Effects of Gibberellic Acid on Rice Germination and Seedling Emergence in Stress Conditions1

Poor emergence resulting in inadequate stand establishment is a major problem associated with direct-seeded rice production, especially for semidwarf cultivars. In order to gain information on the problem, plant type related to gibberellic acid (GA3) and seeding depth was studied. A total of 21 cultivars including 9 semidwarfs and 12 talls had seed treated with GA3 at 25 parts per million (ppm) and evaluated for germination at 50, 55, 60, 65, 72, and 77°F in 1991; and for seedling emergence at planting depths of 0.75 and 2.0 in. over two years. GA3 effectively improved germination and stand establishment and resulted in faster seed germination and seedling emergence. The improvement was greater in stress situations, i.e., semidwarf vs. tall plant type, cold vs. optimal temperature, and deep vs. reasonably shallow planting. The conventional tall cultivars germinated faster and also emerged faster than the semidwarf cultivars, but the semidwarfs gained greater benefits from GA3, especially for germination at cold temperatures and emergence at deep seeding. The emergence speed and stand establishment were greater at the shallow planting than at the deep depth, but the cultivars responded more to GA3 at the deep planting. As a result, GA3 can work as a buffer to maintain good germination along with a healthy stand establishment for direct-seeded rice. The buffer effect would be more important for semidwarf than tall rice, especially when planting early and/or deeply. In other words, treating seed with GA3 would provide a security for the production of direct-seeded semidwarf rice. 1 This is a completed study. AAES Research Series 517