distribution patterns are best explained by the different responses of photosynthetic quantum yields to temperature between C₃ and C₄ plants.

C₄ photosynthesis suppresses photorespiration by operating a CO₂ concentrating mechanism that comes at additional energetic cost. This cost is independent of ambient CO₂ and temperatures. In contrast, photorespiration (and its associated energy cost) increases steeply with temperature in C₃ plants and is highly dependent on CO₂ concentrations. Under saturating irradiance and current ambient atmospheric CO₂ concentration, the threshold temperature where the cost of photorespiration in C₃ plants exceeds that of the CO₂ concentrating mechanism in C₄ plants is estimated around 25^oC. This model provides a physiological basis for understanding today’s contrasting geographic distribution between C₃ and C₄ grasses.

As an example, the C₄ grasses of the northern Australian savannas are relatively un-shaded because of the low tree density and sparse canopy. Light is abundant and since the CO₂ concentration inside C₄ leaves is high, a potentially high rate of light-saturated assimilation can be exploited. Most C₃ species reach light saturation in the range of one-eight to one-half full sunlight (Figure 2.7). In C₄ species, canopy assimilation might not become light saturated even in full sunlight. C₄ plants thus maintain a competitive advantage over C₃ plants in tropical locations, where average daily light receipt is much larger than in temperate zones, and associated with warmer conditions that also favour C₄ photosynthesis (Figure 2.6). Given strong sunlight, warmth and seasonally abundant water, biomass production by C₄ plants is commonly double the rate for C₃ plants. Typically, C₃ plants produce 15–25 t ha^–1 but C₄ plants easily produce 35–45 t ha^–1.

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Physiological characteristics of the C₄ subtypes

As outlined in previous sections, characteristic biochemical, anatomical and physiological traits are associated with each of the three “classical” C₄ subtypes (Table 2.3). However, it should be noted that many C₄ plants have leaf structures that fall outside the “classical” subtype division (eg, NADP-ME tribes, Arundinelleae and Neurachneae). As many as 11 anatomical-biochemical suites have been identified in C₄ grasses. A curious aspect about the subtypes of C₄ grasses is their biogeography. In Australia and elsewhere, NADP-ME grasses are more frequent at higher rainfall, NAD-ME grasses predominate at lower rainfall, while the distribution of PCK grasses is even across rainfall gradients (Hattersley 1992).