Since hyphae colonize grass leaves as they develop on the SAM, hyphae usually are present in all parts of the leaf, including the tip, blade, and sheath. Hyphal strands within leaves typically are seldom branched (Fig. 14-21) in comparison with hyphal growth in meristematic tissues. The extent of leaf blade colonization is dependent on the plant and endophyte genotype (Fig. 14-22) and on the nutrient status of the host (Rasmussen et al., 2007). Some tall fescue cultivars, such as Kentucky 31, have low concentrations of hyphae in leaf blades (Hinton and Bacon, 1985). The amount of hyphae in leaves is temperature dependent, being low in cold winter months (Ju et al., 2006). The reason why the extent of leaf colonization including that of leaf blades varies among host grasses is not fully understood but probably reflects the stage of development of the primordial leaf when hyphae become established. The lower concentration of hyphae in the colder months is possibly due to slower hyphal growth the basal meristematic zone relative to the formation of leaf cells. Hyphae from most associations are rarely present within vascular bundles (Fig. 14-23). For further information about hyphae of Neotyphodium endophytes in vascular bundles see Christensen et al. (2001). Growth of Neotyphodium endophytes in leaf blades is discussed in detail elsewhere (Christensen et al., 1998, 2002; Hinton and Bacon, 1985).
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