Calcium ammonium nitrate applications for improved leaf growth and photosynthetic responses in the CAM orchid Phalaenopsis under elevated CO2 in a greenhouse

Abstract

Phalaenopsis Queen Beer ‘Mantefon’ is an orchid species that absorbs CO2 at nighttime due to its crassulacean acid metabolism. These orchids originated from nutrient–poor environments in tropical forest understories. However, elevated CO2 at nighttime influences plant nitrogen utilization for growth. This study investigated the leaf growth and photosynthetic responses in Phalaenopsis Queen Beer ‘Mantefon’ to varying ambient and elevated CO2 (400 and 800 μmol⋅mol−1, respectively) and four nitrogen−based calcium ammonium nitrate (CAN) levels over 20 weeks: calcium, ammonium, and nitrate levels by 0.00, 0.00, and 0.00 mmol⋅L−1 (CAN0, control), 0.90, 0.55, and 2.97 mmol⋅L−1 (CAN1), 6.73, 1.11, and 6.05 mmol⋅L−1 (CAN2), and 12.80, 1.72, and 9.13 mmol⋅L−1 (CAN3), respectively. The phosphate and potassium levels were consistent across all CAN levels at 2.74 and 4.37 mmol⋅L−1, respectively. Elevated CO2 levels showed no significant impact on the number of leaves in CAN1−3 plants yet increased with those with CAN0. Leaf area significantly increased with elevated CO2 levels by 44 % (CAN0) and 26 % (CAN1). Strong correlations emerged between the number of leaves and leaf area, as well as between leaf area and root−to−shoot dry biomass ratio (R/S). Increasing CO2 levels heightened SPAD chlorophyll in CAN0 plants, and SPAD and F0 showed a strong correlation with elevated CO2 and increasing CAN levels. The total dry biomass decreased in plants with high CAN (CAN and CAN3) compared to those with CAN0, regardless of CO2 levels, and Fv/Fm dropped below 0.616 with high CAN under elevated CO2. Phalaenopsis Queen Beer ‘Mantefon’ demonstrated augmented leaf growth and R/S under limited CAN (CAN0) and elevated CO2 conditions, indicating improved resource allocation potential. The results suggest that supplying CO2 in a greenhouse can effectively improve Phalaenopsis vegetative growth with limited nutrient conditions, which differs from growth aspects under ambient CO2 levels.