Physiological mechanisms contributing to the increased N stress tolerance of tropical maize selected for drought tolerance

An improved response of crop varieties to various stress factors may be associated to constitutive stress tolerance mechanisms that increase yield and yield stability. Increased leaf longevity, increased water and nutrient uptake, greater assimilate supply during grain filling, and increased grain and ear set have been associated with constitutive stress tolerance mechanisms in maize (Zea mays L.). We examined tropical maize for adaptive changes associated with drought tolerance that are sustained under N stress and therefore may indicate constitutive stress tolerance mechanisms. Original and drought-tolerant selection cycles of four populations were evaluated in five experiments differing in N supply at Poza Rica, Mexico between 1992 and 1994. Selection for tolerance to mid-season drought stress consistently increased grain yield across N levels due to an increase in both the number of ears per plant and kernel weight. The number of ears per plant was associated with a shorter anthesis-silking interval (ASI) of drought-tolerant cycles. Reduction in ASI due to selection was greater under N stress as compared to well-fertilized conditions, however, it was not associated with either biomass or N accumulation of plants and cars around flowering. The N content of individual kernels did not change with selection and grain N concentration decreased. Greater kernel weights were likely the result of delayed leaf senescence and increased assimilate supply during grain filling. We conclude that decreased ear abortion and increased assimilate supply during grain filling of maize selected for tolerance to mid-season drought also provide tolerance to N stress and therefore may contribute to increased yield and yield stability. (C) 2002 Elsevier Science B.V. All rights reserved.