The response of winter wheat to waterlogging at different stages of growth outdoors was studied for 3 years using lysimeters containing undisturbed monoliths of sandy h a m and clay soils. The treatments were selected as the most extreme likely to occur in field conditions in Britain, i.e. waterlogging to the soil surface for up to 16 days in October, 42 days in January and February and 6 days in May. Waterlogging in midwinter was common to all experiments. Yield losses from mid-winter waterlogging ranged from 15% when yields were heavy (10 t ha-I), to nil when yields were light (3--4 t ha-1) after the plants had been affected by frost damage in late winter and by take-all. Winter wheat was most sensitive to waterlogging after germination but before emergence. At this stage, 16 days waterlogging killed all seedlings and 6 days waterlogging depressed plant populations to 12% (clay) and 38% (sandy loam) of the control. However, there was vigorous compensatory growth in the remainder of the growing season and yield was only depressed to about 82% of the control. When the crop was waterlogged at any stage after emergence plant populations were not affected; however, winter waterlogging treatments usually depressed shoot numbers and, to a lesser extent, ear numbers at harvest. Shoot survival under waterlogged conditions seemed related to nitrogen availability in the soil. The amount of 'take-all' (C;ueumumiomyces gruminis var. tritici) was increased by waterlogging.

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S U M M A R YThe effects of waterlogging the soil for up to 5 days on the growth of peas at different stages of development were investigated in glasshouse and outdoor experiments. During waterlogging the oxygen content of the soil declined to less than 2% within 2-3 days. Subsequently, leaf senescence was hastened, stem growth rate was slowed and yield was decreased. The effects on yield were mainly attributable to the development of fewer nodes and fewer pods. Waterlogging for 24 h or more just before flowering restricted growth and yield. The effects of waterlogging at earlier and later stages were less marked. Waterlogging at any period after flower buds were visible prevented further increase in root dry weight.

Yields of cereals in Britain vary between sites and between years, and average farm yields are less than half of the calculated potential maximum yield. Yield variation is discussed in relation to soil conditions and root growth. An analysis of yield and its components of winter wheat for several sites over several years shows that yield is strongly correlated with number of grains per ear, and number of ears per unit area. These components of yield are influenced by environmental conditions during the reproductive phase of development, particularly light interception, nitrogen absorption and plant water stress. Crop growth analysis shows that total dry matter production is strongly correlated with light absorption,' which depends on seasonal variation in leaf area index. Leaf expansion in the field is mainly dependent on temperature, plant water stress, and nitrogen absorption. The environmental factors that are believed to have a strong influence on number of grains per ear, number of ears per unit area, and leaf expansion are discussed. Subjects in which further research is needed are identified. These include: effects of restrictions to root growth on absorption of nutrients and water; effects of soil temperature on leaf expansion and nutrient absorption ; the nitrogen requirements of the cereal crop, including the effects of temporary deficiencies of N on the final grain yield.