To type a column in italics (or any other non-standard font form), you must use the syntax >{\cmd}for the Specification column.
Assigning a column type >{\itshape}lcreates a left-selected column in italics.
, iris$Species <- paste0("\\textit{", iris$Species, "}"), , , .
:
\documentclass{article}
\usepackage{array}
\begin{document}
<<xtableItalics, results = "asis">>=
library(xtable)
print(xtable(head(iris), align = c(rep("l", 5), ">{\\itshape}l")))
@
\end{document}
PDF :

, array.
EDIT. , :
print(xtable(head(iris), align = c(rep("l", 5), ">{\\textit\\bgroup}l<{\\egroup}")))
print(xtable(head(iris), align = c(rep("l", 5), ">{\\textcolor{red}\\bgroup}l<{\\egroup}")))
\itshape \textit{}, . \textit{} , , . ( wikibooks.org, .)
, , . lrbox, .