Influences of PAR, temperature and water vapor concentration on gas exchange by ferns

Abstract

The effects of photosynthetically active radiation (PAR), leaf temperature and the leaf‐to‐air water vapor concentration drop on net CO2 uptake and water vapor conductance were surveyed for 14 species of ferns. Most previous studies indicated that ferns have extremely low maximal rates of net CO2 uptake, below 2 umol m−2 s−1, whereas the average maximal rate observed here at 250 C was 7 umol m−2 s−1. Net CO2 uptake reached 90% of saturation at an average PAR (400 to 700 nm) of only 240 umol m−2 s−1, consistent with the typically shaded habitats of most ferns. Maximal CO2 uptake rates were positively correlated with the PAR for 90% saturation (r2=0.59), the chlorophyII per unit leaf area (r2=0.30), the water vapor conductance (r2=0.65), and the CO2 residual conductance (r2=0.69). A higher water vapor conductance (gwv) was correlated with a greater fractional change in gwv as the leaf‐to‐air water vapor concentration drop (Δcwv) was raised from 5to20 g m−3 (r2=0.90). Specifically, for species with low gwv of about I mm s−1 the ratio of gwv at Δcwv= 5 g m−3 to that at Δcwv= 20 g m−3 was near 1, but it was near 2 for species with gwv of about 4 mm s−1. Such a relationship, which can prevent excessive transpiration, has apparently not previously been pointed out in surveys of other plant groups.