Influence of gibberellic acid and semidwarfism on seed vigor and isoenzyme in rice (Oryza sativa L.)

Inadequate stand establishment is a major problem for drill-seeded rice production in the southern states of the U.S., especially for semidwarf cultivars. Rice seedling emergence is a function of mesocotyl and coleoptile elongation influenced by gibberellic acid (GA{dollar}sb3{dollar}). Seed of 21 rice cultivars, including 9 semidwarf and 12 tall, were treated with GA{dollar}sb3{dollar} at 25 mg/kg seed and tested for seed vigor in germination at various temperatures, elongation of mesocotyl and coleoptile, isoenzyme response, seedling emergence and stand establishment at different depths of planting. Three hundred seventy nine accessions of semidwarf germplasm were evaluated for seedling vigor.; GA{dollar}sb3{dollar} promoted germination speed significantly from 10 to 22{dollar}spcirc{dollar}C, and the efficiency was increased with a decrease of temperature. The germination percentage increased from GA{dollar}sb3{dollar} seed treatment at 10 to 16{dollar}spcirc{dollar}C. Semidwarf cultivars germinated less and slower, but had higher response to GA{dollar}sb3{dollar} than tall cultivars. Lengths of mesocotyl and coleoptile for all cultivars except Newbonnet increased with the GA{dollar}sb3{dollar} treatment. Semidwarf cultivars had shorter mesocotyl and coleoptile, but responded more to GA{dollar}sb3{dollar} in comparison with tall cultivars. Esterase isoenzymes E{dollar}sb3{dollar} and E{dollar}sb4{dollar} were induced, and activities of E{dollar}sb5{dollar}, E{dollar}sb6{dollar} and NE{dollar}sb1{dollar} were reduced by GA{dollar}sb3{dollar} with greater reduction in semidwarf cultivars than in tall cultivars. GA{dollar}sb3{dollar}-treated seed emerged faster and had greater stand establishment than the untreated seed in both 1990 and 1991. Seed of semidwarf cultivars had less stand and emerged slower, but responded more to GA{dollar}sb3{dollar} as compared to tall cultivars at two depths of planting. Planting at 0.75{dollar}sp{lcub}primeprime{rcub}{dollar} was significantly better on stand establishment and rate of emergence than at 2.0{dollar}sp{lcub}primeprime{rcub}{dollar}. Thirty-two semidwarf accessions with high seedling vigor were identified, and eight high vigor accessions did not respond to GA{dollar}sb3{dollar}.; This study demonstrates that seedling emergence and stand establishment is a cumulative performance of seed germination and elongation of the mesocotyl and coleoptile. GA{dollar}sb3{dollar} could function as an effective aid for stand establishment and act as a buffer in obtaining good seed germination and seedling emergence, especially for semidwarf cultivars. Activity of E{dollar}sb5{dollar}, E{dollar}sb6{dollar} and NE{dollar}sb1{dollar} was decreased by GA{dollar}sb3{dollar} with the improvement of seedling vigor. Furthermore, breeding for seed vigor is possible and high vigor germplasm is available in the USDA/ARS rice germplasm collection.