Ppd-H1 and HvVRN1 interact to control the development of the MSA under different ambient temperatures. Microscopic changes in MSA development were scored under control (blue; 20°C/16°C, day/night) and high ambient (pink; 28°C/24°C, day/night) temperatures under LDs (16 h light/8 h night) in F3 families derived from a cross between the winter barley Igri and the spring barley Golden Promise. Selected F3 families segregated for Ppd-H1 and HvVRN1 and were fixed for the spring alleles at HvVRN2 (deleted) and HvFT1. Early MSA development was accelerated under high temperature in Ppd-H1/HvVRN1 and delayed in Ppd-H1/Hvvrn1, ppd-H1/HvVRN1, and ppd-H1/Hvvrn1. Significant differences were determined by a two-way ANOVA and a Tukey HSD pairwise comparison test: *P < 0.05, **P < 0.01, ***P < 0.001., n.s = nonsignificant.