Impacts of solar ultraviolet-B radiation on terrestrial ecosystems of Tierra del Fuego (southern Argentina). An overview of recent progress.

The southern part of Tierra del Fuego, in the southernmost tip of South America, is covered by dense Nothofagus spp. forests and Sphagnum-dominated peat bogs, which are subjected to the influence of ozone depletion and to increased levels of solar ultraviolet-B radiation (UV-B). Over the last 5 years we have studied some of the biological impacts of solar UV-B on natural ecosystems of this region. We have addressed two general problems: (i) do the fluctuations in UV-B levels under the influence of the Antarctic ozone 'hole' have any measurable biological impact, and (ii) what are the long-term effects of solar (ambient) UV-B on the Tierra del Fuego ecosystems? In this paper, we provide an overview of the progress made during the first 4 years of the project. We highlight and discuss the following results: (1) ambient UV-B has subtle but significant inhibitory effects on the growth of herbaceous and graminoid species of this region (growth reduction < or = 12%), whereas no consistent inhibitory effects could be detected in woody perennials; (2) in the species investigated in greatest detail, Gunnera magellanica, the inhibitory effect of solar UV-B is accompanied by increased levels of DNA damage in leaf tissue, and the DNA damage density in the early spring is clearly correlated with the dose of weighted UV-B measured at ground level; (3) the herbaceous species investigated thus far show little or no acclimation responses to ambient UV-B such as increased sunscreen levels and DNA repair capacity; and (4) ambient UV-B has significant effects on heterotrophic organisms, included marked inhibitory effects on insect herbivory. The results from the experiments summarized in this review clearly indicate that UV-B influences several potentially important processes and ecological interactions in the terrestrial ecosystems of Tierra del Fuego.

Functional significance and induction by solar radiation of ultraviolet-absorbing sunscreens in field-grown soybean crops.

Colorless phenylpropanoid derivatives are known to protect plants from ultraviolet (UV) radiation, but their photoregulation and physiological roles under field conditions have not been investigated in detail. Here we describe a fast method to estimate the degree of UV penetration into photosynthetic tissue, which is based on chlorophyll fluorescence imaging. In Arabidopsis this technique clearly separated the UV-hypersensitive transparent testa (tt) tt5 and tt6 mutants from the wild type (WT) and tt3, tt4, and tt7 mutants. In field-grown soybean (Glycine max), we found significant differences in UV penetration among cultivars with different levels of leaf phenolics, and between plants grown under contrasting levels of solar UV-B. The reduction in UV penetration induced by ambient UV-B had direct implications for DNA integrity in the underlying leaf tissue; thus, the number of cyclobutane pyrimidine dimers caused by a short exposure to solar UV-B was much larger in leaves with high UV transmittance than in leaves pretreated with solar UV-B to increase the content phenylpropanoids. Most of the phenylpropanoid response to solar UV in field-grown soybeans was induced by the UV-B component (lambda

Ozone depletion and UVB radiation: impact on plant DNA damage in southern South America.

The primary motivation behind the considerable effort in studying stratospheric ozone depletion is the potential for biological consequences of increased solar UVB (280-315 nm) radiation. Yet, direct links between ozone depletion and biological impacts have been established only for organisms of Antarctic waters under the influence of the ozone "hole;" no direct evidence exists that ozone-related variations in UVB affect ecosystems of temperate latitudes. Indeed, calculations based on laboratory studies with plants suggest that the biological impact of ozone depletion (measured by the formation of cyclobutane pyrimidine dimers in DNA) is likely to be less marked than previously thought, because UVA quanta (315-400 nm) may also cause significant damage, and UVA is unaffected by ozone depletion. Herein, we show that the temperate ecosystems of southern South America have been subjected to increasingly high levels of ozone depletion during the last decade. We found that in the spring of 1997, despite frequent cloud cover, the passages of the ozone hole over Tierra del Fuego (55 degrees S) caused concomitant increases in solar UV and that the enhanced ground-level UV led to significant increases in DNA damage in the native plant Gunnera magellanica. The fluctuations in solar UV explained a large proportion of the variation in DNA damage (up to 68%), particularly when the solar UV was weighted for biological effectiveness according to action spectra that assume a sharp decline in quantum efficiency with increasing wavelength from the UVB into the UVA regions of the spectrum.

Perception of solar UVB radiation by phytophagous insects: behavioral responses and ecosystem implications.

Most of our present knowledge about the impacts of solar UVB radiation on terrestrial ecosystems comes from studies with plants. Recently, the effects of UVB on the growth and survival of consumer species have begun to receive attention, but very little is known about UVB impacts on animal behavior. Here we report that manipulations of the flux of solar UVB received by field-grown soybean crops had large and consistent effects on the density of the thrips (Caliothrips phaseoli, Thysanoptera: Thripidae) populations that invaded the canopies, as well as on the amount of leaf damage caused by the insects. Solar UVB strongly reduced thrips herbivory. Thrips not only preferred leaves from plants that were not exposed to solar UVB over leaves from UVB-exposed plants in laboratory and field choice experiments, but they also appeared to directly sense and avoid exposure to solar UVB. Additional choice experiments showed that soybean leaf consumption by the late-season soybean worm Anticarsia gemmatalis (Lepidoptera: Noctuidae) was much less intense in leaves with even slight symptoms of an early thrips attack than in undamaged leaves. These experiments suggest that phytophagous insects can present direct and indirect behavioral responses to solar UVB. The indirect responses are mediated by changes in the plant host that are induced by UVB and, possibly, by other insects whose behavior is affected by UVB.

Prognostic factors in T3,N0-1 glottic and transglottic carcinoma. A multifactorial study of 221 cases treated by surgery or radiotherapy.

OBJECTIVE: To evaluate prognostic factors in patients with T3,N0-1 glottic and transglottic carcinoma treated in a single institution. DESIGN: Retrospective, nonrandomized case series. SETTING: Tertiary case referral centers, ambulatory or hospitalized care. PATIENTS: Two hundred twenty-one consecutive cases of stage III glottic or transglottic squamous cell carcinoma. Tumor stage was T3,N0,M0 in 167 cases and T3,N1,M0 in 54 cases. INTERVENTIONS: Surgery in 176 cases and radiotherapy in 45 cases. MAIN OUTCOME MEASURES: Recurrences and survival (multivariate). RESULTS: Almost 7% of the patients who underwent surgery and 39.6% who had radiotherapy had local recurrences. Recurrences in the neck were seen in 16.4% of the patients who underwent surgery and in 10.5% of those who had radiotherapy. Distant metastases were diagnosed only in patients who underwent surgery (4.6%). The 5-year actuarial overall survival rates were 56.3% in the surgical group and 35.2% in the radiotherapy group (P = .007). Age involvement of pyriform sinus, N stage, and history of tracheostomy were independent prognostic factors for risk of death. CONCLUSIONS: The presence of metastatic lymph nodes, age, and involvement of the pyriform sinus were the important prognostic factors in patients who underwent surgery. A small group of patients with T3,N0,M0 tumors could benefit from radiotherapy, with surgery reserved for recurrence.

Factors influencing contralateral metastasis and prognosis from pyriform sinus carcinoma.

BACKGROUND: The presence of neck metastasis reduces the probability of regional control and survival. The purpose of this study is to identify risk factors for contralateral neck metastasis in a series of 218 patients with pyriform sinus carcinoma. PATIENTS AND METHODS: Tumor stages were: 31 T1-T2, 187 T3-T4, 40 N0, 174 N1-N3, and 4 NX. A total of 182 patients (83%) had metastatic lymph nodes (23 in the contralateral neck). RESULTS: Thirty-five patients presented neck recurrences (27 in contralateral undissected neck). Logistic regression identified T and N stages, epilarynx and posterior pharyngeal wall involvement, hemilarynx fixation, and ipsilateral level 1 metastasis as important predictors of contralateral metastasis. Five-year survival rates were 41% (pN0), 23% (contralateral pN0), and 31% (contralateral positive nodes). CONCLUSIONS: The contralateral side of the neck was the most common site of recurrence. A lateral neck dissection is advisable for the contralateral side of the neck for high-risk patients.

Signaling among neighboring plants and the development of size inequalities in plant populations.

Transgenic tobacco plants that express an oat phytochrome gene (phyA) under control of the cauliflower mosaic virus (CaMV) 35S promoter and display altered photophysiology were used to test the role of light as a vehicle of information in dominance relationships between neighboring plants. Compared with the isogenic wild type, phyA-overexpressing plants showed dramatically reduced morphological responsivity to changes in the red/far red ratio of the incident light and to the proximity of neighboring plants in spacing experiments. In transgenic canopies an increase in stand density caused the small plants of the population to be rapidly suppressed by their neighbors. In wild-type canopies, plants responded to increased density with large morphological changes, and there appeared to be an inverse relationship between the magnitude of this morphological response and the ranking of the individual plant in the population size hierarchy. In these wild-type populations, size inequality increased only moderately with density within the time frame of the experiments. Our results suggest that, in crowded stands, the ability of individual plants to acquire information about their light environment via phytochrome plays a central role in driving architectural changes that, at the population level, delay the development of size differences between neighbors.

Prognostic factors in glottic carcinoma clinical stage I and II treated by surgery or radiotherapy.

INTRODUCTION: Decision making regarding selection of treatment for early glottic carcinoma remains controversial. This study was undertaken to assess the impact of such factors as patient age, stage of tumor, site and size of characteristic of the lesion, and other characteristics relative to disease free and overall survival rates. PATIENTS AND METHODS: The records of 145 consecutive patients with stage I and II glottic carcinomas treated between 1954 and 1990 were reviewed retrospectively. Surgery was performed on 50 patients (34.5%), and irradiation therapy was performed on 95 (65.5%). Cox's regression model was used to estimate the hazard ratios of recurrence and death. RESULTS: Median follow-up was 69.3 months. Death due to cancer or treatment complications occurred in 29 patients, whereas 25 patients died due to causes not related to cancer. Five-year rates for overall survival and disease-free intervals were 94.6% and 70.8%, respectively. Tumor control was achieved by initial surgery or irradiation in 78% and 69.5%, respectively. T stage and vocal cord mobility in this series were not associated with prognosis. Arytenoid involvement intended to indicate a worse prognosis. Other site involvement such as anterior commissure had no prognostic impact. DISCUSSION: Although stage I and II glottic cancers represent a heterogenous group, survival rates after surgery or radiotherapy vary relatively little. Death due to occurred in 17.9% of patients included in this series, whereas 17.2% died due to causes not related to cancer. Local recurrence following irradiation (29.5%) occurred more frequently than following surgery (10%). The choice of treatment modality for stage I and II glottic cancer should be justified by patient preference, involvement of anterior commissure, and impairment of vocal cord mobility and should not be a contraindication to radiotherapy.

On the opportunity cost of the photosynthate invested in stem elongation reactions mediated by phytochrome.

Seedlings of shade-intolerant species react to alterations of the light climate caused by their neighbors with morphological changes that may influence the pattern of resource acquisition and utilization at the whole-canopy level. One such change, the increased stem elongation rate that is triggered by low red (R, 660 nm) to far-red (FR, 730 nm) ratios (R:FR) in dense canopies, might reduce the amount of assimilates available for leaf area expansion or root growth, and in that way affect resource capture by the canopy. We have tested this hypothesis by comparing the growth of both isolated individuals and canopies of the weed Amaranthus quitensis under conditions differing only in the spectral distribution of the incident light. When canopies received the full spectrum of sunlight, the stems were a large proportion (40-57%) of total biomass. Filtering the FR waveband (and hence raising the R:FR ratio to eliminate the neighbors' proximity-signal) resulted in shorter canopies with lighter stems. However, the growth of leaves and roots was not promoted by this treatment, indicating that the opportunity cost of the assimilates invested in the stems was nil or very small. Filtering the FR had no effect on biomass accumulation when plants were grown as isolated individuals. The higher growth of the canopics under full spectrum could be due to a higher light interception or to a higher efficiency of light conversion into biomass. The first possibility is weakened by the observation that filtering the FR had no effect on the dynamics of soil covering by the crops. The second is indirectly strengthened by results of an experiment with isolated plants showing that stem elongation, stem growth, and total plant biomass can be increased by reducing the flux of R light received by the stems without affecting the light climate of the leaves. Further work is needed to distinguish between these two possibilities; whatever the cause, our results show that the elongation responses to decreased R:FR may lead to a net increase in canopy productivity, and do not necessarily have a negative impact on the growth of resource-harvesting organs.

Photomodulation of axis extension in sparse canopies : role of the stem in the perception of light-quality signals of stand density.

A fiber optic probe inserted into plant tissues was used to investigate the effects of canopy density on the light environment in different organs. The red:far-red ratio inside the stem of Datura ferox L. seedlings and the estimated phytochrome photoequilibrium were strongly reduced by the presence of neighbors forming canopies too sparse to cause any mutual shading at the level of the leaves. In such canopies, changes in plant density had little effects on the light regime inside the leaves of the succulent Aeonium haworthii (S.D.) Webb et Berth., particularly when the lamina was kept nearly normal to the direct rays of the sun. In field experiments using D. ferox and Sinapis alba L. seedlings, the elongation of the internodes responded to various types of localized light-quality treatments that simulated different plant densities in sparse canopies. The responses were quantitatively similar to those elicited by changes in plant density. The evidence supports the hypothesis that, in stands formed by plants of similar size, the red:far-red ratio of the light that impinges laterally on the stems is among the earliest environmental cues that allow plants to detect local canopy density and adjust axis extension accordingly.