Physiological and morphological responses of farmer preferred cassava (Manihot esculenta Crantz) cultivars to drought stress in Uganda
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Cassava is the most important staple food crop in the tropics feeding over 800 million people, mainly resource poor small scale-farmers. In Uganda, cassava ranks 2nd to bananas in terms of total production and per capita consumption but ranks 2nd to maize in production area, (UCA, 2010) and it is the leading food security crop. Cassava is commonly cultivated in areas considered marginal for most other crops, with low-fertility soils and annual rainfall of less than 600 mm in semi-arid environments thus making it vulnerable to drought stress. Drought tolerance mechanisms in cassava and strategies for its management developed elsewhere, but no such information was available in Uganda. The main objective of this study was to assess the physiological and morphological responses of cassava cultivars in Uganda to drought. A total of 56 cassava cultivars were collected from 17 districts in Uganda covering the major agro-ecological zones of the country. One check MM96/0686 known to be drought tolerant was acquired from International Institute for Tropical Agriculture (IITA) and included in the collections. The 56 cultivars were evaluated in the field in a randomized complete block design (RCBD) with two replicates in two drought prone areas of Nakasogola and Bullisa and one location with normal rainfall regime at Makerere University Agricultural Research Institute Kabanyolo (MUARIK). Based on their performance 5 month after planting (MAP), 15 cultivars were identified for rain-out shade study at MUARIK. Cultivars contrasting in their response to drought stress were selected based on number of surviving plants after three month of constant drought stress for detailed studies. In the rain-out shade, plants were established in a RCBD with 2 replicates in large polythene bags of 80 cm deep and 60 cm diameter. All experiments received adequate water supply by adding 5L of water once every two days for 1 MAP and thereafter adjusted to two watering regimes 1) stressed treatment (5L of water once every three weeks) and 2) non-stressed treatment (5L of water twice every week). Data was collected on crop establishment, plant height, leaf area and leaf retention, number of fibrous roots, fibrous root length, number of storage roots, fresh storage root weight, stomatal conductance, xylem pressure potential, shoot weight and harvest index. These assessments were done on five plants per cultivar per treatment starting one MAP at a month interval except for root data, shoot weight and harvest index that were collected at harvest. Results from field experiments indicated significant differences among cultivars as evaluated across locations for the number of plants that survived and degree of wilting. Genotype by environment (GxE) interaction was highly significant for number of surviving plants and leaf wilting. The cultivars which had the highest number of plants surviving in Bullisa and Nakasongola until end of the experiment (5 MAP) were considered drought tolerant, while those which had no surviving plant where considered most susceptible to drought stress. In the rain-out shade, cultivars showed significant reductions in morphological traits. Considerable reductions were in plant height (3-35%), shoot biomass (3-60%), leaf longevity (66-84%) and leaf area (38-56%), while significant increases were in fibrous root density (2-56%) and length (4-59%). Drought stress caused a general increase in xylem pressure potential (27-79%) of all the cultivars evaluated, while reducing stomatal conductance (57-86%) of these cultivars. Yield responses to soil moisture deficit also showed significant difference for all parameters measured. Analysis of variance of yield parameters of cultivars across the two water regimes indicated considerable significant differences for storage root number, storage root length, fresh storage root weight and harvest index (HI), while there was no significant difference for storage root circumference. There were also considerable variations in performances of cultivars across the two water regimes for all the yield parameters.Correlation analysis showed a significantly positive relationship of shoot weight with both storage root length (r=0.64*) and circumference (r=0.63*) In addition, leaf longevity had a positive significant relationship with harvest index (r=0.64*). In addition, there was positive significant correlation between leaf area and storage root length (r=0.66*) and storage root weight (r=0.76*) while there was negative significant correlation of leaf wilting with storage root circumference (r=-0.68*). Furthermore, there was also negative significant correlation between fine root number with harvest index (HI) (r=-0.68*) and drought response index (DRI) (r=-0.80**). Basing on the generated data, the major conclusion of this study is that although cassava is known to be drought tolerant, severe moisture stress has significant devastating effects on both vegetative growth and yield of cassava. In addition, it is also concluded that cultivars respond differently to drought stress with some being tolerant (MM96/0686, MM96/4271, omongole) while others were very susceptible to drought stress (Bamunanika, Bao and Nase 12). It can also be concluded that cassava tolerates drought stress by closing stomata, shading leaves, reducing plant growth, increasing fibrous root density and length, but unfortunately all these tolerance mechanisms are at the expense of yield. The existence of significant variations among cultivars in the mechanisms of tolerating drought offers opportunities for genetic improvement for these important traits. Breeders should therefore accumulate genes for drought tolerance in breeding materials to develop cultivars with high yield potential for dry ecologies.