Energy budgets and temperatures of nyctinastic leaves on freezing nights.

Temperatures of exposed horizontal and vertical soybean leaves (Glycine max [L.] Merr. var. Chippewa) were measured on calm, clear nights with temperatures near freezing. Average leaf-air temperature differences for 5 nights were -1.5 C and -1.0 C for horizontal and vertical leaves respectively. The horizontal leaves were cooler than the vertical leaves. The mean of all observed horizontal-vertical leaf temperature differences was -0.5 C with a maximum average for 1 night of -0.8 C, while maximum differences theoretically attainable in similar leaves were calculated to be -1.7 C. No differences were observed in the extent of frost damage in horizontal and vertical leaves. The apparent reduction in frost damage in vertical leaves observed by Charles Darwin was probably caused by his method of using corks to hold the horizontal leaves and not by leaf orientation. Theoretical considerations and the experimental results indicate that nyctinastic leaf movements probably do not provide significant protection from frost for any plants.