Toward improving photosynthesis in cassava: Characterizing photosynthetic limitations in four current African cultivars

Abstract

Despite the vast importance of cassava (Manihot esculenta Crantz) for smallholder farmers in Africa, yields per unit land area have not increased over the past 55 years. Genetic engineering or breeding for increased photosynthetic efficiency may represent a new approach. This requires the understanding of limitations to photosynthesis within existing germplasm. Here, leaf photosynthetic gas exchange, leaf carbon and nitrogen content, and nonstructural carbohydrates content and growth were analyzed in four high‐yielding and farm‐preferred African cultivars: two landraces (TME 7, TME 419) and two improved lines (TMS 98/0581 and TMS 30572). Surprisingly, the two landraces had, on average, 18% higher light‐saturating leaf CO 2 uptake (A sat) than the improved lines due to higher maximum apparent carboxylation rates of Rubisco carboxylation (V cmax) and regeneration of ribulose‐1,5‐biphosphate expressed as electron transport rate (J max). TME 419 also showed a greater intrinsic water use efficiency. Except for the cultivar TMS 30572, photosynthesis in cassava showed a triose phosphate utilization (TPU) limitation at high intercellular [CO 2]. The capacity for TPU in the leaf would not limit photosynthesis rates under current conditions, but without modification would be a barrier to increasing photosynthetic efficiency to levels predicted possible in this crop. The lower capacity of the lines improved through breeding, may perhaps reflect the predominant need, until now, in cassava breeding for improved disease and pest resistance. However, the availability today of equipment for high‐throughput screening of photosynthetic capacity provides a means to select for maintenance or improvement of photosynthetic capacity while also selecting for pest and disease resistance.

The Cassava Source–Sink project: opportunities and challenges for crop improvement by metabolic engineering

“…The generally high Calvin–Benson cycle enzyme activities can also explain the high rates of photosynthesis in cassava. Second, despite the high rates of photosynthesis, cassava appears to be source‐limited because root yield correlated with leaf metabolite profiles rather than metabolite profiles of stems or storage roots (De Souza and Long, 2018; Obata et al ., 2020). Third, compared to greenhouse‐grown plants, field‐grown plants produced more and larger storage roots, which was associated with higher ADP‐glucose pyrophosphorylase (AGPase) activity and lower sucrose levels in storage roots.…”

Physiology, Growth and Yield of Different Cassava Genotypes Planted in Upland with Dry Environment during High Storage Root Accumulation Stage

“…This might be the result of a good distribution of rainfall that could help maintains soil moisture (Figure 3), which supports crop photosynthesis activity. Investigation on the performance of different cassava genotypes by using high-throughput screening of Pn could provide valuable information for breeding to improve cassava yield under various environments [34,35]. According to the comparisons of the Pn values during water-limited condition at 270 and 360 DAP among different genotypes for 2018/2019 (Table 6), Rayong 7 and CMR38-125-77 had higher performance in photosynthesis when compared to the other genotypes (p < 0.01).…”

Growing out of hunger: towards an improved understanding of the water and nutrient limited yield of cassava

“…Nutrient demand was however strongly reduced as dying leaves provided sufficient nutrients. Further, with partial stomatal closure, cassava maintains reasonable CO2 uptake rates, and continues to accumulate dry matter in its storage roots (El-Sharkawy et al, 1992;Rosenthal and Ort, 2012;De Souza and Long, 2018). After the dry period, nutrient uptake and yield increased in all locations, and was larger in optimized fertilizer treatments than in the control and treatments with smaller fertilizer rates, suggesting that soil available pools were larger as well.…”

“…This is vividly demonstrated by the fact that average cassava yield has not increased remarkably in Nigeria over the past five decades (FAOSTAT, 2020a), while over the same period, maize yields in Nigeria increased by 129 % (De Souza et al, 2017). Most breeding programs and efforts to improve cassava yields have prioritized drought tolerance, reducing cyanogenic content, and resistance to pest and diseases (Egesi et al, 2007;De Souza and Long, 2018). Recently, efforts have focused on improving cassava nutritional quality (Montagnac et al, 2009;Adenle et al, 2012).…”

“…The other improved cultivar TMS 30572 was bred for resistance to cassava anthracnose disease, cassava bacterial blight (CBB), cassava mealybug, and cassava green mite (Eke-Okoro and Njoku, 2012). Through bioengineering, it may be possible to substantially increase the photosynthetic capacity of cassava for higher yields (De Souza and Long, 2018). A study by De Souza and Long (2018) on yield assessment and disease resistance of different cassava varieties showed that while TME 419 was only moderately resistant to diseases and pest, the cultivar showed a greater water use efficiency through its slower stomatal conductance, with higher photosynthetic rates than others studied.…”