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Effect of Water Stress on Canopy Senescence and Carbon Exchange Rates in Cotton¹

The work reported here was done as part of an effort to develop a physiological process level data base for the simulation of cotton (Gossypium hirsutum L.) growth and yield. The effects of water stress on vegetative growth and on carbon exchange rate (CER) in cotton canopies were investigated in naturally illuminated, controlled environment cabinets (SPAR units). The control treatment was irrigated daily. In the other treatments, irrigation was stopped just before the appearance of the first blooms. An extra low water treatment was allowed to dry much faster than the low water treatment. Water stress had a strong effect on vegetative growth. Rate of increase in plant height and leaf area declined when midday leaf water potential was lower than –1.4 MPa, and they were almost zero at –2.0 to –2.5 MPa. Water stress had a similar effect on the CER over a wide range of radiation intensities. CER started to decline < 2 weeks after irrigation was stopped, and it was about 50% of the control when midday leaf water potential reached –2.0 to –2.5 MPa. Two different mechanisms are suggested for the effect of water stress on canopy photosynthesis. A direct effect on single leaves may have closed stomata, increased mesophyll resistance, or decreased Hill reaction activity. Additionally, there was the long-term effect of reduced leaf growth, which produced a canopy consisting mainly of old, less photosynthetically efficient leaves.

Key Words: CER • Gossypium hirsutum L. • Leaf growth • Leaf water potential • Plant height

² Visiting scientist (on leave from the Hebrew Univ. of Jerusalem, Faculty of Agric., Rehovot 76100, Israel), research agronomist, USDA-ARS, research associate, Mississippi Agric. and Forestry Exp. Stn., and electronics engineer, USDA-ARS, respectively.

Received for publication June 11, 1984.

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