Analysis of Chemical Composition of Cowpea Floral Volatiles and Nectar

Abstract

Cowpea is one of the 14 species of grain legumes. It rewards insects by producing nectar and it advertises to the pollinators by producing floral volatiles. The volatiles act as cues that guide insect pollinators in terms of pollen and nectar. Floral volatile quality also influences the efficiency of pollination. Genetic manipulations involving selection of varieties with high quality floral volatiles and nectar can therefore increase pollination efficiency and hence cowpea yields. It is believed that efficiency of insect pollination in several food crops is dependent on the quality of floral volatiles. Several molecules, including allozyme, co-dominant and isozyme DNA molecular markers (AFLP, RAPD and SSR) among others are useful in the selection of disease resistant and high yielding food crop cultivars in breeding programmes. Floral volatiles and nectar profiles can therefore act as molecular markers in cowpea breeding programmes. This project involved the collection, analysis and characterization of cowpea floral nectar and volatile composition. The flowers bloomed for one day and nectar was secreted between 6.00 and 10.00 am East African time. The flower sizes in the six selected cultivars were measured, nectar withdrawn using microlitre syringe and its characteristics (volume and sugar composition) examined. The sugar composition of the nectar was analyzed using HPLC, LC-MS and co-injection with authentic standards. Hydro-distillation and static headspace trapping with adsorbents (activated charcoal, reverse-phase, C18 bonded silica and porapak Q) was done in the six selected cultivars and volatiles concentrated using gentle stream of N2 while cooling under ice. GC was used to analyze the composition of floral volatiles and GC-MS for identification of the components. Co-injection with authentic standards was used to confirm identity of the components. Nectar volume varied as a function of time. A correlation between nectar production and time was observed. There were significant quantitative differences in the volumes of nectar produced in the different cowpea cultivars. The highest volume collected (18 μl) was from cultivar 219, with a mean of 7.99 ± 0.78 μl and the lowest recorded value (0.2 μl) was from SP46 with a mean of 3.65 ± 0.59 μl. The cultivars showed similar trends in the rate of reduction in the volumes of nectar produced with time. Sucrose (0.104 ± 0.099 mg), glucose (0.0224 ± 0.006 mg), and fructose (0.0225 ± 0.012 mg) occured frequently in the nectars. Lactose (0.003 ± 0.001 mg), raffinose (0.004 ± 0.002 mg), and mannose (0.006 ± 0.004 mg) were present in trace amounts. The biggest flower size recorded (47 mm) was from cultivar 219 with a mean of 42.62 ± 0.65 mm while the smallest size (10 mm) was from SP46, with a mean of 17.13 ± 0.65 mm. The nectar volume is directly proportional to the flower size. The total number of trapped volatile compounds ranged from 43-109 for headspace trapping and hydrodistillation. Porapak Q trapped the largest number of compounds. Quantitative and qualitative differences in volatile composition of various cultivars were noted. Aliphatic compounds were the most abundant followed by benzenoids, monoterpenes, sesquiterpenes, norisoprenoids and other compounds. The most common cowpea floral volatiles were toluene, 1-hexanol, benzaldehyde, acetophenone, limonene, 1-octen-3-ol, artemisia alcohol and nerolidol. Palmitic acid was the most abundant component of steam distillates. Due to the high protein content, grain legumes should help in reducing protein deficiency cases in developing countries. Improved legume yields should be encouraged to address protein deficiency in children. There is need to intensify production of cowpea by developing more efficient and well adapted varieties with good pest and disease resistance through biotechnology.