Impacts of climate change on cowpea (Vigna unguiculata L. Walp) treated with biostimulants.
Voko, Mxolisi Peter.
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Vigna unguiculata L. Walp, commonly known as cowpea, is a warm-season herbaceous legume considered native in Africa and Asia. The crop is traditionally consumed as both a leafy vegetable and staple pulse. Although the growth behaviour and nutritional composition of V. unguiculata have been explored by the research community, information regarding the plant’s response to biostimulants under abiotic stresses remain limited. Therefore, studies of this nature are pertinent, especially in the presence of climate change which manifests into global warming, drought episodes and dissipating of natural resources. Hence, a better understanding of the effects of temperature and drought stress on V. unguiculata physiology, morphology, nutrition and phytochemistry are important to ensure high yields which is important for meeting the goals of global food security. Firstly, this study investigated the effects of seed priming with biostimulants [vermicompost leachate, VCL (1:20 v/v), commercial seaweed extract Kelpak®, KEL (0.6%) and smoke-water, SW (1:1000 v/v)] and distilled water (dH2O) on V. unguiculata germination and seedling parameters under constant day/night temperatures of 30/30, 35/35 and 40/40 °C in Conviron® plant chambers. In addition, hydroponic experiments were set-up to evaluate biostimulant efficacy on rooting. Secondly, post-germination effects of VCL 1:20 (v/v), KEL (0.6%) and SW 1:1000 (v/v) were evaluated under similar temperature conditions to ascertain the influence on morphological parameters 28 days after sowing (DAS). Thirdly, postgermination effects of VCL 1:20 (v/v), KEL (0.6%) and SW 1:1000 (v/v) were investigated under different watering regimes using greenhouse protocols to ascertain biostimulation influence on growth variables and flowering after 13 weeks. Finally, the effects of VCL 1:20 (v/v), KEL (0.6%) and SW 1:1000 (v/v) were tested on V. unguiculata’s photosynthetic pigments [chlorophyll a, b, (a + b) and carotenoids], carbohydrates, proteins and phytochemicals (total phenolics and flavonoids) grown for 11 weeks under different watering regimes in the greenhouse. Despite the biostimulants not differing significantly with the corresponding controls, KEL and SW induced marked germination at 30, 35 and 40 °C while VCL being potent under 30 and 40 °C. Seed priming (i.e. biostimulant and hydropriming) significantly improved shoot length and root length over non-priming, with biostimulant-priming being more effective under 40 °C by also inducing significantly higher leaf number, fresh weights and seedling areas compared to non-primed controls. At 30 °C, priming with the three biostimulants improved peduncle diameter, fresh weight and established a significant increase on root length and dry biomass over hydropriming. VCL was most effective at promoting shoot length, root elongation and dry weight under 30 °C while KEL was most effective in increasing seedling leaves, shoot length, root number, fresh and dry biomass of plants exposed to 35 °C. Although overall, biostimulant-non-priming was second best after biostimulantpriming, non-priming with biostimulants was able to promote key variables compared to both controls (i.e. non-primed and hydroprimed controls). Hydroponic results revealed that non-priming with VCL and SW increased root number by 3 and 4-fold, respectively, at 40 °C. SW also stands out at enhancing a significant increase in leaf number and seedling area whereas KEL was the most significant solution promoting shoot length and seedling area. At 30 °C, SW-non-priming promoted significant root elongation, improved fresh and dry weights while VCL was best at promoting root number and root length. KEL and SW also exhibited post-germination effects over the control at 40 °C by improving leaf number on a weekly basis. Fresh and dry weight were improved similarly with significant improvements at 30 °C by KEL and SW. Increasing watering regimes from once to thrice a week significantly increased the number of leaves, root length and flower number. The number of nodules, however, did not differ significantly. Restricting watering frequency to once a week significantly increased shoot length, root length and leaf area in SW-treated plants compared to the control. Shoot length and root length of KEL-treated plants were also increased similarly. Remarkably, VCL increased the number of nodules and shoot length by 4 and 3-fold, respectively. Relative leaf weekly growth in SW, VCL and KEL was higher by 1, 4 and 5 leave(s), respectively, after 11 weeks under high water deficits. This foliage increase remained high by more than 4 leaves in KEL and SW-treated plants watered twice and thrice a week. Accompanying increase in number of flowers was only established in SW water-stressed plants. However, raising watering frequency to twice a week increased flower number in SW, VCL and KEL-treated plants by 2, 4 and 7-fold, respectively, compared to the control. This floral increase was still comparatively high by 2, 4 and 2-fold, respectively, in plants watered thrice a week with biostimulants. VCL also induced a marked significant increase on root length, peduncle diameter and dry weights of plants watered thrice a week. Decreasing substrate water availability from thrice to once a week induced a general increase in leaf soluble proteins, total phenolics and flavonoids. This watering transition significantly enhanced root soluble carbohydrates and proteins while root phenolics and flavonoids markedly declined. VCL, KEL and SW promoted leaf carbohydrates coupled with significant increases in those of roots of SW plants compared to the corresponding controls. Remarkably, leaf soluble proteins of biostimulant plants significantly declined to within the ranges of the plants watered twice and thrice a week. Root proteins were significantly greater to those of leaves in high water-stressed plants and statistically the same to those of roots of plants watered twice and thrice a week. Total phenolics and flavonoids of foliage of the biostimulant plants were lowered and relatively the same in the different watering regimes. Root total phenolics were highly inhibited in less watered plants and gradually increased with an increase in watering regimes. Similar trends were established in flavonoids although they were greater than in the corresponding controls. Biostimulant photosynthetic pigments [i.e. chlorophyll a, b, (a + b) and carotenoid contents] did not differ significantly with those of the control in plants watered once a week. However, the three biostimulants were able to improve chlorophyll a and a + b. KEL and SW induced higher increase in chlorophyll b and carotenoid concentrations. The biostimulants increased chlorophyll a in 3-day-watered plants by more than 2-fold. These biostimulants also improved the chlorophyll a + b and carotenoid contents. Both increasing and decreasing trends in compatible solutes (i.e. soluble sugars and proteins), photosynthetic pigments and phytochemicals under water deficits indicated biostimulant-induced capacity in cowpea for osmotic adjustment, drought tolerance or adaptive mechanisms to water stress. These findings demonstrated the biological potential of VCL, KEL and SW to improve germination, seedling/plant growth and yield in legumes even under temperature stress and drought stress. Thus, establishing their stress amelioration properties to offset negative impacts of climate change on plants and yield.