MORPHOLOGICAL AND PHYSIOLOGICAL CONSEQUENCES OF A POSITIVE PLANT INTERACTION

It is known that plants can create acceptable environments for other p lant species that may be unable to physiologically adjust to harsh phy sical conditions in isolation. In this study, we explore the morpholog ical and physiological responses of a positive interaction on a common New England salt marsh shrub, Iva frutescens L. In previous experimen ts, we have shown that Iva benefits from the presence of the black rus h, Juncus gerardi Loisel. Juncus removal in the lower marsh caused inc reases in soil salinity and anoxia, which resulted in lower Iva biomas s, growth, and survival. We hypothesize that the positive effect of Ju ncus on Iva is a consequence of (1) the turf morphology of Juncus shad ing soil surfaces, thereby minimizing salt accumulation, and (2) the i ncreased soil oxygen content surrounding Juncus caused by radial oxyge n loss from its rhizosphere. However, the specific morphological and p hysiological benefits conferred on Iva are unknown. We conducted exper iments in the salt marsh to isolate the effects of Juncus neighbors, w aterlogging, and salt water stress on Iva. We found that Iva, when exp osed to salty, oxygen-deprived soil conditions created in the absence of Juncus, responded more negatively to salinity than waterlogging, al though the interaction between salinity and waterlogging was often sig nificant. Iva without neighbors showed decreases in plant height, leaf density, leaf area, and flower density compared to controls or plants with fresh-water additions. Leaves of Iva without neighbors responded to salt stress and waterlogging by increasing leaf thickness and succ ulence. The increase in soil salinity of plants without neighbors caus ed plants to have correspondingly low water potentials, indicating tha t they were severely water stressed. Physiologically, Iva plants had a low mean photosynthetic rate, transpiration rate, water-use efficienc y, acid stomatal conductance compared to controls or Iva with fresh-wa ter additions. Low water-use efficiency in Iva without neighbors was c orrelated with stomatal closure but also with high intercellular CO2 c oncentrations. The mechanisms behind this apparent anomaly may include both nonstomatal inhibition of photosynthesis as well as patchy stoma tal closure, although it is uncertain which predominates from our data . Our results show that the presence of Juncus, with its superior abil ity to withstand waterlogging and salt stress, can create a hospitable environment for Iva, extending its distribution to lower intertidal h abitats in which it does not have the morphological or physiological p lasticity to normally cope.