Sensitivity of seed germination to water stress in high-altitude populations of a threatened palm species.

Divergence in seed germination patterns among populations of the same species is important for understanding plant responses to environmental gradients and potential plant sensitivity to climate change. In order to test responses to flooding and decreasing water potentials, over 3 years we germinated and grew seeds from three habitats of Euterpe edulis Mart. occurring along an altitudinal gradient. Seed germination and root growth were evaluated under different water availability treatments: control, flood, -0.4 MPa, -0.8 MPa, in the years 2012, 2013 and 2014, and in the final year of the experiment (2014) at -1.0 MPa and -1.5 MPa. Seeds from the montane habitat did not germinate in the flooding treatment. Seed germination of all three habitats decreased in the -1.5 MPa treatment and the montane habitat had lowest germination in this treatment. Time required for half of the seeds to germinate increased up to -0.8 MPa. Seeds from montane habitats germinated more slowly in all treatments. The only difference in seed germination synchrony was an increase in the submontane population under the flooding treatment. However, synchrony decreased at the lowest water potentials. Roots of the montane population were more vigorous in most treatments, except at -0.8 MPa. The unusual ability of these seeds to germinate at low water potentials might be related to early seed germination at the onset of the rainy season, which potentially decreases seed predation pressure. Seeds of the montane population were more sensitive to both types of water stress. A predicted increase in the frequency and intensity of extreme high rainfall or drought events may predispose early stages of this population to adverse factors that might negatively affect population viability with elevational in future climate change scenarios.

Diurnal changes in chlorophyll a fluorescence, CO(2)-exchange and organic acid decarboxylation in the tropical CAM tree Clusia hilariana.

Crassulacean acid metabolism (CAM) plants are dependent on the organic acids that accumulate overnight in the vacuoles as a source of CO(2) during the daylight deacidification period, when stomata are closed and high irradiances generally prevail. We performed an integrative analysis of diurnal changes in gas exchange, chlorophyll fluorescence parameters and organic acid decarboxylation to understand the adjustments in photochemical and non-photochemical processes during the different CAM phases in Clusia hilariana Schlecht., a dominant tree species in the sandy coastal plains of southeastern Brazil. A linear relationship was obtained between the quantum yields of photochemical and non-photochemical quenching, irrespective of the CAM phase and prevailing irradiance. Degradation of malic and citric acids during the midday stomatal closure period could lead to potential CO(2) fixation rates of 23 &mgr;mol m(-2) s(-1), whereas CO(2) losses, measured as CO(2) evolution, corresponded to about 3% of this value. Thus, decarboxylation of malate and citrate provided high internal CO(2) concentrations during phase III of CAM, even though the stomata were closed, allowing optimal utilization of light energy, as indicated by the non-saturating electron transport rates (ETR) in the light response curves, with highest rates of ETR occurring at midday in the diurnal curves. At the transition from phase III to IV of CAM, depletion of internal CO(2) sources and low stomatal conductances, which restricted the supply of exogenous CO(2), reduced the demand for photochemical energy to drive carbon assimilation. This was compensated by increases in thermal energy dissipation as indicated by higher rates of non-photochemical quenching, while high irradiances still prevailed. Shifts in the CAM phases and changes in protective thermal dissipation potential allowed C. hilariana to match changes in PPFD patterns for leaves of different orientations. Evidence that most of the decline in photochemical efficiency was probably related to the fast-relaxing component of non-photochemical quenching is provided by the high values of the quantum yield of photosystem II after 20 min of relaxation in darkness, and an almost complete recovery after sunset. These adjustments in photosynthetic machinery minimized the danger of photo-inhibition in C. hilariana, which is commonly found in fully exposed habitats.