Edwin L. Fiscus

Is increased UV-B a threat to crop photosynthesis and productivity?

It has been suggested that increases in ground-level UV-B, as a result of stratospheric ozone depletion, may have major deleterious effects on crop photosynthesis and productivity. The direct consequences of such effects have been projected by some as a world-wide decrease in crop yields of 20–25%. Further losses, or unrealized gains, have also been suggested as a result of increased UV-B counteracting the beneficial effects of elevated atmospheric CO2. Deleterious UV-B effects may be largely partitioned between damage to the plant genome and damage to the photosynthetic machinery. Direct damage to DNA is a common result of absorption of high energy UV-B photons. However, most plants possess repair mechanisms adequate to deal with the levels of damage expected from projected increases in ground-level UV-B. In addition, most plants have the ability to increase production of UV-absorbing compounds in their leaves as a result of exposure to UV-B, UV-A and visible radiation. These compounds contribute substantially to reducing UV-B damage in situ. It has also been shown that in some plants, under the proper conditions, almost every facet of the photosynthetic machinery can be damaged directly by very high UV-B exposures. However, electron transport, mediated by Photosystem II (PS II) appears to be the most sensitive part of the system. Various laboratories have reported damage to virtually all parts of the PS II complex from the Mn binding site to the plastoquinone acceptor sites on the opposite surface of the thylakoid membrane. However, a critical review of the literature with emphasis on exposure protocols and characterization of the radiation environment, revealed that most growth chamber and greenhouse experiments and very many field experiments have been conducted at unrealistic or indeterminate UV-B exposure levels, especially with regard to the spectral balance of their normal radiation environment. Thus, these experiments have led directly to large overestimates of the potential for damage to crop photosynthesis and yield within the context of 100 year projections for stratospheric ozone depletion. Indeed, given the massive UV-B exposures necessary to produce many of these effects, we suggest it is unlikely that they would occur in a natural setting and urge reconsideration of the purported impacts of projected increases of UV-B on crop productivity.

Increased protein carbonylation in leaves of Arabidopsis and soybean in response to elevated [CO2]

While exposure of C3 plants to elevated [CO2] would be expected to reduce production of reactive oxygen species (ROS) in leaves because of reduced photorespiratory metabolism, results obtained in the present study suggest that exposure of plants to elevated [CO2] can result in increased oxidative stress. First, in Arabidopsis and soybean, leaf protein carbonylation, a marker of oxidative stress, was often increased when plants were exposed to elevated [CO2]. In soybean, increased carbonyl content was often associated with loss of leaf chlorophyll and reduced enhancement of leaf photosynthetic rate (Pn) by elevated [CO2]. Second, two-dimensional (2-DE) difference gel electrophoresis (DIGE) analysis of proteins extracted from leaves of soybean plants grown at elevated [CO2] or [O3] revealed that both treatments altered the abundance of a similar subset of proteins, consistent with the idea that both conditions may involve an oxidative stress. The 2-DE analysis of leaf proteins was facilitated by a novel and simple procedure to remove ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from soluble soybean leaf extracts. Collectively, these findings add a new dimension to our understanding of global change biology and raise the possibility that oxidative signals can be an unexpected component of plant response to elevated [CO2].

Growth of Arabidopsis flavonoid mutants under solar radiation and UV filters

Abstract Growth of the chalcone isomerase defective tt-5 mutant of Arabidopsis thaliana and its Landsberg erecta progenitor were compared under a variety of full spectrum solar radiation conditions to determine if the tt-5 mutant could serve as an adequate subject for studies of the mechanisms of damage by UV-B radiation. An experiment was conducted in the fall of 1995 under open field filter frames using cellulose diacetate and Mylar filters to transmit and exclude natural UV-B irradiation, respectively. Even though growth under these conditions was slow and erratic owing to lack of temperature control, growth suppression as indicated by rosette diameter and harvest fresh weights provided a sensitive indicator of UV-B stress. This experience led to development of temperature-controlled Teflon-covered field chambers that admit up to 88% of the total daily PAR and about 85% of ambient UV-B, omit predators, and provide a generally stable environment for quantitative plant growth studies. The chambers were designed to facilitate the addition of optical filters and/or shade cloth and to accommodate control of the gaseous environment for pollutant and climate change studies and to provide clean air for other experiments. Three additional experiments were conducted in these chambers. Measurements of rosette diameter, weights of various aboveground plant parts, and plant height were evaluated as potential methods of comparing growth sensitivities of the tt-5 mutant to UV-B radiation. The weight of the reproductive parts (flowers and siliques) as a fraction of the total (e.g. harvest index) was consistently and negatively affected by solar UV-B, as was simple plant height. However, in no case, even in the virtual absence of UV-B, was growth of tt-5 comparable to that of Ler. We conclude that the disruption of secondary metabolism in tt-5 has growth implications far beyond the lack of UV-B protection, making it unsuitable as a surrogate for high UV-B experimentation.

Minimal influence of G-protein null mutations on ozone-induced changes in gene expression, foliar injury, gas exchange and peroxidase activity in Arabidopsis thaliana L.

Ozone (O(3)) uptake by plants leads to an increase in reactive oxygen species (ROS) in the intercellular space of leaves and induces signalling processes reported to involve the membrane-bound heterotrimeric G-protein complex. Therefore, potential G-protein-mediated response mechanisms to O(3) were compared between Arabidopsis thaliana L. lines with null mutations in the α- and β-subunits (gpa1-4, agb1-2 and gpa1-4/agb1-2) and Col-0 wild-type plants. Plants were treated with a range of O(3) concentrations (5, 125, 175 and 300 nL L(-1)) for 1 and 2 d in controlled environment chambers. Transcript levels of GPA1, AGB1 and RGS1 transiently increased in Col-0 exposed to 125 nL L(-1) O(3) compared with the 5 nL L(-1) control treatment. However, silencing of α and β G-protein genes resulted in little alteration of many processes associated with O(3) injury, including the induction of ROS-signalling genes, increased leaf tissue ion leakage, decreased net photosynthesis and stomatal conductance, and increased peroxidase activity, especially in the leaf apoplast. These results indicated that many responses to O(3) stress at physiological levels were not detectably influenced by α and β G-proteins.

Soil Microbial Responses to Elevated CO2 and O3 in a Nitrogen-Aggrading Agroecosystem

Climate change factors such as elevated atmospheric carbon dioxide (CO2) and ozone (O3) can exert significant impacts on soil microbes and the ecosystem level processes they mediate. However, the underlying mechanisms by which soil microbes respond to these environmental changes remain poorly understood. The prevailing hypothesis, which states that CO2- or O3-induced changes in carbon (C) availability dominate microbial responses, is primarily based on results from nitrogen (N)-limiting forests and grasslands. It remains largely unexplored how soil microbes respond to elevated CO2 and O3 in N-rich or N-aggrading systems, which severely hinders our ability to predict the long-term soil C dynamics in agroecosystems. Using a long-term field study conducted in a no-till wheat-soybean rotation system with open-top chambers, we showed that elevated CO2 but not O3 had a potent influence on soil microbes. Elevated CO2 (1.5×ambient) significantly increased, while O3 (1.4×ambient) reduced, aboveground (and presumably belowground) plant residue C and N inputs to soil. However, only elevated CO2 significantly affected soil microbial biomass, activities (namely heterotrophic respiration) and community composition. The enhancement of microbial biomass and activities by elevated CO2 largely occurred in the third and fourth years of the experiment and coincided with increased soil N availability, likely due to CO2-stimulation of symbiotic N2 fixation in soybean. Fungal biomass and the fungi∶bacteria ratio decreased under both ambient and elevated CO2 by the third year and also coincided with increased soil N availability; but they were significantly higher under elevated than ambient CO2. These results suggest that more attention should be directed towards assessing the impact of N availability on microbial activities and decomposition in projections of soil organic C balance in N-rich systems under future CO2 scenarios.

Is UV-B a Hazard to Soybean Photosynthesis and Yield? Results of an Ozone-UV-B Interaction Study and Model Predictions

Current levels of tropospheric ozone suppress photosynthesis and yield in soybean. Also, it has been suggested that increased ground-level UV-B as a result of stratospheric ozone depletion may have additional deleterious effects. A three-year field study, conducted in open-top chambers, was undertaken to uncover possible interactions between these putative stressors. Ozone treatments resulted in the expected and well-documented reductions in photosynthesis, yield and acceleration of senescence. However, UV-B treatments not only failed to induce any significant interactions, but did not induce any significant reductions in photosynthesis or yield, even at levels simulating a 35% column ozone depletion. Reconciliation of our data with other predictions of physiological dysfunction and crop losses due to increased UV-B was attempted by examining the models used to predict ground-level UV-B, ground truthing, and critically reviewing the literature. Comparison of ground-based measurements at our location with the Green et al. (9) model, frequently used to predict ground-level UV-B, showed consistent over-predictions of clear-sky UV-B of 32% on an annual basis. We believe that similar over-predictions have led some researchers to underestimate the actual dosages used, with the result that the effects reported would normally only occur at much higher UV-B levels and are much greater than would occur at the reported dosages. Lack of ground-level UV-B monitoring in many experiments has obscured and perpetuated this problem. Also, there generally has been no adjustment of enhancement levels for either season or weather conditions, except where modulated systems have been used, so that effects are additionally exaggerated for these reasons. Interpretation of experimental results is confounded by these four factors (model over-prediction, seasonal changes, weather changes, and failure to monitor UV-B) and made much more difficult when UV-B enhancement experiments are conducted under greenhouse growth conditions. Additional illustrative calculations for greenhouse conditions are included for consideration. Examination of the literature in light of these findings indicates there is little evidence that increased ground-level UV-B, well in excess of current predictions for the next century, will pose any hazard to soybean growth and productivity.

Temperature influences the ability of tall fescue to control transpiration in response to atmospheric vapour pressure deficit

Water availability for turfgrass systems is often limited and is likely to become more so in the future. Here, we conducted experiments that examined the ability of tall fescue (Festuca arundinacea Schreb.) to control transpiration with increasing vapour pressure deficit (VPD) and determined whether control was influenced by temperature. The first study was under steady-state conditions at two temperatures (21 and 27°C) and two VPDs (1.2 and 1.8kPa). At the lower temperature, water use was similar at both VPDs, indicating a restriction of transpiration at high VPD. At 27°C, transpiration control at high VPD was weakened and root growth also declined; both responses increase susceptibility to water-deficit stress. Another series of experiments was used to examine the physiological stability of the transpiration control. Temperature and VPD were adjusted in a stepwise manner and transpiration measured across a range of VPD in the days following environmental shifts. Results indicated that VPD control acclimated to the growth environment, with adjustment to drier conditions becoming evident after ~1 week. Control was again more effective at cool than at hot temperatures. Collectively, the results indicate that transpiration control by this cool season grass is most effective in the temperature range where it is best adapted.

Atmospheric vapor pressure deficit is critical in predicting growth response of "cool-season" grass Festuca arundinacea to temperature change.

There is a lack of information on plant response to multifactor environmental variability including the interactive response to temperature and atmospheric humidity. These two factors are almost always confounded because saturated vapor pressure increases exponentially with temperature, and vapor pressure deficit (VPD) could have a large impact on plant growth. In this study using climate controlled mini-greenhouses, we examined the interacting influence of temperature and VPD on long-term growth of tall fescue (Festuca arundinacea Schreb), a cool season grass. From past studies it was expected that growth of tall fescue would decline with warmer temperatures over the range of 18.5–27°C, but growth actually increased markedly with increasing temperature when VPD was held constant. In contrast, growth declined in experiments where tall fescue was exposed to increasing VPD and temperature was held constant at 21°C. The inhibited growth appears to be in response to a maximum transpiration rate that can be supported by the tall fescue plants. The sensitivity to VPD indicates that if VPD remains stable in future climates as it has in the past, growth of tall fescue could well be stimulated rather than decreased by global warming in temperate climate zones.

Growth of Arabidopsis flavonoid mutant is challenged by radiation longer than the UV-B band

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 earths 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.

Influence of atmospheric vapour pressure deficit on ozone responses of snap bean (Phaseolus vulgaris L.) genotypes

Environmental conditions influence plant responses to ozone (O3), but few studies have evaluated individual factors directly. In this study, the effect of O3 at high and low atmospheric vapour pressure deficit (VPD) was evaluated in two genotypes of snap bean (Phaseolus vulgaris L.) (R123 and S156) used as O3 bioindicator plants. Plants were grown in outdoor controlled-environment chambers in charcoal-filtered air containing 0 or 60 nl l−1 O3 (12 h average) at two VPDs (1.26 and 1.96 kPa) and sampled for biomass, leaf area, daily water loss, and seed yield. VPD clearly influenced O3 effects. At low VPD, O3 reduced biomass, leaf area, and seed yield substantially in both genotypes, while at high VPD, O3 had no significant effect on these components. In clean air, high VPD reduced biomass and yield by similar fractions in both genotypes compared with low VPD. Data suggest that a stomatal response to VPD per se may be lacking in both genotypes and it is hypothesized that the high VPD resulted in unsustainable transpiration and water deficits that resulted in reduced growth and yield. High VPD- and water-stress-induced stomatal responses may have reduced the O3 flux into the leaves, which contributed to a higher yield compared to the low VPD treatment in both genotypes. At low VPD, transpiration increased in the O3 treatment relative to the clean air treatment, suggesting that whole-plant conductance was increased by O3 exposure. Ozone-related biomass reductions at low VPD were proportionally higher in S156 than in R123, indicating that differential O3 sensitivity of these bioindicator plants remained evident when environmental conditions were conducive for O3 effects. Assessments of potential O3 impacts on vegetation should incorporate interacting factors such as VPD.