Waterlogging decreases a plant's metabolism, stomatal conductance (gs) and photosynthetic rate (A); however, some evergreen species show acclimation to waterlogging. By studying both the physiological and morphological responses to waterlogging, the objective of this study was to assess the acclimation capacity of four swamp forest species that reside in different microhabitats. We proposed that species (Luma apiculata [D.C.] Burret. and Drimys winteri J.R. et G. Forster.) abundant in seasonally and intermittently waterlogged areas (SIWA) would have a higher acclimation capacity than species abundant in the inner swamp (Blepharocalyx cruckshanksii [H et A.] Mied. and Myrceugenia exsucca [D.C.] Berg.) where permanent waterlogging occurs (PWA); it was expected that the species from SIWA would maintain leaf expansion and gas exchange rates during intermittent waterlogging treatments. Conversely, we expected that PWA species would have higher constitutive waterlogging tolerance, and this would be reflected in the formation of lenticels and adventitious roots. Over the course of 2 months, we subjected seedlings to different waterlogging treatments: (i) permanent (sudden, SW), (ii) intermittent (gradual) or (iii) control (field capacity, C). Survival after waterlogging was high (≥80%) for all species and treatments, and only the growth rate of D. winteri subjected to SW was affected. Drimys winteri plants had low, but constant A and g during both waterlogging treatments. Conversely, L. apiculata had the highest A and g values, and g increased significantly during the first several days of waterlogging. In general, seedlings of all species subjected to waterlogging produced more adventitious roots and fully expanded leaves and had higher specific leaf area (SLA) and stomatal density (StD) than seedlings in the C treatment. From the results gathered here, we partially accept our hypothesis as all species showed high tolerance to waterlogging, maintained growth, and had increased A or g during different time points of waterlogging. Differences in leaf (SLA) and stomata functioning (gs, StD) plasticity likely allows plants to maintain positive carbon gains when waterlogging occurs. The species-specific differences found here were not entirely related to microhabitat distribution.
The The hypothesis that Embothrium coccineum, a species able to solubilize P through cluster roots (CR) carboxylate exudation, makes P more available in volcanic depositions, thus facilitating the growth of other plant species such as the N-fixing Sophora cassioides was tested. In this work, seedlings of these two tree species were grown alone or co-cultivated for six months in greenhouse conditions with either pumice, a recent volcanic deposition with low P availability, or an organic commercial mix. Upon analyzing the aboveground growth of the two substrate treatments, we found no clear evidence of facilitation. Despite this, substrate type did influence the relative growth and some characteristics of CR and nodules. On the other side, the number of total soil bacteria, but not Rhizobium spp, was greater when E. coccineum and S. cassioidea were grown together than when only one species was grown alone. Nutritional parameters, such as the concentration of nitrogen (N) and phosphorous (P) in roots and total leaf protein content in leaves, indicated that the nutritional content of S. cassioides was greater when co-cultivated with E. coccineum in pumice. Specifically, co-cultivation tended to reduced P limitation and improved N acquisition by S. cassioides. Based on these results, our hypothesis is partially accepted.
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