VARIATION IN GROWTH FORM OF SHOAL GRASS (HALODULE-WRIGHTII) DUE TO CHANGES IN THE SPECTRAL COMPOSITION OF LIGHT BELOW A CANOPY OF TURTLE GRASS (THALASSIA-TESTUDINUM)

Shoal grass, Halodule wrightii, exhibited different morphologies when growing in a monospecific meadow compared to plants growing under a canopy of turtle grass, Thalassia testudinum. When growing under a turtle grass canopy, shoal grass had longer internodes, less frequent branching, and lower aboveground to belowground biomass ratios than when growing in a monospecific meadow. Aquarium studies approximated the observed phenomena in the field study. Rhizome growth of sprigs of shoal grass (mm d-1) was greater for plants grown under 35% light reduction caused by blades of turtle grass than for sprigs grown under ambient light. Sprigs grown under 35% light reduction caused by neutral density screens had rhizome growth rates lower than controls. Greater rhizome growth rates of sprigs grown under turtle grass blades were due to increased internode distances, rather than increased rates of shoot formation. The environmental factor responsible for inducing morphological changes in shoal grass appears to be the ratio between red and far-red light. This ratio declines as light passes through a canopy of blades of turtle grass.