Plants were grown in a greenhouse at temperatures or 18–21/15.5–18◦C (D/N), unless otherwise noted in each experiment depending on the season or in the growth chambers as specified. Daily average greenhouse temperatures and irradiance levels are presented for selected months (Fig. 1). Monthly average photoperiod during the year at Beltsville, MD, USA ranged from ca.10 to 15 h (Hulme, 2011).