Ecophysiological analysis and modeling of genotype by environment by crop management interactions on cotton (gossypium hirsutum l.) in cameroon for the design of ideotypes

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Author

ROMAIN LOISON

CIRAD – France

Abstract

Cotton lint is the first natural fiber used in the world. Cotton provides income to more than 10 million people in West and Central Africa. In Cameroon, it is produced in 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 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-date 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 providing key drought adaptation traits to breed for 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. Then, cultivars AC, L484 and forty-two virtual cultivars with ±20% from L484 parameter values were compared across 99 years of generated weather 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.

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SOWING DATE AND NITROGEN APPLICATION EFFECTS ON PETIOLE NITRATE CONTENT IN CAMEROON

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Studies were conducted in Maroua under Sudano-Sahelian conditions to estimate the effects of sowing date and nitrogen application on petiole nitrate content of cotton. The dry weight and petiole nitrate content of the 4th leaf from the top of the canopy of cotton plants were measured at three day intervals. During the observation period and for all treatments, the petiole had a rapid weight gain phase followed by constant weight phase.  Application of nitrogen on the early sowing date did not significantly increase the petiole nitrate content when compared with the control. On the other hand, the application of nitrogen on the late sowing date and before flowering, increased the petiole nitrate content but this remained low when compared with that of the early sowing date of the same age. The change in nitrate content observed was not linked to the increase in weight of the fourth leaf. In Maroua conditions, the change in petiole nitrate content during the vegetative cycle differs from that described in the literature.

Discussion and Conclusion

Nitrogen application had an effect on the petiole weight when cotton was sown late, which confirms knowledge on the effect of nitrogen. However, the absence of weight differences between control and nitrogen application for the early sowing date indicates that  there was no effect on the growth of the organ. Sowing date and nitrogen application have an important action on the growth of leaves and determining leaf number on the main stem.  The increase in petiole nitrate content while the weight of this organ is also increasing shows that the nitrate content is not at this time affected by the dry matter increase.  The decrease observed during the flowering period seems to be related to important needs of nitrogen at each flower or boll. The late application of nitrogen is not sufficient to increase the level of petiole nitrate content as occurs during the early sowing date. Observations by Richard (1980) are not confirmed when cotton is sown earlier this shows that petiole nitrate content depended more on the quantity of nitrogen applied, sowing date and date of nitrogen application than on the increase of dry matter weight.  This can be helpful in the interpretation of results of the diagnosis.

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NITROGEN, PHOSPHORUS AND POTASSIUM AVAILABILITY IN THE SOIL, PHYSIOLOGY OF THE ASSIMILATION AND USE OF THESE NUTRIENTS BY THE PLANT

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Cotton has an indeterminant growth habit and an extreme sensitivity to adverse environmental conditions which can result in excess fruit abscission. Because of these characteristics, during most of the time this plant has to assume simultaneously flowering, canopy and boll development, until any limiting factors (water, temperature, light or nutrient stress) occurs, producing shedding and limiting lint and seed yields. The effects of N, P and K nutrients on vegetative growth, flowering, fruiting and lint or seed production have often been described. Nevertheless the synchronism of these process implies that it is necessary to supply every nutrient throughout the growth cycle of cotton. Then the mineral nutrition of cotton depends both on the cotton roots ability to explore the soil and on the soil ability to supply N, P and K nutrients.

The exploration of a large volume of soils favours high nutrition in all elements. The failure in root growth can be put down to physical conditions, excess of water, plant diseases and also because of chemical factors: toxicity (aluminium in acid soils) or shortage of plant vigour like water stress, low temperature and solar radiation, bad sanitary conditions etc.

The soils N, P and K supplies are in dynamic equilibrium between available forms (dissolved and easily dissolved substances) and reserves which cannot be absorbed directly. These balances are linked to physical, biological and chemical conditions in soil.

The N, P and K in fertilizers are added to the soil mineral “pool”. Many documents present responses of cotton and/or soil to N, P and K fertilizers. However, there is a large response variability to fertilizers: variability between regions, years, sowing dates, etc.

The cotton price fluctuations and environment problems should encourage better management of fertilizer, using the physiological and agronomical knowledge of N, P and K dynamics. We must search for an economic optimum and take the long term future of the farm into account.  This management must be done firstly by cropping techniques to improve the root’s ability to explore the soil.

Fertilization must also be managed at the historical level. The cotton field benefits or suffers from the previous crop or fallow and from technical practices on the field. Fertilization of cotton also influences the following crops, the sustainability of the fields and pollution.

The fertilization must be managed at a geographical level. Nutrients transfer may occurs from some place in the landscape to another by wind, livestock, stream, harvest removals, etc. on the vertical axis, trees and some cover crops may absorb drained elements from deeper levels of soil and deliver them to the surface.

Then fertilization is now a systems problem and research has to quantify the relationships between all the factors of the fertilization and create models in order to help the farmers to manage their crop.

Conclusion

Agricultural science is currently changing rapidly.  For several years, it has been developing into an ecophysiology.  ECO because it takes into account what is happening within the natural environment and PHYSIOLOGICAL because attempts are being made to understand yield determination through increasingly fine-tuned analyses.  This change has been made possible through the extraordinary advances made in computerisation, which have enabled the modelling of relations and the development of yield determination models that take into account the crop system as a whole.  In fact, the plant and the field are immersed in a set of relations that have to be taken into account to provide farmers with fertilizer recommendations.  Consideration of economics in the advice given is also destined to develop.

Of course, this change is still new and not all the systems have been described, validated or quantified, but it is highly likely that as it proceeds it will completely transform how recommendations for farmers are established.

In many cases, the search for optimum yields has been superseded by that for a response to farmers’ economic and social requirements.

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