Photosynthetic Response of Hybrid Mahogany Grown Under Contrasting Light Regimes

Abstract

In an experimental trial in RIo Piedras, Puerto Rico, saplings of mahogany hybrids (small-leaf #x00D7; big-leaf mahogany) were transplanted into environments with low (6% of full sun), medium (33% of full sun), and high (80% of full sun) light. The saplings showed wide variation in height growth in all environments. We measured photosynthetic response to light and CO2 as well as chlorophyll fluorescence characteristics to determine if physiological traits showed significant variation that might explain the differences in height growth under high light. Mean light-saturated photosynthetic rates were 5.2 ± 0.1 µmolm−2s−1 for plants grown in low light, 5.6 ± 0.2 for medium light, and 6.6 ± 0.2 for high light. Mean light compensation point was 8.5 µmolm−2s−1 in low light, indicating that these hybrids are capable of positive photosynthesis under heavy shade. The shade treatments significantly affected specific leaf mass, N per unit mass, and A max per unit mass, but did not affect A max per unit N. The variation in height growth was apparently explained by the capacity of the plants to acquire nitrogen because shorter plants had lower concentrations of foliar N than did taller ones. Large plants had larger leaf-specific mass than did shorter ones. No strong evidence for chronic photoinhibition in high light was found, although considerable energy was dissipated through photoprotective mechanisms, as shown by reductions in intrinsic Photosystems II efficiency.