Based upon results presented at these meetings by E. A. Vigil in 1991 and J. A. Landivar in 1992, there are several theories concerning the development of white speck in cotton fibers. Some of these theories may be stated as follows: (1) Plant water stress at certain critical periods of plant development leads to aborted seed embryos and immature fibers; (2) Aborted seed embryos and immature fibers are major contributors to white speck in cotton fibers; (3) Measurement of leaf water potential at dawn and mid-day are indicators of plant water stress; and (4) Measurement of soil water content is an indicator of plant water stress. These theories and others were tested in replicated field plots at Perthshire Farms in Bolivar County Mississippi.

Plots were laid out in a field of Robinsville sandy loam both under a center pivot irrigation system and in one corner out of the coverage of the pivot system. The plots were replicated five times. An access tube for the Troxler Soil Moisture Capacatence Probe was installed to a depth of five feet. Starting two weeks after first bloom and for four consecutive weeks, twenty white blooms were tagged in the same row and adjacent to the access tube, both under the pivot and outside of its coverage. At the same times three plants adjacent to the access tube were mapped. In the adjacent rows to the access tube, three most newly expanded leaves were harvested from each rep at dawn and three more at mid-day for leaf water potential measurements. Soil moisture content was determined after the dawn leaf harvesting. The tagged flowers that were retained as bolls were harvested as they opened, approximately four weeks after tagging. Tagging was limited to fruiting branches 1-8 and fruiting positions 1-3.

The opened bolls were kept in separate bags and shipped to the Fiber Quality Laboratory at the USDA-ARS Southern Regional Research Center in New Orleans, LA for mote counting and AFIS testing. The results were then analyzed for fiber quality indicators.

The weather at the field site was monitored throughout the growing season and the crop simulation model, GOSSYM/COMAX, was used to schedule irrigations. The weather was dry for 2-3 week periods of time and then short intervals of rain. However, solar radiation was lower than normal, maximum temperature lower than normal, but minimum temperatures higher than normal during the flowering and boll maturation period. This led to three irrigations, two in July and one in August. However, the crop response to these irrigations was minimal. The irrigated area of the field yielded 831.6 lbs/A and the unirrigated area 812.1 lbs/A. The plant water potentials and fiber quality responses reflected the same trends. Only on August 11, were the plant water potentials significantly different between the irrigated and unirrigated plots. Total mote counts, long fiber mote counts, short fiber content, circularity, immature fiber fraction and micronafis were not significantly different in bolls taken from the irrigated plots compared to bolls from the unirrigated plots. It is hypothesized that the low solar radiation and high night time temperatures led to low photosynthesis and high respiration rates which means a severe carbon stress in the plants. GOSSYM estimates of carbon stress showed that the plants were carbon limiting on 38 days out of 73 days during the boll growth period. This overshadowed the water stress effects. A repeat of these observations in 1994 is planned if funding can be secured.