Emergence response of sunflower (Helianthus annuus) cultivars to supra-optimal soil temperatures

Abstract

Seedling emergence is a sensitive stage during sunflower (Helianthus annuus L.) establishment. Abiotic stresses, such as high soil temperatures (above 40 °C), can cause delayed emergence resulting in uneven stand and ultimately poor yields. Therefore, the emergence and early growth responses of seven sunflower cultivars to four daily supra-optimal soil temperatures (DSST) of 35, 40, 45 and 50 °C were evaluated. The experiment was laid out as a randomised complete block design with four replications repeated twice in a glasshouse. Results clearly indicated that emergence index (EI) declined with an increase in soil temperature. A moderate decline was observed from DSST 35 °C to 45 °C, but from DSST 45 °C to 50 °C the decline was extreme. Root length of seedlings 24 h after planting were inconsistent between plantings. Root length at surface cracking and 48 h after surface cracking indicated that seedling root length at DSST 35 °C was longer than at DSST 45 °C and 50 °C. The lowest dry mass for both plantings was recorded at DSST 50 °C. Morphological differences were also visible for sunflower seedlings at the selected temperatures, but there were no definite visible differences between cultivars. Generally, deformed seedlings were observed at temperatures above DSST 40 °C.