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.
Sylvie Jenni and Gaetan Bourgeois
The Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie (BBCH) identification key was adapted for crisphead lettuce (Lactuca sativa) to facilitate identification of phenological stages and decisions regarding field operations from seeding to harvest maturity. The original system described leaf development based on leaf count from stage 11 (1 leaf) to stage 19 (9 leaf), and head development based on percentage of expected head size reached at maturity from stage 41 to 49. The new coding leaf development stages range from 11 to 29, corresponding to the 1-leaf to 19-leaf stages. The head development stages also ranged from 41 to 49, but phenological stages near commercial maturity from 43 to 49 are now described as a function of head firmness. The important maturity traits of crisphead lettuce include head size and density. Head volume can be estimated from three diameters by using Currence's equation, which takes into account head geometry. The firmness index obtained by hand compression gave a more precise estimate of head density than the density estimate derived from Currence's equation or the sphere equation. Crisphead lettuce development stages and maturity traits can be easily quantified in the field for use in planning field operations and for experimental purposes.
Sylvie Jenni and Katrine A. Stewart
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.
Sylvie Jenni, David de Koeyer, and George Emery
Rib discoloration is a physiological disorder associated with heat stress in crisphead lettuce (Lactuca sativa L.). Rib discoloration resistance was studied in a 2-year field experiment using parental and F2 plant populations from a cross of ‘Emperor’, a resistant cultivar, and ‘Eldorado’, a susceptible cultivar. Rib discoloration was evaluated in terms of incidence (percentage of plants with symptoms) and severity (on a 1–5 scale) and was correlated with maturity traits. The rib discoloration severity ratings for the two reciprocal F2 populations were intermediate between the two parents and were not significantly different, indicating the lack of cytoplasmic inheritance for rib discoloration in ‘Emperor’ and ‘Eldorado’. In both parents and F2 progenies, rib discoloration severity was strongly correlated with stem length, head height, head diameter, and head weight, but not with head density. In the resistant parent, more severe rib discoloration was associated with denser heads, whereas in the susceptible parent, the expression of the disorder was independent of head density. The chi-square tests rejected the hypothesis for monogenic segregation in some plantings but not in others. Resistance to rib discoloration is likely to be controlled by more than one gene with a high heritability (h2 = 0.57, P < 0.0001).
Fahrurrozi Aziz, Katrine A. Stewart, and Sylvie Jenni
Field experiments were conducted during 1997, 1998, and 1999 to determine effects of 10 combinations of mulched minitunnel and thermal water tube on air, soil, and water-tube temperatures and on vegetative growth of `Earligold' netted muskmelon (Cucumis melo L. Reticulatus Group) within the tunnels. Use of mulched minitunnels significantly increased air, soil and water temperatures during the preanthesis phase in all years compared with control treatments. Inclusion of water tubes and venting the tunnels decreased air temperature fluctuations in the tunnels. During the first 10 to 15 days after transplanting, plants grown in nonperforated tunnels had higher relative growth rates (RGRs), net assimilation rates (NARs), and dry weights (DWs) than those grown under perforated tunnels and control plots. Plants in tunnels containing thermal water tubes generally had higher RGRs, NARs, and DWs than those without tubes. During the later part of the experiment, from 11 to 16 days after transplanting until anthesis, however, there were no consistent effects of mulched minitunnels on RGR, NAR, and plant DW. Tunneled muskmelons had significantly higher RGRs, but generally lower NARs than those grown without tunnel. Use of mulched minitunnels significantly increased plant DW at anthesis in 1997, but not in 1998 and 1999. Plants grown in the minitunnels containing a thermal water tube generally had higher RGRs, NARs, and DWs than those without water tubes. Ventilating nonperforated tunnels generally increased RGR, NAR, and plant DW. Plants grown in the tunnels reached anthesis 10 days earlier than those without tunnels.
Fahrurrozi Aziz, Katrine A. Stewart, and Sylvie Jenni
Temperature modification is the most investigated environmental factor considered to affect muskmelon (Cucumis melo L. Reticulatus Group) growth in a mulched minitunnel production system. Until now, effects on CO2 concentrations within the tunnel have been ignored. Experiments on production of `Earligold' netted muskmelon were conducted in 1997, 1998, and 1999 to determine daily CO2 concentrations for 10 mulched minitunnel and thermal water tube combinations. Carbon dioxide concentrations under nonperforated (clear or infrared-blocking polyethylene) tunnels were significantly higher (three to four times) than that of ambient air. Soil respiration under the plastic mulch was primarily responsible for increased CO2 levels in the tunnel. Daily CO2 concentrations in the tunnels varied little during early muskmelon growth, but fluctuated widely as the plants developed. Ventilation significantly decreased CO2 concentrations in the tunnels but levels remained significantly above the control and perforated tunnel treatments. When using mulched minitunnels for muskmelon production, daily CO2 concentrations should be recognized as a significant factor influencing growth.
Sylvie Jenni, Pierre Dutilleul, Stephen Yamasaki, and Nicolas Tremblay
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.
Sylvie Jenni, Pierre Dutilleul, Stephen Yamasaki, and Nicolas Tremblay
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.
Sylvie Jenni, Katrine A. Stewart, Gaétan Bourgeois, and Daniel C. Cloutier
Sylvie Jenni, Daniel C. Cloutier, Gaétan Bourgeois, and Katrine A. Stewart
Growth of `Earligold' muskmelon (Cucumis melo L.), expressed as plant dry weight from transplanting to anthesis, could be predicted using a multiple linear regression based on air and soil temperatures for 11 mulch and rowcover combinations. The two independent variables of the regression model consisted of a heat unit formula for air temperatures, with a base temperature of 14C and a maximum reduced threshold of 40C, and a standard growing-degree day formula for soil temperatures with a base temperature of 12C. Based on 2 years of data, 86.5% of the variation in the dry weight (on a log scale) could be predicted with this model. The base temperature for predicting developmental time to anthesis of perfect flowers was established at 6.8C and the thermal time ranged between 335 and 391 degree days in the 2 years of the experiment.