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Published in Crop Sci 26:112-117 (1986)
© 1986 Crop Science Society of America
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Photosynthetic Rate Control in Sorghum: Stomatal and Nonstomatal Factors

D. R. Krieg and R. B. Hutmacher2

The purpose of this series of experiments was to determine the cause of photosynthetic rate responses to environmental effectors in one grain sorghumh ybrid [Sorghum bicolor (L.) Moench cv. ATX623 x TX430]. Field experiments were conducted over a 4-year period (1980–1983) at two locations using a range irrigation levels, to provide different degrees of plant waters tress. One site had a fine loamy sand soil (fine loamy, mixed, thermic family of Aridic Paleustaifs) with the second site having a clay loam (fine mixed, thermic family of Torrertic Paleustolls). Gas exchange rates of individual leaves were monitored on both a diurnal and seasonal basis. The results indicated that the primary cause of the photosynthetic rate changes due to leaf age, water stress, and photon flux density effects was related to mesophyll problems rather than due to stomatal conductance limitations. Increasing leaf age and environmental stress affected the photosynthetic processes by reducing the CO2 saturated rate (Amax) and the assimilation rate at all intercellular CO2 concentrations evaluated. The assimilation rate per unit photon flux density was also reduced in water-stressed plants. Stomatal conductance responded to photosynthetic rate changes to maintain a relatively constant intercellular CO2 concentration at ambient CO2 levels. The primary control of photosynthetic carbon fixation in sorghum appears to be related to chloroplast activity rather than to CO2 supply due to stomatal conductance.

Key Words: Intercellular CO2 • Stomatal conductance • Leaf water potential • Photon flux density • Growth stage • Leaf age

1 Joint contribution of Texas Tech Univ. and the Texas Agric. Exit. Stn. Financial support was provided by USDA-CRGO grant no. 5901-0410-8-0095-0, Texas Tech Univ. Water Resources Ctr., and Pioneer HiBred Int.

2 Professor of crop physiology and former graduate research assistant. Plant and Soil Science Dep. Texas Tech Univ. Lubbock, Texas 79409.

Received for publication October 26, 1984.




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Copyright © 1986 by the Crop Science Society of America.