Population density and low nitrogen affects yield-associated traits in tropical maize

Worldwide, tropical maize (Zea mays L.) is commonly exposed to low N conditions. Identification of low N tolerance-related traits would help to develop indirect selection for yield and marker assisted selection under stress. Tolerance to high plant population density has been proposed as an alternative breeding strategy to improve stress tolerance in maize. A better understanding of mechanisms underlying tolerance to high plant population density and low N is, however, needed. For this purpose, elite CIMMYT open-pollinated varieties (OPVs), inbred lines, and hybrids were grown under optimal, high plant population density and low N conditions. Yield, yield components, and a set of morpho-physiological traits (secondary traits) were assessed in the different treatments and germplasm types. Emphasis was placed on anthesis-silking interval and traits related to senescence, dry matter partitioning, and ovule and grain number. Association was observed under low N conditions between grain yield and anthesis-silking interval, delayed senescence as expressed by either chlorophyll concentration or the number of green leaves above the ear, and ear/tassel weight ratio. Under optimal, high-plant population density and low N conditions, final grain number depended more on abortion rate than on the total number of ovules at anthesis. Under low N stress, grain yield was significantly negatively correlated with abortion rate. Under high plant population density, a positive association was noted between ovule number and abortion rate, suggesting a source limitation for C products. The effect of stress on yield components and the strength of association between secondary traits and yield varied greatly according to germplasm type.