Aims This research studied effects of light intensity on leaf anatomical characteristics of three alfalfa (Medicago sativa) cultivars (‘Victoria’, ‘Ameristand 201’and ‘Eureka’) with different fall dormancy during overwintering in climatic transitional regions from north to south China. Our objective was to determine leaf anatomical characteristics sensitive to light intensity and correlate fall dormancy and shade tolerance of overwintering alfalfa.
Methods We cultured seedlings of the alfalfa cultivars for 70 days under four light intensity levels: 100%, 43%, 21% and 7% of full natural light intensity. Using the paraffin section method, fluorescence microscopy technique and Image-Pro Plus 6.0, we measured leaf epidermises, mesophyll tissues, and some leaf-integrated characteristics under different shade conditions to examine effects of light intensity on leaf anatomical structures.
Important findings Cuticle thicknesses of lower and upper epidermis and stomatal densities and apertures of the cultivars significantly decreased while thicknesses of upper and lower epidermis increased under with shading. Widths of spongy cell increased and thicknesses of palisade parenchyma, layers of palisade parenchyma and palisade-spongy ratio significantly decreased with decreased light intensity, but thicknesses of spongy parenchyma and widths of palisade cells had different trends among the cultivars. Leaf, mesophyll and midrib thicknesses and cell tense ratio of the cultivars significantly decreased, and vein protuberant degree did not significantly change when the spongy ratio increased significantly with increasing shade. Significant differences in range and plasticity indexes of leaf anatomical characteristics among the cultivars under different shading conditions showed different shade-tolerant mechanisms among them. Pearson correlation analysis showed that stomatal density, thickness of palisade parenchyma, mesophyll thickness, leaf thickness and palisade-spongy ratio were positively correlated with light intensity, which meant that these leaf characteristics may be anatomically sensitive in response to light intensity. ‘Victoria’ had fewer leaf anatomical characteristics correlated with light intensity and also had weaker association to some of these five characteristics than the other cultivars. Results suggested that fall dormancy of alfalfa could be relevant to its shade tolerance during overwintering, and semi-dormancy cultivar of alfalfa has a higher shade tolerance than other cultivars. The order of shade tolerance of three alfalfa cultivars is semi-dormancy cultivar ‘Victoria’ > dormancy cultivar ‘Ameristand 201’ ≥ non-dormancy cultivar ‘Eureka’.