Sangamesh V. Angadi

Establishing a good canola (rapeseed; Brassica napus L.) stand is difficult in the semiarid prairie region of Canada where low temperature, water stress, and soil crusting could result in poor seed bed conditions. A field study was conducted from 1999 to 2001 at Swift Current, SK, Canada, to determine the effect of a range of uniform (5 to 80 plants m⁻²) and nonuniform (seedlings from 1‐m lengths from two adjoining rows were removed and retained alternatively; 10 to 40 plants m⁻²) plant populations on yield and yield components of canola. Canola adjusted seed yield across a wide range of plant populations, although it did not compensate completely for the decreasing populations. Environmental conditions played a significant role in the expression of plasticity of canola. For example, in 2000, with slightly above‐normal growing season precipitation, canola maintained similar yield levels across a wide range of populations (20 to 80 plants m⁻²), while in 2001, with well below normal precipitation, seed yield declined as populations dropped below 40 plants m⁻² Reducing plant population by half from 80 to 40 plants m⁻² did not reduce seed yield when the reduced plant population was uniformly distributed, but reduced yield when the population was nonuniformly distributed. The primary response of canola to lower plant population was increased pods per plant through increased branching and increased pod retention at each node. The number of pods formed on primary and secondary branches increased as population decreased. Seeds per pod and seed weight were stable across populations.

We compared the growth and yield of five Brassica spp. genotypes [two B.juncea (L.) Coss., two B. napus L. and one B. rapa L.] in 11 environments spanning two degrees of latitude and longitude in southern Saskatchewan in 1996–1998. All entries were managed with three levels of N fertility at each site. Plant height for the canola-quality B. juncea breeding line J904316 was 21% greater than the mean of the other genotypes, but shoot biomass did not differ for the same comparison. Cutlass Oriental mustard consistently yielded greatest across all environments while the yield of J904316 was highly variable among environments. The mean seed yield of Cutlass was 12% greater than the highest-yielding B. napus cultivar and 32% greater than the B. rapa cultivar, and had a greater harvest index. Earlier genotypes yielded greater than late-flowering and maturing genotypes within B. juncea and B. napus, but the earliest genotype, Maverick (B. rapa), yielded lower than all other genotypes. All Brassica spp. genotypes were responsive to N fertilization, and high total available N (average = 109, range = 91 to 131 kg N ha^-1) was generally required to maximize seed yield, but not shoot biomass, under the conditions of this study. Pod density was the primary yield component influenced by N level. This study illustrates the potential adaptive advantage of B. juncea on the semiarid northern Great Plains. Key words: Brassica, canola, genotype, mustard, N fertility, semiarid, northern Great Plains

Development of early maturing short stature sunflower (Helianthus annuus L.) cultivars followed the success of semi‐dwarf cereals. Although a number of breeding programs have recently released dwarf sunflower cultivars for production in western Canada, the drought adaptability of dwarf cultivars has not been studied. Therefore, field studies were conducted under rainfed conditions at two Manitoba locations during 1994 and 1995 to compare seasonal water relations of ‘Sunwheat‐103’ (SW‐103), a dwarf hybrid, and ‘AC‐Aurora’ (Aurora), a dwarf open pollinated cultivar, with ‘IS‐6111’, a standard height hybrid. The dwarf hybrid, SW‐103, experienced less stress (up to 0.44 MPa higher leaf water potential) than the standard height hybrid, IS‐6111. Genotypic variation for osmotic adjustment was observed in both 1994 and 1995. SW‐103 and Aurora respectively had up to 0.42 and 0.39 MPa lower osmotic potential at full turgor compared with IS‐6111. Aurora displayed greater osmotic adjustment per unit decrease in leaf water potential than other cultivars. All cultivars maintained positive pressure potential during the study and whenever significant differences in pressure potential were observed, SW‐103 usually had higher pressure potential than other cultivars. IS‐6111 maintained higher photosynthetic rate and stomatal conductance than the dwarf cultivars. These results suggest that under conditions of adequate water supply, the tall hybrid had the highest productivity despite experiencing the lowest leaf water potential and osmotic adjustment. The drought tolerance advantages of shorter statured cultivars identified here may be more important to productivity maintenance under limited soil water conditions.

Early-maturing, short-stature sunflower (Helianthus annuus L.) cultivars improve the adaptability of sunflower to the short growing season of western Canada. However, the agronomic potential of the recently developed dwarf cultivars in comparison to standard-height sunflower is yet to be understood. Multi-environment field studies, consisting of space-planted trials, where interplant competition was low, and agronomy trials, where plants were grown at commercial population densities, were conducted in southern Manitoba to compare the yield formation of dwarf hybrids (sunwheats; SW-101 and SW-103), and dwarf open-pollinated c ultivars (sunola; Aurora and Sierra) with that of standard-height cultivars (IS-6111 and SF-187). In space-planted trials, dry matter (DM) accumulation and water use efficiency for dry matter production (WUE_DM) decreased as plant height decreased. In agronomy trials, differences in DM and WUE_DM among the different height classes were masked. The diversion of assimilate from stem to head depended on the genetic background, while the efficiency of utilising assimilate in the head for seed production was lower in dwarf cultivars. Only one of the four dwarf cultivars (SW-103) displayed a higher harvest index than IS-6111. Higher seed yield for the standard-height cultivar, IS-6111, and the dwarf open-pollinated cultivar, Aurora, compared to other cultivars was attributed to both greater DM and improved DM partitioning. However, variations observed among the dwarf cultivars for DM accumulation and partitioning revealed that the dwarfing gene was not a limiting factor for breeding a dwarf sunflower cultivar with better partitioning of DM along with better yield potential. Key words: Dwarf sunflower, Sunola, seed yield, biomass, water use efficiency

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 heat 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 mm more compared with dwarf hybrids and dwarf open pollinated cultivars, respectively) was attributed to deeper rooting depth (average 0.2 and 0.4 m 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.