Agrometeorological indices for predicting maize (Zea mays L.) phenology and yield under climate change in tropical and subtropical regions: A review

Maize (Zea mays L.) is one of the most important cereal crops and its productivity is highly responsive to meteorological variability throughout its life cycle. This review synthesises evidence on five agrometeorological indices, growing degree days (GDD), heat use efficiency (HUE), photothermal quotient (PTQ), radiation use efficiency (RUE) and crop water stress index (CWSI), that govern maize phenology and yield in tropical and subtropical South Asia. Cumulative GDD from sowing to maturity for tropical hybrids ranges from 870 to 1100 GDD and warming trends in peninsular India are accelerating this accumulation, shortening kharif growing seasons by 8 to 14 days by 2041–2060 and advancing reproductive development. Under mid-century warming, PTQ during rabi grain filling is projected to decline by 5–15 % and CWSI during flowering to rise from a baseline of 0.25–0.35 to 0.45–0.65, jointly threatening kernel set and kernel weight, with rain-fed yield declines of 8–22 % projected without adaptation. The review addresses implications for sowing dates, crop duration and heat-stress exposure and identifies key gaps: the lack of integrated multi-index models calibrated for tropical maize genotypes, limited remote-sensing-based index estimation across space and time and insufficient genotypic characterisation of index response functions. Recommendations for research and operational practices for climate-resilient maize production are provided.