Application of Ice to Phalaenopsis Orchid Roots Does Not Affect Quantum Efficiency of Photosystem II

The use of ice to irrigate plants has been an accepted practice for over 25 years. Recently, a company that recommends this practice for watering orchids was challenged by an orchid society to prove that ice was not detrimental to orchids. To address this question, we conducted two different studies using chlorophyll fluorescence measurements to determine if this procedure causes root damage: 1) The internal temperature of Phalaenopsis orchid roots was monitored using a micro-thermocouple placed deep inside the stele of the root as an ice cube was placed directly on the root and 2) the quantum efficiency of the photosynthetic spongy parenchyma of the orchid root was assessed in a time study where roots were immersed in a bath filled with anti-freeze. The temperature of the anti-freeze solution was lowered two degrees every hour until the roots showed signs of freezing. Freezing was evident when the exothermic reaction of ice crystal formation expressed itself as a jump in the tissue temperature. Freeze damage was also assessed by determining the quantum efficiency of photosystem. Our results showed that when ice cubes were placed directly onto the root surface, internal root temperatures dropped quickly, but never below 2 °C. In the second study, roots did not show freeze-induced damage until the temperature of the water bath reached -7 °C, at which time the internal temperature of the roots reached -2 °C. The quantum efficiency of photosystem II dropped from about 0.75 in healthy roots to 0.17 in freeze-damaged roots. This drop in quantum efficiency is likely due to the cell membranes in the roots being compromised by ice crystal formation. The below 0° C freezing point may be due to sugars and other solutes in the root tissue. In summary, the internal temperature of roots with an ice cube placed on top did not drop below 2 °C (38.6 °F), while freeze-induced damage in the roots was seen only at root temperatures below -2 °C. There is no evidence of root damage based on the quantum efficiency of photosystem II when ice is placed directly on the root. Long-term studies with the use of ice cubes compared to conventional watering are being conducted to look at how ice cubes may affect plant health and flower longevity long term.