Transgenic Sunflower - Genetics, Genomics and Breeding of Crop Plants - page 295

In plants, ammonium absorption, which is an alternative pathway to
the nitrogen cycle, is performed through the glutamine synthetase (GS)
enzyme. However, in darkness and with a low available C:N ratio, some
variants of asparagine synthetase (AS) enzyme, coded by HAS1 and HAS1.1
genes provisionally store N as asparagine, thereby preventing ammonium
intoxication (Herrera Rodriguez et al. 2004). In GM plants, AS can substitute
GS under conditions that limit its activity (such as in Medicago truncatula;
Carvalho et al. 2000) and act as an alternative N-storing metabolic pathway
(as in Nicotiana tabacum ; Ferrario-Méry et al. 2002). AS expression in GM
sunflower may therefore improve N-metabolism and contribute to a more
efficient use of this element.

Table 9-3 Research articles about sunflower crop mineral nutrition published during
five years (CAB 2002-2007).

Group Element Articles

# Detail # %

Macronutrientes 4 N-P-K-S 19 46

only N 1 6 3 9

Micronutrients 6 B-Fe-Zn-Ca-Mg-Si 1 5 3 7

only B 9 2 2

Heavy metals 2 Cd-Se 3 7

Others 3 inorganic amend (lime, gypsum) 3 7

salt, salinity 2 5

rhizosphere, mycorrizae 2 5

Total mineral nutrition 1 5 4 1 100

9.4.2 Weed Control
Although no-till adoption in sunflower would be limited by the lack of wide
range herbicide-tolerant varieties, weed interference seems less attractive to
researchers than pests and diseases ( Table 9-4 ) . Crop-related weeds comprise
at least 86 species, among which the most outstanding is the genus
Orobanche , especially O. cumana (broomrape) a hemiparasitic difficult-to-
control weed, widespread in several European regions.
At present, control strategies for broomrape tend to use a specific genetic
mechanism obtained from wild species (Fernández Martínez et al. 2000;
Labrousse et al. 2004; Velasco et al. 2007). However, the continuous emergence
of new races of the weed shows evidence that a process of constant renewal
of resistant sources is required to maintain these control strategies.
Some herbicides that are members of the imidazolinone and sulfonylurea
families (Group B) inhibiting hydroxyacetic acid synthetase enzyme (AHAS)
are useful for controlling broomrape (Alonso et al. 1998). Genes conferring
tolerance to these herbicides (Baumgartner et al. 1999; Kolkman et al. 2004)
found in wild sunflower populations in the State of Kansas in the US were