Temperature Effects on Leaf Anatomy, Phenolic Acids, and Tissue Digestibility in Tall Fescue1

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Abstract
Increasing growth temperature lowers digestibility of tall fescue (Festuca arundinacea Schreb.) cell walls, but specific tissue responses have not been fully elucidated. The objectives of our research were to determine the effect of increasing growth temperature on leaf anatomy, histological reactions for lignin, alkali‐extractable phenolic acids, and the digestion of different tissue types by rumen microorganisms after 4, 10, 24, or 48 h. Plant material was grown in a controlled environment in which temperature was changed sequentially from 13/10°C (T1), to 20/18°C (T2), to 30/27°C (T3), and back to 13/10°C (T4). Two consecutive harvests of uppermost leaf blades following 28 days growth were made at each temperature, and analyses were performed on leaf blades. Histological reactions for lignin and leaf anatomy were similar for all treatments but with higher amounts of lignified vascular tissue in T3. At incubation times less than 48 h, leaf tissues from plants grown at T1 or T4 were degraded, as determined by scanning electron microscopy, to a greater extent than those at T2 or T3. However, after 48 h, tissues from plants grown at T2 were clearly the most degraded, and the order for all digestible tissues was as follows: T2 > T4 > T1 > T3. From 786 to 854 g kg−1 of the phenolic acids extracted from forage with 1 M NaOH were trans‐p‐coumaric and trans‐ferulic acids. Increases in growth temperature tended to increase levels of these and other phenolic acids, with T3 resulting in the highest level; amounts at T1 were lower (P≤10.05) than those at T2 through T4. Increased temperature resulted in a lower digestibility of easily degraded tissues without causing a marked change in leaf anatomy or histological reaction for lignin. Increased levels of p‐coumaric and ferulic acids up to T3 suggest that these compounds may contribute to limiting the digestibility of leaf blade tissues at higher temperatures.