Joachim H. J. R. Makoi

Effect of legume plant density and mixed culture on symbiotic N2 fixation in five cowpea (Vigna unguiculata L. Walp.) genotypes in South Africa

A field experiment involving two plant densities (83,333 and 166,666 plants per hectare), two cropping systems (monoculture and mixed culture) and five cowpea (Vigna unguiculata L. Walp.) genotypes (3 farmer-selected varieties: Bensogla, Sanzie and Omondaw, and 2 breeder-improved cultivars: ITH98-46 and TVuI509) was conducted for two years in 2005 and 2006 at Nietvoorbij (33°54S, 18°14E), Stellenbosch, South Africa, to evaluate the effect of these treatments on the growth and symbiotic performance of cowpea. The results showed that, of the five cowpea genotypes, plant growth and N2 fixation were significantly greater in the three farmer-selected varieties (Sanzie, Bensogla and Omondaw) relative to the two improved cultivars (ITH98-46 and TVuI509). Furthermore, plant growth and symbiotic performance (measured as tissue N concentration, plant N content,15N natural abundance and N-fixed) were significantly (P<-50.05) decreased by both high plant density and mixed culture (intercropping). However, the %Ndfa values were significantly (P<-50.05) increased by both high plant density and mixed culture compared to low plant density or monoculture (or monocropping). Whether under low or high plant density, the cv. Sanzie was found to accumulate significantly greater total N per plant in both 2005 and 2006, followed by the other two farmer varieties relative to the improved cultivars. Similarly, the actual amount of N-fixed was much greater in cv. Sanzie, followed by the other farmer varieties, under both low and high plant density. The data also showed better growth and greater symbiotic N yield in cowpea plants cultivated in monoculture (or low plant density) relative to those in mixed culture (or high plant density). Our data suggest that optimising legume density in cropping systems could potentially increase N2 fixation in cowpea, and significantly contribute to the N economy of agricultural soils in Africa.

Photosynthesis, water-use efficiency and δ13C of five cowpea genotypes grown in mixed culture and at different densities with sorghum

A field experiment involving two planting densities (83,333 and 166,666 plants per ha), two cropping systems (monoculture and mixed culture) and five cowpea [Vigna unguiculata L. (Walp.)] genotypes was conducted at Nietvoorbij (33°54S, 18°14E), Stellenbosch, South Africa, to select cowpea material with superior growth and water-use efficiency (WUE). The results showed significantly higher photosynthetic rates, stomatal conductance and transpiration in leaves of plants at low density and in monoculture due to greater chlorophyll (Chl) levels relative to those at high density and in mixed culture. As a result, C concentration in leaves and the amount of C, P, K, Ca, Mg, Fe, Cu, Zn, Mn, and B accumulated in shoots at low density and under monoculture were also much higher. Even though no marked differences in photosynthetic rates were found between and among the five cowpea genotypes, leaf C concentration and shoot C, P, K, Ca, Mg, Fe, Cu, Zn, Mn, and B contents differed considerably, with Sanzie exhibiting the highest C concentration and C, P, K, Ca, Mg, Fe, Cu, Zn, Mn, and B contents in shoots, followed by Bensogla and Omondaw, while ITH98-46 and TVu1509 had the lowest shoot concentration and contents of C, P, K, Ca, Mg, Fe, Cu, Zn, Mn, and B. WUE (calculated as photosynthate produced per unit water molecule transpired) was significantly greater in plants at low density and monoculture relative to those at high density and in mixed culture. Isotopic analysis revealed significant differences in δ13C values of sorghum [Sorghum bicolor L. (Moench.)] and cowpea, with higher δ13C values being obtained for plants at low density and in monoculture relative to those at high density or in mixed culture. The five cowpea genotypes also showed significant differences in δ13C values, with Sanzie exhibiting the most negative value (i.e. low WUE) and ITH98-46, the least negative δ13C value (i.e. high WUE). Whether measured isotopically or from gas-exchange studies, sorghum (a C4 species) exhibited much higher WUE relative to cowpea (a C3 species). Both correlation and regression analyses revealed a positive relationship between WUE from gas-exchange studies and δ13C values from isotopic analysis of cowpea and sorghum shoots.

Allelopathy as protectant, defence and growth stimulants in legume cereal mixed culture systems

Abstract In Africa, yield loss and low income to farmers is exacerbated by pests and notorious weeds manifested in different cropping systems. The fact that crop species may be allelopathic to common weeds, pathogens and insect pests has attracted considerable attention as an alternative strategy for weed and pest management in small-scale and low-input mixed culture systems. The use of crop plants with greater allelochemical production could limit the need for agrochemicals and conventional herbicides for plant protection and defence. As a result, plant growth and grain yield will be increased, leading to greater profit margins for farmers. This review discusses the possible role(s) of allelopathy as a key player in protection, defence and growth stimulants in legume cereal intercropping systems.

Elevated levels of acid and alkaline phosphatase activity in roots and rhizosphere of cowpea (vigna unguiculata L. Walp.) genotypes grown in mixed culture and at different densities with sorghum (Sorghum bicolor L.)

The aim of this study was to assess P acquisition efficiency in 5 cowpea genotypes in mixed culture and at different plant densities using assays of acid and alkaline phosphatase activity and measurement of P in organs. Five cowpea genotypes (2 improved cvv. ITH98-46 and TVu1509, and 3 farmer-selected varieties, namely Bensogla, Sanzie, and Omondaw) were grown in the field at 2 planting densities (83 333 and 166 666 plants/ha) under monoculture and mixed culture with sorghum during 2005 and 2006. Fresh plant roots and rhizosphere soils were collected during the 2 years of experimentation, and assayed for acid and alkaline phosphatase activity. P concentrations in root tissue and rhizosphere soil were also determined using inductively coupled plasma-mass spectrometry. The data for 2005 and 2006 were similar, and therefore pooled for statistical analysis. Our results showed that raising cowpea density from 83 333 to 166 666 plants/ha significantly increased both acid and alkaline phosphatase activity in the rhizosphere, just as mixed culture (or intercropping) also increased the acid and alkaline phosphatase activity in cowpea rhizosphere soil. High plant density and mixed culture (or intercropping) also raised the acid phosphatase activity in fresh roots of cowpea plants. The increased enzyme activity in roots and rhizosphere soil resulted in significantly improved P nutrition in cowpea, greater plant growth, and higher grain yield in the farmer-selected varieties, especially cv. Sanzie. This suggests that field-grown legumes can be screened for high P acquisition efficiency by assaying for acid and alkaline phosphatase activities.

Changes in rhizosphere concentration of mineral elements as affected by differences in root uptake and plant growth of five cowpea genotypes grown in mixed culture and at different densities with sorghum.

Aims: To evaluate the effect of planting density and cropping systems on the changes in rhizosphere concentration and uptake of mineral elements of five cowpea genotypes (i.e. Bensogla, ITH98-46, Sanzie, TVu1509 and Omondaw). Study Design: 3-factorial randomized complete block design. Original Research Article American Journal of Experimental Agriculture, 4(2): 193-214, 2014 194 Place and Duration of Study: Nietvoorbij (33o54S, 18o14E), Stellenbosch, South Africa during 2005 and 2006 summer seasons. Methodology: A field experiment involving two cowpea plant densities (83,333 and 166,666 plants.ha), two cropping systems (monocropping and intercropping) and five cowpea genotypes (i.e. Bensogla, ITH98-46, Sanzie, TVu1509 and Omondaw). Results: The data for 2005 and 2006 were similar, and therefore pooled for statistical analysis. The concentrations of P, K, S, Na, Cu, and Zn were lower in rhizosphere of cowpea relative to bulk soil, while those of Ca and Mg were greater in the rhizosphere compared with bulk soil. With sorghum, only K, S, and Na were lower in the rhizosphere, in contrast to P, Ca, Mg, Cu, and Zn, which were higher in the rhizosphere. These differences in mineral concentration were due to alteration in rhizosphere pH, which was increased by cowpea but unchanged by sorghum. The data also showed that high plant density (166,666 plants.ha) and mixed culture significantly decreased rhizosphere soil pH, resulting in low availability of P, K, Ca, Mg, Na, S, Fe, Cu, Zn, Mn and B in the rhizosphere of cowpea and sorghum compared with low plant density (83,333 plants.ha) or monocropping. The results also showed significant differences in rhizosphere concentration of minerals between and among the five cowpea genotypes, with cv. Sanzie consistently indicating much lower levels of P and Ca as a result of higher root uptake, which was evidenced by the higher tissue content of P, K, Ca, Mg, Na, S, Fe, Zn, Mn and B in cv. Sanzie. Conclusion: N2-fixing cowpea significantly lowered the concentration and increased the uptake of mineral elements from the rhizosphere soil relative to sorghum.

Symbiotic N Nutrition and Yield of Cowpea Genotypes under Different Plant Densities and Cropping Systems

Symbiotic legumes are an important component of the cropping systems in tropical environments because of their ability to transfer fixed-N to non-legume crops. The total amount of N-fixed per unit area in intercropping systems can be low due to sub-optimal legume plant density. The choice of legume cultivar can also influence the potential contribution of fixed-N to the cropping system. This study assessed the effect of plant density and different cowpea genotypes on N2 fixation in a sorghum-based cropping system.