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Jennifer R. DeEll, Robert K. Prange, and Dennis P. Murr

Chlorophyll fluorescence was evaluated as a rapid and nondestructive technique to detect low-O2 or high-CO2 stress in apples (Malus domestica Borkh.) during storage. `Marshall' McIntosh apples were held for 5, 10, 15, 20, or 25 days at 3C in 1) standard O2 (2.5% to 3%) and low CO2 (<1%), 2) low O2 (1% to 1.5%) and low CO2 (<1%), 3) standard O2 (2.5% to 3%) and standard CO2 (4% to 4.5%), or 4) standard O2 (2.5% to 3%) and high CO2 (11% to 12%). Only 10% of the apples had skin discoloration after 5 days in 1% to 1.5% O2; 80% developed skin discoloration after 20 days in low O2. Small desiccated cavities in the cortex, associated with CO2 injury, developed in 10% of the apples after 20 days in 11% to 12% CO2. Five days in 1% to 1.5% O2 or 11% to 12% CO2 caused variable fluorescence (Fv) of apple fruit to decrease compared to those held in standard atmospheres. Additional exposure did not significantly affect Fv in either the low-O2 (1% to 1.5%) or high-CO2 (11% to 12%) treatment. Our results suggest that chlorophyll fluorescence techniques can detect low-O2 and high-CO2 stress in apples before the development of associated disorders.

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Peter M.A. Toivonen

Chlorophyll “a” fluorescence (Fvar) was compared with respiration and vitamin C content of broccoli [Brassica oleracea L. (Botrytis group)] during storage at 1C. The amplitude of the Fvar maxima declined in a similar manner as respiration and vitamin C content. Fvar was highly correlated with respiration (r = 0·83, P > = 0·0001). The correlation of Fvar with vitamin C content was weaker (r = 0·42, P > = 0·0002). The results demonstrate that Fvar is an indicator of postharvest changes in broccoli and that Fvar can be used as a nondestructive indicator of early changes in tissue condition (i.e., degree of freshness) of broccoli in storage.

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Sarah E. Bruce and D. Bradley Rowe

Propagation failures of Taxus are often attributed to cutting collection from stock plants of poor quality. If a quick, reliable method of determining the potential rooting of cuttings based on the condition of a specific stock plant was available for propagators, rooting success could be predicted before investing time, labor, and resources. Our studies examined chlorophyll fluorescence (Fv/Fm) as a potential tool for stock plant selection, assessment of storage conditions, and measurement of stress over the course of propagation. Ten cultivars of Taxus × media (Taxus baccata L. × T. cuspidata Sieb. & Zucc.) were used: Brownii, Dark Green Pyramidalis, Dark Green Spreader, Densiformis, Densiformis Gem, Hicksii, L.C. Bobbink, Runyan, Tauntoni, and Wardii. Storage condition treatments consisted of desiccation (low, medium, and high), duration (34, 70, and 107 days), and temperature (–30, –2.5, 0, 2.5, 5, 10, and 20 °C). Cultivars differed in Fv/Fm initially as well as over time. Correlations were not found between initial stock plant Fv/Fm and rooting percentage, number of roots, root dry weight, or root length, indicating that Fv/Fm is not a reliable indicator of stock plant propagation potential. Short storage duration at –2.5 to 2.5 °C was found to be ideal. Fv/Fm could detect substandard storage conditions only at temperature and desiccation extremes. Although chlorophyll fluorescence measurements do not appear to be a practical method of predicting adventitious rooting, there is a potential for assessing cutting or plant quality before shipping.

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Angela K. Durhman, D. Bradley Rowe, and Clayton L. Rugh

Green roofs, or vegetative or living roofs, are an emerging technology in the United States. Because environmental conditions are often more extreme on rooftops, many xerophytic plants, especially Sedum, are ideal for extensive green roofs because they are physiologically and morphologically adapted to withstand drought. A greenhouse experiment was conducted to determine the effect of watering regimens on plant stress as measured by chlorophyll fluorescence (Fv/Fm), biomass accumulation, substrate moisture, and evapotransipiration on succulent plants of Sedum acre L., S. reflexum L., S. kamtschaticum ellacombianum Fisch., and non-Crassulacean acid metabolism (CAM) plants of Schizachyrium scoparium Nash and Coreopsis lanceolata L. Plants were grown at a substrate depth of 7.5 cm. Results indicate even after the 4-month period, Sedum spp. survived and maintained active photosynthetic metabolism to a greater extent than Schizachyrium and Coreopsis. Furthermore, when Sedum was watered after 28 days of drought, chlorophyll fluorescence (Fv/Fm) values recovered to values characteristic of the 2 days between watering (DBW) treatment. In contrast, the non-CAM plants required watering frequency every other day to survive and maintain active growth and development. Regardless of species, the greatest increase in total biomass accumulation and fastest growth occurred under the 2 DBW regimens.

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D.H. Willits and M.M. Peet

Chlorophyll fluorescence was measured under both laboratory and greenhouse conditions in an effort to develop a quick, reliable, and inexpensive laboratory procedure capable of predicting heat stress experienced by tomato (Lycopersicon esculentum Mill.) under greenhouse conditions. The laboratory tests consisted of measurements of the ratio of variable to maximal chlorophyll fluorescence (Fv/Fm) performed on leaf discs taken from whole tomato leaves and placed on a temperature controlled plate. Comparisons were made with greenhouse measurements of the same parameter conducted on intact leaves of whole plants exposed to different temperature treatments imposed by manipulation of the aerial environment of the greenhouse. Dark adaption periods ranging from 15 min to all day in the greenhouse and temperature exposure periods ranging from 5 min to 60 min in the laboratory were compared to find the best correlation between the two tests. Best agreement was obtained with 60 min treatment times in the laboratory and 60 min dark adaption periods in the greenhouse. Fv/Fm decreased quadratically with increasing leaf temperature in a similar fashion in both tests, suggesting that the laboratory approach can adequately predict plant response to greenhouse heat stress.

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Thomas E. Marler and Michael V. Mickelbart

The influence of drought stress on leaf gas exchange and chlorophyll fluorescence characteristics of field-grown papaya (Carica papaya L.) plants was determined under a range of incident light fluxes and times of day. These data may aid in improving management systems for papaya production which minimize detrimental effects from suboptimal environmental conditions. Water was withheld from field-grown `Red Lady' plants in one study and `Tainung #2', `Red Lady', and `Sunrise' plants in a second study until soil matric potential was -60 to -70 kPa. Drought-stressed plants exhibited reduced net CO2 assimilation (ACO2) above light saturation, photosynthetic photon flux (PPF) at which light saturation for ACO2 occurred, and apparent quantum yield compared to well-watered plants. The light compensation point of drought-stressed plants was greater than that of well-watered plants. Leaf chlorophyll fluorescence characteristics were not influenced by drought stress. The daily pattern of leaf gas exchange was dependent on climatic conditions. For sunny days, ACO2, stomatal conductance of water (gs), and water use efficiency of well-watered plants were maximal at mid-morning, declined during midday, and then partially recovered during late afternoon. In drought-stressed plants, leaf gas exchange was relatively constant after a brief early morning maximum. On overcast days, the responses of gas exchange variables in relation to time of day followed smooth bell-shaped patterns regardless of the level of drought stress. Combined with previously published data, these results indicate that the influence of drought stress on gas exchange is highly dependent on time of day, ambient sky conditions, plant size, and speed with which drought stress occurs.

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Mark A. Walker, Dale M. Smith, K. Peter Pauls, and Bryan D. McKersie

The chilling tolerance of commercial Lycopersicon esculentum cultivars (H2653, H722), Solanum lycopersicoides, an F1 hybrid of S. lycopersicoides × Sub-Arctic Maxi, and 25 BC2F2 lines of L. hirsutum × H722 (backcrossed twice to H722) was evaluated using a chlorophyll fluorescence assay. The ratio of the initial to the peak fluorescence (Fo: Fp) measured from fully expanded leaves was chosen as an indicator of plant health. Chilling induced an increase in Fo: Fp that was correlated with the sensitivity of the plant to low-temperature stress. Values of Fo: Fp remained low for cold-treated S. lycopersicoides and the F1 hybrid, which showed few symptoms of chilling-related damage, whereas the commercial cultivars, which were essentially intolerant to low temperatures, had large increases in Fo: Fp. A full range of Fo: Fp values was measured in the 25 BC2F2 lines, indicating that some chilling tolerance from the L. hirsutum parent was expressed by plants in these populations.

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Terri Starman and Leonardo Lombardini

A study was conducted to characterize the morphological and physiological responses of four herbaceous perennial species subjected to two subsequent drought cycles. Lantana camara L. `New Gold' (lantana), Lobelia cardinalis L. (cardinal flower), Salvia farinacea Benth. `Henry Duelberg' (mealy sage), and Scaevola aemula R. Br. `New Wonder' (fan flower) were subjected to two consecutive 10-day drought cycles. Growth response, leaf gas exchange, and chlorophyll fluorescence were measured during the experiment. The morphology of L. cardinalis and L. camara was not affected by drought, while S. farinacea had reductions in plant height and leaf area and S. aemula had reductions in dry weight. Overall, plant growth and development continued even when substrate water content was reduced to 0.13 mm3·mm-3, which indicated a level of substrate water below container capacity was sufficient for greenhouse production of these species. The drought treatments had little effect on the photochemical efficiency (Fv/Fm) of Photosystem II. An increase in minimal fluorescence (Fo) was observed in S. aemula on the last day of the second cycle. Drought treatment caused increased leaf-level water use efficiency (WUE) at the end of the first cycle in L. cardinalis and S. aemula, but not in L. camara and S. farinacea. Plants of L. camara, S. farinacea, and S. aemula that had received drought during both cycles became more water use efficient by the end of the second cycle, but L. cardinalis did not.

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M.S. Tian, A.B. Woolf, J.H. Bowen, and I.B. Ferguson

Hot water treatments (HWTs), at a range of temperatures (43 to 55C) and durations (10 sec to 30 min), were applied to floret groups of `Shogun' broccoli (Brassica oleracea L. var italica) directly after harvest. Floret groups were then stored at 20C in the dark for 3 days. A range of optimal treatments was found in which yellowing was markedly reduced, and heat damage (water soaking and decay) did not occur. Chlorophyll fluorescence measurements indicated that in the optimum treatment that prevented yellowing the Fv/Fm ratio following HWT decreased immediately and was maintained at a constant level for the next 3 days. A further experiment examined the effect of HWT durations up to 20 min at 47C on fluorescence and yellowing. Longer durations of HWTs (>5 min) progressively reduced yellowing and the Fv/Fm ratio. From these three experiments a HWT of 47C for 7.5 min was selected as the best treatment. This treatment consistently reduced yellowing for up to 5 days. A decrease in the Fv/Fm ratio may be a useful indicator of broccoli florets response to hot water treatments.

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S. Khanizadeh, J.R. DeEll, and N. Hakam

Frost tolerance of flower buds is one of the most important characteristics of strawberry cultivars that produce fruit early or very early in the season. The objective of this study was to evaluate chlorophyll fluorescence (CF) as a suitable rapid method to assess spring frost injury of strawberry flowers. More specifically, to determine if there was a relationship between a decrease in CF and the appearance of visual symptoms (visual expression of necrosis (VEN) based on the amount of dark, damaged, and/or water soaked tissue of the pistil) due to frost. Sixty-six strawberry genotypes with varying levels of chilling susceptibility were used. The plants were grown in a greenhouse under a 16-h light period at 20-22 °C during the daytime and 16-18 °C at night. For the CF and VEN measurements, the plants were stored at -3 °C for 24 h followed by 24 h in the greenhouse. The CF measurements were made on dark-adapted tissue, using the Fv/Fm test of an OS-500 modulated fluorometer. For the VEN method, the flowers which had dark, damaged, and/or water soaked pistils were counted. The results showed that variable fluorescence (Fv) decreased as the temperature was lowered. The spring frost resistant cultivars maintained Fv at a stable level and had a smaller regression slope (ß1), whereas the susceptible cultivars showed a very dramatic decrease in Fv. The CF method gave results that correlated with the VEN results. The strong relationship between chilling tolerance determined via visual and florescence techniques supports the use of CF in selecting resistant spring frost selections in a breeding program. The use of CF will allow the breeder not only to select for spring frost-resistant selections independently of environmental changes, but also to select frost resistant seedlings prior to planting in the field.