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Identifying Turgor Responses of Water-Stressed Cotton to Rapid Changes in Net Radiation Using Spectral Analysis¹

The kinetics of stem shrinkage and swelling in response to changes in the radiation loading of field-grown cotton (Gossyplum hirsutum L.) plants were examined by time series spectral analysis. This technique involves determination in the frequency domain of the relationship between two time series events. The type and degree of relationship a.re assessed by computing the squared coherency, phase, and gain spectra relating events in the measured net radiation and stem diameter data sets. The squared coherency spectrum shows the statistical strength of the relationship between events in the two time series, while the grain spectrum indicates the amount of response, or the mm of change in stem diam. eter resulting from a unit change in radiation intensity at each frequency. The phase spectra show the relative positions between maximum points in the two time series

Plants growing under conditions of water stress respond to changes in ,radiation much more rapidly than those growing on well-watered, deep soils. Although low frequency (slow) shrinkage response may be caused factors affecting both evapotranspiration from the leaves and root water uptake, high frequency (rapid) shrinkage or swelling responses are probably controlled exclusively by alterations in the rate of water loss because of the water storage capacity of the plant itself.

Key Words: Stomata • Transpiration • Roots • Microclimate • Time-series spectral analysis • Kinetics • Water movement • Gossypium hirsutum L.

² Assistant professor, Dep. of Geography, Indiana Univ., Bloomington, Ind.; Soil scientist, USDA, Auburn, A1a., respectively.

Received for publication December 22, 1977.