Strawberry (Fragaria ·ananassa Duch.) Growth and Productivity as Affected by Temperature

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 weremeasured. 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. Strawberry cultivars grown in specific areas are adapted to the day length and temperatures of that region. Nevertheless, heat stress is one of the challenges that face strawberry production. Reduction in plant growth by high temperatures is well established in horticultural crops such as tomato (Solanum lycopersicum L.) (Adams et al., 2001), grape (Vitis spp.) (Chaumont et al., 1997), and strawberry (Renquist et al., 1983). Damage to crops by high temperatures has been reported in many regions around the world and Kansas is no exception; temperatures above 35 C are common during the growing season. High temperature adversely affects vegetative growth and fruit quality of tomato (Adams et al., 2001; Mulholland et al., 2003) and reproductive systems of peanut (Arachis hypogaea L.) (Vara-Prasad et al., 1999). Heat stress affects photosynthesis, which is highly sensitive to thermal inhibition (Henning and Brown, 1986) whether stress occurs early or late in the growing season. High temperature inhibits thylakoid activities, especially near the photosystem II (PSII) reaction center (Berry and Björkman, 1980). There have been reports on responses of strawberry cells and whole plants to various temperatures. Strawberry cells subjected to 30 C grew slowly and did not proliferate normally in suspension cultures (Zang et al., 1997). Strawberry vegetative growth (Hellman and Travis, 1988), root growth (Fukuda and Matsumoto, 1988), fruit set (Nishiyama et al., 2003), pollen viability (Ledesma and Sugiyama, 2005), fruit weight (Mori, 1998), fruit quality (Polito et al., 2002), and leaf protein expression (Gulen and Eris, 2004; Ledesma et al., 2004) were negatively affected by high temperatures. However, strawberry plants resistant to high temperature have the ability to maintain high rates of photosynthesis, stabilize proteins, and synthesize new proteins (Gulen and Eris, 2004). It has been established that the critical temperature range for strawberry growth inhibition is between 35 and 40 C and that development of runners is inhibited by 3 d of exposure to 40 C (Hellman and Travis, 1988). Nevertheless, the mechanism involved in heat stress is not well defined, and information related to strawberry varietal response to heat stress manifested in direct effect on photosynthetic rate and indirect effect on the photosynthesis process is limited. Annual temperature fluctuation occurs frequently in Kansas from late spring through midsummer, a period characterized by high temperatures and long exposure that severely impact growth and production of strawberries. In this respect, our objective was to investigate the effects of low, moderate, and high temperatures on physiological characteristics and productivity of Chandler and Sweet Charlie strawberry plants. Specific objectives were to identify the optimal temperatures for photosynthesis and chlorophyll fluorescence, determine the influence of high temperature on photosynthesis and efficiency of PSII, and to assess growth response and productivity of strawberry plants under three temperature treatments. Materials and Methods Plant materials and treatments. Two new June-bearing strawberry cultivars were selected to replace the old cultivars in Kansas. Chandler and Sweet Charlie are chosen for their early production, high yield, and quality fruits. Plug plants of Chandler and Sweet Charlie (Davon Crest Farms LLC, Hurlock, Md.) strawberry (Fragaria ·ananassa) were planted 6 Aug. 2003 in polyethylene pots (16.25 · 16.25 · 12.5 cm) containing a mixture of 1 soil:1 peatmoss:1 perlite (by volume). Each pot had six drainage holes to facilitate water drainage. Plants were grown for 4 weeks under greenhouse conditions of 22/17 ± 3 C day/night (D/N) temperatures, 50 ± 10% relative humidity (RH), and 16/8-h light/dark (L/D) photoperiods with 500 mmol m s PPF density (PPFD) (400–700 nm, measured with LI-188B Integrating Quantum/Radiometer/Photometer and LI190sB sensor; LI-COR, Inc., Lincoln, Nebr.) on a horizontal plane above the plant canopy. Supplemental light was provided by Hydrofarm grow lights with 400-W, high-pressure sodium, S-51-type lamps (Hydrofarm Products, Petaluma, Calif.). Plants were irrigated Received for publication 24 May 2006. Accepted for publication 9 July 2006. With our appreciation, funding for this study was provided by the Initiative for Future Agricultural Food System, U.S. Department of Agriculture (IFAFS/USDA), and Kansas Agricultural Experiment Station. This is contribution no. 06-265-J from the Kansas State Agricultural Experiment Station. To whom reprint requests should be addressed; e-mail [email protected]. Professor, Agronomy Department. HORTSCIENCE VOL. 41(6) OCTOBER 2006 1423 as needed with deionized distilled water to full pot capacity (Olson et al., 2000). Full pot capacity was determined by saturating the dry soil with water and then measuring the increase in weight after water drained by gravity from the pot. Moisture was maintained by weighing the pots and replenishing water to full pot capacity. Plants were fertilized weekly with a commercial fertilizer containing 300 mg L nitrogen (N), 250 mg L phosphorus (P), and 220 mg L potassium (K) (Miracle-Gro; Scotts MiracleGro Products, Port Washington, N.Y.). Four weeks later, and before temperature treatments, the most recently fully expanded leaflet was measured to determine gas exchange, relative chlorophyll content, and chlorophyll fluorescence. Nine plants of each cultivar were randomly distributed into one of the three growth chambers (Conviron CMP 3244, Asheville, N.C.) set at 20/15 ± 1 C, 30/25 ± 1 C, or 40/35 ± 1 C D/N temperatures, 16/8-h L/D photoperiods with 550 mmol m s PPFD, and 70% RH. Growth chamber lights were turned off between 10:00 PM and 6:00 AM. Plants were watered as needed to full pot capacity with deionized distilled water. Pest control was carried out according to strawberry pest control recommendations (Kadir et al., 2006). Gas exchange, chlorophyll fluorescence, and relative chlorophyll content of the most recently fully expanded leaflet were measured at weekly intervals for 4 weeks between 9:00 AM and 11:00 AM. Open flower number was recorded and percentage of dead flower was calculated at weekly intervals. Total fruit yield and fruit quality were recorded as the strawberry plants grow; vegetative and root growth was determined after 4 weeks of exposure. Leaf gas exchange measurement. Gas exchange was measured with an LI-6400 open system portable photosynthesis meter (LI-COR Inc.). Net CO2 assimilation rate (A), stomatal conductance (gs), transpiration rate (E), intercellular CO2 concentration (Ci), and leaf temperature were determined. Instantaneous water use efficiency (WUE) was calculated as the ratio between the photosynthetic and transpiration rates (A/E). A leaf sample was placed in the leaf chamber (6.0 cm) and exposed to 550 mmol m s PPFD and a CO2 concentration of 400 mL L. Data were recorded after 30 to 45 s, when CO2 and gS stabilized. Leaf chlorophyll fluorescence measurement. Leaf chlorophyll fluorescence was determined by using a pulse-modulated fluorometer (Fluorescence Monitoring System [FMS-1]; Hansatech Instruments Ltd., Norfolk, U.K.). The FMS-1 requires no dark adaptation of the leaf because it uses modulated fluorometry to separate actinic light from the fluorescence signal. During measurements, a tissue sample is exposed to a pulsed amber LED light source causing excitation of a pulsed fluorescence signal in the absence of actinic light. The machine was operated in the Fv/Fm mode, and the fluorescence was measured with a photodiode in the 710to 760-nm range. The fluorometer probe was placed 5 mm away from the leaf, and measurements were made at a steady state of 2000 mmol m s and a saturating state of 5000 mmol m s for 0.8 s. Fluorescence parameters of initial fluorescence (Fo), maximal fluorescence (Fm), variable fluorescence (Fv = Fm – Fo), and quantum yield efficiency of PSII (Fv/Fm) were recorded from the fluorescence LCD display. Three readings from locations between the veins were averaged to represent one observation. Relative chlorophyll content measurement. An indirect index of chlorophyll content was measured with a leaf chlorophyll meter (SPAD-501; Minolta Corp., Osaka, Japan) at weekly intervals. Three SPAD measurements (38 mm total leaf area) from locations between the veins were averaged to represent one observation. Plant growth and productivity. Individual plants were harvested after 4 weeks of temperature treatments and leaf, crown, and runner numbers were recorded. Total leaf area (LA) per plant was measured with the LI-3100 leaf area meter (LI-COR Inc.). Roots were extracted from the soil, washed with deionized distilled water, and placed on paper towels in the greenhouse for one day. Leaves, shoots (crowns and petioles), and roots were dried at 70 C ± 2 for 72 h and were weighed. Weekly observation of flower development was conducted in each growth chamber. Open flower number was recorded and percentage of dead flower was calculated based on total flower number per week in each temperature. Percentage of dead flower was determined by dividing total number of flower by number of dead flower and multiplying by 100. Fig. 1. Effect of three temperature regimes (20/15, 30/25, or 40/35 C day/night [D/N] and 16/8-h photoperiod for 4 weeks) on net CO2 assimilation rate (A) (A.1 and B.1), gS (gs) (A.2 and B.2), intercellular CO2 (Ci) (A.3 and B.3), and instantaneous water use efficiency (WUE) (A.4 and B.4) of newly fully developed leaflet on 4-week-old Chandler (A 1–4) and Sweet Charlie (B 1–4) plants. Measurements were obtained at CO2 concentration of 400 mL L at weekly intervals. Vertical bars through data points are standard errors; values smaller than symbols are not shown. Data represent means of nine plants grown in the same temperature regime. 1424 HORTSCIENCE VOL. 41(6) OCTOBER 2006 Fruits were harvested at full maturity (more than 90% skin color). Fruit yield per plant and average fresh fruit weight were recorded. Soluble solids concentration (SSC) was measured from extracted juice with a handheld sugar refractometer (Atago No. 41385; Leslie Ratay, New Gardens, N.Y.) and a sucrose scale calibrated at 20 C. Three external fruit color measurements were taken with a Chromameter (CR-310; Minolta, Japan) according to the Hunter a , L , b system and the results were averaged. The machine was calibrated with a white standard (L = 97.83, a = –0.38, b = 1.94). The ‘‘a’’ value represents greenish to redness, lightness coefficient ‘‘L’’ represents brightness and darkness, and ‘‘b’’ value represents blueish to yellowish. Hue value (h) measures fruit color intensity and was calculated from a combination of Hunter ‘‘a’’ and ‘‘b’’ values (Hunter and Harold, 1987); the lowest h represents the greatest degree of red skin color. The experiment was a randomized complete block design with a factorial arrangement of cultivar · temperature · time. Temperature treatments were replicated two times; for the second replication, strawberry plants were planted in the greenhouse on 10 Jan. 2004; there was no interaction between treatments and replications; thus, data are averaged across replications. Temperature treatment response was determined by analysis of variance according to General Linear Models Procedure (SAS Institute, Cary, N.C.). Least significant differences (LSD) among means were tested at P = 0.05. Standard errors of the means were calculated. Person correlation coefficients (r) between selected parameters were established. Appropriate data were subjected to regression analysis to establish the coefficient of determination (r) for the best model. Results and Discussion Gas exchange. Temperature and duration of exposure significantly interacted with cultivar to influence gas exchange parameters (Fig. 1). High temperature (40/35 C) was more detrimental to photosynthesis than moderate or low temperature (20/15 or 30/ 25 C). Net CO2 assimilation rates (A) of Chandler (Fig. 1, A.1) and Sweet Charlie (Fig. 1, B.1) in 20/15 C were the same as those of plants in 30/25 C, although a 15% decline was observed in Sweet Charlie after 4 weeks of exposure to 30/25 C. Regardless of the cultivar, plants grown in 40/35 C showed early decline in A rates. One and 2 weeks of exposure reduced A rates by 44% and 20% in Sweet Charlie and Chandler , respectively, compared with the control. Photosynthetic rate in 40/35 C at the end of the experiment was practically the same for both cultivars, but Chandler maintained significantly higher A rates for at least 3 weeks of high temperature than Sweet Charlie . These results indicate that photosynthetic rate in Chandler remained higher at warm temperatures than that in Sweet Charlie ; the former might survive longer exposure to high temperature. Initially, gS (gs) of both cultivars was the same, but reduction occurred in the 40/35 C treatment after 2 and 3 weeks of exposure in Sweet Charlie (Fig. 1, B.2) and Chandler (Fig. 1, A.2), respectively. Four weeks of exposure to high temperature reduced gs in Sweet Charlie by 92% comparedwith 72% in Chandler . Nevertheless, the gs of Chandler was significantly higher than that of Sweet Charlie at the end of the experiment, which might indicate that gs of Chandler is relatively less responsive to high temperature than that of Sweet Charlie . In both cultivars, a decline in gs at high temperature did not correspond to a decline in A rates. Different reduction trends of A rate and gs at high Fig. 2. Effect of high temperature (40/35 C day/night [D/N] and 16/8-h photoperiod for 4 weeks) on leaf temperature (bars) and transpiration rate (E) (circles) of newly fully developed leaflet on 4-week-old Chandler (solid circles and bars) and Sweet Charlie (open circles and bars) plants. Measurements were obtained at CO2 concentration of 400 mL L at weekly intervals. Data represent means of nine plants grown in the same temperature regime. Fig. 3. Relationship between duration of exposure to high temperature (40/35 C day/night [D/N]) and 16/8-h photoperiod for 4 weeks) and transpiration rate (E) of newly fully developed leaflet on 4-weekold Chandler (•) and Sweet Charlie (X) plants. Measurements were obtained at CO2 concentration of 400 mL L at weekly intervals. Lines represent linear regression analysis of the means. Data represent means of nine plants. HORTSCIENCE VOL. 41(6) OCTOBER 2006 1425 temperature suggest that high temperature directly acts on limitation factors related to photosynthesis processes other than gs activities. This agrees with earlier report that gs usually is not involved in temperature effects on photosynthesis (Paulsen, 1994). High temperature affects photosynthetic processes through nonstomatal activities such as enzymatic activity during photosynthesis (Medrano et al., 2003). Low temperature (20/15 C) had no influence on intercellular CO2 (Ci) concentrations in either cultivar. However, in 30/25 and 40/35 C, Ci was reduced 1 week earlier in Sweet Charlie (Fig. 1, B.3) than in Chandler (Fig. 1, A.3). In contrast to Sweet Charlie , Chandler maintained significantly higher Ci concentrations for at least 3 weeks at 30/25 C. A linear decline for Sweet Charlie and Chandler in 40/35 C was recorded after 1 and 2 weeks of exposure, respectively. After 4 weeks of high temperature, reduction was more severe in Sweet Charlie (87%) than Chandler (68%) compared with the control plants. Results indicate that timing of Ci reduction in both cultivars coincided with a decrease in A rates but not gs. This suggests that decline in Ci, which results in a decline in A rate, might also be related to factors other than gS such as a decrease in mesophyll conductance (CandolfiVasconcelos and Koblet, 1991). Instantaneous WUE measures the efficiency of carbon fixation per unit water loss. For both cultivars, the rate of carbon gain was almost always higher than water loss (higher WUE) in 20/15 and 30/25 C than in 40/35 C. The declining trend of WUE at high temperature was similar to the trend of A rates and Ci concentrations in both cultivars; WUE in Sweet Charlie (Fig. 1, B.4) decreased by 79% after 1 week and by 64% in Chandler after 2 weeks of high temperature (Fig. 1, A.4). Chandler had 46% higher WUE after 4 weeks of high temperature than Sweet Charlie . These results suggest that Chandler has higher A to E ratio under high temperature condition than Sweet Charlie . Responses of leaf temperatures and transpiration rates (E) to 40/35 C are shown in Figure 2. No treatment impact on leaf temperatures or E rates was observed for 20/15 C, whereas 30/25 C exerted a trend (data not shown) similar to that for 40/35 C. Leaf temperatures increased in both cultivars after 1 week of exposure to high temperature; average leaf temperature was 39.3 C. Trends of reduction in transpiration were similar to those of gs for both cultivars; E rate was reduced after 2 and 3 weeks in Sweet Charlie and Chandler , respectively. Chandler plants showed constant E rates for 2 weeks but declined thereafter by an average of 16%. Leaf temperature, however, remained constant throughout the duration of the experiment with an average of 38.5 C. In contrast, Sweet Charlie plants showed an average decline in E rate of 28% after 2 weeks, which resulted in an average of 2.1 C increase in leaf temperature. After 4 weeks of exposure, Chandler leaves transpired 20% more and leaf temperature was 4 C cooler than Sweet Charlie leaves. Results showed that Sweet Charlie had higher leaf temperature and lower E rate under high temperature than Chandler . Transpiration rate of Sweet Charlie was negatively related to leaf temperature (r = –0.90) and positively related to gs (r = 0.95); nonetheless, E rate of Chandler was not significantly related to leaf temperature (r = 0.57) and was less related to gs rate (r = 0.86) than Sweet Charlie . This suggests that different mechanisms for heat resistance might have been adopted by each cultivar. In contrast to Sweet Charlie , Chandler leaves under heat stress managed to maintain constant gs, resulting in cooler leaf temperature as a result of high transpiration. When transpiration rate of Sweet Charlie (Fig. 3) was regressed against duration of exposure, there was a negative linear relationship between transpiration and time. The relationship explained most (97%) of the variation in E rate with increasing time of exposure. Chandler , on the other hand, showed a negative linear relationship between transpiration and time although with low r (0.40). Fig. 4. Effect of three temperature regimes [20/15, 30/25, or 40/35 C day/night (D/N) and 16/8-h photoperiod for 4 weeks] on variable fluorescence (Fv) and quantum yield efficiency of photosystem II (Fv/Fm) ratio of newly fully developed leaflet on 4-week-old Chandler (A 1–2) and Sweet Charlie (B 1–2) plants. Chlorophyll fluorescence was measured as described in the ‘‘Materials and Methods’’ at weekly intervals. Vertical bars through data points are standard errors; values smaller than symbols are not shown. Data represent means of nine plants grown in the same temperature regime. Fig. 5. Effect of three temperature regimes (20/15, 30/25, or 40/35 C day/night [D/N] and 16/8-h photoperiod for 4 weeks) on relative chlorophyll content (SPAD value) of newly fully developed leaflet on 4-week-old Chandler and Sweet Charlie plants. Chlorophyll content was measured with the leaf chlorophyll meter at weekly intervals. Vertical bars through data points are standard errors; values smaller than symbols are not shown. Data represent means of nine plants grown in the same temperature