The Collaborative International Dictionary
Plano-
Plani- \Plan"i-\, Plano- \Plan"o-\ [L. planus. See Plane, a.] Combining forms signifying flat, level, plane; as planifolious, planimetry, plano-concave.
Plano-
Plano- \Pla"no-\ See Plani-.
Douglas Harper's Etymology Dictionary
Wiktionary
plano-
pre. (alternative form of plani- English)