Carbon isotope discrimination and gas exchange in Coffea species grown under different irradiance regimes

Abstract

Two coffee species (Coffea arabica L. cv. Catuai Vermelho and Coffea canephora L. cv. Apoata) were grown in pots placed outdoor under three irradiance regimes, full sunlight (S), 50% of shade (50% S) and 80% of shade (80% S), and evaluated for stable carbon isotope discrimination (Δ), photosynthetic gas exchange, and growth. The rate of CO 2 assimilation in S and 50% S plants was higher in C. arabica than in C. canephora, but there was no difference between the two species in 80% S. Both species showed similar response to the irradiance regimes. CO 2 assimilation rate at 80% S was 0.49 and 0.58 of CO 2 assimilation in full sunlight, respectively, for C. arabica and C. canephora. There was no significant difference in CO 2 assimilation between S and 50% S plants for either species. Stomatal conductance in C. arabica increased with the irradiance level, but the increase was less evident than that observed for CO 2 assimilation. Consequently, the CO 2 assimilation/stomatal conductance ratio (A/g), expressing intrinsic water use efficiency, was greater in the S plants and 50% S plants than in the 80% S plants. Irradiance regimes did not affect stomatal conductance in C. canephora, which had a lower A/g ratio than C. arabica for the same irradiance regime. Leaf Δ values were lower in S plants than in 80% S plants for both species, but there was no difference between S and 50% S treatments. The decrease in Δ was associated with a greater increase in the CO 2 assimilation rate (106%) than in the stomatal conductance (54%) in C. arabica. In C. canephora the decrease in Δ was associated with an increase in CO 2 assimilation (73%), because stomatal conductance values were not influenced by the irradiance regime. Leaf tissue Δ was negatively correlated with instantaneous measurements of CO 2 assimilation (r = - 0.92; P = 0.0109) and with stomatal conductance (r = - 0.80; P = 0.0570). Full sunlight grown plants exhibited higher leaf nitrogen content than 80% S plants. The increase in leaf nitrogen content with increasing irradiance levels was strongly correlated with CO 2 assimilation rate (r = 0.99, P = 0.0007) and negatively with Δ (r = - 0.94, P = 0.0071). Additional index terms: CO 2 assimilation rate, growth, leaf N, stomatal conductance.