Gibberellin A3 Treatment of Seeds, Container Volume, Substrate pH, and Nitrogen Source and Rate Influence Growth of Containerized Seabeach Amaranth (Amaranthus pumilus)

Abstract

Seeds of seabeach amaranth (Amaranthus pumilus Raf.), a species federally listed as ‘threatened,’ were stratified (moist-chilled) for 90 days at 4C (39F) or treated with a solution of the potassium (K) salt (K-salt) of gibberellin A3 (K-GA3) at 1000 mg·liter−1 (ppm) for 24 hr. After treatment, both groups of seeds were sown in containers of two volumes, 139 or 635 cm3 (9 or 39 in3) with a substrate of peat:pine bark (1:1, v/v) amended with one of two rates of pulverized dolomitic lime [2.24 or 4.48 kg·m−3 (3.8 or 7.6 lb·yd−3)]. Containers were maintained in a greenhouse. After seedling emergence, seedlings were fertilized with a 20N-4.4P-16.6K (20N-10P205-20K20) acidic, water soluble fertilizer or a 15N-2.2P-12.5K (15N-5P205-15K20) basic, water soluble fertilizer applied thrice weekly at nitrogen (N) application rates (NARs) of 75, 150, 225, or 300 mg·liter−1. The study was terminated 8 weeks after seeds were sown and data recorded. Regardless of fertilizer, acidic or basic, top dry weight and leaf area of seabeach amaranth increased linearly with increasing NAR. Maximum top dry weight and leaf area occurred with N at 300 mg·liter−1, whereas root dry weight was unaffected by NAR. Both fertilizers increased electrical conductivity (EC) linearly with increasing NAR, and EC values of 1.15 to 1.18 dS·m−1 were adequate for maximum top growth or leaf area. Substrate pH decreased linearly with increasing NAR 21, 43, and 57 days after initiation. Top and root dry weights and leaf area were greatest for seedlings derived from seeds treated with K-GA3. Large containers yielded top and root dry weights and leaf area 61, 33, and 57% greater, respectively, than smaller containers. Top N concentration increased linearly with increasing NAR for acidic and basic fertilizers with N concentrations of 58.4 and 50.4 mg·g−1, respectively, at maximum top dry weight. Although top nutrient content of N increased linearly with NAR, top N content was unaffected by either rate of lime or type of fertilizer.