The influence of ammonium/nitrate ratio on the chlorophyll content and induction of chlorosis under low‐pressure sodium (LPS)‐lamps was examined in two cultivars ofChrysanthemum morifoliurnRamat. ('Mountain Snow’ and ‘Fiesta') found previously to differ in sensitivity to water stress. Plants were grown in a peat moss‐vermiculite mix under controlled environments for 39 days under LPS lamps at a photosynthetic photon flux of 320 umol s‐1m‐2. Plants were fertilized twice daily with one of three complete nutrient solutions containing nitrogen at a NH4/NO3ratio: of 0/100; 50/50; or 100/0. Significant differences in the severity and rate of development of chlorosis were observed depending upon the genotype and NH4/NO3ratio. Drought‐tolerant ‘Fiesta’ showed little chlorosis after 39 days of treatment while drought‐sensitive ‘Mountain Snow’ exhibited marked bleaching of the leaves as the percentage of NH4in the solution increased. Bleaching appeared first in the youngest leaves on the main shoot and lateral shoots and proceeded basipetally. Increasing the NH4/NO3ratio from 0/100 to 100/0 hastened the onset of chlorosis in the primary and secondary leaves of NH4‐sensitive ‘Mountain Snow’ by 8–9 days. After 39 days, both cultivars displayed chlorosis, even at 100% NO3, indicating that this damage was probably not caused by NH4treatment alone, but was enhanced by high NH4. These findings thus demonstrate an interaction between spectral quality and NH4nutrition in growth chamber‐grown chrysanthemum.