Evaluation of morphophysiological, biochemical and yield responses of Bambara groundnut landraces to sodium chloride salinity

Abstract

Soil salinity limits crop growth, development and productivity in agricultural soils worldwide and contributes to food insecurity. It is induced by accumulation of high levels of particularly sodium and chloride ions within the rooting zone. Bambara groundnut {Vigna subterranea (L.) Verdc} is underutilized and fairly neglected indigenous African food legume with potential to alleviate food and nutritional insecurity in tropical regions of Africa. The crop has potential to be grown in semi-arid areas or under irrigation, both of which offer potentially saline conditions. The effect of sodium chloride (NaCl) salinity on growth and physiology of this plant continues to attract research, more so on the locally grown landraces. There are still unanswered questions on the effect of NaCl salinity on photosynthetic pigments, compatible solutes, nodulation and yield. Therefore, the overall objective of the study was to evaluate morphophysiological, biochemical and yield responses of Bambara groundnut landraces in response to NaCl salinity. The specific objectives were to determine the effects of NaCl salinity on growth, gas exchange (transpiration rate (Tr), stomatal conductance (gs) and C02 assimilation rate(Cr)), mineral nutrients (sodium, potassium and calcium), proline, leaf pigments (chlorophyll a, b, total chlorophyll and carotenoids), nodulation and yield parameters in Bambara. Experiments were laid out in a greenhouse at Maseno University in a completely randomized design, involving 15 factors of 5 NaCl salinity treatments: (0/control, 2, 4, 6 and 8) in dSm-1, and 3 Bambara groundnut landraces. There were 3 replications. Ten large similar sized seeds of three local landraces: red seed coat (RSC), white (WSC) and black (BSC) were each sown in a 20-liter pot containing moist loam soil with pH of 4.7. All the seeds were coated with Bradyrhizobium strain USDA 110, to enhance biological nitrogen fixation of the seedlings. Thinning was done 7 days after emergence, leaving 5 plants per pot. NaCl salinity treatments commenced 7 days after thinning. Data collection commenced on the 3rd day after initiating salinity treatments and was repeated after every 2 or 4 weeks. Plant height (PH), root length (RL), width and length of leaf were measured using a meter rule, and leaves and branches were counted. Seedling fresh and dry weights were measured using electronic weighing balance. Leaf area (LA), root to shoot ratio (R:S) and percent water content (%WC) were calculated. Gas exchange parameters were determined using portable infra-red gas analyzer. Plant mineral nutrient, proline and leaf pigment content were determined using atomic absorption spectrometry, colorimetric assay and spectrophotometer respectively. Nodules, pods and seeds were counted. Data were subjected to analysis of variance using the SAS Statistical Computer Package and separation of means using the Least Significance Difference at 5% level. Plant growth parameters, PH, LA, leaf number, shoot and root fresh weights of the three landraces significantly (p≤0.05) reduced as NaCl salinity increased. Increase in salinity had adverse effect on leaf pigments, and number of pods and seeds in all the landraces. Salinity did not influence root dry weight (RDW) of the RSC and BSC landraces. There were significant (p ≤ 0.05) interactions between NaCl salinity treatments and Bambara such that the number of branches and Na+ content increased in all the landraces. Salinity increased shoot dry weight (SDW) in salinity treatments of 2, 4 and 6 dSm-1 in RSC landrace, RDWin salinity treatments of 4, 6 and 8 dSm-1 in WSC landrace, RL in salinity treatment of 2 dSm-1in all the landraces, R: S ratio in salinity treatment of 4 dSm-1 in WSC and BSC land races, and 2, 4 and 6 dSm-1 in RSC landrace, and %WC in salinity treatment of 2 dSm-1 in WSC and 4 dSm-1in BSC landrace. Salinity also increased K+ and Ca 2+in salinity treatment of 2 dSm-1in all the landraces, and carotenoid content in salinity treatment of 2 dSm-1 in WSC landrace. Cr, gs, Tr and number of nodules significantly increased in salinity treatment of 2 dSm-1 in all the landraces however Cr, Tr and number of nodules also increased in RSC landrace atsalinity treatment of 4 dSm-1. Increased proline content in all the landraces under salinity could be an indicator of salt tolerance due to osmotic adjustment. Salinity negatively influenced morphophysiological, biochemical and yield responses of Bambara groundnut landraces. The RSC landrace responded better to NaCl salinity on average followed by BSC and least was WSC. The three landraces can do well where soil NaCl salinity reaches 4dSm-1 electrical conductivity.