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The model of Farquhar, von Caemmerer and Berry is the standard in relating photosynthetic carbon assimilation and concentration of intercellular CO(2). The techniques used in collecting the data from which its parameters are estimated have been the object of extensive optimization, but the statistical aspects of estimation have not received the same attention. The model segments assimilation into three regions, each modeled by a distinct function. Three parameters of the model, namely the maximum rate of Rubisco carboxylation (V(c max)), the rate of electron transport (J), and nonphotorespiratory CO(2) evolution (R(d)), are customarily estimated from gas exchange data through separate fitting of the component functions corresponding to the first two segments. This disjunct approach is problematic in requiring preliminary arbitrary subsetting of data into sets believed to correspond to each region. It is shown how multiple segments can be estimated simultaneously, using the entire data set, without predetermination of transitions by the investigator. Investigation of the number of parameters that can be estimated in the two-segment model suggests that, under some conditions, it is possible to estimate four or even five parameters, but that only V(c max), J, and R(d), have good statistical properties. Practical difficulties and their solutions are reviewed, and software programs are provided.

The heterotrimeric G-protein complex provides signal amplification and target specificity. The Arabidopsis (Arabidopsis thaliana) Gβ-subunit of this complex (AGB1) interacts with and modulates the activity of target cytoplasmic proteins. This specificity resides in the structure of the interface between AGB1 and its targets. Important surface residues of AGB1, which were deduced from a comparative evolutionary approach, were mutated to dissect AGBl-dependent physiological functions. Analysis of the capacity of these mutants to complement well-established phenotypes of Gβ-null mutants revealed AGB1 residues critical for specific AGB1-mediated biological processes, including growth architecture, pathogen resistance, stomata-mediated leaf-air gas exchange, and possibly photosynthesis. These findings provide promising new avenues to direct the finely tuned engineering of crop yield and traits. [ABSTRACT FROM AUTHOR]

Ozone-sensitive and -tolerant individuals of cutleaf coneflower (Rudbeckia laciniata L.) were compared for their gas exchange characteristics and total non-structural carbohydrates at Purchase Knob, a high elevation site in Great Smoky Mountains National Park, USA. Photosynthesis and stomatal conductance decreased with increased foliar stipple. Sensitive plants had lower photosynthetic rates for all leaves, except the very youngest and oldest when compared to tolerant plants. Stomatal conductance decreased with increasing leaf age, but no ozone-sensitivity differences were found. Lower leaves had less starch than upper ones, while leaves on sensitive plants had less than those on tolerant plants. These results show that ambient levels of ozone in Great Smoky Mountains National Park can adversely affect gas exchange, water use efficiency and leaf starch content in sensitive coneflower plants. Persistence of sensitive genotypes in the Park may be due to physiological recovery in low ozone years. [Copyright &y& Elsevier]

The inhibitory effects of tropospheric O3 on crop photosynthesis, growth, and yield have been documented in numerous studies over the past 35 years. In large part, the results of this research supported governmental regulations designed to limit tropospheric O3 levels to concentrations that affected crop production at economically acceptable levels. Recent studies have brought into question the efficacy of these concentration-based O3 standards compared with flux-based approaches that incorporate O3 uptake along with environmental and biotic factors that influence plant responses. In addition, recent studies provide insight into the biochemical mechanisms of O3 injury to plants. Current interpretations suggest that upon entry into the leaf intercellular space O3 rapidly reacts with components of the leaf apoplast to initiate a complex set of responses involving the formation of toxic metabolites and generation of plant defence responses that constitute variably effective countermeasures. Plant species and cultivars exhibit a range of sensitivity to O3, evident as heritable characteristics, that must reflect identifiable biochemical and molecular processes that affect sensitivity to O3 injury, although their exact makeup remains unclear. Ozone clearly impairs photosynthetic processes, which might include the effects on electron transport and guard cell homeostasis as well as the better-documented effects on carbon fixation via decreased Rubisco activity. Translocation of photosynthate could be inhibited by O3 exposure as well. Further, the influence of tropospheric O3 needs to be considered when assessing potential effects of rising concentrations of atmospheric CO2 on crop production. Advances in O3 flux modelling and improved understanding of biochemical and molecular effects of O3 on photosynthetic gas exchange and plant defence processes are leading to more complete, integrated assessments of O3 impacts on crop physiology that continue to support the rationale for maintaining or improving current O3 air quality standards as well as providing a basis for development of more O3-tolerant crop lines. [ABSTRACT FROM AUTHOR]

Growth, seed yield and accumulation of ultraviolet (UV)-absorbing compounds were studied in chalcone isomerase-defective tt-5 mutant of Arabidopsis thaliana and its Landsberg erecta (Ler) progenitor under full-spectrum solar radiation and a series of filters which attenuated progressively larger portions of the UV-B and UV-A radiation bands. The purpose was to determine: (1) whether or not the tt-5 mutant could be induced to grow more or less normally, given adequate protection from damaging UV in the presence of high levels of photosynthetic active radiation (PAR) so that it could be used as a surrogate for mechanistic high UV studies; (2) whether the generalized plant action spectrum or the alfalfa DNA damage action spectrum would best describe the observed responses; and (3) if the traditional Mylar (polyester) filter provides an adequate control for UV damage studies. Maximum rosette diameter (MRD), plant height and fresh weight at harvest and seed yield were measured, along with absorbance of leaf extracts at 300 nm and accumulation of total phenolics before and after exposure to UV. Three types of UV filters were used: cellulose diacetate (CD), which non-selectively transmits all the UV reaching the earth''s surface; Mylar, which cuts off UV below about 320 nm; and polyvinyl chloride (PVC) which cuts off UV below about 340 nm. Generally, Ler showed no significant growth effects under any of the treatments except for plant height which was reduced in Mylar and CD when compared to PVC. Conversely, tt-5 generally exhibited progressive decreases in all the measures of plant growth with PVC resulting in the best growth, Mylar treatments showing significant reductions and CD treatments even greater reductions. It was clear that even under these circumstances: the disruption to secondary metabolism in tt-5 makes it unsuitable for mechanistic studies of high UV-B damage; the alfalfa DNA action spectrum seemed the best correlated with observed responses and suggests a significant damaging radiation band which is not affected by stratospheric ozone; and since the damaging radiation extends beyond the Mylar cut-in, this material will not provide an adequate control for UV damage studies. [Copyright &y& Elsevier]

Understanding the impact of pollutant ozone (O3) is a concern for agricultural production. This work was undertaken as the first comparative study of the effects of O3 on the photosynthetic processes and yield of three snap bean (Phaseolus vulgaris L.) genotypes with known differences in sensitivity to O3 (S156, R123 and R331). Previous information showed R123 and R331 to be tolerant and S156 sensitive. The purpose was to identify physiological subsystems that may mediate those differences in sensitivity. Plants were grown in environmentally controlled field chambers with four levels of O3 (0, 15, 30 and 60nmolmol−1). Net assimilation (A) and fluorescence were measured throughout the growing season and yield data were collected at physiological maturity. All genotypes were tolerant of low O3 (<30nmolmol−1) but the highest O3 significantly reduced the yield in all three, with R331 and S156 being equally sensitive on a unit exposure basis. Yield reductions were correlated with A, especially during pod filling. No genotype showed any significant response of stomatal conductance (g s) indicating equal O3 fluxes into the leaves in all genotypes. Mesophyll conductance (g m) was affected in S156 only, where it was reduced by 55% at 60nmolmol−1 O3. There was an upward trend in F 0, and a downward trend in the variable fluorescence ratio (F v/F m) with increasing O3 for S156 but not for the other genotypes. S156 was the only genotype to show significant decreases in photochemical quenching (q p) and R123 the only one to show significant decreases in non-photochemical quenching (q n). The sequence of loss of Rubisco content and/or activity and changes in g m, F 0, and F v/F m could not be resolved in time and may all have been the result of generalized tissue destruction rather than sequential attack on individual subsystems. S156 had the highest photosynthetic rate in clean air but appeared to have no significant capacity to protect Rubisco from attack or to up-regulate Rubisco activity at high O3, thus there was no reserve capacity, while R123 was able to maintain both Rubisco activity and A within narrow ranges. These data suggest that S156 has comparatively little anti-oxidant capacity and/or is deficient in its ability to regulate Rubisco activity. For future studies the best contrasts for resolving questions of physiological sensitivity to O3 would be obtained from R123 and S156. [Copyright &y& Elsevier]