Low Irradiance Combined with High Relative Humidity Induce Leaf Tipburn in Eustoma grandiflorum

Leaf tipburn (TB) of Eustoma grandiflorum (Raf.) Shinn. is a calcium-deficient disorder. Low irradiance combined with high relative humidity would reduce transpiration stream and thus may induce this disorder. Vegetative plants of Eustoma ‘King of Orchid’ with five fully expanded leaf pairs were placed in a growth room at 25/18℃ and received two treatments: high irradiance (HI; 285 µmol·m^-2·s^-1 photosynthetic photon flux, PPF) combined with moderate relative humidity (MRH; mean 74% RH), and low irradiance (LI; 212 µmol·m^-2·s^-1 PPF) combined with high relative humidity (HRH; mean 91% RH). Artificial lighting was provided by high-pressure sodium lamps for 12 h per day. Plants for both treatments had microscopically visible sepals 11 days after treatments (DAT). Leaf tipburn occurred at 18 DAT only in plants under LI and HRH conditions. Consistently lower transpiration was found in the first to third leaves under flower bud at LI and HRH, whereas no significant difference in stomatal density and stomatal conductance between treatments. Plants under LI and HRH conditions produced expanding leaves with reduced vasculature density that may limit calcium transport to the leaf tip.