Desiccation tolerance in the resurrection plant Barbacenia graminifolia involves changes in redox metabolism and carotenoid oxidation.

Desiccation tolerance in vegetative tissues enables resurrection plants to remain quiescent under severe drought and rapidly recover full metabolism once water becomes available. Barbacenia graminifolia is a resurrection plant that occurs at high altitudes, typically growing on rock slits, exposed to high irradiance and limited water availability. We analyzed the levels of reactive oxygen species (ROS) and antioxidants, carotenoids and its cleavage products, and stress-related phytohormones in fully hydrated, dehydrated, and rehydrated leaves of B. graminifolia. This species exhibited a precise adjustment of its antioxidant metabolism to desiccation. Our results indicate that this adjustment is associated with enhanced carotenoid and apocarotenoids, α-tocopherol and compounds of ascorbate-glutathione cycle. While α-carotene and lutein increased in dried-leaves suggesting effective protection of the light-harvesting complexes, the decrease in ß-carotene was accompanied of 10.2-fold increase in the content of ß-cyclocitral, an apocarotenoid implicated in the regulation of abiotic stresses, compared to hydrated plants. The principal component analysis showed that dehydrated plants at 30 days formed a separate cluster from both hydrated and dehydrated plants for up to 15 days. This regulation might be part of the protective metabolic strategies employed by this resurrection plant to survive water scarcity in its inhospitable habitat.

Water retention and metabolic changes improve desiccation tolerance in Barbacenia graminifolia (Velloziaceae).

Barbacenia graminifolia is a Velloziaceae species endemic to the campos rupestres in Serra do Cipó, Minas Gerais state (Brazil). This biome is characterised by high irradiance and limited water conditions. Unlike other resurrection plants, B. graminifolia can maintain a high hydric status (>80%) after 28 days of water suppression before desiccation. We investigated the physiological and metabolic mechanisms associated with structural changes that allow B. graminifolia to maintain hydration under a prolonged water deficit and to recover after desiccation. After 30 days of water deficit, desiccated plants exhibited chlorophyll degradation, a 178.4% and 193.7% increase in total carotenoids and MDA, respectively, and twice the CAT and APX activity compared to hydrated plants. The metabolite profile showed increased amino acids, carbohydrates, saturated fatty acids and benzoic acids during dehydration, while trichloroacetic acid cycle acids were higher in hydrated and rehydrated plants. Anatomical and ultrastructural data corroborated the physiological and metabolic changes and revealed the presence of mucilaginous cells with high water retention capacity. Our data indicated that combined strategies of assimilatory metabolism shutdown, accumulation of compatible solutes and antioxidant compounds, increase in hydrophilic molecules, changes in the composition of membrane lipids and remodelling of cell organelles conditioned the efficiency of B. graminifolia in delaying water loss, tolerating further desiccation and quickly recovering after rehydration. These attributes evidence that this species is well adapted to cope with adverse environmental conditions, mainly directing the metabolism to an efficient antioxidant response and improving its capacity to retain water during the dry season.

Assimilatory deficit and energy regulation in young Handroanthus chrysotrichus plants under flooding stress.

Flooding negatively influences the growth and development of several plant species. Here, we show that the flood tolerance of young Handroanthus chrysotrichus plants involves growth deficit, carbon assimilation reductions, starch remobilization, and energy regulation. The effect of hypoxia was evaluated in a controlled experiment consisting of plants subjected to normoxia and water-logging, with later recovery. We measured morphological changes, gas exchange, photosynthetic pigments, soluble carbohydrates and starch contents, the activity of the enzymes alcohol dehydrogenase (ADH), and pyruvate decarboxylase (PDC), and ATP and ADP levels. While control plants showed normal appearance and growth, flooded plants exhibited a drastic decrease in growth, necrosis of some root tips, hypertrophic lenticels on the stems, and foliar chlorosis. Oxygen deprivation in root cells led to a significant decrease in stomatal conductance. The low Amax rates caused a decline in foliar soluble sugar content at 20 days and a subsequent increase in the leaves and roots, coinciding with starch degradation at 40 days. We also observed increases of 220.5% in ADH and 292% in PDC activities in the roots at 20 and 40 days of flooding. The activation of anaerobic metabolism in stressed plants was an essential mechanism for ATP regulation in both tissues used to maintain a minimal metabolism to cope with hypoxia to the detriment of growth. The post-stress recovery process in H. chrysotrichus occurred slowly, with gas exchange gradually resumed and anaerobic metabolism and sugar content maintained to improve energy regulation.

Influence of water limitation on the competitive interaction between two Cerrado species and the invasive grass Brachiaria brizantha cv. Piatã.

Invasive grasses inhibit the growth of other plant species, and water deficit is one of the major competition problems for native vegetation. We evaluated whether the presence of Brachiaria brizantha cv. Piatã has a negative influence on the competition for water and nutrients between Anadenanthera macrocarpa and Anadenanthera colubrina (Angico species). The interspecific competition was evaluated using a randomized experimental design with the following treatments: 1) free competition (FC), in which the native species were cultivated without the grass presence and 2) under competition (UC), in which the native species grew together with the invasive grass for 120 days. We analysed the water relationships in the two species, the effect of water limitation on the antioxidant stress, the nutritional content of shoots and roots, the relative competition intensity (RCI) and growth. The presence of Piatã grass reduced the soil moisture causing a decrease of 21.9% and 29.5% in the relative water content (RWC) of leaves for A. macrocarpa and A. colubrina, respectively. For the two Angico species, the quantum efficiency of Photosystem II (ΦPSII) decreased with reduction of RWC leaf, resulting in the H2O2 increase (57.5% at day 30 for A. colubrina and 38.8% at day 120 for A. macrocarpa). The oxidative stress was evidenced by the increase in the superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities in leaves and roots of both young native trees. In the UC treatment, reductions in water uptake also led to a decrease in root absorption of N, P, K, a Mg and low transport of these nutrients to the leaves of both Angico species. A. macrocarpa and A. colubrina showed less growth caused by limitation of water uptake, but the joint activity of the physiological and biochemical adjustments provided competitive ability.

Mechanisms of desiccation tolerance in the bromeliad Pitcairnia burchellii Mez: biochemical adjustments and structural changes.

Rocky outcrops represent the diversity center of vascular desiccation tolerant (DT) plants. Vegetation in this environment is exposed to an extended dry season and extreme conditions due to rocky soils and high sun exposure. In this study, we demonstrated that Pitcairnia burchellii, a bromeliad from rocky outcrops, tolerates intense desiccation for about 90 days due to strategies as accumulation of compatible osmolytes and antioxidant substances together with leaf morphological changes. In dehydrated plants, an increase in antioxidant activity was observed and the vacuolization of parenchyma cells was accompanied by proline accumulation in leaves and rhizomes. Precursors related to phenylpropanoid pathway increased significantly during plant dehydration. Accordingly, increases in anthocyanin and phenolic contents as well as lignin deposition were observed in leaves of dehydrated plants. Cell divisions and a decrease in stored starch were observed in the rhizomes indicating starch mobilization. Anatomical analyses revealed the presence of a more developed water-storage tissue in dehydrated leaves. During desiccation, leaves curl upwards and the adaxial V deep water-storage tissue is supported by two larger lateral vascular bundles. Cell wall folding and an increased proportion of arabinose-containing polymers was observed in leaves under dehydration, suggesting increasing of cell wall flexibility during desiccation. Such biochemical and morphological changes are consistent with the ability of P. burchellii to tolerate intense desiccation and behave as a resurrection species.

Physiological and biochemical changes attenuate the effects of drought on the Cerrado species Vatairea macrocarpa (Benth.) Ducke.

Drought is considered the main abiotic stress because it influences the distribution of plant species and limits the productivity of ecosystems. The aim of this study was to evaluate the effects of drought on physiological and biochemical parameters during the initial development of Vatairea macrocarpa, a native cerrado species. Plants were subjected to daily watering (control); suppression of watering during 90 days with field capacity (fc) 50% and 25% and then followed by rewatering. Relative leaf water content (RWC), gas exchange, photosynthetic pigments content, carbohydrate and amino acids content, antioxidant activities and growth were recorded. The RWC decreased according to the soil water restriction, causing reduction in stomatal conductance and decrease of 76.4% in net photosynthesis in plants submitted to 25% fc. Water restriction decreased the chlorophyll content, however increased carotenoid content and also improved the antioxidant activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT). In addition, high levels of sugars (sucrose, raffinose) and amino acids (proline, tryptophan, valine, glutamine and GABA) were detected in drought stressed plants, contributing to osmoregulation and as sources of carbon and nitrogen after rehydration. Decreases in carbon assimilation promoted a reduction of the leaf area, however an increase in the root surface area was observed. After rewatering, the analized parameters became similar to the control plants indicating that the severe water stress did not impair the survival of young plants. Instead, adjustments were made to protect them against drought such as the maintenance of the assimilatory metabolism at minimal levels.