Digestibility-related histological attributes of vegetative organs of barrel medic (Medicago truncatula Gaertn.) cultivars

An anatomical analysis of vegetative organs of barrel medic (Medicago truncatula Gaertn.) cultivars was performed using anatomical and stereological methods. The aim was to determine histological bases of the genotypic differences for digestibility, calculate tissue volume densities (Vv), examine the variability and assess tissue proportions along the stem maturity gradient. As the cultivars showed similar proportions of leaf tissues, potential variations in digestibility should be assigned to structural differences observed for the stems. Phloem and xylem Vv increased linearly with stem maturation, whilst Vv of epidermis, mechanical and parenchyma tissue significantly decreased. Secondary xylem was the tissue with the highest potential impact on digestibility. Cultivars ‘Mogul’ and ‘Jemalong’ had low proportion of lignified stem tissues, whilst ‘Parabinga’ and ‘Caliph’ did not show favourable anatomical characteristics. More advantageous proportions of thick-walled and thin-walled cells were recorded in M. truncatula stems, compared to alfalfa (M. sativa). Therefore, examined cultivars are comparable in anatomical quality parameters to this wellknown forage crop. Leaf parameters did not provide variability necessary for the improvement of forage quality. Stem parameters showed higher within-cultivar variability, which could be useful as a starting point in breeding towards improvement of digestibility. As the secondary xylem was the lignified stem tissue with the highest Vv, we recommend the improvement of M. truncatula digestibility through the reduction of its proportion.