Photosynthetic rates of two species of malvaceae, Malvaviscus arboreus var. drummondii

We examined two co-occurring species of Malvaceae in the savanna of central Texas to determine their photosynthetic response to varying levels of light. Abutilon theophrasti had a mean (± 1 SD) density of 4 ± 4 plants/[m.sup.2] in the open-grassland phase of the savanna, and a density of 1 ± 2 plants/[m.sup.2] under canopy of woody mottes. Malvaviscus arboreus var. drummondii was not in the open-grassland phase and had a density of 3 ± 6 plants/[m.sup.2] under canopy of woody mottes. Mean midday levels of light in the open and canopy were significantly different at 2,004 versus 192 µmol/[m.sup.2]/s, respectively. Maximum photosynthetic rate of A. theophrasti (34.6 ± 3.6 µM C[O.sub.2]/[m.sup.2]/s) occurred at a photosynthetic-flux density of 2,000 ± 0.0 µM/[m.sup.2]/s and was significantly greater than the maximum photosynthetic rate of M. arboreus var. drummondii (14.8 ± 2.2 µM C[O.sub.2]/[m.sup.2]/s), which occurred at a photosynthetic-flux density of 1,350 ± 173.0 µM/[m.sup.2]/s. Light saturation, light-compensation point, dark respiration rates, stomatal conductance, and transpiration rates for A. theophrasti were higher than rates of M. arboreus var. drummondii. These species have significant differences in most gas-exchange measurements, reflecting differences in their habitats. Based on these differences, M. arboreus var. drummondii is a sun-shade intermediate and A. theophrasti is a sun plant. However, maximum-photosynthetic-rate values and levels of light at maximum photosynthetic rate suggest that M. arboreus var. drummondii would do well in edge or partially shaded habitats.