Gas exchange characteristics of leaves of four species of grain amaranth

Abstract

Four species of the C4 genus Amaranthus, A. cruentus, A. caudatus, A. hypochondriacus and A. hybridus, were grown at Salt Lake City, Utah for the determination of gas exchange characteristics of recently matured leaves. The response of CO 2 assimilation to leaf temperature, measured at high quantum flux, was very similar in all species, each exhibiting a temperature optimum of approximately 37°C. Above about 40°C, all species showed high temperature inhibition, manifested as an irrversible time-dependent decline in CO 2 assimilation. All four exhibited high rates of net CO 2 assimilation exceeding 40 μmol m −2 s −1 at quantum fluxes of 2.0 mmol m −2 s −1, but none was completely light-saturated at such intensities. Leaf conductance to water vapor declined with decreasing quantum flux in all species, but less rapidly than assimilation, resulting in increasing values of internal CO 2 concentration ( c i). Quantum yield values in three species ( x=0.048±0.003 mol( CO 2/mol(photons))) were similar to those reported previously for NAD-malic enzyme type C4 dicots, but leaves of A. hybridus, which were deep purple in color, exhibited a lower quantum yield, presumably due to light absorption by betacyanin pigments not involved in photosynthesis. All four species had net photosynthesis versus c i responses typical of C4 species, with a steep initial linear response to c i followed by a fairly abrupt transition to saturation in the region of 100–150 μl l −1. Under near-optimal measurement conditions and high quantum flux, all species maintained c i values close to the region at which saturation was reached ( x=148±17 μl l −1). Data pooled acrossspecies relating net photosynthesis measured at 2.0 mmol m −2 s −1 quantum flux and 35°C to Kjeldahl nitrogen content revealed a linear relationship, the slope of which was not significantly different ( P=0.05) from those reported in two otudies employing C4 species.