Mind the gap

A new classification system for water-gap complexes in 16 plant families.

Water Gaps Physical dormancy (PY) is caused by a water-impermeable palisade cell layer(s) in seed or fruit coats along with tightly sealed chalaza and micropyle openings. PY has been demonstrated or inferred to occur in species of 18 angiosperm plant families, and it is unknown in gymnosperms. Seeds of some species with PY also have physiological dormancy (PD); hence, they are considered to have combinational dormancy (CD). The breaking of PY involves disruption or dislodgement of ‘water-gap’ structures causing the seeds/fruits to become permeable. The water-gap region is a morpho-anatomically specialized area and it differs from the rest of the seed or fruit coat. The location, anatomy, morphology and origin of water gaps can differ between and even within families. Twelve different water-gap regions in seven families have been characterized previously. However, the water gaps previously have not been characterized in many others.

A recent paper in Annals of Botany examines the seed/fruit coat anatomy and morphology in 15 plant species in seven families and reports ten new water-gap complexes. Complexes consist of: (1) an opening that forms after PY is broken; (2) a specialized structure that occludes the gap; and (3) associated specialized tissues. Based on the location, anatomy and morphology, there are 24 different kinds of water-gap regions in 16 families. New names were assigned to certain water gaps that had been previously reported in the literature for clarity and to avoid ambiguity. Circular water gaps with plug-like structures occluding the opening were given the name ‘oculus’ (eye), circular or narrow linear water gaps occluded by lid-like structures formed from palisade cells were named ‘gap’ and narrow linear water gaps were named ‘slit’.

This new classification recognizes three basic types (I, II and III) of water-gap complex which are further sub-divided into simple and compound water-gap complexes based on the number of openings involved in the initial water uptake. The outcomes of this study provide a basis for developing an identification key for different kinds of water-gap complexes in PY seeds and fruits.

Identification and characterization of ten new water gaps in seeds and fruits with physical dormancy and classification of water-gap complexes. Ann Bot (2013) 112 (1): 69-84. doi: 10.1093/aob/mct094
Physical dormancy (PY) occurs in seeds or fruits of 18 angiosperm families and is caused by a water-impermeable palisade cell layer(s) in seed or fruit coats. Prior to germination, the seed or fruit coat of species with PY must become permeable in order to imbibe water. Breaking of PY involves formation of a small opening(s) (water gap) in a morpho-anatomically specialized area in seeds or fruits known as the water-gap complex. Twelve different water-gap regions in seven families have previously been characterized. However, the water-gap regions had not been characterized in Cucurbitaceae; clade Cladrastis of Fabaceae; subfamilies Bombacoideae, Brownlowioideae and Bythnerioideae of Malvaceae; Nelumbonaceae; subfamily Sapindoideae of Sapindaceae; Rhamnaceae; or Surianaceae. The primary aims of this study were to identify and describe the water gaps of these taxa and to classify all the known water-gap regions based on their morpho-anatomical features. Physical dormancy in 15 species was broken by exposing seeds or fruits to wet or dry heat under laboratory conditions. Water-gap regions of fruits and seeds were identified and characterized by use of microtome sectioning, light microscopy, scanning electron microscopy, dye tracking and blocking experiments. Ten new water-gap regions were identified in seven different families, and two previously hypothesized regions were confirmed. Water-gap complexes consist of (1) an opening that forms after PY is broken; (2) a specialized structure that occludes the gap; and (3) associated specialized tissues. In some species, more than one opening is involved in the initial imbibition of water. Based on morpho-anatomical features, three basic water-gap complexes (Types-I, -II and -III) were identified in species with PY in 16 families. Depending on the number of openings involved in initial imbibition, the water-gap complexes were sub-divided into simple and compound. The proposed classification system enables understanding of the relationships between the water-gap complexes of taxonomically unrelated species with PY.