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Effects of temperature on the growth of lettuce (Lactuca sativa L.) and the implications for assessing the impacts of potential climate change

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Stands of lettuce were grown from a single transplanting in the field within two double-walled polyethylene tunnels along which temperature gradients were imposed. Plant dry weight was initially higher in the warmer plots, but maximum dry weight of
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  Temperature and lettuce growth 330 ppmv was assumed, in accordance with data of Baker and Allen (1993). Dry weight accounting for C0 2 concentration (W 3) was calculated from W 2 of equation (4), such that: (5) in which C is C0 2 concentration (ppmv) and z is a constant calculated from the regression of percentage increase in dry weight and C0 2 concentration which has the value 0.0684 (s.e. = 0.014). Daily increase in dry weight of lettuces (allowing 2 days growth check after transplanting of plants of 0.13 g dry weight) were calculated from daily records of mean air temperature and incident radiation for Rothamsted (51.8°N latitude, 0.5°W longitude) in 15 of the years between 1968 and 1991 using equations (2)-(5) (the years 1970, 1976, and 1981-1987 were omitted due to the absence of radiation data). The daily increase in temperature calculated near to this site (51 .25°N latitude, 0.5°W longitude) for six scenarios of climate change were then added to these baseline temperature data. The scenarios used were : three composite time-dependent scenarios for the years 2010, 2030, and 2050 (assuming a Businessas-usual (A) policy emissions scenario (Houghton et al., 1990)), and three GCM equilibrium 2 x C0 2 scenarios (derived from the Geophysical Fluid Dynamics Laboratory, GFDL; United Kingdom Meteorological Office low resolution, UKMO-L ; and the Goddard Institute of Space Studies, GISS, general circulation models). More details of these scenarios are provided by Barrow (1993). The mean temperature increase and C0 2 concentration calculated using these six scenarios for this growth period at a site near to Rothamsted are shown in Table 1. The daily radiation integral for the baseline period was also used for each scenario. The dry matter content of lettuces of more than 50 g fresh weight was found to be constant at 4.5 per cent. The calculated daily increase in dry weight of lettuce was then used to forecast maximum plant fresh weight at 4.5 per cent dry matter, and time to attain a fresh weight of 200 g for each of the 15 years using each of the six scenarios. Table 1. Summary of the mean temperature increase relative to the baseline climate, and COz- concentration for the present and climate change scenarios for the growing period 15 August to 2 7 September. Baseline A2010 A2030 A2050 GFDL UKMO GISS Temperature 0 0.5 1.2 1.9 5.6 5.6 4.3 increase (°C) C0 2 concn. 353 400 458 539 560 560 560 (ppmv) Vol. 2, 0° 4- 1993 307 RESULTS Growth o lettuce Mean air temperature during crop growth differed between plots at each end of the tunnels by 5.5 and 4.1 °C (range 15.6 to 21.1 °C, and 14.7° to 18.8 °C) in tunnels 1 and 2, respectively. Mean air temperature for the harvest intervals was between 10.7 and 22.3 °C. Mean ambient air temperature over this period was 16.6 °C. The initial increase in dry weight was more rapid in the warmer than the cooler plots: dry weight (calculated from each fitted function) was a positive function of mean temperature at 14 d from transplanting p < 0.01). However, the maximum dry weight of lettuce grown at the cooler end of the tunnel, although achieved more slowly, was ultimately greater than that grown under warmer conditions. Accordingly, the relationship between dry weight and mean temperature was negative for final maximum dry weight p < 0.01). Mean relative growth rate, calculated for each harvest interval, declined linearly with time from transplanting at all temperatures. Also, there was an interaction between time from transplanting and mean temperature for each harvest interval such that the relationship between mean relative growth rate and temperature changed during ontogeny from a positive function soon after transplanting to a negative one at about 40 d from transplanting (Figure 1). Thus, the i:l 0,3 bl 0,2 Q) § .s::: 0,1 e Ol Q) > 0 Oi a: Time from transplanting d) Temperature (oC) Figure 1. Relationship between mean relative growth rate (•), time from transplanting (t), and mean temperature en for that harvest interval. The fitted response plane is described by R = 0.179 + 0.39 X 10- 3 t + 10.39 X 10- 3 T -0.613 X l0- 3 t.T, R 2 = 0.835, 91 df The position of each observation from the plane is denoted by a bar.  308 Table 2. Optimum values of the parameters of equations 2) and 3). Parameter Value Ro 0.35 g g- 1 d-1 ef 685 °Cd a 0.037 oc l b 0.005 MJ- 1 m- d-1 optimum temperature for relative growth rate appeared to decline from more than 23 oc soon after transplanting to less than 10 oc at final harvest. The optimum values for the parameters of equations (2) and (3) for plants grown in two plots are shown in Table 2. The increase in dry weight predicted using these equations with these parameter values reproduced both the rapid increase in plant weight observed in the warmer plot, and the higher final dry weight observed in the cooler plot (Figure 2a). The mean daily relative growth rates predicted were initially larger in the warmer plots, but then declined more rapidly compared with values predicted for the cooler plot. This trend followed that observed for estimates of mean relative growth rate (Figure 2b). Parameter values for equations (2) and (3), optimised for two plots of lettuce, provided an accurate prediction of the dry weights observed independently in the remaining 14 plots (Figure 3). No systematic deviations between observed and predicted weights were detected over the range of observations from 2.0 g to 14.8 g dry weight, except for some of the heaviest plants for which the model overestimated plant weight. The main variable in the model was time. Predictions using the model based on time alone (given by the fitted relationship between the logarithm of dry weight and time for all plants grown in the two plots from which model parameters were estimated) accounted for 84.5 per cent of the variance of the independent observations, compared with 87.1 per cent once temperature and radiation were added ; a decrease in the residual sum of squares of 17 per cent (83 df). Effect o climate change Mean maximum fresh weight estimated using the baseline temperatures for 15 years and the current C0 2 concentration was 262 g (CV = 4.0 per cent). This mean increased progressively using the timedependent scenarios, although increase in yield was not significant with the A2010 and A2030 scenarios (Figure 4). Calculated maximum fresh weight for the year 2050 was only 4.7 per cent greater compared with the baseline climate, with a small decrease in variation about the mean (CV = 3.8 per cent). In contrast, maximum fresh weight was less for all equilibrium 2 x C0 2 scenarios compared with the baseline -b bl 12 9 8 Q) § r: Cl 4 Q) > jj a: 0 0,3 -b 0,2 bl Q) § r: 0,1 0 0, Q) > jj a: 0 T.R. Wheeler et al. 0 10 20 30 40 Time from transplanting Figure 2. Relationship between plant dry weight a), relative growth rate b) and time from transplanting for lettuces grown in plots 1 •) and 7 .A.) of tunnel 1. The fitted functions repre sent predictions using equations 2) -  4) and the values of the parameters shown in Table 2 for plots 1 7 - - -). Each observation for dry weight is the mean of nine plants. § :c Cl Qi C:-  0 0 Q) tl 6 Q) :: 16 12 8 4 0 4 8 12 16 Observed dry weight g) Figure 3. Comparison of actual dry weight of lettuces from 14 independent plots within the tunnels with those predicted using equations 2) -  4) with the value of the parameters presented in Table 2. The line shown represents exact agreement between observation and prediction. Eur. J Agron.  Temperature and lettuce growth climate. The greatest reduction in yield was 5.5 per cent using the GFDL scenario, although differences among these three GCM 2 x C0 2 scenarios were not significant (Figure 4 ). Variation about these means was increased compared to the baseline (for example, for the GFDL scenario, CV = 5.6 per cent). 300 § .E 200 l iii : : .c /) lL 100 Baseline A2010 A2050 UKMO-L A2030 GFDL GISS Figure 4. Mean maximum fresh weight of lettuces calculated using the baseline temperature and radiation record for 15 years at Rothamsted between 1968 and 1991) and six scenarios of climate change. Each bar represents the mean± one s.e. The mean estimated time for lettuces to reach a fresh weight greater than 200 g was 35 days from transplanting using the baseline climate and current C0 2 concentration (CV = 6.1 per cent). 30 Ol 0 0 C\J 20 .8 Q) E i= 10 Baseline A201 0 A2050 UKMO-L A2030 GFDL GISS Figure 5. Mean time from transplanting to 200 g fresh weight calculated using the baseline temperature and radiation record for 15 years at Rothamsted between 1968 and 1991) and six scenarios of climate change. Each bar represents the mean ± one s.e. Vol. 2, n° 4- 1993 309 This time was significantly reduced using all six climate change scenarios except A2010 (Figure 5). Using the three time-dependent scenarios, the mean time to 200 g declined progressively from 33 d for the year 2010 to 30 d for the year 2050. Mean time to 200 g in all three equilibrium 2 x C0 2 scenarios was 26 d (CVs were 4.7, 4.9 and 4.9 per cent, respectively). DISCUSSION Increasing temperature resulted in a rapid increase in early dry weight accumulation in lettuce but reduced final weight. This response accounts for both the positive correlation reported between temperature and lettuce weight during early growth (Hicklenton and Wolynetz, 1987), and the negative relationship reported for crisphead lettuce between temperature and head weight and head density at final harvest (Wurr and Fellows, 1991 ; Wurr et al., 1992). Similar examples have been reported for soyabean (Hadley et al., 1994) and pearl millet Pennisetum typhoides S. and H.) (Squire et al., 1984). The effect of temperature on final crop yield is dependent on whether a crop is determinate or indeterminate (Goudriaan and Unsworth, 1990). The response observed here for lettuce is characteristic of that for determinate crops, in which an increase in temperature usually results in a reduction in final yield (Squire, 1990 ; Goudriaan and Unsworth, 1990). The relationship between relative growth rate and temperature changed during ontogeny. Although Ri was a function of temperature, equation (3), the decline in potential relative growth rate (RP) during ontogeny resulted in an apparent decline in the temperature optimum for relative growth rate of lettuce during crop growth. This is exactly the response predicted previously using equations (2) and (3) to describe the growth of soy abean (Hadley et al., 1994 , and thus provides evidence in lettuce to support the hypothesis that instantaneous relative growth rate is a function of current temperature and developmental thermal time. The equations for the effects of temperature and incident radiation on crop growth described and validated here for lettuce may provide a framework for a simple and robust model to assess the effects of potential climate change on crops. The model, as optimized here, can only be used at this plant density for a well watered and fertilized crop ; further experiments at different sowing dates are required to extend the application of the model. However, the potential effect of different climate change scenarios on outdoor butterhead lettuce production can be assessed within the limits of these assumptions. The overall effect of potential climate change on the yield of many determinate crops will partly be a
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