Effect of Gibberllic Acid, Thiourea and Potassium Nitrate on Seed Germination and Embryo Growth of Cupressus sempervirens var. pyramidalis and C. sempervirens var. horizontalis

Cupressus sempervirens, the climacteric forest vegetation of El-Jable El-Akhdar (Libya), has two varieties: pyramidalis and horizontalis; both exhibit pronounced seed dormancy which participates in extinction of their natural vegetation. The present study investigates the effect of seed pretreatment with GA3 and thiourea at concentrations of 0, 50, 100, 200 and 400 mg/L, and with KNO3 at concentrations of 0 and 0.2% on seed germination and embryo growth of the two varieties of C. sempervirens. Seed germinability was higher in horizontalis variety than in pyramidalis, but rate of germination as well as embryo extension was comparable in the two varieties. Both seed germinability and embryo growth were higher in GA3 than in thiourea. Rate of germination - in terms of mean germination time - was non-significantly affected by treatments (variety, type of chemical and chemical concentration). By contrast, germinability and embryo extension increased with the increase in concentrations of GA3 and thiourea up to an optimum of 50-100 mg/L, beyond which they progressively declined. The exact optimum concentration, the magnitude of promotion at that optimum and the magnitude of retardation beyond the optimum varied according to variety and the chemical used. KNO3 at a concentration of 0.2% had limited effect on seed germination of the two varieties as well as embryo extension of horizontalis variety compared with that of GA3 and thiourea but increased embryo extension of pyramidalis variety to more than twice the control.