Deying Li, Lei Han, Nick E. Christians, and David D. Minner

Low oxygen (hypoxia) is one of the soil-related stresses that affect grasses. Understanding physiological responses of grasses to hypoxia is an important part of their management. The objectives of these studies were to observe morphological changes and aerenchyma formation of tall fescue (Festuca arundinacea Schreb.) subjected to hypoxic conditions and to investigate the role of ethylene in these morphological and anatomical responses. ‘Finelawn’ tall fescue was subjected to water-saturated condition to induce hypoxia. Tall fescue growing in well drained soil was subjected to 1.5, 2.5, and 4 µL L^-1 ethylene, and 0.3, 0.45, and 0.6 mM 2-chloroethyl phosphonic acid (ethephon). A second study involved the application of 0.1, 0.5, and 1 mM Ag⁺ and 1, 5, and 10 µM Co⁺⁺ to ‘Finelawn’ tall fescue subjected to water saturation. In the third study, ‘Finelawn’ tall fescue was treated with 0.3 mM ethephon and then treated with 0.01, 0.05, and 0.1 mM Indole Acetic Acid (IAA). Aerenchyma formed in the cortical tissue of the root- enlongation zone when the plants were kept under water-saturated conditions and when they were treated with exogenous ethylene. Internal atmosphere ethylene levels increased in plants under water-saturated condition. Ag⁺ and Co⁺⁺ applications inhibited the formation of aerenchyma in plants subjected to water saturated conditions. The results indicate that internal ethylene was responsible for the aerenchyma formation. Increased adaxial leaf angle was observed in plants grown under hypoxic conditions and in plants treated with exogenous ethylene. Ag⁺ and Co⁺⁺ did not change leaf adaxial angle, whereas IAA inhibited it in plants treated with exogenous ethylene or maintained under hypoxic conditions.

Unsaturated control 21.7 19.6 15.8 14.2

Water saturation 30.7 26.1 16.6 18.5

Ethephon + 0.3 mM 41.2 36.0 28.5 7.0

Ethephon + 0.4 mM 37.8 29.2 25.4 9.4

Ethephon + 0.7 mM 33.1 26.4 23.2 10.3

† Leaves are numbered from the bottom upward.

‡ Three plants per pot.

Unsaturated control 23.4 17.4 0.43 0.21 10.0

0.35 mM ethephon 31.8 25.4 0.58 0.22 10.8

0.35 mM ethephon

0.35 mM ethephon

† Leaves are numbered from the bottom upward.

‡ Three plants per pot.

§ Water saturation.

Table 3. Interaction between the effects of IAA and ethephon in the morphological changes of 'Finelawn' tall fescue plants subjected to water-saturated conditions.

Control 22.7 21.0 22.2

0.01 mM IAA 25.9 25.5 19.5

0.05 mM IAA 25.0 22.5 20.8

0.1 mM IAA 24.5 21.5 23.1

Water saturation 49.9 28.0 29.3

Water saturation + 0.01 mM IAA 53.2 28.7 36.7

Water saturation + 0.05 mM IAA 45.8 23.2 29.8

Water saturation + 0.1 mM IAA 35.4 19.2 27.0

0.3 mM ethephon 35.2 28.3 25.8

0.3 mM ethephon + 0.01 mM IAA 25.7 24.0 22.4

0.3 mM ethephon + 0.05 mM IAA 27.2 25.5 20.1

0.3 mM ethephon + 0.1 mM IAA 29.2 21.0 21.4

† Leaves are numbered from the bottom upward. First leaf is not included because the first tiller emerged.

‡ Three plants per pot.