Root system and water use patterns of different height sunflower cultivars

Dwarf sunflower (Helianthus annuus L.) cultivars have agronomic and management benefits over conventional standard height sunflower cultivars. However, the effects of reduced plant stature on root systems and water extraction characteristics are not known. Therefore, field trials were conducted at two locations in western Canada during 1994 and 1995 to compare root system characteristics and water extraction patterns of dwarf hybrids [two Sunwheat hybrids: 'Sunwheat-101' (SW-101) and 'Sunwheat-103' (SW-103)], and dwarf open pollinated cultivars [two Sunola cultivars, 'AC-Aurora' (Aurora) and 'AC-Sierra' (Sierra)] with standard height hybrids (IS-6111 and SF-187). Reducing plant height reduced rooting depth (by 0.20 to 0.60 m), root length density (by 6.6 m m(-2)) and root distribution in SW-103 compared with IS-6111, while no differences were observed in these traits between IS-6111 and Aurora. The soil water depletion front velocities of sunflower cultivars were influenced by accumulated beat units, suggesting a temperature effect on rooting characteristics. IS-6111 (0.14 to 0.19 cm growing degree d(-1)) and Aurora (0.15 to 0.17 cm growing degree d(-1)) had significantly higher depletion front velocities compared with SW-103 (0.09 to 0.11 cm growing degree d(-1)). Greater soil water depletion by standard height hybrids in agronomy trials (average 59 and 79 min more compared with dwarf hybrids and dwarf open pollinated cultivars, respectively) was attributed to deeper rooting depth (average 0.2 and 0.4 in more compared with dwarf hybrids and dwarf open pollinated cultivars, respectively) and more efficient water extraction, although a longer growth duration may also have been a factor.