The rapid elongation (~2.5 cm in 20 days) that occurs in fiber initials suggests that fiber elongation may be very sensitive to water stress. However, bolls have water potentials that are 3 to 5 bars higher than leaves. Although water stress can limit early fiber development, severe water stress (greater than -26 bars) is required before significant decreases in fiber length are observed (Grimes and Yamada, 1982). Moderate water stress reduces yields due to abscission of floral buds and bolls, and reduced flowering (Cook and El-Zik, 1993). To further explore the effect of water stress on fiber development and final fiber quality, we subjected plants to moderate and high levels of water stress. Plants were grown in 12 l pots in an outside growing area. Plants were watered daily with half-strength Hoagland's nutrient solution. Bolls were tagged twice-weekly from 50% first flowering. Water stress treatment was imposed after 50% first flowering to maintain ~1.8 MPa midday leaf water potential (moderate) and ~2.25 MPa midday leaf water potential (severe). Control plants maintained midday leaf water potentials near 1.24 MPa and were well watered. Plants were rewatered 21 days after initiation of water stress. At 4 day intervals after imposition of water stress, bolls that were 4 and 7 days old on day of treatment were harvested. Fiber length and fresh and dry weights were determined for the immature bolls. At frequent intervals during stress treatment and after rewatering, leaf water potentials were determined using a Sholander pressure chamber. Leaf photosynthetic rates were measured with a LiCor 6200 gas exchange system. Leaf, air and boll temperatures were determined using fine-gauge thermocouples placed throughout the canopy. Although leaf water potentials and photosynthetic activities were significantly reduced by both water stress levels, no differences were observed in fiber elongation. A significant difference in canopy air temperature was observed in plants under water stress conditions. Thus, a potentially more significant impact of water stress is the increased heat load to the plant due to the inability to adequately cool through transpiration, resulting in greater boll temperatures. Additional constraints due to reduced photosynthetic rates under water stress conditions may limit carbon uptake, inducing greater boll and square abscission.

Cook, CG, El-Zik, KM 1993 Field Crops Res 33:411-421 Grimes, DW, Yamada, H 1982 Crop Sci 22:134-139