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  • Author or Editor: Sylvie Jenni x
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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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Quebec vegetable growers are increasingly using agricultural plastics (plasticulture) not only for gains in crop yield, earliness, and quality, but also for weed control and water and fertilizer conservation. Curcubitaceae include heat-loving crops that respond well to plasticulture. Melons are among the most responsive of all crops because they are sensitive to both low soil and air temperatures and to wind, but are very tolerant of high temperatures. The objective of this project was to develop a bioeconomic model that will predict the yield and timing of a melon crop under a number of mulch/tunnel combinations, evaluate the profitability of each production regime, and establish the optimal combinations that will maximize profit and continuity of supply over an extended growing season. A compartment model representing state, rate, and driving variables will be presented.

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To investigate whether brown bead can be reduced by various cultural practices, a 3-year field study was conducted on a 600-acre broccoli (Brassica oleracea L., Italica group) farm in southwestern Quebec. Factors studied included N fertilization, soil series, previous crop, season of bed forming, or planting method. Four N treatments were randomly applied to two blocks in 41 fields of `Everest' broccoli: 85-0-0, 85-54-0, 85-54-54, and 85-54-108; the first number indicating N (kg·ha-1) applied before planting; the second, N applied 5 weeks after planting; and the last, N applied 7 weeks after planting. Over the 3-year study, brown bead affected 11% of the broccoli heads and accounted for one-third of the rejects. Brown bead severity on individual heads was described on a 0-8 scale. Plots with greater N applications (i.e., 85-54-54, 85-54-108) had significantly (P < 0.001) lower proportions of plants with brown bead compared with plots with lower N applications. Brown bead incidence reacted similarly from year to year to N fertilization and soil type. However, fertilization interacted with soil type. The less N was applied, the more soil effect was important. Soil effect was maximum at a low N level (85-0-0) with 2.5 times more plants showing brown bead in the Saint Blaise series than in the Sainte Rosalie series. Bed type, previous crop, or planting type did not affect the incidence of brown bead.

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In order to investigate their relationships with brown bead, a data set composed of 48 variables characterizing the developmental rate, climate, and nutrients in the soil and in the tissues of heads of broccoli (Brassica oleracea L., Italica group) was collected from 328 plots (41 experimental fields over 3 year× 4 N fertilization level× 2 blocks). The four N treatments were 85-0-0, 85-54-0, 85-54-54, and 85-54-108, the first number indicating the N level (kg·ha-1) applied before planting; the second, N level applied 5 weeks after planting; and the last, N level applied 7 weeks after planting. Broccoli plants were either direct-seeded (26 experimental fields) or transplanted (15 experimental fields). Whether direct-seeded or transplanted, fast-developing broccoli plants showed a lower incidence of brown bead. More particularly, heads of transplanted broccoli plants experiencing warmer temperatures had a lower brown bead incidence and severity. A regular supply of water decreased the incidence and severity of the physiological disorder in both direct-seeded and transplanted broccoli plants. Low levels of Ca and high levels of Mg and K in mature broccoli head tissues were associated with a higher incidence of brown bead. Multiple-regression models were developed to predict the percentage of broccoli heads with brown bead for direct-seeded plants (R 2 = 0.76; n = 104), and for transplanted plants (R 2 = 0.69; n = 44). For direct-seeded broccoli, solar radiation between the button stage (head diameter of 2.5 cm) and maturity (head diameter of 10 cm), as well as soil and tissue Mg content, were among the first variables to enter the regression models. In general, more solar radiation and less precipitation translated into more heads showing brown bead symptoms. For transplanted broccoli plants, the minimum temperature from the button stage to maturity was a key variable in the prediction of the percentage of heads with brown bead and the corresponding index of severity.

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The heat-unit system, involving the sum of daily mean temperatures above a given base temperature, is used with processing pea (Pisum sativum L.) to predict relative maturity during the growing season and to schedule planting dates based on average temperature data. The Quebec pea processing industry uses a base temperature of 5 °C to compute growing-degree days (GDD) between sowing and maturity. This study was initiated to verify if the current model, which uses a base temperature of 5 °C, can be improved to predict maturity in Quebec. Four pea cultivars, `Bolero', `Rally', `Flair', and `Kriter', were grown between 1985 and 1997 on an experimental farm in Quebec. For all cultivars, when using a limited number of years, a base temperature between 0.0 and 0.8 °C reduced the coefficient of variation (cv) as compared with 5.0 °C, indicating that the base temperature used commercially is probably not the most appropriate for Quebec climatic conditions. The division of the developmental period into different stages (sowing until emergence, emergence until flowering, and flowering until maturity) was also investigated for some years. Use of base temperatures specific for each crop phase did not improve the prediction of maturity when compared with the use of an overall base temperature. All years for a given cultivar were then used to determine the base temperature with the lowest cv for predicting the time from sowing to maturity. A base temperature from 0 to 5 °C was generally adequate for all cultivars, and a common base temperature of 3.0 °C was selected for all cultivars. For the years and cultivars used in this study, the computation of GDD with a base temperature of 3 °C gave an overall prediction of maturity of 2.0, 2.4, 2.2, and 2.5 days based on the average of the absolute values of the differences for the cultivars Bolero, Rally, Flair, and Kriter, respectively.

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Four snap bean (Phaseolus vulgaris L.) cultivars, Goldrush, Teseo, Labrador, and Flevoro, were grown in irrigated fields of southern Quebec between 1985 and 1998. Data on phenology collected from these fields were used to determine which base temperature would best predict time from sowing to maturity. The optimal base temperature was 0 °C for `Goldrush', `Teseo', and `Labrador' and 6.7 °C for `Flevoro'. Adjusting different base temperatures for intermediate developmental stages (emergence, flowering) did not improve the prediction model. All years for a given cultivar were then used to determine the base temperature with the lowest coefficient of variation (CV) for predicting the time from sowing to maturity. A common base temperature of 0 °C was selected for all cultivars, since `Flevoro' was not very sensitive to changes in base temperature. This method improved the prediction of maturity compared with the conventional computation growing-degree days (GDD) with a base of 10 °C. For the years and cultivars used in this study, calculating GDD with a base of 0 °C gave an overall prediction of maturity of 1.7, 1.5, 2.0, and 1.4 days based on average absolute differences, for `Flevoro', `Goldrush', `Teseo', and `Labrador', respectively.

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Combinations of mulch/rowcover systems were tested using 'Earligold' melon (Cucumis melo L.) transplanted between 7 and 9 May during each of 3 years. The mulches used were black (B), wavelength-selective green (G), or clear (C). In 1993 and 1994, these three mulches were combined with two rowcover systems, either a clear perforated polyethylene (PO; 500 holes/m2) or a clear nonperforated polyethylene with a water-filled tube (UT, thermal tube). Controls consisted of mulch-alone treatments. In 1995, only the green wavelength-selective mulch was tested. A clear mulch combined with an infrared-blocking polyethylene with a thermal tube (IT) and a spunbonded polypropylene agrotextile (AO) were also tested. The highest air temperatures, sometimes >40 °C, were recorded under the CIT and BUT treatments. Perforated tunnels were less efficient in increasing daytime air temperatures, particularly during windy conditions, than the nonperforated tunnels. When combined with all tunnel types, the wavelength-selective mulch produced effects intermediate between those of the clear and black mulches for air and soil temperatures, chilling injury, and days to flowering of perfect flowers. In 1994, only those plants grown with a clear mulch combined with an infrared-blocking or standard nonperforated tunnel with a thermal tube (CIT and CUT) survived seven sequential nights with temperatures between 1.6 and 5.8 °C. Plants in the nonperforated tunnels flowered first, had the heaviest biomass at anthesis, and gave the highest early yields, both in terms of fruit number and mass. However, total yields did not differ significantly between perforated and nonperforated tunnels. Plants produced smaller fruits in treatments that resulted in earlier flowering, i.e., nonperforated tunnels.

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A lightweight agrotextile floating rowcover (10 g·m−2) designed for insect control was evaluated for its potential to reduce carrot weevil [Listronotus oregonensis (Le conte)] damage and to improve germination and carrot (Daucus carota L.) yield. The floating rowcover had no effect on total emergence and spread on emergence time but decreased emergence time by 0.5 day. Although floating rowcovers generally increased fresh weight of carrot leaves and roots during early development, no effect was detected late in the season and at harvest time. Carrot weevil damage of uncovered plants was 0.4 tunnels per root in 2006 and 2.0 tunnels per root in 2005. In both years, covering carrots with a floating rowcover for a period of 35 days after sowing reduced carrot weevil damage by 65% to 75%. In most years with low or medium carrot weevil infestation, the use of a rowcover could eliminate the use of insecticide to control this pest.

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