Air temperature and humidity affect petunia ornamental value

Petunias are a representative bedding plant known for their tolerance for hot and dry climates. However, petunia growth and ornamental value as bedding plants in a year-round hot but humid climate such as that in tropical regions is not well studied. In order to evaluate the adaptability of petunias in hot and humid climates, petunia ‘Madness Red’ was treated with 50% relative humidity (RH) at an air temperature (AT) of 25°C, and with 80% RH at 25°C, 30°C, and 35°C AT. The results showed that at 25°C AT, 80% RH briefly delayed flowering but partially improved shoot growth and ornamental value variables such as plant weight, leaf area, and flower size and longevity compared to 50% RH. At 80% RH, increased AT significantly promoted flowering time and number, but also caused a slight decline in plant development parameters like biomass, flow size, and flower color. Conversely, a rapid decline in plant development was observed at 35°C AT only, indicating heat damage symptoms such as anatomical distortion of the leaf surface. The ornamental value of bedding plants was determined by flower production and longevity during the landscaping period. Petunia ornamental value was optimal at 30°C AT, even though 25°C AT is generally reported to be suitable for plant development in bedding petunias. These results indicate that humid conditions improve the high temperature adaptability of ornamental value in petunia ‘Madness Red’ which can thus be used as a street landscaping plant in tropical regions.