Plant growth, yield, and fruit quality of two strawberries (Fragaria ×ananassa Duch.)—`Chandler' and `Sweet Charlie'—grown under high tunnels (HTs) were compared with that of field plants during 2002–03 and 2003–04 growing seasons. Plug plants were planted in mid-October 2002 and mid-September 2003 on raised beds covered with black polyethylene mulch. Microclimate of the HTs protected strawberry crowns from winter damage and advanced fruit production 5 weeks earlier than that of plants grown under field conditions. From December to February, average minimum and maximum crown temperatures under the HTs were 5 and 12 °C warmer than those of the field crowns, respectively. The earliest HT fruit were harvested on 7 Apr. 2003 and 11 Mar. 2004. Yield and fruit quality under the HTs were superior to that of field-grown plants. HT plants, especially `Sweet Charlie', bloomed earlier than did field plants, but `Chandler' produced higher yield than `Sweet Charlie' late in the season. Larger fruit with higher soluble solids concentration (SSC) were produced inside the HTs than outside. HT `Sweet Charlie' fruit were sweeter than `Chandler' fruit, but `Chandler' produced larger fruit. Larger leaf area, greater number of leaves and shoot biomass, more branch-crowns, and fewer runners were developed under HTs than field conditions. Total leaf area, leaf production, total shoot biomass, and number of branch-crowns of HT `Chandler' were greater than HT `Sweet Charlie'. Results of this study indicate that strawberry plants under HTs were not only precocious, but also produced higher yields and superior quality to that of field plants. HT conditions suppressed runner growth, but enhanced branch-crown development.

Thermotolerance of photosynthesis and productivity in `Chandler' and `Sweet Charlie' strawberry plants (Fragaria ×ananassa Duch.) exposed to three temperature regimes was studied. Net CO2 assimilation rate (A), variable chlorophyll fluorescence (Fv), efficiency of photosystem II (Fv/Fm), relative chlorophyll content, plant growth, and fruit yield and quality were measured. High temperature (40 °C day/35 °C night) was more detrimental to photosynthesis and productivity than the moderate or low temperature (30/25 or 20/15 °C). Net CO2 assimilation rate in both cultivars was markedly reduced by 40/35 °C, although there was slight decline in `Sweet Charlie' at 30/25 °C. `Chandler' maintained significantly higher A rates than `Sweet Charlie' for at least three weeks of heat stress, indicating that `Chandler' might tolerate longer exposure to high temperature. In parallel to the decrease in A rate, intercellular CO2 concentration (Ci) and instantaneous water use efficiency (WUE) were significantly decreased at high temperature. `Chandler' leaves were cooler and transpired more than `Sweet Charlie' leaves, suggesting that each cultivar adopted different heat resistance mechanisms at 40/35 °C. There were changes in Fv and Fv/Fm with increasing temperature, indicating irreversible damage to photosystem II at 40/35 °C might have occurred. The trend of reduction in stomatal conductance (gS) in both cultivars at high temperature did not coincide with the reduction in A rates. Decline in A rates at high temperature was more related to changes in Fv/Fm than to gS activity. The optimal temperature for vegetative growth was 30/25 °C. Reduction in A rate at high temperature resulted in reduction in total leaf area (LA), shoot, root, and leaf biomasses. Strawberry roots were more responsive than shoot growth to temperatures above 20/15 °C. Fruit yield for `Chandler' was higher at 20/15 °C than at 30/25 °C, suggesting that `Chandler' might have a higher source-to-sink relationship at 20/15 °C than at 30/25 °C. Fruit skin color was temperature dependent only for `Chandler'. A quadratic relationship between flower development and duration of exposure to 30/25 °C for both cultivars was observed; more than two weeks of 30/25 °C can be detrimental to flower development. Regardless of the cultivar and duration of exposure, 40/35 °C was the temperature regime most detrimental to fruit set.

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Damage and capacity to recover of photosystem II (PSII) from long exposures to heat stress were investigated in grapes using chlorophyll fluorescence. Two wine grapes, Vitis aestivalis Michx. cv. ‘Cynthiana’ and French-American hybrid ‘Vignoles’ (Vitis L. hybrid), were exposed to a sudden heat shock (SHS) and a gradual heat shock (GHS) at 40/35 °C. After heat stress, plants were moved to a greenhouse to allow PSII to recover from heat treatments. Changes in maximum quantum efficiency of PSII, indicated by the ratio of variable fluorescence and maximum fluorescence (Fv/Fm), were observed after 3, 6, and 12 days of heat stress and after 3, 7, 14, and 21 days recovery periods of damage to PSII. Total leaf area (LA) and leaf, shoot, and root biomass were determined at the end of the experiment. Regardless of the heat treatment, increasing duration of exposure to high temperature caused a decline in Fv/Fm in both cultivars. Heat stress treatments also caused a progressive decline in LA as well as leaf and shoot biomass. Maximum quantum efficiency of PSII was observed after 3 days of exposure in both cultivars, regardless of the heat stress treatment. ‘Vignoles’, however, showed higher PSII photochemical efficiency 12 days after heat exposure. GHS was less detrimental to PSII compared with SHS heat treatment. The damaged PSII of ‘Vignoles’ recovered faster than that of ‘Cynthiana’. A positive relationship was observed between Fv/Fm and LA of plants exposed to heat treatments. Based on Fv/Fm values, this study indicates that PSII of ‘Vignoles’ is more thermostable and can recover faster than that of ‘Cynthiana’ leaves, regardless of the heat treatment. These results suggest that ‘Vignoles’ is generally more heat-tolerant than ‘Cynthiana’ and changes in Fv/Fm ratio under heat stress conditions could be a good indicator for screening heat-resistant grape cultivars.

Field experiments were conducted at the Irrigated Agricultural Research and Extension Center, Prosser, Washington, in 1992 and 1993 to study the effects of simulated chlorsulfuron (2 chloro‐N [[(4 methoxy‐6 methyl 1,3,5‐tnazin 2 yl)amino]‐carbonyl]benzenesulfonamide) drift at different reproductive growth stages of three cherry (Prunus avium L ) cultivars, Rainier, Chinook, and Bing Branches of each cultivar were treated at side‐green, full bloom, postbloom, and pit hardening growth stages with 0, 251 1 × 10 7, 83 8 × 10 7, 27 9 × 10−7, 9 3 × 10−7, and 3 1 × 10−7 M chlorsulfuron In single‐exposure expetiments, Chinook was adversely affected by the lower levels of chlorsulfuron Bing was not affected by levels of chlorsulfuron below 27 9 × 10 7 M The yield and quality of fruit decreased significantly with the increase in chlorsulfuron concentrations applied at full bloom and postbloom stages Reduction in yield was correlated with the severity of injury symptoms on leaves In the multiple exposure experiments, fruit yield, fruit size, and color were significantly reduced by increasing chlorsulfuron concentration and number of exposures Average firmness of fruit increased with increases in chlorsulfuron concentration The data suggested that multiple exposures of a susceptible cherry cultivar to low levels of chlorsulfuron at full bloom and postbloom stage can reduce fruit yield and delay maturity of cherries while increasing fruit firmness