The effect of carbon dioxide, temperature and water stress on growth, yield and seed quality of sunflower (Helianthus Annuus L.)

Sunflower (Helianthus annuus L.) is currently the third most resilient grain crop grown in Southern Africa and the region's most important oilseed crop. Its low cost of production and capacity to yield consistently in adverse weather conditions makes it a highly desirable crop for farmers to cultivate. However, with climate change, crop productivity will be difficult to maintain. Not only will climate change affect seed yield, but it will also affect the morphology and physiology of the following generations. The aim of the study was therefore to determine the effects of environmental factors on sunflower growth, yield, and seed quality. This was accomplished by investigating abiotic factors such as elevated CO2, temperature and water stress. Sunflower (cultivar AGSUN8251) was grown under varying treatments for each factor which were as follows: CO2 (ambient, 412 ppm and elevated, 700 ppm), temperature (22˚C, 28˚C and 35˚C) and water stress (45%, 65% and 90% of field capacity (FC)). The trials were laid out in a completely randomised design and were repeated. Plant height was monitored throughout the growing period and yield components were measured at harvest. The quality testing of the progeny seeds harvested from the different treatments included moisture content (MC), thousand seed weight (TSW), unhulled and hulled seed weight, seed germination and vigour (accelerated ageing and electrical conductivity tests). The results from conviron trials revealed that elevated CO2 enhanced growth by increasing the sunflower height in both trials. In terms of yield and seed quality of the progeny seeds, the trials showed variable results. In trial 1, number of seeds produced per head (NOS), total number of seed per head (TNOS), seed weight per head (SW), head weight with (HWS) and without (EHW) seeds, head diameter (HD) and belowground biomass accumulation were significantly reduced owing to elevated CO2. In trial 1, the two CO2 levels had no effect on the number of leaves produced and the accumulation of aboveground biomass. Under elevated CO2 levels, all sunflower yield components increased significantly in trial 2. The progeny TSW, hulled and un-hulled seed weight, seed germination, and vigour (accelerated ageing) were significantly reduced in trial 1 under elevated CO2, but the opposite was observed in trial 2. The results from the glasshouse trial investigating the effect of temperature stress on sunflower showed that an increase in temperature reduced sunflower height in both trials. An increase in temperature, significantly decreased the NOS, TNOS, SW and biomass accumulation in trial 1. In trial 2, a rise in temperature resulted in a decrease in the NOS, SW, and HD only at 35˚C. The number of hulls, TNOS and EHW were significantly higher at 35˚C compared to 22˚C and 28˚C. The overall progeny seed germination and vigour were not influenced by temperature stress in both trials. Field trials were conducted to evaluate the effect of water stress on sunflower. Sunflower height was decreased at 90% FC in trial 1, whereas in trial 2 sunflower height was not influenced by water stress. Trial 1 results showed that only seed weight per head and head diameter were significantly reduced by water stress. TNOS per head was reduced at 45% compared to 90% but there were no significant differences between 90% and 65% FC and between 65% and 45% FC. In trial 2, water stress was observed to reduce all sunflower yield components, but the reduction was not significant between 45 and 65% FC. The progeny seed quality was not influenced by water stress exposure of the mother plant. The results of this study suggest that the increase in CO2 levels will positively influence the growth and yield of sunflower while temperature and water stress will negatively influence it. However, all these factors will not influence the progeny seed quality. The findings of this study could be used to obtain more data to improve crop performance and seed quality in order to give better agronomic recommendations to farmers.