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Wood properties influence the leaf life span (LL) of tree crowns. As lignin is an important component of wood and the water transport system, we investigated its relationship with embolism resistance and the LL of several tree species in a seasonally dry tropical ecosystem. We determined total lignin and the monomer contents of guaiacyl (G) and syringyl (S) and related them to wood traits and xylem vulnerability to embolism (Ψ ) for the most common species of the Brazilian semiarid, locally known as Caatinga. Leaf life span was negatively related to Ψ and positively related to S : G, which was negatively related to Ψ . This means that greater S : G increases LL by reducing Ψ . Lignin content was not correlated with any variable. We found two apparently unrelated axes of drought resistance. One axis, associated with lignin monomeric composition, increases LL in the dry season as a result of lower xylem embolism vulnerability. The other, associated with wood density and stem water content, helps leafless trees to withstand drought and allows them to resprout at the end of the dry season. The monomeric composition of lignin (S : G) is therefore an important functional wood attribute affecting several key functional aspects of tropical tree species in a semiarid climate.

ABSTRACT:Hydrogen peroxide (H 2 O 2 ) is an essential signaling molecule that mediates plant responses against several biotic and abiotic stresses. H 2 O 2 pretreatment has emerged as a signaling way, inducing salt stress acclimation in plants. Here, we analyzed the effects of H 2 O 2 leaf pretreatment on the non-enzymatic defense system (ascorbate and glutathione), plant growth, relative water content (RWC), relative chlorophyll content, H 2 O 2 content, and gas exchange in maize plants under NaCl stress. The results showed that salinity reduced the leaf area and shoot and root dry mass as compared to control, and the leaf spraying with H 2 O 2 significantly improved the growth of salt stressed plants. Photosynthesis and transpiration, stomatal conductance and intercellular CO 2 concentration were strongly decreased by salinity after 7 and 14 days of salt exposure; however, the decrease was lower in plants sprayed with H 2 O 2 . The improved gas exchange in H 2 O 2 -sprayed stressed plants correlated positively with higher RWC and relative chlorophyll content and lower leaf H 2 O 2 accumulation under NaCl stress conditions. Ascorbate and glutathione did not play any obvious effects as non-enzymatic antioxidants in the ROS scavenging. In conclusion, the salt tolerance induced by H 2 O 2 leaf pretreatment is attributed to a reduction in the H 2 O 2 content and maintenance of RWC and chlorophyll in maize leaves. These characteristics allow maize plants to maintain high rates of

An effective strategy for re-establishing K+ and Na+ homeostasis is a challenge for the improvement of plant performance in saline soil. Specifically, attempts to understand the mechanisms of Na+ extrusion from plant cells, the control of Na+ loading in the xylem and the partitioning of the accumulated Na+ between different plant organs are ongoing. Our goal was to provide insight into how an external nitrogen source affects Na+ accumulation in Sorghum bicolor under saline conditions. The NH4+ supply improved the salt tolerance of the plant by restricting Na+ accumulation and improving the K+/Na+ homeostasis in shoots, which was consistent with the high activity and expression of Na+/H+ antiporters and proton pumps in the plasma membrane and vacuoles in the roots, resulting in low Na+ loading in the xylem. Conversely, although NO3--grown plants had exclusion and sequestration mechanisms for Na+, these responses were not sufficient to reduce Na+ accumulation. In conclusion, NH4+ acts as an efficient signal to activate co-ordinately responses involved in the regulation of Na+ homeostasis in sorghum plants under salt stress, which leads to salt tolerance.

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The salt overly sensitive (SOS) pathway is the only mechanism known for Na extrusion in plant cells. SOS pathway activation involves Ca-sensing proteins, such as calcineurin B-like (CBL) proteins, and CBL-interacting protein kinases (CIPKs). In this signalling mechanism, a transit increase in cytosolic Ca concentration triggered by Na accumulation is perceived by CBL (also known as SOS3). Afterward, SOS3 physically interacts with a CIPK (also known as SOS2), forming the SOS2/SOS3 complex, which can regulate the number downstream targets, controlling ionic homeostasis. For instance, the SOS2/SOS3 complex phosphorylates and activates the SOS1 plasmalemma protein, which is a Na/H antiporter that extrudes Na out of the cell. The CBL-CIPK networking system displays specificity, complexity and diversity, constituting a critical response against salt stress and other abiotic stresses. In a study reported in the journal Plant and Cell Physiology, we showed that NH induces the robust activation of transporters for Na homeostasis in root cells, especially the SOS1 antiporter and plasma membrane H-ATPase, differently than does NO. Despite some studies having shown that external NH ameliorates salt-induced effects on ionic homeostasis, there is no evidence that NH per se or some product of its assimilation is responsible for these responses. Here, we speculate about the signalling role behind glutamine in CBL-CIPK modulation, which could effectively activate the SOS pathway in NH-fed stressed plants.