Abstract:

This study explored the patterns of change in stem characters and mechanical traits in Commelinaceae plants grown under different light intensities. Ramets of Commelina purpurea， Tradescantia fluminensis ‘Variegata’， T. zebrina and T. fluminensis ‘Vairidia’ propagated from stem cuttings were used as experimental material. Shading net was used to create a series of five light levels of 100% natural light intensity， 75% natural light intensity， 50% natural light intensity， 25% natural light intensity and 5% natural light intensity. Stem morphological characters， anatomical structure and mechanical traits were measured under the different light levels. It was found that： As the light intensity decreased， plant internode length of the four taxa increased significantly （P<0.05）. Conversely， decrease in light intensity led to significant decrease in the specific stem weight （P<0.05）. Apart from C. purpurea， which showed no significant change， stem biomass was greatest at medium light levels in the taxa studied. Different light intensities had different effects on stem epidermis thickness， cortical thickness， vascular bundle diameter， medulla diameter and stem diameter. However， the relationship between internal tissue structure and stem thickness remained relatively stable across the range of light intensities. Reduced light intensity led to a significant increase in cortical cell length and medulla cell length of the four taxa （P<0.05）， but also to a significant decrease in the cortical and medulla cell widths （P<0.05）. The resistance to bending and compressive strength of all taxa and the organizational density of C. purpurea， T. zebrina and T. fluminensis ‘Vairidia’ significantly decreased （P<0.05） when light intensity decreased. Pearson correlation analysis showed that specific stem weight， epidermis thickness， cortical thickness， vascular bundle diameter， medulla diameter， stem diameter， cortical cell width and medulla cell width were all significantly correlated with resistance to bending （P<0.01）. Medulla cell width was significantly correlated with organizational density （P<0.01）， while specific stem weight， epidermis thickness， medulla diameter and medulla cell width were significantly correlated with compressive strength （P<0.01）. These results indicate that the four studied taxa were able to obtain limited light resources in low-light environments by increasing their stem biomass and plant internode length. The increase of stem biomass has obvious limitations. These changes led to an unbalanced state with the increased plant height and internodes. This not only resulted in decreases of specific stem weight and changes of stalk anatomical structure， but also reduced stem rigidity， organizational density and compressive strength. A membership function analysis showed that the stalk characteristics of T. fluminensis ‘Variegata’ and T. zebrina were superior under a range of light intensities， followed by the T. fluminensis ‘Vairidia’， while C. purpurea was the worst.

Key words: light intensity, Commelinaceae, stem characters, mechanical traits