Introduction Intercropping offers numerous benefits that can be leveraged to increase crop yields. This study assessed gas exchange parameters, leaf chlorophyll content, water-use efficiency (WUE), carbon assimilation (C assimilation), and shoot dry-matter yield of six selected legumes grown under monocropping and intercropping (maize-legume) in Eswatini. Methods Instantaneous water-use efficiency (WUEi) was estimated as the ratio of photosynthetic rate to stomatal conductance from gas-exchange measurements, while the δ 13 C natural abundance technique was used as an integrated estimate of WUE. Results Legumes with high photosynthetic rates, stomatal conductance, and leaf chlorophyll content also exhibited high WUEi and vice versa; this finding warrants further exploration to increase crop production under water-stress conditions. Legumes grown under monocropping had high photosynthetic rates (19.66 μmol CO 2 m −2 s −1 ) and leaf chlorophyll content (2.98 mg g −1 ) when compared to legumes grown under maize-legume intercropping (16.43 μmol CO 2 m −2 s −1 and 2.38 mg g −1 , respectively), an observation that highlights the impact of different cropping systems on these parameters. The low photosynthetic rates and leaf chlorophyll content observed under maize–legume intercropping can be attributed to reduced light penetration due to maize shading. The observed lower photosynthesis and chlorophyll under intercropping are consistent with shading by maize; such conditions could reduce symbiotic N acquisition, but N₂ fixation was not directly measured here. As expected, the test legumes recorded low C:N ratios, with values between 12.67 and 21.44 g g −1 , which are within the range reported for most symbiotic legumes. A low C:N ratio in nodulated legumes is usually due to increased N 2 fixation and can promote faster decomposition of residues when used as green manure. Furthermore, sole legumes exhibited less negative shoot δ 13 C (−27.85 ± 0.06‰)—indicative of higher integrated WUE—whereas intercrops had more negative δ 13 C (−28.60 ± 0.07‰), consistent with lower integrated WUE. Discussion The combined use of gas-exchange measurements and δ 13 C natural abundance effectively discriminated between high- and low-water-use-efficient plants. Thus, these results provide insights into the impact of the cropping system on gas-exchange parameters, leaf chlorophyll content, and WUE of the six selected legumes.
Ngwenya et al. (Thu,) studied this question.