Physiological Responses to Hypoxia and Manganese in Eucalyptus Clones with Differential Tolerance to Vale do Rio Doce Shoot Dieback

Abstract

ABSTRACT:Vale do Rio Doce shoot dieback (VRDSD) is an anomaly whose cause seems to be associated with hypoxic conditions and their consequences (excess Mn and Fe) triggered by elevation of the water table in areas with poor drainage. Different plants have distinct survival strategies under this form of stress. The objective of this study was to understand the physiological responses involved in the differential tolerance of eucalyptus clones to VRDSD and their relationship to hypoxia and excess Mn. A hydroponic experiment was carried out using a 2 × 2 × 2 factorial arrangement, two eucalyptus clones with different levels of tolerance to VRDSD (sensitive Urograndis hybrid -1213; and the tolerant Rio Claro hybrid -Eucalyptus grandis x unknown -2719), two concentrations of O 2 (8 and 4 mg L -1 ), and two Mn concentrations (1.39 and 300 mg L -1 ) in a randomized block design (RBD) with three replicates. Forty-day-old clones were maintained in Clark nutrient solution for 30 days. After this period, the treatments were applied for 11 days. Plant gaseous exchange shoot and root production, and the quantity of enzymes related to oxidative stress in leaves and roots were evaluated. In the tolerant clone, reactive oxygen species (ROS) were produced under hypoxic conditions, accompanied by reduction in production of dry matter, malondialdehyde (MDA), and in activity of the enzyme alcohol dehydrogenase (ADH). However, this clone had greater production of superoxide dismutase (SOD) under these conditions, an enzyme responsible for detoxification of ROS, which acts as part of the Low Oxygen Quiescence Syndrome (LOQS). In contrast, sensitive clones did not exhibit expressive reductions in growth or changes in the leaf/root ratio. These clones formed large quantities of adventitious roots and had high levels of MDA and ADH and low levels of SOD. Therefore, sensitive clones appear not to be prepared for detoxification of ROS and other toxic metabolites, but rather adopt morphological escape mechanisms, the Low Oxygen Escape Syndrome (LOES), in response to hypoxia. Thus, the period of soil waterlogging may cause the death of large numbers of roots in sensitive clones, limiting their ability to absorb water and nutrients and culminating in the death of these plants. Excess Mn seems to aggravate the damage caused by hypoxia, but it is not the causal agent of VRDSD.

FOLIAR COMPOSITION OF Eucalyptus urograndis IN TWO SITES FERTILIZED WITH N, P AND K IN SOUTHERN BRAZIL

“…The values can be considered normal as the plants did not show apparent symptoms of Mn toxicity and the growth was adequate (BASSACO et al, 2018). Reinforcing the indication of normality, Harguindeguy et al (2018) found values between 300 and 5,083 mg kg -1 in Eucalyptus seedling leaves developed under controlled oxygenation condition and Mn availability, without being able to associate this variation with growth of the two studied clones.…”

Physiological and biochemical responses of Eucalyptus seedlings to hypoxia

“…Short-term hypoxia is likely to increase across South America, particularly on compacted and/or soils with shallow profiles (40-50 cm deep) where drainage is blocked. This results from changes in rainfall seasonality and the increase of rainfall under climate change (Harguindeguy et al 2017;Gonçalves et al 2016;Feng et al 2013). Plant tolerance to hypoxia is a result of morphological and physiological adaptations (Kreuzwieser and Rennenberg 2014).…”

“…Intra-specific variation in tolerance to hypoxia is observed among clones of Eucalyptus, particularly within the hybrid E. urograndis (a hybrid between E. urophylla and E. grandis) (Harguindeguy et al 2017). Despite this occurrence, the physiological and/or chemical basis for this variation is not known.…”