The effect of different carbon dioxide, temperature and watering regimes on plant growth and seed quality of soybean (Glycine max (L.) Merr.)

Abstract

Soybean (Glycine max {L.} Merr.) is an important crop for humans and livestock alike and commonly cultivated in South Africa. However, changing environmental factors like increasing carbon dioxide (CO2), increasing temperatures as well as decreased water availability may affect soybean production in terms of yield and seed quality. Commercial, and more so smallholder farmers, typically re-use the seed from one harvest in the next season’s planting and therefore it is important to determine soybean seed quality in the face of changing environmental factors. In this study, the effect of elevated CO2 and temperatures as well as the effect of reduced irrigation on growth, yield and seed quality of soybean was investigated in three separate experimental trials. Each treatment involved half the seeds being inoculated with Bradyrhizobium japonicum, with applications of superphosphate and KCl (potassium chloride), while the other half remained uninoculated, but received superphosphate, KCl, and LAN (limestone ammonium nitrate) applications. Pot trials were conducted in two Conviron plant growth chambers set at 412 ppm (ambient CO2) and 700 ppm (elevated CO2) with 26 replicates of each treatment. Reduced water availability (replenishing 45%, 65% or 90% of plant available water [PAW]) was conducted in field trials with four replications of each treatment. Lastly, pot trials were conducted in a glasshouse set at 22°C and 28°C as well as a Conviron plant growth chamber set at 35°C with 26 replicates. All followed a complete randomised block design and the trials were repeated. Plant emergence as well as flowering and pod formation date was recorded. Plant height and net assimilation readings were recorded throughout the growing period for the water and CO2 trials. Measurements of seed and pod yield, and root (with the exception of field trial root measurements) and aboveground dry mass were taken at harvest. Seed quality testing of harvested progeny seeds included thousand seed weight (TSW), seed germination and vigour (accelerated ageing {AA} and electrical conductivity {EC} tests). Data was statistically analysed for variance using Statistical Analysis Software (SAS) version 9.4 where a General Linear Model (GLM) procedure was used to perform an Analysis of Variance (ANOVA). The means were separated using Tukey’s Least Significant Difference (LSD) test (P= 0.05). In the first trial there were no significant differences between seed number, pod yield, final plant height and dry mass of roots or aboveground parts between the elevated and ambient CO2 treatment. There was an increase in the seed number and pod yield but not the plant height, net assimilation rate or root and aboveground dry weight in the second trial. Nitrogen fertiliser application also improved the plant height, aboveground and root mass as well as the seed and pod yield and number in both trials. Standard germination and germination after AA both were not significantly different between elevated and ambient treatments in the first trial but were significantly higher in the second trial for the 412 ppm treatment. Standard germination was not significantly different between fertiliser and Bradyrhizobium in the first trial but was higher for Bradyrhizobium in the second trial. The electrical conductivity (EC) for all the trials was under 25 μS.cm-1.g-1 indicating high vigour. The results indicate that Bradyrhizobium inoculation can possibly improve seed quality of progeny seeds. Furthermore, fertiliser application and elevated CO2 levels improved yield. Optimal irrigation (90% PAW) led to the highest seed yield and quality. There seemed to be no differences in yield or seed quality between the two water stressed (45% and 65% PAW) treatments, implying that water stress, regardless of the severity, had adverse effects on soybeans. The final height of soybeans did not differ between irrigation treatments, with the exception of the 90% PAW treatment in trial 2, which were significantly shorter. Notably, the application of nitrogen fertiliser demonstrated increased soybean height, net assimilation rate before flowering, dry mass of aboveground parts and TSW. Regarding the temperature trials, the seed yield improved when the mother plant was subjected to continuous moderate temperatures of 22°C and inoculated with Bradyrhizobium. When subjected to higher temperatures (i.e. 28°C), Bradyrhizobium inoculation also improved seed yield. A high TSW may have led to a higher germination percentage. The germination tests revealed that soybean grown under 22°C produced seeds with higher vigour. The EC tests were under 25 μS.cm-1.g-1 for all treatments, which indicated high vigour, therefore allowing no differentiation for seed vigour in this context. The Bradyrhizobium treatment had higher seed vigour (following the AA test) than the fertiliser treatment. The 35°C treatment yielded no viable seed. These findings may therefore indicate that the effects of changing environmental conditions may negatively influence soybean yield and seed quality, and may therefore affect the food security. Further research on the combined effect of water and temperature stress as well as increased CO2 levels on soybean yield and seed quality is recommended.