Phenotypic approaches to drought in cassava: review

Abstract

Cassava is an important crop in Africa, Asia, Latin America, and the Caribbean. Cassava can be produced adequately in drought conditions making it the ideal food security crop in marginal environments. Although cassava can tolerate drought stress, it can be genetically improved to enhance productivity in such environments. Drought adaptation studies in over three decades in cassava have identified relevant mechanisms which have been explored in conventional breeding. Drought is a quantitative trait and its multigenic nature makes it very challenging to effectively manipulate and combine genes in breeding for rapid genetic gain and selection process. Cassava has a long growth cycle of 12–18 months which invariably contributes to a long breeding scheme for the crop. Modern breeding using advances in genomics and improved genotyping, is facilitating the dissection and genetic analysis of complex traits including drought tolerance, thus helping to better elucidate and understand the genetic basis of such traits. A beneficial goal of new innovative breeding strategies is to shorten the breeding cycle using minimized, efficient or fast phenotyping protocols. While high throughput genotyping have been achieved, this is rarely the case for phenotyping for drought adaptation. Some of the storage root phenotyping in cassava are often done very late in the evaluation cycle making selection process very slow. This paper highlights some modified traits suitable for early-growth phase phenotyping that may be used to reduce drought phenotyping cycle in cassava. Such modified traits can significantly complement the high throughput genotyping procedures to fast track breeding of improved drought tolerant varieties. The need for metabolite profiling, improved phenomics to take advantage of next generation sequencing technologies and high throughput phenotyping are basic steps for future direction to improve genetic gain and maximize speed for drought tolerance breeding.

Genetic variability in storage root bulking of cassava genotypes under irrigation and no irrigation

“…This observation was contrary to the prediction by Hershey [23] and Okogbenin et al [17] who indicated that productivity in cassava at six MAP can be used as a criterion to screen for high-yielding cassava varieties because early fresh storage root yielders are the higher yielders at later stages of growth. Genotypes that had low storage root yield at 6 months but high yield at 12 MAP might have used the above ground part as the major sink before partitioning materials into storage roots.…”

“…Though most crops have specific phases in their phenology that can be targeted for improvement against drought, it has not been easy to do so for cassava due to differential partitioning of dry matter into the above ground biomass and roots [4,7]. High-yielding cassava cultivars have a high bulking rate over a long period of time, whereas cultivars with intermediate and low storage root yield have a low bulking rate of shorter duration or lower bulking rate for longer duration [17,23].…”

“…Early storage root yield (bulking) in cassava has also been identified as a very important trait in drought tolerance [14,17]. In some cases, earliness enables escape from late season droughts, pests and diseases and is perceived as a key control strategy for cassava brown streak disease (CBSD), which is currently epidemic in east, central and southern Africa [8,16,18,19].…”

“…This calls for an in-depth study into the relationship between the growth pattern of storage roots and productivity in cassava under limited moisture and well-watered conditions to assess the pattern on dry matter accumulation. Careful exploitation of the reported genetic variation in the rate and pattern of accumulation of dry matter in the storage roots [17,23] has to be made to develop early-bulking cassava varieties with acceptable yields in savannah ecologies.…”

Agrometeorological models for estimating sweet cassava yield

“…One of the reasons for the high tolerance of cassava during dry periods is its highly sensitive stomatal system. In low moisture soil conditions, the plant saves water by controlling transpiration; however, long periods with severe dry spells during the cycle decrease yield (Oliveira et al 1982, Okogbenin et al 2013. Air temperatures in October and November, during phases of tuber bulking, vegetative growth and the beginning of starch accumulation were the other variables that had a large influence on sweet cassava yield, with a direct relation (+).…”