Photosynthetic characteristics of Cymbidium ‘Red Fire’ and ‘Yokihi’ at different developmental stages

Abstract

Photosynthetic characteristics of two Cymbidium hybrids, ‘Red Fire’ and ‘Yokihi’, were investigated at different developmental stages. The net CO2 assimilation rate (An) as a function of incident photosynthetic photon flux (PPF), intercellular CO2 concentration (Ci), and relative humidity (RH) in a leaf chamber head was determined in a greenhouse with 1- and 2-year-old plant leaves. The maximum net CO2 assimilation rate (Anmax) in response to PPF of 1-year-old ‘Red Fire’ was 6.11 μmol·CO2·m−2·s−1 at 700 μmol·m−2 ·s−1 PPF, whereas it was 4.9 μmol·CO2·m−2·s−1 at 500 μmol·m−2·s−1 PPF in ‘Yokihi’. Under low light intensity (below 250 μmol·m−2 ·s−1 PPF), An was more pronounced in ‘Yokihi’ than in ‘Red Fire’. In 2-year-old Cymbidium, however, the maximum An was less than 2 μmol·CO2·m−2·s−1 in both cultivars at the same CO2 condition of 600 μmol·CO2·mol−1 air. The An increased in the leaves of 2-year-old plant under an elevated CO2 condition up to 1,500 μmol·CO2·mol−1 air with increasing PPF up to about 800 μmol·m−2·s−1. The An increased with increasing Ci and showed the highest value when Ci was over 1,000 and 2,000 μmol·mol −1 air in ‘Red Fire’ and ‘Yokihi’, respectively. The carboxylation efficiency of ‘Red Fire’ and ‘Yokihi’ were 0.044 and 0.036 μmol·CO2·m−2·s−1, respectively, while An of ‘Yokihi’ was higher than that of ‘Red Fire’ at above 1,000 μmol·mol−1 air Ci. The An tended to increase with RH, increasing from 15 to 70% in both hybrids. The Anmax of ‘Yokihi’ was observed at over 70% of RH, whereas 45% of RH was sufficient enough to show Anmax in ‘Red Fire’. The results shows that the photosynthetic characteristics of Cymbidium ‘Red Fire’ and ‘Yokihi’ can be increased by controlling the PPF, CO2, and RH conditions and the optimal conditions for maximum photosynthetic efficiency vary by cultivar, developmental stage, and greenhouse conditions.