Injury to plants due to environmental stress is mainly caused by the reactive oxygen species that attack biomolecules and membranes. To prevent this damage plants have evolved an antioxidative system consisting of low molecular weight antioxidants such as ascorbate, a-tocopherol, glutathione and carotenoids, and of enzymes such as catalase, ascorbate peroxidase (APX) and glutathione reductase (GR). Since competition among plants decreases the availability of moisture and nutrients, it may initiate oxidative stress in plants. We studied the effect of intraspecific and interspecific competition on oxidative stress tolerance of cotton under adequate supply of moisture and nutrients in greenhouse. Two species of cotton (Gossypium hirsutum L., cultivar Delta Pine 5415, and Gossypium barbadense L., cultivar Pima S-7) were grown alone or three plants per pot for intraspecific competition. One plant of each of cotton species was grown with two plants of spurred anoda [Anoda cristata (L.) Schlecht.], an important cotton weed in the southern United States, for interspecific competition. A single plant of spurred anoda was maintained per replication. These treatment combinations were replicated six times. Net photosynthesis and photosynthetic quantum yield were not influenced by planting design or species at three weeks after planting. Chlorophyll a+b content was greater in A. cristata and G. hirsutum than G. barbadense. Carotenoid content was highest in A. cristata and higher in G. barbadense than G. hirsutum. Two cotton species had constitutively greater catalase and APX activities than spurred anoda while G. hirsutum had greater GR activity. When half of the experimental plants were subjected to a six-day drought, APX activity increased by 62% in G. hirsutum and 66% in G. barbadense; catalase and GR were unaffected. The levels of low molecular weight antioxidants as influenced by competition and drought will be presented.

Key words: Anoda cristata, competition, drought, Gossypium barbadense, Gossypium hirsutum, oxidative stress tolerance