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Sylvie Jenni

Rib discoloration in crisphead lettuce (Lactuca sativa) has been successfully induced by applying heat stress. Two studies were conducted to determine the effect of short periods (3 and 5 days) of high temperatures (35/25 °C and 35/15 °C day/night temperatures) at various developmental stages (at heading, and at 1, 2, and 3 weeks after heading) on rib discoloration incidence and severity. Lettuce (cv. Ithaca) was most sensitive to heat stress 2 weeks after heading: applying 35/25 °C or 35/15 °C day/night temperatures for 3 or 5 days resulted on average in 46% of mature heads with rib discoloration symptoms. Stressing plants at earlier or later stages resulted in significantly lower incidences of the disorder, with only 4% to 17% plants showing symptoms. More leaves were affected by the disorder when heat stress was applied 2 weeks after heading than when the stress was applied earlier or later. Night temperature and stress duration had no effect on the incidence and severity of rib discoloration. Up to eight leaves, located between the first and fifteenth leaves acropetal to the cap leaf, showed symptoms. This report establishes a direct relationship between rib discoloration and heat stress, proposes a new method to help lettuce breeders screen germplasm for rib discoloration tolerance, and supports the development of tools for predicting the occurrence of rib discoloration in the field according to meteorological data.

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Xuetong Fan and James Mattheis

Carrots, broccoli, and lettuce were treated with air, continuous ethylene, 1-methylcyclopropene (MCP), or a combination of MCP before continuous ethylene. The respiration rate of ethylene-treated carrots reached a maximum 4 days after treatment and remained higher compared to controls through 16 days at 10 °C. Ethylene treatment also resulted in an accumulation of isocoumarin. Treating carrots with MCP before ethylene exposure inhibited the increase in respiration rate and accumulation of isocoumarin. MCP treatment reduced broccoli respiration and yellowing compared to controls, indicating that ethylene is involved in the senescence of broccoli. Ethylene exposure stimulated respiration and yellowing of broccoli. Treatment with MCP before continuous ethylene exposure negated the ethylene effects. MCP also inhibited respiration and russet spotting of lettuce stored in ethylene-containing atmospheres. The results indicate MCP can be used to block ethylene-induced isocoumarin accumulation (associated with bitterness) in carrots, yellowing in broccoli, and russet spotting in lettuce.

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Teresa Eileen Snyder-Leiby and Shixiong Wang

‘Honeycrisp’ is a relatively new apple cultivar (Malus ×domestica Borkh.) with a unique crisp fruit texture that makes it highly desirable. However, the leaves often develop a zonal chlorosis that resembles potato leafhopper damage. Other researchers have determined that the symptoms correlate with decreasing crop load rather than leafhopper damage. This study investigates the possibility that the zonal chlorosis is related to the buildup of starch grains causing rupture of chloroplasts. Transmission electron microscopy was used to document ultrastructural changes in chloroplasts in trees with three different crop loads from the beginning of the season to the end of the season. In trees with extremely low crop loads, we observed abnormalities of chloroplast membrane structure and accumulation of larger and more numerous starch grains. These appear before the appearance of chlorotic symptoms. By early August, the chloroplasts from chlorotic regions of leaves on trees with light crop loads are completely disrupted and abnormally large starch grains are found in place of chloroplasts. As the season progresses, trees with moderate and heavy crop loads showed less severe chloroplast disruption and smaller starch grain accumulation than trees with light crop load.

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Jennifer R. DeEll and Robert K. Prange

This paper reports preliminary results on the postharvest quality and storage characteristics of several scab-resistant apple cultivars. `Novaspy', `Moira', `Priscilla', `Novamac', `Nova Easygro', `Prima', and `Macfree' were stored for 3 months at 3C in air or standard controlled atmosphere (CA; 4.5% CO2 and 2.5% O2) in 1990 and for 4 months at 0C in air, standard CA, or low-O, CA (LO; 1.5% CO2 and 1.5% O2) in 1991. `Moira', `Prima', and `Priscilla' had very limited storage life. `Moira' was susceptible to bitterpit, scald, core browning, vascular breakdown, and storage rots. `Prima' was susceptible to core browning and vascular breakdown and had a high incidence of storage rots in air storage. `Priscilla' had several defects as a result of insect damage and was susceptible to bitterpit and scald. `Novaspy' stored very well and had virtually no physiological disorders or storage rots. `Novamac, `Nova Easygro', and `Macfree' developed few storage rots and were essentially at the end of their storage life after 4 months, regardless of storage conditions. Firmness in `Novamac' decreased substantially in all storage atmospheres, while `Nova Easygro' and `Macfree' were susceptible to core browning and scald.

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Katsumi Ohyama, Koji Manabe, Yoshitaka Omura, Toyoki Kozai, and Chieri Kubota

To evaluate the potential use of a 24-hour photoperiod for transplant production in a closed system, tomato (Lycopersicon esculentum Mill.) plug transplants were grown for 17 days either under a 24-hour photoperiod with a photosynthetic photon flux (PPF) of 200 μmol·m-2·s-1 or under a 16-hour photoperiod with a PPF of 300 μmol·m-2·s-1, resulting in the same daily integrated PPF (17.3 mol·m-2). Air temperatures were alternated between 28 °C during the first 16 hours and 16 °C for the subsequent 8 hours of each day. Fresh weight, dry weight and leaf area were 41%, 25%, and 64% greater, respectively, under the 24-hour photoperiod than under the 16-hour photoperiod. Physiological disorders (e.g., chlorosis and/or necrosis) were not observed under the 24-hour photoperiod, probably due to the alternating air temperature. Floral development of plants originating from both treatments did not differ significantly. Electric energy use efficiency of the closed system was 9% greater under the 24-hour photoperiod than under the 16-hour photoperiod. These results suggest that using a 24-hour photoperiod with relatively low PPF can reduce both initial and operational costs for transplant production in a closed system due to the reduction in the number of lamps.

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P. Guy Lévesque, Jennifer R. DeEll, and Dennis P. Murr

Sequential decreases or increases in the levels of O2 in controlled atmosphere (CA) were investigated as techniques to improve fruit quality of `McIntosh' apples (Malus ×sylvestris [L.] Mill. var. domestica [Borkh.] Mansf.), a cultivar that tends to soften rapidly in storage. Precooled fruit that were harvested at optimum maturity for long-term storage were placed immediately in different programmed CA regimes. In the first year, CA programs consisted of 1) `standard' CA (SCA; 2.5–3.0% O2 + 2.5% CO2 for the first 30 d, 4.5% CO2 thereafter) at 3 °C for 180 d; 2) low CO2 SCA (2.5–3.0% O2 + 2.5% CO2) at 3 °C for 60 d, transferred to low O2 (LO; 1.5% O2 + 1.5% CO2) at 0 or 3 °C for 60 d, and then to ultralow O2 (ULO; 0.7% O2 + 1.0% CO2) at 0 or 3 °C for 60 d; and 3) ULO at 3 °C for 60 d, transferred to LO at 0 or 3 °C for 60 d, and then to SCA or low CO2 SCA at 0 or 3 °C for 60 d. In the second year, the regimes sequentially decreasing in O2 were compared with continuous ULO and SCA. After removal from storage, apples were held in ambient air at 20 °C for a 1-week ripening period. Fruit firmness was evaluated after 1 and 7 d at 20 °C, whereas the incidence of physiological disorders was assessed only after 7 d. Lowering the temperature while decreasing O2 was the best CA program with significant increased firmness retention during storage and after the 1-week ripening period. Reduced incidence of low O2 injury in decreasing O2 programs and absence of core browning at the lower temperature were also observed.

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C.B. Watkins and F.W. Liu

-cut slice quality because of maintenance of texture and slow browning ( Kim et al., 1993 ). Market demand for ‘Empire’ apples is high and the industry would like to store the fruit for at least 10 months. A number of physiological disorders limit the storage

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Luiz C. Argenta, Xuetong Fan, and James P. Mattheis

apple cultivars include reduction of ethylene production and respiration rates, slower progression of ripening processes, including softening, loss of acidity, yellowing, volatile production, and reduced development of some physiological disorders ( Fan

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Xingbin Xie, Todd Einhorn, and Yan Wang

loss for many european pear cultivars ( Franck et al., 2007 ). In the present study, two types of IB were identified: senescent core breakdown (SCB) and brown heart (BH). SCB is a physiological disorder typically found in early maturing european pears

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O.L. Lau

In a 2-year study, the benefits and risks of an initial low O2 stress treatment (ILOS; 0.04 kPa O2 for 10 days) as a supplement to 1.5 or 0.7 kPa O2 storage for controlling scald in `Starkrimson Delicious' apples (Malus ×domestica Borkh.) were evaluated. The fruit were picked from 15 orchards and harvested over five successive weeks to generate a wide range in maturity. Storage in 0.7 kPa O2 did not adequately control scald in fruit picked at starch index between 1.0 and 2.1 (10% to 35% scald), but reduced watercore-induced breakdown in fruit picked at starch index ≥2.4 (0–9 scale). The ILOS treatment gave a statistically significant but commercially nonsignificant scald control benefit to fruit held in 1.5 kPa O2 in 1 year, but not to fruit held in 0.7 kPa O2. ILOS did not increase alcoholic taste, but increased skin purpling in 0.7 kPa O2-stored fruit from the final harvest in 1 year. ILOS decreased flesh firmness in fruit picked at starch index ≥1.7 and increased watercore-induced breakdown in fruit picked at starch index ≥2.1 in both years.