The phenomenon of photoperiodism in plants was discovered by 

The observation of Garner and Allard (1920) that a variety of tobacco, maryland mammoth, flowered when the relative length of the day was shorter than the length of the dark period led to the discovery of photoperiodism. They used the term 'photoperiodism' to designate the response of the organism to the relative length of day and light and photoperiod to designate the favourable length of the day for each plant.