Rhizome, root/sediment interactions, aerenchyma and internal pressure changes in seagrasses

© Springer International Publishing AG, part of Springer Nature 2018. Life in seawater presents several challenges for seagrasses owing to low O 2 and CO 2 solubility and slow gas diffusion rates. Seagrasses have evolved numerous adaptations to these environmental conditions including porous tissue providing low-resistance internal gas channels (aerenchyma) and carbon concentration mechanisms involving the enzyme carbonic anhydrase. Moreover, seagrasses grow in reduced, anoxic sediments, and aerobic metabolism in roots and rhizomes therefore has to be sustained via rapid O 2 transport through the aerenchyma. Tissue aeration is driven by internal concentration gradients between leaves and belowground tissues, where the leaves are the source of O 2 and the rhizomes and roots function as O 2 sinks. Inadequate internal aeration e.g., due to low O 2 availability in the surrounding water during night time, can lead to sulphide intrusion into roots and rhizomes, which has been linked to enhanced seagrass mortality. Under favourable conditions, however, seagrasses leak O 2 and dissolved organic carbon into the rhizosphere, where it maintains oxic microzones protecting the plant against reduced phytotoxic compounds and generates dynamic chemical microgradients that modulate the rhizosphere microenvironment. Local radial O 2 loss from belowground tissues of seagrasses leads to sulphide oxidation in the rhizosphere, which generates protons and results in local acidification. Such low-pH microniches can lead to dissolution of carbonates and protolytic phosphorus solubilisation in carbonate-rich sediments. The seagrass rhizosphere is also characterised by numerous high-pH microniches indicative of local stimulation of proton consuming microbial processes such as sulphate reduction via root/rhizome exudates and/or release of alkaline substances. High sediment pH shifts the sulphide speciation away from H 2 S towards non-tissue-penetrating HS - ions, which can alleviate the belowground tissue exposure to phytotoxic H 2 S. High sulphide production can also lead to iron and phosphorus mobilization through sulphide-induced reduction of insoluble Fe(III)oxyhydroxides to dissolved Fe(II) with concomitant phosphorus release to the porewater. Adequate internal tissue aeration is thus of vital importance for seagrasses as it ensures aerobic metabolism in distal parts of the roots and provides protection against intrusion of phytotoxins from the surrounding sediment

Model-assisted evaluation of crop load effects on stem diameter variations and fruit growth in peach

Key message: The paper identifies and quantifies how crop load influences plant physiological variables that determine stem diameter variations to better understand the effect of crop load on drought stress indicators. Stem diameter (D (stem)) variations have extensively been applied in optimisation strategies for plant-based irrigation scheduling in fruit trees. Two D (stem) derived water status indicators, maximum daily shrinkage (MDS) and daily growth rate (DGR), are however influenced by other factors such as crop load, making it difficult to unambiguously use these indicators in practical irrigation applications. Furthermore, crop load influences the growth of individual fruits, because of competition for assimilates. This paper aims to explain the effect of crop load on DGR, MDS and individual fruit growth in peach using a water and carbon transport model that includes simulation of stem diameter variations. This modelling approach enabled to relate differences in crop load to differences in xylem and phloem water potential components. As such, crop load effects on DGR were attributed to effects on the stem phloem turgor pressure. The effect of crop load on MDS could be explained by the plant water status, the phloem carbon concentration and the elasticity of the tissue. The influence on fruit growth could predominantly be explained by the effect on the early fruit growth stages