Photosynthetic efficiency and biochemical responses of vegetable-type soybean cultivars under drought stress
Hlahla, Jeremiah Mpumelelo
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Climate change accelerates drought, which negatively impacts plant growth and development by altering the normal metabolic activities, resulting in poor crop yield. Soybean, (Glycine max L. Merrill), also known as edamame is native to South-East Asia and is popular in many parts of the world because of its high nutritional value, health, and economic benefits. The crop was recently introduced to South Africa because of its potential to improve food security. However, there is lack of information on its physiological and biochemical responses to drought. Previous studies on drought-stressed edamame demonstrated that some cultivars, especially the better-adapted ones, had induced accumulation of the total soluble sugars. Since the production of the sugars emanates mainly from the process of photosynthesis, this study was conducted to elucidate how edamame responds to drought stress by focusing on the photosynthetic efficiency, quantification of the non-structural (i.e., soluble sugars) and structural carbohydrates (i.e., non-soluble sugars), including lignin and total phenols. Also, it was important to determine the relationships between the photosynthetic efficiency parameters and all the carbohydrate responses. The information from this study is very crucial as it can be used to identify the physiological and biochemical markers of drought tolerance, which is critical for screening edamame cultivars for drought tolerance. Such information can ultimately be employed in the selection of cultivars to be included in the edamame drought tolerance breeding programs. The six edamame cultivars were germinated and grown in the greenhouse under controlled conditions. Drought stress was applied at the third trifoliate (V3) leaf stage by withholding irrigation to reach 30% WHC. The photosynthetic efficiency parameters such as chlorophyll fluorescence and stomatal conductance were determined using non-invasive methods. The sampling for all the physiological and biochemical parameters was done at flowering (R2) and pod filling (R4) stages. The photosynthesis pigments were quantified using a spectrophotometer. The non-structural carbohydrates (glucose, sucrose, trehalose, and starch) were quantified according to the instructions on the Megazyme kits. The structural carbohydrates were quantified using spectroscopy, Fourier-transform infrared (FTIR), and X-ray diffraction (XRD). The statistical analysis was done using Genstat software. Drought stress increased the quantum efficiency (Fv/Fm) of AGS429 and UVE17 by 10% and 9% respectively at the pod filling stage respectively. The Performance index on absorption basis (PIabs) of AGS429 and UVE17 were also increased by 72% and 71% respectively at the pod filling stage. The overall photosynthetic performance of UVE17 was reduced by drought stress because this cultivar showed a total performance index (PItotal) decrease of about 28% at the pod filling stage. The cultivars, AGS354 and AGS429 had a 21% and 26% increase in chlorophyll b content at pod filling. The carotenoid content was significantly reduced in UVE17 at pod filling, showing that this cultivar had the least antioxidative defence against the reactive oxygen species (ROS) produced under drought stress. Carotenoids (CRDs) positively correlated with chlorophyll-a (Chl-a), PIabs, and PItotal, which could result in the protection of photosystems. Additionally, at the pod filling stage, UVE17 had a 59% reduction in the stomatal conductance, suggesting a poor rate of CO2 uptake and assimilation under drought stress. According to the studied photosynthesis parameters, UV17 is a poor-performing cultivar under drought stress. Drought stress-induced the accumulation of trehalose, sucrose, and starch at pod filling, but decreased glucose content of most cultivars, except UVE7. Starch had a significant positive correlation with Ch-a and the total seed mass per plant, showing its importance in improving yield under drought stress in edamame. The cell wall studies showed that drought stress did not have any significant effect on the total phenolic content of all cultivars. However, the acid-soluble lignin was significantly increased in most cultivars except UVE14 and UVE17. The FTIR and XRD results suggest that the UVE14 cell wall was the most intact during drought stress followed by UVE8 and AGS429. In conclusion, the CRDs, stomatal conductance, performance indexes of the photosystems, starch, hemicellulose, and lignin are essential physiological-biochemical mechanisms of drought tolerance in edamame. These parameters could be used in the edamame breeding programs for drought tolerance screening. AGS429 and UVE14 are better performing cultivars under severe drought stress and should be included in the drought tolerance breeding programs.