Water relations of different height sunflower (Helianthus annuus L.) cultivars

Recently developed dwarf sunflower cultivars have the potential of increasing sunflower acreage in the Canadian prairie. However, no information is available on the adaptability of dwarf sunflowers to different agro-climatic situations. In addition, dwarf cultivars differ in plant architecture. Therefore, information on variations within dwarf cultivars is equally important. Field and green house experiments were conducted during 1993 to 1996 to understand the effect of reduced plant stature on the water extraction ability, drought tolerance and productivity of sunflower cultivars. Dwarf open pollinated cultivars (sunola; cv. Aurora and Sierra), and dwarf hybrids (sunwheats; cv. SW-103 and SW-101) were compared to standard height hybrids (cv. IS-6111 and SF-187).;The ability to extract soil water decreased with reduction in plant stature. The dwarf cultivars; had a smaller tap root diameter compared to the standard height cv. IS-6111. Rooting depth, extraction front velocity and root distribution were higher in IS-6111 compared to SW-103, although they were similar between IS-6111 and Aurora. Thus, the dwarfing gene and/or the genetic background of the cultivar played a role in determining the root system of sunflower cultivars. Greater water extraction ability of taller cultivars than dwarf cultivars was attributed to differences in growth duration, and a deeper root system. Tall cultivars were particularly efficient in extracting water below 110 cm soil depth.;The dwarf hybrid, SW-103, consistently had higher leaf water potential, which was frequently higher than that for IS-6111. However, Aurora had leaf-water potentials lower, or at best, similar to IS-6111. The strategy adopted to overcome water stress differed between height classes. The standard height cv. IS-6111, under stress, allowed leaf water potential to drop lower and depended more on the root system to extract more soil water. In contrast, the dwarf cultivars sensed water stress early and initiated osmotic and stomatal regulation and depended less on their root system. Genotypic differences in osmotic adjustment per unit of water stress were observed. Under the moderate water stress conditions encountered in the present study, which represented the long term weather trends of the region, all height classes of sunflower maintained positive turgor. However, the standard height cv. IS-6111 maintained higher stomatal conductivity and photosynthesis than the dwarf cultivars.;The lower plant height of dwarf cultivars did not consistently translate to an increased harvest index. Only one out of four cultivars had a higher harvest index than both standard height hybrids. Seed yield was more dependent on cultivar than on stature. Aurora and IS-6111 produced similar yields. The photosynthates conserved by dwarf cultivars were retained by the complex head in some cultivars. These results suggest that more breeding work is needed to develop an ideotype with water extraction ability of IS-6111, stress responses of dwarf cultivars, small head and thin stern of Aurora and better retranslocation of photosynthates to seed of SW-103.;Thus, short stature sunflower cultivars have lower water extraction ability but higher drought tolerance compared to standard height sunflower cultivars. However, productivity under moderate stress conditions depends more on cultivar than on plant stature.