J. I. L. Morison

Growth and yield of winter wheat (Triticum aestivum) crops in response to CO2 and temperature

Crops of winter wheat (Triticum aestivum L. cv. Hereward) were grown within temperature gradient tunnels at a range of temperatures at either c. 350 or 700 μmol mol -1 CO 2 in 1991/92 and 1992/93 at Reading, UK. At terminal spikelet stage, leaf area was 45 % greater at elevated CO 2 in the first year due to more tillers, and was 30 % greater in the second year due to larger leaf areas on the primary tillers. At harvest maturity, total crop biomass was negatively related to mean seasonal temperature within each year and CO 2 treatment, due principally to shorter crop durations at the warmer temperatures. Biomass was 6-31% greater at elevated compared with normal CO 2 and was also affected by a positive interaction between temperature and CO 2 in the first year only. Seed yield per unit area was greater at cooler temperatures and at elevated CO 2 concentrations. A 7-44 % greater seed dry weight at elevated CO 2 in the first year was due to more ears per unit area and heavier grains. In the following year, mean seed dry weight was increased by > 72 % at elevated CO 2 , because grain numbers per ear did not decline with an increase in temperature at elevated CO 2 . Grain numbers were reduced by temperatures > 31°C immediately before anthesis at normal atmospheric CO 2 in 1992/93, and at both CO 2 concentrations in 1991/92. To quantify the impact of future climates of elevated CO 2 concentrations and warmer temperatures on wheat yields, consideration of both interactions between CO 2 and mean seasonal temperature, and possible effects of instantaneous temperatures on yield components at different CO 2 concentrations are required. Nevertheless, the results obtained suggest that the benefits to winter wheat grain yield from CO 2 doubling are offset by an increase in mean seasonal temperature of only 1.0 °C to 1.8 °C in the UK.

Including the heat storage term in sap flow measurements with the stem heat balance method

The importance of the change in stem temperature and therefore the heat storage term in the stem heat balance method of measuring sap flow is examined. Results from a range of measurements on a model stem, potted sunflower plants in a glasshouse, and Guiera senegalensis shrubs in the Sahel, Niger, are presented. A novel analysis of the heat balance in zero flow conditions allows the accurate determination of the gauge radial conductance and the stem segment heat capacity, both of which are required for accurate sap flow measurement with good dynamic resolution in low flow conditions. In high sap flow conditions the change in heat storage constitutes only a small component of the balance, and can be neglected, especially for small stems. The improved accuracy and dynamic resolution for stems of any size if heat storage is included allowed the measurement of low night-time flows during rehydration, and of redistribution of water between stems of G. senegalensis bushes in the field following rain.

Estimation of transpiration by single trees: comparison of sap flow measurements with a combination equation

Abstract Sap flow estimates for whole trees (scaled from measurements on selected branches using the heat balance method) were compared with estimates of transpiration based on porometry in a study of poplar trees in an agroforestry system in the south of the UK. Sap flow showed good agreement with the transpiration rate estimated using the Penman-Monteith equation with measured stomatal conductance (R2 = 0.886) on six selected days during the season. The dominant environmental variable influencing transpiration was the vapour pressure deficit, as the “aerodynamic term” in the Penman-Monteith equation accounted for more than 70% of daily total transpiration, with the rest due to the “radiation component”. Stomatal conductance, estimated by inverting the Penman-Monteith equation from continuous measurements of sap flow over 55 days, was used to determine the parameters for a multiplicative stomatal conductance model. For an independent data set there was better agreement between measured sap flow and transpiration predicted from the stomatal conductance (R2 = 0.90) than for calculated and predicted stomatal conductance (R2 = 0.51).

Yield and partitioning in crops of contrasting cultivars of winter wheat in response to CO2 and temperature in field studies using temperature gradient tunnels

Diverse cultivars of winter wheat ( Triticum aestivum L.) were grown in the field in 1993/94 and 1994/95 at Reading UK in temperature gradient tunnels at normal atmospheric ( c . 370) or elevated CO 2 concentration ( c . 700 μmol CO 2 mol −1 air). In 1993/94, grain yield of cv. Avalon was insensitive to mean temperature (between 8·8 and 10·9°C), while elevated CO 2 increased yield by 1·3 t ha −1 (12·6%). In all other cultivars, warming reduced grain yield and CO 2 increased grain yield. In 1993/94, in cvs Galahad and Mercia the effects of CO 2 and temperature on yield were additive. However, for cv. Hereward in both years and for cv. Soissons in 1994/95, there were negative interactions between the effects of CO 2 and temperature on yield: the maximum benefit of doubling CO 2 to grain yield, 4·5 and 2·7 t ha −1 (65 and 29%) respectively, occurred at cooler temperatures; there was no benefit from doubling CO 2 (i.e. 0%) once the temperature had increased above the seasonal mean by 2·2–2·6°C in cv. Hereward and by 1·3°C in cv. Soissons. The beneficial effect of doubling CO 2 on grain yield in cvs Galahad, Hereward, Mercia and Soissons was negated by an increase in mean seasonal temperature of only 0·7–2·0°C. Warming decreased root dry mass at anthesis in 1994/95 while it increased at elevated CO 2 (49 and 186%, coolest and warmest regime, respectively). Carbon partitioned to roots declined progressively with warming, while at elevated CO 2 there was an average of 56% increase in allocation to roots. The relative impacts of both CO 2 and temperature were greater on root dry mass than on either grain yield or total above-ground biomass, while the effects on grain and biomass yield varied considerably between cultivars, suggesting that the impact of rising CO 2 and temperature are likely to be dependent on cultivar.

Modelling the impact of climatic warming on winter cereal development

Abstract Predictions of climate change suggest modification of developmental calendars for temperate crops, particularly those grown over winter. The AFRC-Wheat developmental model was used with 92 and 105 year daily temperature records from Edinburgh and Oxford, respectively, to examine the sensitivity of modelled winter wheat development to both past interannual temperature variations and arbitrary temperature changes. The model is driven by accumulated temperatures, but modified by vernalization requirement and photoperiod responses. Using the historic data, the duration from sowing to the stages of double-ridges and anthesis is approximately linearly related to mean temperature, and is more responsive at the northern site of Edinburgh than at Oxford. The duration from sowing to maturity is only weakly related to seasonal mean temperature. This is because the time course of temperatures within the season is critical in determining development rates, therefore seasons with equivalent mean temperatures can show very different wheat development calendars. This emphasizes the limitations of current climatic scenarios with little seasonal detail for agricultural impact predictions. The model does not appear to work with temperatures more than 1°C lower than present conditions at Edinburgh. Arbitrary warming by 2 and 4°C resulted in substantial shortening of the growing season (by 30 and 49 days at Edinburgh), implying major losses of potential yield, unless varieties or sowing dates are modified so that the development calendar is fitted to the insolation pattern.

Sap flow measurements from stem heat balances: a comparison of constant with variable power methods

Abstract Two methods of sap flow measurement using the stem surface heat balance approach were compared: the variable power (VP) and constant power (CP) methods. Comparisons were carried out on woody stems in a glasshouse in the UK ( Ficus benjamina ) and in the field during the rainy season in Niger ( Guiera senegalensis ). Substantial errors occurred if changes in stem temperature were neglected in the CP method. The simple VP method used neglected conductive losses, but was less affected by changes in stem temperature than the CP method. Therefore, at low flows the VP method overestimated sap fluxes, but at high flows it had a better dynamic response than the CP method. Daily totals of sap flow over 9 days in G. senegalensis stems with leaf areas of 1.3–2.3 m 2 ranged from 2 to 9 kg day −1 , and the two methods agreed well, although there was a consistent 8–12% higher estimate by the CP method.

Effects of CO2, temperature and their interaction on the growth, development and yield of cauliflower (Brassica oleracea L. botrytis)

Abstract Stands of summer cauliflower were grown within polyethylene-covered tunnels along which a temperature gradient was imposed. Two tunnels were maintained at either normal or elevated CO 2 concentrations. At the last harvest (88 days from transplanting) no interaction between CO 2 and temperature on total biomass was detected. The total dry weight of plants grown at 531 μmol mol −1 CO 2 was 34% greater than those grown at 328 μmol mol −1 CO 2 , whereas a 1 °C rise reduced dry weight by 6%. From serial harvests the radiation conversion coefficient was 2.01 g MJ −1 and 1.42 g MJ −1 at 531 μmol mol −1 CO 2 and 328 μmol mol −1 CO 2 , respectively, but was not greatly affected by differences in temperature. No effect of either CO 2 or temperature on the canopy light extinction coefficient was detected. The rate of progress towards curd initiation increased to a maximum at 15.5 °C, and declined thereafter. Provided the effect of temperature was accounted for, CO 2 enrichment did not affect the time of curd initiation. From serial harvests after curd initiation, the logarithm of curd weight or diameter were negative linear functions of mean temperature from initiation. Increases in curd weight and diameter at 531 compared with 328 μmol mol −1 CO 2 were greater at warmer temperatures (27% at 13 °C compared with 47% at 15 °C, 57 days after initiation). Effects of CO 2 on curd diameter were less than those on curd dry weight because the curd dry matter content was greater at 531 compared with 328 μmol mol −1 CO 2 . Thus, the effects of elevated CO 2 concentrations on fresh weight based yield parameters of cauliflower were less than the increase in total dry matter production.

Changes in growth and radiation use by lettuce crops in relation to temperature and ontogeny

A crop of lettuce was grown within two polyethylene-covered tunnels along each of which a temperature gradient was imposed. Between transplanting and harvest maturity the mean temperature differed by up to 5.5°C along the tunnels. Initial increase in plant dry weight was more rapid in the warmer plots. Similarly, maximum ground cover was attained earlier in warmer plots. However, dry weights at the final harvest were greater in plants from cooler plots. Accumulated dry matter was linearly related to accumulated total intercepted radiation, and this relationship differed among plots (P < 0.001). When these data were analysed over each harvest interval, the slope of this relationship, the radiation conversion coefficient, varied systematically during vegetative growth (P < 0.001). Estimates of the radiation conversion coefficient compared after set thermal periods among all plots increased progressively with thermal time from 0.33 g MJ−1 soon after transplanting to a peak of 1.39 g MJ−1 after 400°Cd, and thereafter declined progressively to a minimum of 0.33 g MJ−1 at 650°Cd. Reanalysis of published data for both lettuce and other field crops revealed further examples in which similar ontogenetic trends in radiation conversion coefficient were apparent.

Developmental and tillering responses of winter wheat ( Triticum aestivuni) crops to CO2 and temperature

Winter wheat (Triticum aestivum L., cv. Hereward) was grown in the field within four double-walled polyethylene-covered tunnels along which near-linear temperature gradients were imposed at normal atmospheric or at an elevated CO 2 concentration (c. 700 μmol mol -1 CO 2 ) in 1991/92 and in a further experiment in 1992/93. Development was more rapid the warmer the temperature. In 1991/92 an increase in mean seasonal temperature of 3.5 °C reduced the duration from sowing to harvest maturity (the stage when grain moisture content reduced naturally to 15-18 %) by c. 38 days, and reduced the duration from the double ridge stage to harvest maturity by c. 34 days. A similar difference resulted from only 1.6 °C warming in 1992/93. Although the range of mean seasonal temperatures differed between years, the relation between temperature and rate of development from sowing to harvest maturity was common to both years (base temperature, -0.8 °C; thermal time 2410 °C d). Carbon dioxide concentration had no effect on this relation or on that between temperature and the rate of development from sowing to the double ridge stage and from the double ridge stage to harvest maturity. Carbon dioxide enrichment increased tillering substantially in 1991/92; there were 200 more shoots m -2 at terminal spikelet formation in crops grown at elevated compared to normal CO 2 (additional shoots were principally coleoptile tillers and those developing after tiller 2) and this difference was reduced to 100 shoots m -2 approaching harvest maturity (additional shoots remaining were those developing after tiller 2). In contrast, no effect of CO 2 enrichment on tillering was detected at any stage of development in 1992/93. The number of tillers per plant at terminal spikelet formation was a linear function of main stem dry weight at this developmental stage; this relationship was not affected by year or CO 2 . As CO 2 enrichment increased main stem dry weight in the first year only, when main stem dry weights at normal CO 2 were only one half of those values determined in the following year, it is concluded that any benefit of increase in CO 2 concentration to tillering in winter wheat may be greatest in those crop production environments where main stem dry weights at terminal spikelet are least and vice versa.

Postharvest sprouting of onion bulbs grown in different temperature and C02 environments in the UK

SummaryThe effects of different mean growing season temperatures and C02 concentrations during bulb production on postharvest bulb sprouting in a common storage environment at Reading, UK, was examined in two cultivars of the Rijnsburger type of onion (Allium cepa L.). Crops were grown in the field in temperature gradient tunnels maintained at either 374 or 532 ppm C02. At crop maturity, cohorts of bulbs were harvested, transferred to a constant temperature room (at an average of 11.6°C) and the subsequent duration to sprouting recorded. The duration to the onset of sprouting (expressed as days in storage until the first bulb sprouted) was not affected by cultivar, mean growing season temperature or CO2 concentration, and was 165 d. The subsequent rate of sprouting (expressed as bulbs per day) was a positive linear function of mean growing season temperature, but no effects of CO2 or cultivar were detected. Mean rate of sprouting increased from an average of 0.036 bulbs per day at 12.3°C to 0.093 bulbs pe...