An Open-Circuit Infrared Gas Analysis System for Measuring Aquatic Plant Photosynthesis at Physiological pH

A temperature-controlled assimilation chamber and gas circuit are described which allow steady-state measurement of photosynthesis by aquatic plants in media containing bicarbonate ions. Gas entering the assimilation chamber is divided into fine bubbles by passage through a sintered glass disc which forms the chamber floor. Rapid achievement and subsequent maintenance of chemical and phase equilibria are ensured by sustaining a high gas flow through the chamber via a recirculating loop in the gas circuit. Incorporation of this unit in a conventional open infrared gas analysis system allowed accurate and sensitive measurements to be made over wide ranges of pH and CO2 concentration.

A comparison of barley cultivars with different leaf inclinations

Under conditions of adequate moisture an erect-leaf barley cultivar, Lenta, responded to an increase in density (resulting from doubling of the sowing rate) with increases in dry matter production and in grain yield, whereas the cultivar Research, which has long lax leaves, responded with decreases in dry matter production and yield. In a study of canopy structure and its effects on light interception and utilization, it was found that in Research, with a leaf area index (LAI) of 6.1, the leaves were concentrated near the canopy surface and a relatively small proportion of the above-crop light penetrated through this layer. The net crop photosynthesis of this canopy (measured in a field assimilation chamber) was 3.8 g CO2/m2.hr when visible radiation was 313 W/m2. With Lenta (LAI 7.0) on the other hand, leaves were concentrated in the middle layers of the canopy and the light was more evenly distributed throughout the canopy. The net crop photosynthesis with the same radiation as for Research was 4.3 g CO2/m2.hr. The relative rates of photosynthesis at various levels in the canopies were determined by introducing 14CO2 into the assimilation chambers enclosing the cultivars and observing where the 14C was fixed. With Research most of it was localized near the canopy surface while with Lenta most of it was near the centre of the canopy. Of the 14CO2 taken up, 7 % was fixed in the leaf sheaths of Research and 12% in those of Lenta.

Photosynthesis of wheat under field conditions. V.* The effect of sugar elevation on the distribution of photosynthetic activity within the crop canopy

The distribution of photosynthetic activity within the canopy of a wheat community was measured at different times of the day Plants In the field were covered by a Perspex assimilation chamber and exposed to 14CO2 and light for 15 min., then harvested and dissected. The relative specific activity and dry weight of the parts, and the calculated total carbon dioxide uptake of the community were then used to determine the distribution of photosynthesis within the community. The contribution of the lower leaves and leaf sheaths increased with increasing elevation of the sun up to 67°, while the photosynthesis of the flag leaves approached a maximum when the elevation of the sun was only 28°. The relative contribution of the ears to total photosynthesis decreased from 33 to 13% as the solar elevation increased from 17 to 67°. __________________ *Part IV, Aust. J. Agric. Res., 22: 1 I (1971).

Photosynthesis of wheat under field conditions. III. Seasonal trends in carbon dioxide uptake of crop communities

Photosynthesis of two wheat cultivars grown in the field was examined during three seasons by use of a portable field assimilation chamber. There were large differences in dry weight, leaf area, and carbon dioxide uptake between seasons. Variations in carbon dioxide uptake by the community were related mainly to changes in leaf area index (LAI). There were changes in carbon dioxide uptake per unit LAI with time, and between the two cultivars in the first season, but the effects of these changes were small compared with the effects of LAI. Differences in grain yield were correlated with LAI and carbon dioxide uptake in the period after anthesis.

Photosynthesis of wheat under field conditions. II.* Effect of defoliation on the carbon dioxide uptake of the community

A field assimilation chamber was used to measure the carbon dioxide uptake of wheat plants in the field before and after defoliation or ear removal. At anthesis and 10 days later only the top three leaves were effective in photosynthesis. Removal of the two leaves below the flag leaf reduced photosynthesis of the community by 25–28% and further removal of the flag leaf reduced the community photosynthesis by an additional 24–30%. The estimated contribution of the ears depended on the technique of measurement and varied from 7 to 15%. After removal of the leaf laminae and ears, the stems and leaf sheaths assimilated carbon dioxide at a rate of 44% of that of the whole community. For defoliated plants, photosynthesis of the ears and stems increased with increasing elevation of the sun, which indicated a greater efficiency of photosynthesis with an increased horizontal component of radiation. This was opposite to the response of the undefoliated community. The techniques of estimating the contribution of individual organs to total photosynthesis of the wheat plant are discussed in relation to variability of results and the effects of compensation. * Part I, Aust. J. Agric. Res., 1968, 19: 711.

Corrigendum - Photosynthesis of wheat under field conditions. II. Effect of defoliation on the carbon dioxide uptake of the community

A field assimilation chamber was used to measure the carbon dioxide uptake of wheat plants in the field before and after defoliation or ear removal. At anthesis and 10 days later only the top three leaves were effective in photosynthesis. Removal of the two leaves below the flag leaf reduced photosynthesis of the community by 25–28% and further removal of the flag leaf reduced the community photosynthesis by an additional 24–30%. The estimated contribution of the ears depended on the technique of measurement and varied from 7 to 15%. After removal of the leaf laminae and ears, the stems and leaf sheaths assimilated carbon dioxide at a rate of 44% of that of the whole community. For defoliated plants, photosynthesis of the ears and stems increased with increasing elevation of the sun, which indicated a greater efficiency of photosynthesis with an increased horizontal component of radiation. This was opposite to the response of the undefoliated community. The techniques of estimating the contribution of individual organs to total photosynthesis of the wheat plant are discussed in relation to variability of results and the effects of compensation.