The relative effect of a freeze-thaw cycle on photosynthesis, respiration, and ion leakage of potato leaf tissue was examined in two potato species, Solanum acaule Bitt. and Solanum commersonii Dun. Photosynthesis was found to be much more sensitive to freezing stress than was respiration, and demonstrated more than a 60% inhibition before any impairment of respiratory function was observed. Photosynthesis showed a slight to moderate inhibition when only 5 to 10% of the total electrolytes had leaked from the tissue (reversible injury). This was in contrast to respiration which showed no impairment until temperatures at which about 50% ion leakage (irreversible injury) had occurred. The influence of freeze-thaw protocol was further examined in S. acaule and S. commersonii, in order to explore discrepancies in the literature as to the relative sensitivities of photosynthesis and respiration. As bath cooling rates increased from 1°C/hour to about 3 or 6°C/hour, there was a dramatic increase in the level of damage to all measured cellular functions. The initiation of ice formation in deeply supercooled tissue caused even greater damage. As the cooling rates used in stress treatments increased, the differential sensitivity between photosynthesis and respiration nearly disappeared. Examination of agriculturally relevant, climatological data from an 11 year period confirmed that air cooling rates in the freezing range do not exceed 2°C/hour. It was demonstrated, in the studies presented here, that simply increasing the actual cooling rate from 1.0 to 2.90C/hour, in frozen tissue from paired leaflet halves, meant the difference between cell survival and cell death.While the effects of freezing on a number of essential cellular and subcellular processes have been extensively studied (1 1, 13, 14), there has been little effort to relate systematically the sequential development of injury to several of these processes during the imposition of realistic freezing and thawing stress on an intact tissue system. To the contrary, many of the investigations in this area rely solely on results obtained with isolated systems, such as chloroplasts or thylakoid membranes, which can lead to conclusions that are often non consistent with those derived from intact tissue (8,9).

The effect of light intensity on antioxidants, antioxidant enzymes, and chlorophyll content was studied in common bean (Phaseolus vulgaris L.) exposed to excess Mn. Leaves of bean genotypes contrasting in Mn tolerance were exposed to two different light intensities and to excess Mn; light was controlled by shading a leaflet with filter paper. After 5 d of Mn treatment ascorbate was depleted by 45% in leaves of the Mn-sensitive genotype ZPV-292 and by 20% in the Mn-tolerant genotype CALIMA. Nonprotein sulfhydryl groups and glutathione reductase were not affected by Mn or light treatment. Ten days of Mn-toxicity stress increased leaf ascorbate peroxidase activity of cv ZPV-292 by 78% in low light and by 235% in high light, and superoxide dismutase activity followed a similar trend. Increases of ascorbate peroxidase and superoxide dismutase activity observed in cv CALIMA were lower than those observed in the susceptible cv ZPV-292. The cv CALIMA had less ascorbate oxidation under excess Mn-toxicity stress. Depletion of ascorbate occurred before the onset of chlorosis in Mn-stressed plants, especially in cv ZPV-292. Lipid peroxidation was not detected in floating leaf discs of mature leaves exposed to excess Mn. Our results suggest that Mn toxicity may be mediated by oxidative stress, and that the tolerant genotype may maintain higher ascorbate levels under stress than the sensitive genotype.

The growth and tuberization of Norland potatoes were studied under five different temperatures and two photoperiods. Treatment levels included 12, 16, 20, 24, and 28 C with either a 24-h (continuous light) or a 12-h photoperiod at 400 micromoles m-2 s-1 PPF. Plants were grown in 6-liter containers and harvested at 56-days-age. Stem length increased with increasing temperature under both photoperiods. The highest tuber yield occurred at 16 C under the 24-h photoperiod (755 g/plant) and at 20 C under the 12-h photoperiod (460 g/plant). Little or no tuber formation occurred at 28 C under either photoperiod or at 24 C under continuous light. As with tuber yield, the highest total plant dry weights also occurred at 16 C under the 24-h photoperiod and at 20 C under the 12-h photoperiod. Harvest index (tuber dry weight to total dry weight ratio) decreased with increasing temperatures and with continuous light. Results indicate that good growth and tuberization can occur under continuous light, and that increasing the photoperiod form 12 to 24 h effectively decreased the optimal temperature for tuber formation from near 20 C to 16 C. Alternatively, the results imply that a cooler temperatures, the potato becomes less obligate for dark period stimulation of tuberization.

. 1997. Photosynthetic function, lipid peroxidation and ␣-tocopherol content in spinach leaves during exposure to UV-B radiation. Can. J. Plant Sci. 77: [453][454][455][456][457][458][459]. Spinach (Spinacia oleracea L. 'Meridian') plants were irradiated for 9 h d -1 for 12 d with ultra-violet-B (UV-B) radiation [13.5 kJ m -2 d -1 of biologically effective UV-B radiation (UV-B BE ) + 350 µmol m -2 s -1 of photosynthetically active radiation (PAR)] or PAR (350 µmol m -2 s -1 ) to determine if incipient UV-B-induced inhibition of photosynthetic function occurred in photosystem 2 (PS2) reaction centers or in the lipid matrix of thylakoid membranes. Chlorophyll concentrations (leaf area basis) in UV-B-irradiated spinach leaves were significantly lower than in PAR plants after 4, 8 and 12 d of exposure. A significant UV-B-induced rise in initial fluorescence (Fo) at days 8 and 10 was accompanied by a decline in the photochemical efficiency (Fv/Fm) ratio, indicating a reduction in the efficiency of PS2 energy harvesting and trapping. When based upon the chlorophyll content of thylakoid membranes, UV-B radiation did not alter lipid peroxidation but did increase α-tocopherol content compared with the PAR treatment during the first 8 of the 12 exposure days. These data suggest that incipient UV-B damage to the photochemical apparatus did not occur in PS2 reaction center complexes nor in the lipid matrix of the thylakoid membranes. Photosynthetic function was more susceptible to UV-B-induced impairment than thylakoid membrane lipids were to UV-B radiation damage. 'Meridian') ont été exposés 9 h par jour pendant 12 jours à un rayonnement photosynthétiquement utile (RPU) de 350 µmol -2 s -1 ou à ce même rayonnement RPU + 13,5 kJ m -2 j -1 de rayonnement UV-B biologiquement effectif (UV-B BE ). L'objet était de déter-miner si un début d'inhibition de fonction photosynthétique par les UV-B se produi dans les centres de réaction du PS2 ou dans les lipides de la membrane des thylakoïdes. Après 4, 8 et 12 jours d'exposition, les concentrations de chlorophylle (par unité de surface foliaire) étaient significativement plus basses dans les plantes irradiées par les UV-B que dans celles recevant seulement le RPU. La hausse significative de la fluorescence initiale (Fo) due au UV-B observé aux jours 8 et 10 était suivie d'une baisse du rapport d'efficience photochimique Fv/Fm, signe d'une réduction de l'efficacité de la récolte et de la capture de l'énergie dans le PS2. Calculé en fonction de la teneur en chlorophylle de la membrane des thylakoïdes, le rayonnement UV-B ne modifiait pas la peroxydation des lipides mais provoquait l'accroissement des teneurs en α-tocophérol par rapport au traitement RPU dans les 8 premiers jours d'exposition. Il découle de ces observations qu'il n'y a pas eu de début d'endommagement par les UV-B dans les centres de réaction du PS2, ni dans les lipides de la membrane des thylakoïdes. La fonction photosynthétique était plus sensible aux effets négatifs des UV-B que les lipides de la membrane des thylakoï...

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