Effects of environmental and plant factors on foliar freezing resistance in tropical grasses. I. Precondition factors and conditions during freezing

Abstract

The foliar freezing resistance of Setaria anceps cv. Narok and Panicum maximum var. trichoglume cv. Petrie was studied under controlled freezing in the laboratory. Maximum foliar hardening in both species was achieved in 8 days at a night temperature of 10°C or less. A reduction in relative humidity from 100 to 79 % during freezing at –5°C caused 80 and 88 % decreases in foliar damage (as measured by percentage foliage death) in P. maximum and S. anceps respectively. Reducing the cooling rate during a freezing cycle significantly increased the amount of foliar damage to both species but there was no significant effect of reheating rate. An increase in freezing duration from 0.5 to 4 hr increased foliar damage by 35 and 58 % at –2.5° and by 45 and 36 % at –3.5° in S. anceps and P. maximum respectively. Phosphorus and potassium applications had no effect on frost damage, but increased nitrogen application rates and nitrogen concentrations in the tissues were related to greater frost damage. A decrease in plant water status prior to frosting caused a significant decrease in foliar frost damage in P. maximum. Except for atmospheric humidity, a decrease in the minimum freezing temperature by 2° had a larger effect on foliar damage than had other plant or environmental factors. S. anceps demonstrated a greater frost tolerance than P. maximum in all experiments.