Incidence of controlled water restriction on density and chemical profiles in tree rings of Araucaria araucana seedlings

Abstract

Abstract Background Dehydration of plant tissues caused by water stress affects the dynamics of the lateral cambium, the rate of cell division and differentiation in cell lumen size, wall thickness and wall chemical properties. Based on evidence that ongoing climate change projects longer and more intense water stress conditions, forest dynamics and decline are predicted to be affected by increases in drought intensity and frequency. To verify the interaction of these phenomena, we described the modifications in the anatomy and mineral contents of annual growth rings of Araucaria araucana seedlings subjected to water stress treatments, using X-ray densitometry and X-ray fluorescence techniques. Results Severe water stress conditions during the growing period produced narrower tree rings, with reduced cell lumen size (5–7 µm) and higher tracheid reinforcement factor values, but with lower wood density. Plants subjected to moderate water stress generated intra-annual density fluctuations coinciding with periods of decreased soil moisture. Under the precept that the essential nutrients play a relevant role in the functioning of trees, we found evidence of element allocation and concentration in response to drought. Calcium and phosphorus concentration increased significantly as stress becomes more severe, but with small differences between early- and latewood. On the contrary, potassium and sulfur presented lower values in the most stressed plants, and manganese had the lowest values only for moderate water stress. Finally, S/Ca and K/Ca molar ratios decreased, while Ca/Mn increased as stress becomes more severe. Conclusions Our findings suggested that A. araucana seedlings invested resources aimed at increasing structural components of the cell wall to prevent cavitation. This would maintain metabolism and cell growth even in unfavorable environmental conditions. Furthermore, the imbalance of manganese and calcium and their consequent ratio (Ca/Mn) could be linked to early adaptive signals to avoid dieback.