2.2.4  Environmental physiology

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C4 species frequently occur in regions of strong irradiance. For example, the C4 grasses of savannas of northern Australia are relatively unshaded because of the low tree density and sparse canopy. Light is abundant and since the CO2 con-centration 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 2.6). 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. Added to this superior light-saturated capacity, C4 plants achieve higher nitrogen and water use efficiencies due to their CO2-concentrating mechanism and absence of photorespiration. 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.