The root barrier to radial O 2 loss (ROL) is a trait enabling waterlogging tolerance of plants. The ROL barrier restricts O 2 diffusion to the anoxic soil so that O 2 is retained inside root tissues.We hypothesised that the ROL barrier can also restrict radial diffusion of other gases (H 2 and water vapour) in rice roots with a barrier to ROL. We used O 2 and H 2 microsensors to measure ROL and permeability of rice roots, and gravimetric measurements to assess the influence of the ROL barrier on radial water loss (RWL).The ROL barrier greatly restricted radial diffusion of O 2 as well as H 2 . At 60 kPa pO 2 , we found no radial diffusion of O 2 across the barrier, and for H 2 the barrier reduced radial diffusion by 73%. Similarly, RWL was reduced by 93% in roots with a ROL barrier.Our study showed that the root barrier to ROL not only completely blocks radial O 2 diffusion under steep concentration gradients but is also a diffusive barrier to H 2 and to water vapour. The strong correlation between ROL and RWL presents a case in which simple measurements of RWL can be used to predict ROL in screening studies with a focus on waterlogging tolerance.

Flooding is an environmental stress that leads to a shortage of O2 that can be detrimental for plants. When flooded, deepwater rice grow floating adventitious roots to replace the dysfunctional soil-borne root system but the features that ensure O2 supply and hence growth of aquatic roots have not been explored. We investigate the sources of O2 in aquatic adventitious roots and relate aerenchyma and barriers for gas diffusion to local oxygen gradients, as measured by microsensor technology, to link O2 distribution in distinct root zones to their anatomical features. The mature root part receives O2 exclusively from the stem. It has aerenchyma that, together with suberin and lignin depositions at the water-to-root and cortex-to-stele interfaces, provides a path for longitudinal oxygen movement toward the tip. The root tip has no diffusion barriers and receives O2 from the stem and floodwater resulting in improved aeration of the root tip over mature tissues. Local formation of aerenchyma and diffusion barriers in the mature root channel O2 towards the tip which also obtains O2 from the floodwater. These features explain aeration of floating roots and their ability to grow under water.

Hypoxic floodwaters can seriously damage seedlings. Seed dormancy could be an effective trait to avoid lethal underwater germination. This research aimed to discover novel adaptive dormancy responses to hypoxic floodwaters in seeds of Echinochloa crus-galli, a noxious weed from rice fields and lowland croplands.• Hypoxic floodwaters interfere with E. crus-galli seed seasonal dormancy changes. Dormancy-breaking signals are overridden during hypoxic floods, drastically decreasing underwater germination. In addition, results indicate that a fraction of E. crus-galli seeds perceive dormancy-breaking signals under hypoxic water and germinate immediately after aerobic conditions are regained, a hazardous yet less competitive environment for establishment.

A key trait conferring flood tolerance is the ability to grow adventitious roots as a response to submergence. The genetic traits of deepwater rice determining the development and characteristics of aquatic adventitious roots (AAR) had not been evaluated.We used near-isogenic lines introgressed to test the hypothesis that the impressive shoot elongation ability of deepwater rice linked to quantitative trait loci 1 and 12 also promote the development of AAR.The deepwater rice genotype NIL-12 possessed expanded regions at the stem nodes where numerous AAR developed as a response to submergence. Two types (AR1 and AR2) of roots with distinct timing of emergence and large differences in morphological and anatomical traits formed within 3 (AR1) to 7 (AR2) d of submergence. The mechanical impedance provided by the leaf sheath caused AR2 to emerge later promoting thicker roots, higher elongation capacity and higher desiccation tolerance. Upregulation of key genes suggests a joint contribution in activating the meristem in AAR enhancing the development of these in response to submergence.The morphological and anatomical traits suggested that AR2 is better adapted to long-term flooding than AR1. We therefore propose that AR2 in deepwater rice functions as an evolutionary defence strategy to tackle periodic submergence.

Excess water can induce flooding stress resulting in yield loss of crops, even in wetland plants such as rice. However, traits from species of wild Oryza have already been used to improve tolerance to abiotic stress in cultivated rice. This study aimed to establish root responses to sudden soil flooding among 8 wild relatives of rice with different habitat preferences benchmarked against 3 genotypes of O. sativa. Plants were raised in hydroponics, mimicking drained or flooded soils, to assess the plasticity of adventitious roots. Traits included were apparent permeance (PA) to O2 of the outer part of the roots, radial water loss (RWL), tissue porosity, apoplastic barriers in the exodermis and root anatomical traits. These were analysed using a plasticity index and hierarchical clustering based on principal components analysis. For example, O. brachyantha, a wetland species, possessed very low tissue porosity compared to other wetland species, whereas dryland species O. latifolia and O. granulata exhibited significantly lower plasticity compared to wetland species and clustered in their own group. Most species clustered according to growing conditions based on PA, RWL, root porosity and key anatomical traits, indicating strong anatomical and physiological responses to sudden soil flooding.