479
the local landrace (Table 2). This corresponds with previous findings that cowpea
and soybean nodulation is greatly dependent on the crop genotype. Similar results
were recently reported among common bean genotypes (Argaw and Muleta 2018;
Motaroki et al. 2018) which could be could be attributed to differences in genetic
makeup of the various genotypes.
There was a significant difference (p < 0.0001) in shoot and root dry weight
between the farms. Cowpea crops grown in soils from farm 8 recorded the highest
shoot and root dry weights, while those grown in soils from farms 10 and 3 had the
lowest shoot and root dry weights, respectively. Moreover, root dry weights were
significantly different (p = 0.04) between the cowpea varieties, where variety C1
and C2 had the highest shoot and root dry weights (Table 2). Although cowpea varieties C3 and C4 had a high number of nodules, this did not translate to higher shoot
dry weights. This could be attributed to the presence of rhizobia isolates that are
parasitic (deleterious) to the plant and do not fix nitrogen effectively. This is corroborated by Jida and Assefa (2011), who reported that rhizobia effectiveness cannot be sufficiently determined by nodule dry weight and nodule number; hence, in
the rhizobia legume association, rhizobia may infect the host plant but may not be
effective in nitrogen fixation.
Relationship Between Soil Characteristics, Cowpea Nodulation and Plant
Growth Parameters
Soil characteristics varied across the farms. This may explain the differences
observed in shoot dry weights, root dry weights, nodule number and nodule dry
weights. Ulzen et al. (2016) reported that a deficiency of nutrients in soil, and other
soil parameters such soil pH and temperature, affected nitrogen fixation and ultimately nodulation in cowpeas. This is also consistent with the findings by Slattery
and Pearce (2002) who, after working with 6 legumes as trap hosts for rhizobia
(field pea [Pisum sativum], faba bean [Vicia faba], lentil [Lens culinaris], vetch
[Vicia sativa], chickpea [Cicer arietinum] and lupin [Lupinus angustifolius]),
reported that soil characteristics such as soil pH, nutrients, temperature and osmotic
stress affect the symbiotic relationships between rhizobia and legumes.
Based on RDA, available phosphates in the soil had a great influence on nodule
number and nodule dry weight (Fig. 1). Nitrogen fixation process requires about
25–30 mole of ATP per mole of nitrogen fixed. As a result, nodules provide a strong
sink for phosphorus. Moreover, phosphorus increases the overall nutrition of legume
crops including nodule number and shoot and root dry weights. According to Bargaz
et al. (2012), high available P in the soil stimulates nodulation in legumes, thus helping the plants overcome the inhibitory effects of high N on nodulation.
Potassium availability also correlated closely with high shoot and root dry
weights. The shoot dry weights and root dry weights also closely correlated with
available phosphates and the nodule number. However, the shoot and root dry
weights were closely influenced by the additive effect of soil pH, calcium ions,
sodium ions, nitrogen and soil organic matter (Fig. 1).
Exploiting Arbuscular Mycorrhizal Fungi-Rhizobia-Legume Symbiosis to Increase…
the local landrace (Table 2). This corresponds with previous findings that cowpea
and soybean nodulation is greatly dependent on the crop genotype. Similar results
were recently reported among common bean genotypes (Argaw and Muleta 2018;
Motaroki et al. 2018) which could be could be attributed to differences in genetic
makeup of the various genotypes.
There was a significant difference (p < 0.0001) in shoot and root dry weight
between the farms. Cowpea crops grown in soils from farm 8 recorded the highest
shoot and root dry weights, while those grown in soils from farms 10 and 3 had the
lowest shoot and root dry weights, respectively. Moreover, root dry weights were
significantly different (p = 0.04) between the cowpea varieties, where variety C1
and C2 had the highest shoot and root dry weights (Table 2). Although cowpea varieties C3 and C4 had a high number of nodules, this did not translate to higher shoot
dry weights. This could be attributed to the presence of rhizobia isolates that are
parasitic (deleterious) to the plant and do not fix nitrogen effectively. This is corroborated by Jida and Assefa (2011), who reported that rhizobia effectiveness cannot be sufficiently determined by nodule dry weight and nodule number; hence, in
the rhizobia legume association, rhizobia may infect the host plant but may not be
effective in nitrogen fixation.
Relationship Between Soil Characteristics, Cowpea Nodulation and Plant
Growth Parameters
Soil characteristics varied across the farms. This may explain the differences
observed in shoot dry weights, root dry weights, nodule number and nodule dry
weights. Ulzen et al. (2016) reported that a deficiency of nutrients in soil, and other
soil parameters such soil pH and temperature, affected nitrogen fixation and ultimately nodulation in cowpeas. This is also consistent with the findings by Slattery
and Pearce (2002) who, after working with 6 legumes as trap hosts for rhizobia
(field pea [Pisum sativum], faba bean [Vicia faba], lentil [Lens culinaris], vetch
[Vicia sativa], chickpea [Cicer arietinum] and lupin [Lupinus angustifolius]),
reported that soil characteristics such as soil pH, nutrients, temperature and osmotic
stress affect the symbiotic relationships between rhizobia and legumes.
Based on RDA, available phosphates in the soil had a great influence on nodule
number and nodule dry weight (Fig. 1). Nitrogen fixation process requires about
25–30 mole of ATP per mole of nitrogen fixed. As a result, nodules provide a strong
sink for phosphorus. Moreover, phosphorus increases the overall nutrition of legume
crops including nodule number and shoot and root dry weights. According to Bargaz
et al. (2012), high available P in the soil stimulates nodulation in legumes, thus helping the plants overcome the inhibitory effects of high N on nodulation.
Potassium availability also correlated closely with high shoot and root dry
weights. The shoot dry weights and root dry weights also closely correlated with
available phosphates and the nodule number. However, the shoot and root dry
weights were closely influenced by the additive effect of soil pH, calcium ions,
sodium ions, nitrogen and soil organic matter (Fig. 1).
Exploiting Arbuscular Mycorrhizal Fungi-Rhizobia-Legume Symbiosis to Increase…