Phenology, heat unit requirement and heat use efficiency of African marigold under year-round transplanting conditions of Punjab, India

The present study was conducted during 2021-22 and 2022-23 to study crop phenology, heat unit requirement and heat use efficiency (HUE) in African marigold under year-round transplanting at Punjab Agricultural University, Ludhiana, Punjab, India. The highest heat use efficiency for seed and flower production were recorded in crop transplanted in rainy season particularly in the month of August. Higher growing degree days (GDD) and photothermal units (PTU) for attaining bud visibility and 50 % flowering stages were recorded under spring and summer transplanting (February to June) due to longer day length conditions which forced the crop to witness shorter seed filling period with lesser seed yield and HUE. Optimum seed yield period in marigold (rainy season transplanting), however, recorded lesser GDD and PTU for attainment of bud visibility and 50 % flowering as compared to summer transplanting dates and consequently had longer seed filling period resulting in higher seed yield. Notably, GDD and PTU for seed filling period recorded higher values in rainy season transplanted crop as compared to crop transplanted in other seasons of year. Correlation studies were conducted to understand the role of weather variables for high seed yield obtained under rainy season transplanting. Seed yield in African marigold recorded the highest value of correlation with HUE for seed production (r2= 0.978) followed by GDD for seed filling period (r2= 0.810), HUE for flower production (r2= 0.787) and PTU for seed filling period (r2= 0.774), respectively. Apparently, mean temperature and sunshine hours during seed filling period are the most important determinants of seed yield in African marigold.