Morphology and stomatal density of developing Arceuthobium americanum (lodgepole pine dwarf mistletoe) fruit: a qualitative and quantitative analysis using environmental scanning electron microscopy

Abstract

Arceuthobium americanum Nutt. ex Engelm., the lodgepole pine dwarf mistletoe, is a dioecious parasitic flowering plant infecting lodgepole pine (Pinus contorta subsp. latifolia (Engelm.) Critchfield) in the Pacific Northwest, compromising timber value by stunting the host’s growth. The plant disperses its seeds by explosive discharge, and thus understanding its reproduction is an integral step toward managing its spread. The life cycle of Arceuthobium americanum occurs over five to six years, and the fruit matures over two consecutive growing seasons, dispersing in the second. Using low-vacuum environmental scanning electron microscopy (eSEM), we examined branching architecture as well as the morphology and anatomy of fruits during their second year of development up to explosive discharge. We found that branching patterns shifted from opposite-decussate to verticillate after several years’ growth. In addition, the perianth parts (sepals), style, and stigma persisted through the season, and the fruit’s abscission zone developed slightly distal to the end of the pedicel. Stomata, found only on the sepals, were located in crypts surrounded by two subsidiary cells, and stomatal density significantly decreased during development. The decline in stomatal density along with the thick pericarp cuticle may function in retaining water inside the fruit to facilitate discharge and (or) provide a heating mechanism through reduced transpiration.

Diverse early dwarf mistletoes (Arceuthobium), ecological keystones of the Eocene Baltic amber biota

“…1J, 2G, 2H, 4C, 4E, 8E, 10D). As already mentioned, extant Arceuthobium fruits show a clear division into an upper (distal) and lower (proximal) part (Wilson and Calvin, 1996; Ziegler and Ross Friedman, 2017). This clear division is also present in fruits of fossil species A. johnianum (Fig.…”

“…The distal portion comprises between 21–45% of the total fruit length. The pedicellate fruits are ovoid, oblong, or elliptic in shape and decussately arranged (rarely whorled) (Gill, 1935; Kuijt, 1955; Hawksworth and Wiens, 1972, 1996b, c; Wilson and Calvin, 1996; Ziegler and Ross Friedman, 2017).…”

“…We suggest that the partly indistinct fruit division as well as the large distal portion observed in A. obovatum is due to the immaturity of these fruits, which is also indicated by their straight pedicels. In extant dwarf mistletoes, the fruit pedicel is erect, and the junction line is still indistinct during the fi rst year of fruit maturation, while only ripe fruits possess an elongated recurved pedicel whereby the distal fruit end points downward ( Gill, 1935 ;Kuijt, 1955 ;Hinds et al, 1963 ;Ziegler and Ross Friedman, 2017 ).…”

“…Besides the viscous cell layer surrounding the seed, the stomatal density on the distal fruit part is of great signifi cance for releasing the seed. Th e density of the stomata declines with fruit maturation and thus, minimizes water loss of the fruit, contributing indirectly to an increasing hydrostatic pressure of the entire fruit ( Ziegler and Ross Friedman, 2017 ). Th e repositioning of the pedicel from erect to recurve improves the height and distance for the fi nal explosive seed discharge ( Hinds et al, 1963 ).…”