Resource use efficiency of C4 grasses with different evolutionary origins

Abstract

C4 photosynthesis has evolved to overcome the limitations of ancestral C3 photosynthetic pathway by operating a CO2 concentration mechanism. Following the depletion of CO2 concentration ([CO2]) in the atmosphere 30 million years ago, the efficiency of the C3 pathway was reduced especially under the prevailing high atmospheric and soil aridity, all of which promote photorespiration. Under most physiological conditions, photorespiration results in the loss of energy and carbon fixed by C3 photosynthesis. C4 photosynthesis is an adaptation that overcomes photorespiration and improves carbon efficiency. C4 plants are more productive and resource use efficient than the more prevalent C3 plants. At present, C4 plants constitute 3% of the world's species and C4-dominated grasslands contribute up to 20% of global primary productivity. About 50% of C4 species are grasses with 15 distinct evolutionary origins distributed over 370 genera and 4600 species. Further, C4 plants are divided into three biochemical subtypes following the major C4 acid decarboxylase in the bundle sheath cells: NAD malic enzyme (NAD-ME), NADP-ME and PEP carboxykinase (PCK). These C4 subtypes are closely associated with particular taxa, and some taxa have multiple evolutionary origins. In addition, various physiological and ecophysiological traits have been associated with the biochemical subtype or evolutionary lineage of the C4 grasses. Most physiological studies were undertaken using a small number of C4 species under current ambient [CO2] which does not reflect the low [CO2] environment under which C4 grasses have evolved. Therefore, this PhD project was conducted to compare the physiological efficiencies of diverse C4 grass species under conditions that promote high rates of photorespiration, which contributed to the physiological pressure that led to the evolution of the CO2 concentrating mechanism (CCM) in land plants.

Date of Award	2015
Original language	English