Leaf gas-exchange characteristics of sixteen cycad species

Abstract

Leaf gas exchange characteristics for 16 species of cycad were determined under field conditions in Miami, Fla. Net CO2 assimilation (A(CO2)) ranged from 4.9 μmol · m-2 · s-1 for Lepidozamia peroffskyana Regel to 10.1 μmol · m-2 · s-1 for Zamia furfuracea L. fil. in Aiton. Stomatal conductance to H2O (g(s)) was more variable, ranging from 85 mmol · m-2 · s-1 for Cycas seemannii A. Br. to 335 mmol · m-2 · s-1 for Encephalartos hildebrandtii A. Br. and Bouche. Transpiration (E) ranged from 1.7 mmol · m-2 · s-1 for Cycas chamberlainii W.H. Brown and Keinholz to 4.8 mmol · m-2 · s-1 for Encephalartos hildebrandtii. Highly variable E was more controlling of water-use efficiency than the less-variable A(CO2). The difference between air and pinnae temperature ranged from 0.3 to 1.6 °C and was inversely related to mean g(s) among the species. The values within geographic regions representative of the native habitats of the species were highly variable. For example, two of the African species exhibited the highest and lowest values of water-use efficiency in the survey. Leaf gas exchange for the four largest species with arborescent growth form was less than that for the three small species with subterranean or short bulbous growth form. The diurnal variation in leaf gas exchange for Zamia furfuracea exhibited a two-peaked pattern with a distinct midday depression in A(CO2) and g(s). The ratio dark respiration to maximum A(CO2) for Zamia furfuracea was 0.04. As a group, the values for A(CO2) and g(s) for these cycads ranked at the lower end of the range for all plants species.