Pioneering research on C₄ photosynthesis: implications for crop water relations and productivity in comparison to C₃ cropping systems.

摘要

This review summarizes the history of the discoveries of the many anatomical, agronomical and physiological aspects of C₄ photosynthesis (where the first chemical products of CO₂ fixation in illuminated leaves are four-carbon dicarboxylic acids) and documents the scientists at the University of Arizona and the University of California, Davis, who made these early discoveries. These findings were milestones in plant science that occurred shortly after the biochemical pathway of C₃ photosynthesis in green algae (where the first chemical product is a three-carbon compound) elucidated at the University of California, Berkeley, and earned a Nobel Prize in chemistry. These remarkable achievements were the result of ground-breaking pioneering research efforts carried out by many agronomists, plant physiologists and biochemists in several laboratories, particularly in the USA. Numerous reviews and books written in the past four decades on the history of C₄ photosynthesis have focused on the biochemical aspects and give an unbalanced history of the multidisciplinary/multiinstitutional nature of the achievements made by agronomists, who published much of their work in field study journals such as Crop Science. Most notable among the characteristics of the C₄ species that differentiated them from the C₃ ones are: (I) high optimum temperature and high irradiance saturation for maximum leaf photosynthetic rates; (II) apparent lack of CO₂ release in a rapid stream of CO₂-free air in illuminated leaves in varying temperatures and high irradiances; (III) a very low CO₂ compensation point; (IV) lower mesophyll resistances to CO₂ diffusion coupled with higher stomatal resistances, and, hence, higher instantaneous leaf water use efficiency; (V) the existence of the so-called "Kranz leaf anatomy" and the higher internal exposed mesophyll surface area per cell volume; and (VI) the ability to recycle respiratory CO₂ by illuminated leaves. Recent research conducted at CIAT with the tropical root crop, cassava, revealed that it is endowed with a high photosynthetic capacity that is intermediate between C₃ and C₄ species. Yield under stressful environment was correlated with leaf photosynthetic rate, as measured in the field, PEPC activity and with leaf photosynthetic nitrogen use efficiency.