Photosynthesis across African cassava germplasm is limited by Rubisco and mesophyll conductance at steady-state, but by stomatal conductance in fluctuating light

De Souza, Amanda and Yu, Wang and Orr, Douglas John and Carmo-Silva, Ana Elizabete and Long, Stephen (2020) Photosynthesis across African cassava germplasm is limited by Rubisco and mesophyll conductance at steady-state, but by stomatal conductance in fluctuating light. New Phytologist, 225 (6). pp. 2498-2512. ISSN 0028-646X

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Abstract

Sub‐Saharan Africa is projected to see a 55% increase in food demand by 2035, where cassava (Manihot esculenta) is the most planted crop and a major calorie source. Cassava yield has not increased significantly for 13 years. Improvement of genetic yield potential, the basis of the first Green Revolution, could be increased by improving photosynthetic efficiency. First, the factors limiting photosynthesis and their genetic variability within extant germplasm must be understood. Biochemical and diffusive limitations to leaf photosynthetic CO2 uptake under steady‐state and fluctuating light in thirteen farm‐preferred and high‐yielding African cultivars were analyzed. A cassava leaf metabolic model was developed to quantify the value of overcoming limitations to leaf photosynthesis. At steady‐state, in vivo Rubisco activity and mesophyll conductance accounted for 84% of the limitation whereas under non‐steady‐state conditions of shade to sun transition stomatal conductance was the major limitation contributing resulting in an estimated 13% and 5% losses in CO2 uptake and water use efficiency, across a diurnal period. Triose phosphate utilization, while sufficient to support observed rates, would limit improvement in leaf photosynthesis to 33%, unless improved itself. The variation of carbon assimilation among cultivars were three times greater under non‐steady‐state compared to steady‐state, pinpointing important overlooked breeding targets for improved photosynthetic efficiency in cassava.