Nutrients regulate formation of tuberous storage roots in cassava Per-Olof Lundquist1, Monika Kähr1, Elizabeth Balyejusa Kizito1,2 and Anna Westerbergh1 1Dept of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, Uppsala, Sweden 2Med Biotech Laboratories, Kampala, Uganda

Dept of Plant Biology and Forest Genetics Swedish University of Agricultural Sciences Uppsala, Sweden Funding from The Swedish Agency for International Development Cooperation (Sida). Collaborations with CIAT, Colombia, and researchers in Uganda. Cassava research groups: Anna Westerbergh - genetic diversity, quantitative genetics, QTL mapping Christer Jansson, Chuanxin Sun - starch metabolism and gene regulation Linley Karltun - food security earlier Urban Gullberg - genetic diversity, quantitative genetics 4 PhDs, students from Uganda and Malawi, BIO/EARN project

The experiments were done for two reasons: Nutrients regulate formation of tuberous storage roots in cassava The experiments were done for two reasons: Evaluate how plant cultivation techniques affect plant growth and storage root formation in greenhouse conditions. Learn about how storage root formation is regulated in relation to availability of mineral nutrients. Hypothesis excess mineral nutrients (e.g. NPK) prevents formation of storage roots. .

Our growth conditions and treatments in short Computer-controlled growth unit Rooted cuttings of cassava (one clone of offspring of the selfed accession MTAI 8) Small pot (0.25 l) Big pot (9.5 l) Nutrient solution highly diluted and continuously sprayed to create a mist. Substrate: mineral wool Nutrient solution added at regular intervals. Nutrients available for the plant determined by the water-holding capacity of the substrate. Nutrient content in the solution determined as conductivity and pH every 10 min and adjusted immediately to give essentially free access to nutrients. Nutrient added at a programmed rate.

Theory about plant growth Plants will grow exponentially if they have free access to nutrients. Growth rate limited by genetic capacity in the given environment. Relative growth rate (RGR) = increase in biomass per unit biomass per unit of time Expressed as “mg g-1 d-1“ or as “% per day”.

Nutrient addition and plant growth Relative addition rate (RAR) Control plant growth at a specified rate, by adding nutrients frequently and at a defined rate. For example, if nutrients are added at a relative addition rate of 10 % increase per day, the plants can be controlled to grow at the same relative growth rate (RGR).

Nutrient additions in our experiments Big pots Small pots Becomes nutrient-limited at some point in time. Pots: Every 3rd day, constant amount every time (100 ppm N, 20 P, 86 K). Difference in added amounts of nutrients between big and small pots is due to different water-holding capacity. Free access Nutrient limitation, RAR 10% Nutrient limitation, RAR 10%- 5% Growth units: Programmed to be: Free access (FA) to nutrients FA - 10 % exponential increase FA - 10 – 5 % exponential increase

Results: Nutrient availability Growth rate The response of the plants in the growth units was as expected for nutrient-limited plants. Allocation to total root biomass

Results: Shoot growth Shoot weight at harvest Time: 49 days Time: 49 days 34 d 19+34 d 19+12+22 d = 52 d = 52 d

Storage roots expressed per shoot weight FA FA-10-5 Decreasing nutrient availability Storage roots

Conclusions Production of storage roots is variable depending on nutrient availability. Storage roots are formed when mineral nutrients become limiting for overall plant growth. Hypothesis that there could be an optimum fertilizer level and/or pot size that would allow good shoot growth with high photosynthetic capacity that when the fertilizer is used up would allow a high production of storage roots and starch loading of storage roots. Vegetative shoot and root growth has higher sink-strength for the carbon produced through photosynthesis compared to storage roots when mineral nutrients are freely available. Overall, the findings are consistent with cassava being cultivated in the field in soils with low nutrient contents.

Conclusions, continued For controlled studies of storage root traits and other cassava traits, defined model cultivation systems are useful. - Relevant for any kind of phenotype characterization (biochemical analyses, gene expression, development, morphology etc).

Acknowledgements Technical help from Mr Jan Parsby, SLU. Funding from The Swedish Agency for International Development Cooperation (Sida).

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