Samuel H. Taylor, Douglas J. Orr, Elizabete Carmo-Silva, and Stephen P. Long

Interventions to increase crop radiation use efficiency rely on understanding 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: B. rapa, B. oleracea and B. 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. 

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