Physiological characteristics of cassava tolerance to prolonged drought in the tropics: implications for breeding cultivars adapted to seasonally dry and semiarid environments

Abstract

The paper summarizes research conducted at International Center for Tropical Agriculture (CIAT) on responses of cassava to extended water shortages in the field aided by modern gas-exchange and water-relation techniques as well as biochemical assays. The aim of the research was to coordinate basic and applied aspects of crop physiology into a breeding strategy with a multidisciplinary approach. Several physiological characteristics/traits and mechanisms underpinning tolerance of cassava to drought were elucidated using a large number of genotypes from the CIAT core germplasm collection grown in various locations representing ecozones where cassava is cultivated. Most notable among these characteristics are the high photosynthetic capacity of cassava leaves in favorable environments and the maintenance of reasonable rates throughout prolonged water deficits, a crucial characteristic for high and sustainable productivity. Cassava possess a tight stomatal control over leaf gas exchange that reduces water losses when plants are subjected to soil water deficits as well as to high atmospheric evaporative demands, thus protecting leaves from severe dehydration. During prolonged water deficits, cassava reduces its canopy by shedding older leaves and forming smaller new leaves leading to less light interception, another adaptive trait to drought. Though root yield is reduced (but much less than the reduction in top growth) under water stress, the crop can recover when water becomes available by rapidly forming new canopy leaves with much higher photosynthetic rates compared to unstressed crops, thus compensating for yield losses with final yields approaching those in well-watered crops. Cassava can extract slowly water from deep soils, a characteristic of paramount importance in seasonally dry and semiarid environments where deeply stored water needs to be tapped. Screening large accessions under seasonally dry and semiarid environments showed that yield is significantly correlated with upper canopy leaf photosynthetic rates, and the association was attributed mainly to nonstomatal (anatomical/biochemical) factors. Parental materials with both high yields and photosynthetic rates were identified for incorporation into breeding and selection programs for cultivars adapted to prolonged drought coupled with high temperatures and dry air, conditions that might be further aggravated by global climate changes in tropical regions. Key words: agriculture, breeding, canopy, carboxylation enzymes, C 3 -C 4 intermediate, ecophysiology, growth, leaf photosynthesis, Manihot sp., PEP carboxylase, productivity, stomata, water stress, yield Características fisiológicas da tolerância da mandioca a secas prolongadas nos trópicos: Implicações para o melhoramento de cultivares adaptadas a ambientes semi-áridos e sazonalmente secos: Este artigo sumaria resultados de pesquisas conduzidas no Centro Internacional para Agricultura Tropical (CIAT), sobre as respostas da mandioca a períodos prolongados de déficit hídrico, no campo, centrando-se em...

Towards understanding factors that govern fertilizer response in cassava: lessons from East Africa

“…In addition the crop has the ability to recover from a seasonal drought period and compensate for its adverse effects through an increase in leaf canopy area and by higher photosynthetic rates in the newly developed leaves (El-Sharkawy 2007, 2004Alves 2002;De Tafur et al 1997). Thus, water stress before 3-4 MAP reduces the response to fertilizer as it limits the formation of additional sink (i.e.…”

Productivity and drought tolerance of cassava cultivars in the Coastal Tablelands of Northeastern Brazil

“…Above mentioned cultivars, 'BRS Poti Branca', had the highest average assimilation rate (Table 2). According to EL-SHARKAWY (2007), the photosynthetic process is not inhibited on cassava plants, which are subjected to water deficit because of its inherent tolerance to drought. It is noteworthy also that 'BRS Kiriris' plants lost less water by evaporation and; therefore, had greater efficiency in water use, while 'BRS Jarina' lost more water by evaporation.…”

Transpiração e crescimento foliar de plantas de mandioca em resposta ao deficit hídrico no solo

“…Quando se compara esse valor com os obtidos para outras culturas agrícolas (Sinclair & Ludlow, 1986;Amir & Sinclair, 1991;Muchow & Sinclair, 1991;Ray & Sinclair, 1997;Ray et al, 2002;Davatgar et al, 2009), confirma-se que a mandioca tem meio de controle estomático mais eficiente do que a maioria delas (El-Sharkawy et al, 1989;Alves & Setter, 2000;El-Sharkawy, 2007), o que lhe confere uma posição de destaque como cultura tolerante ao deficit hídrico. Em razão das mudanças climáticas projetadas para o século à frente, no qual eventos extremos como ondas de calor e períodos sem chuva devem se intensificar (Intergovernmental Panel on Climate Change, 2007), e de que a década de 2000 foi caracterizada por uma alta frequência de períodos sem chuva ), a cultura da mandioca tenderá a ganhar destaque pela sustentabilidade, especialmente das pequenas propriedades familiares do Rio Grande do Sul.…”

“…Em condições de deficit de água no solo ou de alta demanda evaporativa da atmosfera, as plantas de mandioca apresentam controle estomático bastante eficiente (El-Sharkawy et al, 1989;Alves & Setter, 2000, 2004El-Sharkawy, 2007). Contudo, a redução da abertura estomática pode limitar a taxa de difusão de CO 2 para o interior da folha, com efeitos diretos sobre a fotossíntese e o crescimento (Alves, 2002;El-Sharkawy, 2007).…”

“…Contudo, a redução da abertura estomática pode limitar a taxa de difusão de CO 2 para o interior da folha, com efeitos diretos sobre a fotossíntese e o crescimento (Alves, 2002;El-Sharkawy, 2007). Outra característica de adaptação ao deficit hídrico, em plantas de mandioca, é a redução da área foliar pela perda das folhas basais mais velhas (El-Sharkawy, 2007). Variedades de mandioca podem diferir quanto à tolerância ao deficit hídrico, o que abre oportunidade para seleção genética quanto a esse caráter.…”