Measurements in marine stratocumulus over the northeast Pacific help scientists unravel the mysteries of this important cloud regime.T he stratocumulus-topped boundary layer (hereafter the STBL), which prevails in the subtropics in regions where the underlying ocean is much colder than the overlying atmosphere, is thought to be an important component of the climate system. Perhaps most striking is its impact on the radiative balance at the top of the atmosphere. The seasonally averaged net cloud radiative forcing from the STBL has been estimated to be as large as 70 W nr 2 (Stephens and Greenwald 1991), more than an order of magnitude larger than the radiative forcing associated with a doubling of atmospheric C0 2 . This means that even rather subtle sensitivities of the STBL to changes in the properties of the atmospheric aero-

Maize cultivars with improved grain yields under nitrogen (N) stress are desirable for sub‐Saharan African maize growing environments. This study assesses N uptake, N utilization, and the genotype × environment (G × E) interaction of 16 tropical maize (Zea mays L.) hybrids differing in grain yield under low‐N conditions. Hybrids were evaluated under low‐N, medium‐N, and high‐N at Harare, Zimbabwe, in 2003 and 2004 and at Kiboko, Kenya, in 2003. At maturity, N accumulation in the aboveground biomass ranged from 47 to 278 kg N ha−1 in various experiments. Grain yields ranged from 1.5 to 4.3 Mg ha−1 and 10.6 to 14.9 Mg ha−1 for the same experiments, respectively. Significant G × E interactions were observed which became more pronounced as the difference in N stress intensity between two environments increased. High grain yield under low‐N was consistently associated with higher postanthesis N uptake, increased grain production per unit N accumulated, and an improved N harvest index. Additive main effect and multiplicative interaction analysis identified hybrids with specific adaptation to either low‐N or high‐N environments. Several hybrids produced high yields under both low‐N and high‐N conditions. More detailed studies with these hybrids are required to examine the underlying physiological mechanisms contributing to the N‐use efficiency.

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