| Cotton (Gossypium sp.) is highly susceptible to weed competition, and therefore herbicides are extensively used in this crop. Although registered for selective broadleaf weed and grass control in cotton, prometryn application may injure susceptible cultivars. Differential prometryn tolerance among cotton cultivars has not been explained by root absorption, translocation, and metabolism of the herbicide, its compartmentalization into lysigenous glands, or photosynthetic electron transport rates. Prometryn interferes with photosynthetic electron transport and induces reactive oxygen species (ROS), which cause oxidative stress and plant death. Higher constitutive or stress-induced enzymatic and non-enzymatic antioxidant levels in cells are involved in protecting plants against environmentally-induced oxidative stress. Thus, prometryn-tolerant Pima S-7 (Pima) (Gossypium barbadense L.) and -susceptible Deltapine 5415 (DP) (G. hirsutum L.) were used to determine the involvement of enhanced enzymatic and non-enzymatic antioxidant mechanisms in conferring prometryn tolerance to Pima compared to DP. These same mechanisms were examined for salt-induced stress as well. Antioxidant enzymes evaluated were ascorbate peroxidase (APX), glutathione reductase (GR), and catalase (CAT). Antioxidants evaluated were beta-carotene, lutein, alpha-tocopherol, and de-epoxidation state of xanthophyll cycle pigments (DPS). Visual symptoms of prometryn damage were less pronounced in Pima compared to DP, confirming differential tolerance. Pima possessed higher constitutive DPS and photosynthetic quantum yield levels, while DP showed higher constitutive GR. Higher DPS and quantum yield in Pima account for a more efficient dissipation of excess energy compared to DP, and therefore probably contribute to protection against prometryn. Other antioxidant mechanisms did not explain differential prometryn tolerance between these cultivars. Physiologically, both cultivars responded similarly to salt stress, and there were no differences in constitutive levels of antioxidant enzymes or protective mechanisms between them. After salt induction, beta-carotene was higher in DP, and APX activity and lutein content were higher in Pima, but results were not consistent. Except for beta-carotene, salt stress did not induce differential antioxidant or energy dissipation mechanisms between Pima and DP. There was no cross tolerance to salt stress nor did salinity stress induce protective mechanisms in either cultivar. |
