Seeds of sunflower (Helianthus annuus L.) from the isogenic
cultivar HA-89 (Semillas Cargill SA, Seville, Spain) were surface-
sterilized in 1% (v/v) hypochlorite solution for 15 min. After rinsing
in distilled water, the seeds were imbibed for 3 h and then sown
in plastic trays containing a 1:1 (v/v) mixture of perlite and ver-
miculite. Seeds were germinated and plants grown in a growth
chamber with a 16 h photoperiod (400 -
mol m−2 s−1 of photo-
synthetically active radiation supplied by “cool white” fluorescent
lamps supplemented by incandescent bulbs) and a day/night
regime of 25/19 ◦C and 70/80% relative humidity. Plants were
irrigated daily with a nutrient solution containing 10 mM KNO3
(Hewitt, 1966).