GrainGenes

II.25. Carbon dioxide compensation values of barley trisomics.

Robert G. McDaniel. Department of Agronomy and Plant Genetics, University of Arizona, Tucson, Arizona 85721, U.S.A.

Carbon dioxide compensation values of intact barley leaves were measured using a closed gas system in an infra-red gas analyser. Plant material was greenhouse-grown within a temperature range of 15 to 30°C. Plants were measured at approximately 60 days of age. Upper portions of leaves were inserted in a plexiglass chamber which was then sealed with clay. After the system was equilibrated with standard gas of known CO₂ concentration, the system was closed and leaf gas exchange was allowed to proceed until a stable meter reading was obtained. During this 10 to 25 minute period, CO₂ concentration in the chamber was reduced to the compensation value of the leaf under conditions of measurement.

Representative data of a series of experiments are given in Table 1. These experiments indicate much higher CO₂ compensation values for barley trisomics than for diploid counterparts. A temperature effect on compensation values may also be seen, as has been reported by numerous workers; most recently in barley by Herath and Ormrod (Plant Physiol. 49:443, 1972). A "dicoccoides" type wheat, showing intermediate CO₂ compensation values under these conditions, is also included for comparison.

Table 1. Carbon dioxide compensation values and photosynthetic rates of barley diploids and primary trisomics as influenced by temperature and kinetin treatment. Light intensity: ca. 5 x 10⁵ erg. cm^-2.sec^-1. Photosynthesis measured at ~35 C.

The data in Table 1 are also illustrative of previous reports indicating inferior photosynthetic rates of barley trisomics in comparison with normal diploids (McDaniel, BGN 1:29, 1971). Additionally, kinetin (10^-6M) treatment has been shown to enhance photosynthetic rates of barley trisomics (McDaniel, BGN 2:51, 1972), an effect which is also noted at the high temperatures used in present experiments. My work indicates that the kinetin response of barley trisomics is temperature sensitive, and is most readily observed under conditions of high temperature stress.

This effect may be related to high temperature-induced destruction of endogenous cytokinins, or by an impairment of their transport from roots in barley trisomics. Alteration of stomatal diameter can not be discounted as contributing to high compensation values observed with trisomics, although this was not a factor in earlier experiments at high CO₂ concentrations.

At the biochemical level, the interpretation of data on compensation values, hormone response and photosynthetic rates of barley trisomics may relate to an insufficiency of Photosystem I. This may be similar to the case described by Alberte, et al. in pale-green cotton mutants (Agron. Absts. p 29, 1972). My data, in fact, show chlorophyll and other pigment differences between trisomics and diploids much as they have been described in cotton. Investigations on the biochemical basis of barley trisomic response to light, temperature, and hormones are continuing.

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