Effect of High Temperature Stress On Floral Development and Yield of Cotton

Global climate change is projected to cause substantial losses in crop productivity by the end of the twenty-first century. Environmental stress during floral development is a major reason for the disparity between actual and potential yields. In Gossypium hirsutum L., canopy growth and flower survival are severely inhibited at temperature regimes in excess of the optimal day/night temperature regime of 30/20°C, which commonly occur in the US Cottonbelt during reproductive development. Much of the sensitivity of reproductive organs to heat stress has been attributed to the sensitivity of the microgametophyte to temperature extremes. Due to the inability of mature pollen grains to acclimate to adverse temperatures, numerous studies have focused on pollen tube elongation responses to high temperature using in vitro systems, showing an optimal temperature range (28 to 32°C) well below the maximum daily temperatures experienced by cotton plants during the flowering period (38°C). In contrast with in vitro systems pollen performance and fertilization efficiency in vivo is also dependent upon the biochemical status of the pistil. Using an in vivo approach we have shown that the energy demands of growing pollen tubes or developing gametophytes cannot be met under heat stress due to decreased source leaf activity, and that a calcium-augmented antioxidant response in heat stressed pistils interferes with enzymatic superoxide production needed for normal pollen tube growth. Although numerous studies have illustrated the need for antioxidant enzymes in acquired photosynthetic thermotolerance, information on their possible role in promoting innate thermotolerance is lacking. We have concluded that maintaining a sufficient antioxidant enzyme pool prior to heat stress, is an innate mechanism for coping with rapid temperature increases that commonly occur under field conditions. Studies are underway to screen and select cotton germplasm for improved thermotolerance.