The temperature response of net CO 2 assimilation rate (A), the rate of whole-chain electron transport, the activity and activation state of Rubisco, and the pool sizes of ribulose-1,5-bisphosphate (RuBP) and 3-phosphoglyceric acid (PGA) were assessed in sweet potato (Ipomoea batatas) grown under greenhouse conditions. Above the thermal optimum of photosynthesis, the activation state of Rubisco declined with increasing temperature. Doubling CO 2 above 370 mbar further reduced the activation state, while reducing CO 2 by one-half increased it. At cool temperature (,16°C), the activation state of Rubisco declined at CO 2 levels where photosynthesis was unaffected by a 90% reduction in O 2 content. Reduction of the partial pressure of CO 2 at cool temperature also enhanced the activation state of Rubisco. The rate of electron transport showed a pronounced temperature response with the same temperature optimum as A at elevated CO 2 . RuBP pool size and the RuBPto-PGA ratio declined with increasing temperature. Increasing CO 2 also reduced the RuBP pool size. These results are consistent with the hypothesis that the reduction in the activation state of Rubisco at high and low temperature is a regulated response to a limitation in one of the processes contributing to the rate of RuBP regeneration. To further evaluate this possibility, we used measured estimates of Rubisco capacity, electron transport capacity, and the inorganic phosphate regeneration capacity to model the response of A to temperature. At elevated CO 2 , the activation state of Rubisco declined at high temperatures where electron transport capacity was predicted to be limiting, and at cooler temperatures where the inorganic phosphate regeneration capacity was limiting. At low CO 2 , where Rubisco capacity was predicted to limit photosynthesis, full activation of Rubisco was observed at all measurement temperatures.

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In many legumes, the nitrogen fixing root nodules produce H2 gas that diffuses into soil. It has been demonstrated that such exposure of soil to H2 can promote plant growth. To assess whether this may be due to H2-oxidizing microorganisms, bacteria were isolated from soil treated with H2 under laboratory conditions and from soils collected adjacent to H2 producing soybean nodules. Nineteen isolates of H2-oxidizing bacteria were obtained and all exhibited a half-saturation coefficient (Ks) for H2 of about 1 ml l(-1). The isolates were identified as Variovorax paradoxus, Flavobacterium johnsoniae and Burkholderia spp. using conventional microbiological tests and 16S rRNA gene sequence analysis. Seventeen of the isolates enhanced (57-254%) root elongation of spring wheat seedlings. Using an Arabidopsis thaliana bioassay, plant biomass was increased by 11-27% when inoculated by one of four isolates of V. paradoxus or one isolate of Burkholderia that were selected for evaluation. The isolates of V. paradoxus found in both H2-treated soil and in soil adjacent to soybean nodules had the greatest impact on plant growth. The results are consistent with the hypothesis that H2-oxidizing bacteria in soils have plant growth promoting properties.

1993. The effect of exposure to enhanced UV-B radiation on the penetration of monochromatic and polychromatic UV-B radiation in leaves of Brassiea napus. -Physiol. Plant. 87: 249-255.Using quartz optical fibres, penetration of both monochromatic (310 nm) and polychromatic UV-B (280-320 nm) radiation in leaves of Brassica napus L. (cv. Ceres) was measured. Plants were grown under either visible light (750 ^mol m -s ' photosynthetically active radiation) or with the addition of 8.9 kJ m"-day~' biologically effective UV-B (UV-B^E) radiation. Results showed that of the 310 nm radiation that penetreated the leaf. 90% was within the initial one third of the leaf with high attenuation in the leaf epidermis, especially in UV-treated plants. Polychromatic UV-B radiation, relative to incident radiation, showed a relatively uniform spectral distribution within the leaf except for collimated radiation. Over 30% of the UVscreening pigments in the leaf, including flavonoids. were found in the adaxial epidermal layer, making this layer less transparent to UV-B radiation than the abaxial epidermis, which contained less than 12% of the UV-screening pigments.UVsereening pigments increased by 20% in UV-treated leaves relative to control leaves. Densely arranged epicuticular wax on the adaxial leaf surface of UV-treated plants may have further decreased penetration of UV-B radiation by reflectance. An increased leaf thickness, and decreases in leaf area and leaf dry weight were also found for UV-treated plants.

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