Ecophysiological analysis and modeling of genotype by environment by crop management interactions on cotton (Gossypium hirsutum L.) in Cameroon for the design of ideotypes

Cotton lint is the first natural fiber used in the world. Cotton provides income to more than 10 million persons in West and Central Africa. In Cameroon, it is produced under rainfed conditions and water shortage is the major abiotic factor limiting yield and lint quality. In this context, a breeding program was initiated in 1950 by IRCT (Institut de Recherches du Coton et des Textiles Exotiques) to increase lint yield, fiber quality and disease resistance. After 60 years, this program has released more than 20 cultivars. However, seed cotton yield has been levelling off for more than thirty years. This study analyzed growth and development of main cultivars released from 1950 to 2009 to evaluate genetic gain including drought adaptation traits indirectly bred for. It also analyzed genotype by environment by crop management interactions (GEI) under water limited conditions in order to use a cotton simulation model in Cameroonian conditions. Then, crop simulation model was used to design cotton ideotypes under Cameroonian cropping conditions. An application of this work was in identifying the measurements of key drought adaptation traits to breed for in order to create cultivars that better withstand water stress. Firstly, phenotype evolution over breeding time and its interaction with cropping conditions in Cameroon was evaluated on cotton development, growth (including roots), yield, and fiber quality. Ten major cultivars were studied under rainfed conditions (field) and controlled conditions (greenhouse and phytotron). Classical GEI analysis of variance of cultivars and regression over their respective year of release were done. The results showed that the breeding program succeeded in improving cotton lint yield and the potential of fiber quality when the crop reached physiological maturity before the end of the rainy season. In late season drought, breeding reduced the fiber quality (fiber length, uniformity and strength). Most of the development and growth variables did not change with time, except the number of leaves which reduced. Breeding created cultivars with better potential fiber production and quality, but with reduced plasticity to sub-optimal environments and access to soil water. Secondly, an analysis of GEI for ecophysiological traits conferring a good response to drought was done in good and water limited conditions for a subset of four cultivars. The results indicated that water deficit had a negative impact on almost all plant functions, both under field and controlled environments. The recent cultivar L484 bred for the driest production area had the fastest development, thickest leaves with most chlorophyll and thus maintained the highest level of photosynthesis and transpiration per unit of leaf area in water-limited conditions. In these conditions, L484 had the highest radiation use efficiency and water use efficiency maintenances. Despite these traits this cultivar did not show any improvement in terms of biomass, harvest index and cotton yield across water conditions. Cotton breeding program in Cameroon succeeded in providing a cultivar (L484) better adapted to local conditions, with a higher stability and faster development coupled with a strategy of growth maintenance, without any improvement in yield. Thirdly, the crop simulation model DSSAT CROPGRO-Cotton was used in order to design ideotypes with higher yield than existing cultivars. Field experiments in Cameroon were used to constitute the minimum dataset for the crop model calibration. Forty-two virtual cultivars were generated by increasing or decreasing by 20% from genetic parameters values of cultivar L484 on phenology, photosynthesis, and leaf morphology. Then, cultivars AC, L484 and these virtual cultivars were compared across 99 years of generated weather from WGEN model in two locations. Compared to L484, the cotton ideotypes in Cameroonian rainfed conditions had reduced emergence to anthesis duration, longer reproductive duration, higher level of photosynthesis maximum with thicker leaves, and smaller leaves for Far North region or bigger ones for North region.