In 2 years the distribution of radioactivity recovered in entire shoots of field-grown winter wheat was determined at various times after exposing the top two leaves (flag leaf or second leaf) to 14C0, for 30 s. In 1976 when 14C was supplied to either leaf 14 days before anthesis, 30% was in the ear at anthesis. Less than 5% was in the leaf exposed to I4CO,. The remainder was equally divided between the stem above and below the flag-leaf node when the flag leaf had been exposed, and was mainly in the lower part of the stem when the second leaf had been exposed. By maturity the proportion in the stem had decreased; 20% of the total activity was in the grain and 30% was still in the ear structures.When 14C was supplied 10 days after anthesis, the proportion in the ear 24 h later ranged from 42 to 69% of that in the whole shoot when the flag leaf was exposed, and from 6 to 28% when the second leaf was exposed. At maturity these proportions increased to 92 and 85% when the 14C had been supplied to flag leaves and second leaves respectively. When 14C was supplied 25 days after anthesis to either flag leaves or second leaves, more than 90% of the activity was in the mature ears. Less than 5% of the 14C remaining at maturity from any treatment was still in the leaf exposed to 14C0,. Between 2 and 6% of 14C supplied after anthesis was in the nongrain parts of the ear.The proportion of the 14C in the ear was greatest for the semi-dwarf varieties Maris Fundin and Hobbit, less for Maris Huntsman, and least for Cappelle-Desprez. These varietal differences were large 24 h after exposure to 14C0,, especially in 1976. They were small and rarely significant at maturity.Nitrogen fertiliser up to 210 kg N ha-* had negligible effects on the distribution of 14C, although it greatly increased growth and yield, especially in 1975.

S U M M A R YWheat plants were kept in a growth room at 15 "C from 7 days after anthesis until maturity and their ears were warmed to 20 or 25 "C for various periods. Continuous warming initially increased the growth rate of the ear and decreased that of the stem, but hastened senescence of the ear and decreased final grain yield by decreasing dry weight per grain. Warming the ears increased the movement of 14C and nitrogen to them from the leaves and stem during the early stages of grain growth but decreased it later. Warming the ears for only the first 10 or 20 days also hastened ear senescence; grain yield decreased progressively with increase in duration of the warm period. All effects were greater at 25 than at 20 "C.The effects of changing ear temperature from 15 to 20 "C were independent of the temperature of the rest of the plant. Altering the humidity of the air around the ears by 4 7 mb at constant temperature had no effect on ear growth or senescence.Warming all of the plant except the ears from 15 to 20 "C increased ear growth slightly during early grain growth and decreased it later, irrespective of ear temperature : stem dry weight, leaf area and net photosynthetic rate of flag leaves and green stems were decreased and dark respiration rate of stems was increased.

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Eight amounts of nitrogen ranging from 0 to 210 kg N/ha were applied to two tall and one semi-dwarf variety of winter wheat in the spring of 1975 and 1976. The tall varieties were Cappelle-Desprez and Maris Huntsman; the semi-dwarf variety was Maris Fundin in 1975 and Hobbit in 1976. Interactions between varieties and nitrogen were few and small compared with the main effects. All varieties produced their maximum grain yields with 180 kg N/ha. The yield of the semi-dwarf varieties, but not the others, decreased slightly with more nitrogen.Cappelle-Desprez yielded less grain than the other varieties in both years. In 1975 the yields of Maris Fundin and Maris Huntsman were similar and in 1976 Hobbit yielded more than Maris Huntsman. The varieties had similar numbers of ears at maturity and similar patterns of tillering. The semi-dwarf varieties had most grains per spikelet, and hence grains per ear, and Cappelle-Desprez had least. The semi-dwarf varieties had the smallest grains. The semi-dwarf varieties had less straw than the other varieties and hence the largest ratios of grain to total above-ground dry weight. The decrease in dry weight of stem and leaves between anthesis and maturity was similar for all varieties. In 1975 the efficiency of the top two leaves plus top internode in producing grain was the same for all varieties, but in 1976 Hobbit was more efficient than the other two. There were some small differences between varieties in nutrient uptake that were not related to differences in growth. Maris Fundin tended to have a greater phosphorus and potassium content than the tall varieties. Hobbit contained slightly less nitrogen than the tall varieties at maturity, and had a smaller concentration of nitrogen in the grain.Applying 210 kg N/ha doubled grain yield in 1975. Applying nitrogen resulted in a large increase in number of ears and a small increase in number of grains per ear due to the development of more fertile spikelets per ear. Nitrogen decreased dry weight per grain, especially of the semi-dwarf varieties. With extra nitrogen, straw dry weight at maturity, shoot dry weight at anthesis and leaf area were all increased relatively more than grain yield, and stems lost more dry weight between anthesis and maturity than without nitrogen. The year 1976 was exceptionally dry and nitrogen had only small effects in that it affected neither straw dry weight nor number of ears but slightly increased grain yield by increasing the number of spikelets and number of grains per spikelet. It also increased leaf area proportionately to grain yield. In 1975 nitrogen increased evaporation of water from the crop before anthesis but decreased it after anthesis, even though it continued to increase the extraction of water from below 90 cm.

The percentage (P) of incident photosynthetically active radiation (PAR) reaching the ground was measured in winter wheat crops throughout the season, and related to projected area index (L) of leaves, plus sheaths and stems when present. Between unfolding of the first leaf and appearance of the flag leaf the mean value for the exponential extinction coefficient (K) was O46±0-007. The value did not vary with the growth stage of the wheat and was scarcely affected by experimental treatments, apart from effects resulting from the presence of senescent tissues that intercepted light but whose area was not included in L. After anthesis, in crops having between 340 and 540 ears/m 2 , between 68 and 89 % of incident PAR penetrated to below the ears. Relation between the further penetration to the base of the crop canopy and the area of green plus senescent leaf and stem tissue gave K = 0'50. The relation did not change significantly between anthesis and complete senescence.The relation between P and L was similar whether P was obtained from instantaneous measurements made near midday, 24 h integrals of PAR or percentage ground cover measured photographically.