The present invention relates to a method of selecting a tomato plant comprising exposing a tomato plant and determining whether said plant exhibits tolerance to exposure to continuous light, wherein said tolerance is indicated by decreased level of chlorosis upon said exposure, and further determining whether said plant exhibits a significant increase in yield when exposed to continuous light. The invention further relates to a method for increasing the yield of a tomato plant line, said method comprising selecting a tomato plant by the method of the invention; crossing the thus selected plant with a plant of a said tomato plant line to produce offspring plants; optionally backcrossing or selfing said offspring plant to produce further offspring plants, and selecting from said offspring plants a plant having tolerance to continuous light and having increased yield as compared to said plant of the original tomato plant line.