The Effect of Oxygen and Turbulence on Elongation of Coleoptiles of Submergence-Tolerant and -Intolerant Rice Cultivars

Abstract

Coleoptiles of rice (Oryza saliva L. ) elongated more rapidly in stagnant solution than in water-saturated air: elongation rates in aerated solution were intermediate. Elongation was stimulated between 170% to 390% in stagnant solution in 17 cultivars screened. On this basis, a relatively submergence-intolerant cultivar and two submergence-tolerant cultivars were studied further. When seedlings were grown in a range of oxygen concentrations imposed by bubbling solutions rapidly with gas, germination and early coleoptile growth (up to 5.0 mm) were inhibited at low oxygen concentrations, particularly in the submergence-intolerant cultivar. Weight per unit length declined with time in this cultivar, even in mild oxygen deficits. However, later stages of coleoptile elongation were unaffected by low oxygen supply: elongation proceeded at control rates even in anoxia. It is concluded that processes unique to germination and early stages of coleoptile elongation (possibly cell division) have greater oxygen requirements than cell extension. Rates of elongation in stagnant solution were much faster than those in any other treatment, including bubbled solutions at similar oxygen concentrations (0.125 mmol O₂ I₋₁). Therefore, elongation appears to be stimulated by accumulation of endogenous compounds which are dispersed in bubbled solutions and in air. The principal volatile compound was almost certainly ethylene, since Ag+ (an antagonist of ethylene) reduced the rate of elongation in stagnant solution by 53%. Differences in turgor pressure were only partly responsible for rapid coleoptile elongation observed in stagnant solution: there must have also been differences in cell wall properties between coleoptiles grown in stagnant and bubbled solution. The rapid elongation in stagnant solution hastens coleoptile emergence from the solution, thus establishing an adequate oxygen supply for the seedling. Leaf and root growth then proceeds.