Definition of "substomatal"

In the substomatal chamber all the signals that regulate the interaction between fungus and host are present along the entire structure of the rust fungus, from the vesicle to the first haustorium which, in the dikaryotic stage, is the first intracellular structure (Mendgen et al., 1988; Heath, 1989, 1995).

2005, Ming Hao Pei, Alistair R. McCracken, editors, Rust Diseases of Willow and Poplar, CABI, page 170

This reduction took place along with a partial stomata closure and storage of CO 2 in substomatal cavities, independently of PPFD. Ecophysiological and cytological data indicated that stomatal and mesophyll processes may be involved in reducing A values and that there is a close co-ordination between them.

2015 August 14, “Functional Leaf Traits and Diurnal Dynamics of Photosynthetic Parameters Predict the Behavior of Grapevine Varieties Towards Ozone”, in PLOS ONE‎