Abstract
Interventions to increase crop radiation use efficiency rely on understanding of how biochemical and stomatal limitations affect photosynthesis. When leaves transition from shade to high light, slow increases in maximum Rubisco carboxylation rate and stomatal conductance limit net CO2 assimilation for several minutes. However, as stomata open intercellular [CO2 ] increases, so electron transport rate could also become limiting. Photosynthetic limitations were evaluated in three important Brassica crops: Brassica rapa, Brassica oleracea and Brassica napus. Measurements of induction after a period of shade showed that net CO2 assimilation by B. rapa and B. napus saturated by 10 min. A new method of analyzing limitations to induction by varying intercellular [CO2 ] showed this was due to co-limitation by Rubisco and electron transport. By contrast, in B. oleracea persistent Rubisco limitation meant that CO2 assimilation was still recovering 15 min after induction. Correspondingly, B. oleracea had the lowest Rubisco total activity. The methodology developed, and its application here, shows a means to identify the basis of variation in photosynthetic efficiency in fluctuating light, which could be exploited in breeding and bioengineering to improve crop productivity.
Highlights
The continued growth of the global human population and its increasing urbanisation will lead to increased pressure on farming systems over the half century, and increased productivity on the land we are already using will be crucial to minimize the environmental impacts (Tilman, Balzer, Hill & Befort 2011)
Leaf level responses to photosynthetic photon flux density (PPFD) (Fig. 1) showed mean values of Asat Rd, and θ that were highest for B. rapa, slightly lower for B. napus, and lowest for B. oleracea (Fig. 1)
Φ was greater in B. oleracea and B. napus than in B. rapa
Summary
The continued growth of the global human population and its increasing urbanisation will lead to increased pressure on farming systems over the half century, and increased productivity on the land we are already using will be crucial to minimize the environmental impacts (Tilman, Balzer, Hill & Befort 2011). In this context, it is essential to understand photosynthetic efficiency because it fundamentally affects the productivity and efficiency of resource use by crops. The major focus of studies of crop photosynthetic efficiency has been under lightsaturating steady-state conditions These are rare for crop leaves in the field or glasshouse
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