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SUMMARYCrops 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 CO2 in 1991/92 and 1992/93 at Reading, UK. At terminal spikelet stage, leaf area was 45% greater at elevated CO2 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 CO2 treatment, due principally to shorter crop durations at the warmer temperatures. Biomass was 6–31% greater at elevated compared with normal CO2 and was also affected by a positive interaction between temperature and CO2 in the first year only. Seed yield per unit area was greater at cooler temperatures and at elevated CO2 concentrations. A 7–44% greater seed dry weight at elevated CO2 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 CO2, because grain numbers per ear did not decline with an increase in temperature at elevated CO2. Grain numbers were reduced by temperatures > 31 °C immediately before anthesis at normal atmospheric CO2 in 1992/93, and at both CO2 concentrations in 1991/92. To quantify the impact of future climates of elevated CO2 concentrations and warmer temperatures on wheat yields, consideration of both interactions between CO2 and mean seasonal temperature, and possible effects of instantaneous temperatures on yield components at different CO2 concentrations are required. Nevertheless, the results obtained suggest that the benefits to winter wheat grain yield from CO2 doubling are offset by an increase in mean seasonal temperature of only 1·0 °C to 1·8 °C in the UK.

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There are two basic physiological prerequisites for seeds to survive in soil: germination must be avoided by dormancy or quiescence, while viability must be maintained. The remarkable longevity of some individual seeds is legendary. This chapter reviews some of these accounts, assessing these long-lived individuals in the context of the overall seed population.

Photoperiod and temperature influence the time from sowing to tassel initiation (TI) in maize (Zea mays L.), with appreciable genetic differences in relative sensitivity to these factors. Three experiments were run to examine these photothermal responses and genetic differences within 12 diverse cultivars chosen to represent germplasm adapted to temperate, tropical highland, tropical lowland, and subtropical environments. Photoperiod influenced the rate of progress toward TI in all cultivars (P < 0.05), with a threefold differences in sensitivity among them. In three of the more photoperiod‐sensitive cultivars (all adapted to the tropics or sub tropics), the value of the critical photoperiod, above which progress to TI is delayed by increase in photoperiod, was close to 13 h d−1. Temperature also influenced rate of progress toward TI. For many cultivars the optimum temperature (To) for rate of progress towards TI was appreciably cooler than the range from 30 to 34 °C that is generally assumed to apply throughout development from sowing to anthesis in maize; estimated values of To in a 12‐h photoperiod (Exp. 3) varied from 19 to 22 °C in the tropical highland cultivar H‐32 to 31 °C in the tropical lowland cultivar Tuxpeño Crema I Cycle 18. In the five cultivars examined, inherent earliness (i.e., time taken to TI in optimum environments) was almost identical, despite considerable differences in To, while relations between rate of progress to TI and suboptimal temperatures were curvilinear, with similar base values (mean 9.4 °C). Curvilinear relations were also detected at supraoptimal temperatures in three of the five cultivars. Curvilinearity in each of these relations (in contrast with the linear relations reported in many other crops) may be compatible with symptoms of damage to maize plants exposed continuously to extreme temperatures (e.g., 12 and 37 °C).