Assessing drought resistance in selected Vigna unguiculata lines using phenotypical and physiological criteria under controlled conditions

Abstract

The natural resources in Africa are under enormous strain to support its human and animal population. Drought is one of the major factors contributing to this dilemma. In the selection of crops it is important to use plants that are well adapted to the climatic - and growth conditions in order to secure food production. One crop that can fulfil this niche is cowpeas (Vigna unguiculata). This crop originated in Africa and is well adapted to a variety of growth conditions. Cowpeas are also part of the Fabaceae family, which means it can act as a source of plant protein, but apart from other good qualities, also has the unique quality of being a nitrogen fixing plant. This means it can also play a role in soil improvement. Cowpeas are regarded as a drought resistant crop but a lot of variation occurs within the genotype. The problem that needed to be solved by this study was to find selection methods that could be used to screen large numbers of plants for drought resistance and still give accurate results. The research project was premised on the belief that intra-specific differences in response to drought could be used to enhance the understanding of the physiological and phenotypical basis of drought resistance and that this knowledge could be used to identify practical screening parameters. This knowledge contribution will help with the expansion of the crop and thus be of economic importance. The selection of the right genotype will be of value for commercial farmers. It will also have a positive impact on subsistence farmers in areas with low precipitation or varied rainfall patterns. For this study five cowpea lines were selected. Two of the lines were obtained from the International Institute of Tropical Agriculture (UTA) in Nigeria, and are known to be drought resistant (IT96D-602) and drought susceptible (TVu7778). These two lines were used as indicator lines. The other three lines were selected on the grounds of their high (IT90K59 and IT92K258-9) or low (Chappy) yield obtained under field conditions in South Africa. These selected cowpea lines were subjected to drought stress by withholding water after which various screening methods were used to determine the levels of drought resistance. These included phenotypical observations in the greenhouse as well as biochemical and physiological screening in the laboratory. The experiments conducted in the greenhouse was mainly to determine the root architecture of the different lines as well as screening of seedlings for drought resistance using the woodenbox technique of Dr. Singh of UTA, Nigeria.

Other screening methods used were assessment of PSII function by chlorophyll fluorescence, chlorophyll a en b concentrations, cell membrane stability, relative water content, leaf water potential and free proline levels. The triphenyltetrazolium chloride reduction assay for determining viability / vitality under drought stress conditions was also included as well as the determination of the activity of the enzymes of the antioxidative system. The enzyme activity was measured to see if this is a strategy used by cowpeas to handle the adverse conditions of drought stress. The yield of the plants in the greenhouse and the size of the leaves of stressed and control plants were also determined. These plants were monitored over a 17 days of drought stress period. In most cases the results of the different screening methods showed a very good correlation and also confirmed the drought ranking reported by the IITA. The screening methods for which the data showed the best correlation with each other were the JIP-test ( chlorophyll fluorescence measurements), chlorophyll a & b levels, the RWC, the woodenbox technique, yield of the greenhouse plants and root architecture. The determination of the free proline levels and L WP showed a relatively good correlation with the above mentioned screening methods. The TTC reduction assay, leaf area measurements as well as the ascorbate peroxidase activity all agreed on the ranking of IT96D-602 as the number one or two line but the ranking of the other lines varied a lot according to these three tests. The other two screening tests of the enzymes of the antioxidative system (SOD and GR) and the CMS did not show differences in the drought resistance levels of the cowpea lines. The information gathered in this study contributes towards a better understanding of the physiological and morphological basis of drought resistance in cowpeas. Although all the plants had similar cell membrane stability, the plants of IT96D-602 were able to maintain a higher relative water content and water potential than the plants of all the other lines. This improved the viability of the plants, which was evident from the TTC reduction assay and the chlorophyll fluorescence values. The fact that IT96D-602 was experiencing lower stress levels than the other lines was also evident from the lower free proline levels and the preservation of more Chl a & b molecules than the other plants. Some of the qualities that were detected in IT96D-602, which might have contributed to the water retention, were

smaller leaves and a more upright growth form. IT96D-602 also possesses a more extensive root system than the other lines, which will help with water uptake. According to our results, methods suitable for screening large numbers of cowpea plants for drought resistance, are the chlorophyll fluorescence (HP-test), the free proline levels and the woodenbox screening for drought resistance at the seedling stage. The relative water content has also proven to be a good measure of water stress. The cowpea lines used in this study could successfully be ranked according to their drought resistance using above-mentioned techniques. The ranking was as follows: IT96D-602 > IT90K59 > IT92K258-9 > Chappy > TVu7778.