A rhythm in the flowering response of photoperiodically‐induced Pharbitis nil to agents affecting cytosolic calcium and pH

Previous work with modulators of Ca²⁺ and pH has indicated that elevated levels of cytoplasmic Ca²⁺ and pH are required during the first 4‐5 h of the dark period for successful floral induction in Pharbitis nil Chois cv. Violet. In the present study we further examined the effect of modulators of Ca²⁺ and pH by supplying them at various times prior to the inductive dark period. Peaks of inhibition by the Ca²⁺ chelator, EGTA, were observed in seedings treated 8, 18‐and 34 h before the start of the dark period. When seedlings of slightly different ages (within one diurnal cycle) were treated with EGTA, maximum inhibition was always obtained in plants treated 8 h before the start of the dark period. Peaks of inhibition by the acidifying agents, salicylic acid and Na‐propionate, were observed at ‐2 to ‐6 h and at ‐10 to ‐ 14 h. Treatment with the alkalizing agent, trisodium citrate, enhanced the flowering response with maximum enhancement at ‐6 to ‐8 h and at ‐ 18 to ‐ 20 h. We hypothesize that treatment with modulators starts an oscillation in endogenous levels of Ca²⁺ and pH. The levels of Ca²⁺ and pH prevailing at the commencement of the inductive dark period will influence the ability of the plant to perceive or to respond to the photoperiodic induction.