Cedar glades in central Texas consist of intercanopy bare patches in Juniperus woodlands. These bare patches have shallow soils where limestone bedrock is at or near the surface, plant cover is low, and are dominated by herbaceous vegetation. An herbaceous species found in these bare patches is a summer annual, Heliotropium tenellum, which grows from May to October. Abiotic and biotic factors that potentially influence the distribution of H. tenellum were examined in the field. Photosynthetic rates, stomatal conductance, and transpiration rates of five H. tenellum plants were measured at midday. Water potential of ten H. tenellum plants was measured at pre-dawn.

Results/Conclusions

There were significant differences in photosynthetic rates at different light levels (One way ANOVA; Tukey-Kramer HSD; p < 0.0001). The maximum net photosynthetic rate was 27.17 ± 2.75 μmolCO₂/m²/s.  There was no significant difference between stomatal conductance and the light levels tested (One-way ANOVA; p = 0.1796). There were significant differences in transpiration rates at different light levels (One-way ANOVA; Tukey-Kramer HSD; p = 0.0011). Mean water potential of H. tenellum was –0.84 ± 0.07 MPa. Sun plants typically have higher photosynthetic rates allowing them to grow at high levels of radiation. The maximum net photosynthetic rate and associated light curve variables suggest that H. tenellum is a sun plant. Furthermore, based on its high water potential, H. tenellum does not appear to be under water stress. This may help explain the ability of H. tenellum to grow in the hottest, driest part of the season in these intercanopy bare patches.