Summary• The effect of different durations of waterlogging and subsequent drainage is described for 3-wk-old wheat ( Triticum aestivum ) plants.• In a pot experiment, plants were subjected to waterlogging to the soil surface for 0, 3, 7, 14, 21 or 28 d, and then drained to allow recovery for up to 25 d.• During waterlogging, the seminal root system stopped growing. Adventitious roots grew to a maximum length of c . 150 mm. Leaf nitrogen concentration was severely decreased by waterlogging. When waterlogged pots were drained, seminal root mass did not recover to control values, even when waterlogging lasted only 3 or 7 d, due to death of existing apices and no initiation of new lateral roots. By contrast, adventitious roots resumed elongation after drainage. By the end of the experiment, shoot mass remained two-to threefold lower in plants from all waterlogged treatments compared with continuously drained controls, due to lower tiller numbers and shorter final leaf lengths in previously waterlogged plants.• The results demonstrate that even short periods (as little as 3 d) of waterlogging have considerable long-term effects on the growth of young wheat plants.

The growth reduction of wheat (Triticum aestivum L.) during and after waterlogging stress depends on the depth of water from the soil surface. In a pot experiment with 3-week-old plants, soil was waterlogged for 14 d at the surface, or at 100 or 200 mm below the surface, and pots were then drained to assess recovery. A fully drained treatment kept at field capacity served as control. During waterlogging, the relative growth rate of roots decreased more than that of shoots (by 6–27% for shoots, by 15–74% for roots), and plant growth was reduced proportionally as the water level was increased. Light-saturated net photosynthesis was reduced by 70–80% for the two most severe waterlogging treatments, but was little affected for plants in soil waterlogged at 200 mm below the surface. The number of adventitious roots formed per stem in plants grown in waterlogged soil increased up to 1.5 times, but the number of tillers per plant was reduced by 24–62%. The adventitious roots only penetrated 85–116 mm below the water level in all waterlogging treatments. Adventitious root porosity was enhanced up to 10-fold for plants grown in waterlogged soil, depending on water level and position along the roots. Porosity also increased in basal zones of roots above the water level when the younger tissues had penetrated the waterlogged zone. Fourteen days after draining the pots, growth rates of plants where the soil had been waterlogged at 200 mm below the surface had recovered, while those of plants in the more severely waterlogged treatments had only partially recovered. These findings show that the depth of waterlogging has a large impact on the response of wheat both during and after a waterlogging event so that assessment of recovery is essential in evaluating waterlogging tolerance in crops.

Four maize (Zea mays L.) hybrids were grown hydroponically for 4 weeks with 20 m ammonium or nitrate as the sole nitrogen source. Dry matter production was strongly depressed by ammonium nutrition in the hybrid Helga relative to plants grown on nitrate, and moderately decreased in the hybrid Melina. Ammonium had no inhibitory effect on total yield in the other two hybrids (Ramses and DK 261). The relative growth rate (RGR) of roots and shoots of the sensitive hybrid Helga decreased significantly under ammonium nutrition during the first 2 weeks of the experiment, while at the end of the experiment nitrogen form had no effect on the RGR in any of the four hybrids. The strong reduction in RGR of Helga in the early seedling stage was correlated with the accumulation of twice the concentration of free ammonium in the shoot tissue relative to the other hybrids. Helga was therefore unable to sufficiently detoxify ammonia in the roots. Root concentrations of water soluble carbohydrates (WSC) in Helga and Melina in the early seedling stage did not differ under ammonium and nitrate nutrition. In contrast, Ramses and DK 261 both had elevated WSC concentrations in ammonium-fed roots. It is hypothesized that a sufficient supply of carbon skeletons for ammonium assimilation in the roots is required for maximum growth under high ammonium concentrations, and that there is genotypic variability in this physiological trait.

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