Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Abstract Only
Publication Acceptance Date: January 4, 2005
Publication Date: January 7, 2005
Citation: Burke, J.J. 2005. A new bioassy to determine the onset of water stress in cotton[ABSTRACT]. National Cotton Council Beltwide Cotton Conference. Technical Abstract: Water-deficit stress reduces cotton yields and negatively impacts fiber quality. Unfortunately, current methods for determining water stress are cumbersome and do not lend themselves to the high throughput needs of modern cotton breeding programs. The present study describes the development and field evaluation of a new technique to determine the onset and magnitude of water-deficit stress. The technique capitalizes on the very nature of a 'source' leaf to provide developing 'sinks' with energy required for growth. Source leaves under non-stressed conditions accumulate photosynthate throughout the day and mobilize the stored photosynthate to the new growth during the night. Little stored photosynthate remains in the source leaf at sunrise and the daily cycle of accumulation and mobilization of photosynthate begins again. Water-deficit stress reduces new growth and lessens the demand on the source leaves. This results in greater quantities of photosynthate remaining in the source leaves at sunrise. The bioassay evaluates the ability of leaf tissue from source leaves harvested at sunrise to withstand a prolonged high respiratory demand. Source leaves from non-stressed plants have little photosynthate remaining in the morning and quickly succumb to the respiratory challenge; while source leaves from water-deficit stressed plants have greater photosynthate stores and are able to withstand the respiratory challenge for a greater length of time. At sunrise, a leaf punch was harvested from a source leaf (in cotton this is the fifth mainstem leaf from the top) using a cork borer and rubber stopper. This was repeated on five separate plants. The punches were transferred to a well in a 24-well microtiter plate that had been 1/2 filled with water. This processes was repeated until samples from all treatments had been harvested. Upon returning to the lab, the punches were placed on moistened 3 MM filter paper in a Pyrex baking dish, covered with Glad Cling Wrap, and placed in the dark in a VWR Model 2005 incubator (Sheldon Manufacturing, Inc., Cornelius, OR) set to 39C. The samples were evaluated hourly following being placed in the 39C incubator. Chlorophyll fluorescence was analyzed using an Opti-Science OS1-FL Modulated Fluorometer (Tyngsboro, MA). The loss of fluorescence yield of photosystem II over time at 39C was used as a relative measure of the stress level of the plant. The least stressed plant had the fastest decline in fluorescence yield because the tissue was unable to maintain cell viability with the prolonged heat treatment. The results of two years of field studies showed that this is a highly sensitive assay that can be used to detect water-deficit stress on a daily basis. Hundreds of samples can be measured within a few minutes, thereby lending this technique useful to cotton breeding programs.