A field comparison of sunflower (Helianthus annuus) and sorghum (Sorghum bicolor) in a long drying cycle. II. Plant water relations, growth and yield

Abstract

Growth and water use were studied in field-grown sunflower and sorghum, mainly in a long drying cycle, though other water treatments were included. When water was plentiful, sorghum was the more productive, but in the drying cycle the order was reversed, largely because sunflower extracted more water. This, together with a smaller leaf area and close control of transpiration rate through stornatal sensitivity to air humidity, ensured a far more favourable leaf water status in sunflower, contributing to its much faster net assimilation rate. In view of the species' different photosynthetic pathways and the large difference in water use efficiency generally associated with these pathways. an unexpected outcome was the smallness of this difference. In the drying cycle, sunflower had the further advantages of a strongly determinate habit and a phenology which was unaffected by drought, even when very severe. Consequently, all plants flowered. In sorghum, drought arrested development, which proved disadvantageous in a continued drought, but turned to dramatic advantage in a late break, when sorghum grain yield increased by 75%, while sunflower responded not at all.