Maybe this? "Melatonin synthesis in rice seedlings in vivo is enhanced at high temperatures and under dark conditions due to increased serotonin N-acetyltransferase and N-acetylserotonin methyltransferase activities"
"The opposite effect occurred during the night, in which the positive effect of darkness on melatonin synthesis was counteracted by the negative effect of a low temperature."
https://doi.org/10.1111/jpi.12111