Seasonal influence, heat unit accumulation and heat use efficiency in relation to maize grain yield in Pakistan

Variations in ambient temperature affect crop yield by modifying the duration of pheno-logical phases and physiological processes. An experimental study was carried out at the Maize and Millets Research Institute (MMRI), Yusafwala, Sahiwal, Pakistan, to determine the seasonal effects of temperature on indigenous and exotic maize (Zea mays L.) hybrids based on morphological, phenological, physiological and grain quality traits in four different growing seasons: kharif 2016 and 2017, and spring 2017 and 2018. Seven indigenous and three exotic hybrids were sown in a randomized complete block design with a split plot arrangement, in three replications. Significant differences between hybrids and growing seasons were found for grain yield, related traits and temperature indices (cumulative heat units, photothermal index and heat use efficiency). Correlation analysis disclosed a significant positive relationship between grain yield and net photosynthetic rate (0.854, P <= 0.01), number of grains per ear (0.624, P <= 0.01) and heat use efficiency (0.980, P <= 0.01) in spring seasons. During kharif, net photosynthetic rate (0.675, P <= 0.01) and heat use efficiency (0.996, P <= 0.01) contributed significantly to grain yield, whereas number of grains per ear (-0.146, not significant) had no significant impact on grain yield. Cumulative heat units and heat use efficiency resulted the temperature indices with the greatest influence on grain yield, and should be considered during the selection of parents to develop high-yielding, climate-smart maize hybrids. Indigenous maize hybrids showed higher yields and were more heat tolerant than exotic hybrids, and the spring sowing season appeared to be the most suitable for the cultivation of maize crops.