Flooding events are detrimental for most terrestrial plant species. The flooded environment severely restricts plant gas exchange due to the diffusion barrier presented by water and, compounding the restricted diffusion of O2 and CO2, are the low light conditions imposed by turbid floodwaters. The resulting effects on respiration and photosynthesis are primary factors leading to plant mortality upon submergence (Bailey-Serres & Voesenek, 2008). Terrestrial wetland plants like rice, utilize specific anatomical and morphological traits to improve underwater photosynthesis and internal aeration and prolong survival (Mommer et al., 2006; Colmer et al., 2011). These traits and their beneficial effect on underwater photosynthesis have been extensively studied, albeit under controlled laboratory conditions. Floodwaters, however, can be highly variable in terms of O2 and CO2 concentrations, light levels and temperature and these variables can have a significant influence on the extent to which tolerance traits can improve plant survival and growth. Therefore, next to performing physiological and molecular studies, under controlled laboratory conditions, in situ field measurements of the implications of adaptive physiological processes and traits during flooding are of utmost importance. To this end, Winkel et al. in this issue of New Phytologist (pp. 1193–1203) investigated the importance of underwater photosynthesis and leaf gas films, for internal aeration of submerged rice plants under field conditions.
‘…underwater photosynthesis, persistent gas films on leaves and aerenchyma in shoot and root tissues … strongly contribute to survival of rice when completely submerged.’
Flooding tolerant plant species overcome the unfavorable conditions imposed by submergence using two contrasting strategies, escape or quiescence, involving a suite of traits that help avoid or endure the stress, respectively (Bailey-Serres & Voesenek, 2008; Voesenek & Bailey-Serres, 2009). In both situations, the ability to photosynthesize underwater can greatly prolong survival by providing O2 and carbohydrates for respiration and growth (Colmer et al., 2011). Leaf traits in particular can significantly improve underwater photosynthesis and thereby contribute to submergence tolerance. Some terrestrial wetland plants produce new ‘semi-aquatic’ leaves when submerged. These acclimated leaves are thinner and have features such as thinner cuticles and re-oriented chloroplasts that improve gas exchange and thereby photosynthesis underwater (Mommer et al., 2004, 2005). Other terrestrial wetland species have hydrophobic leaf surfaces that retain a microlayer of gas when submerged. These leaf gas films enlarge the water–gas interface and improve gas exchange with consequent positive effects on plant underwater photosynthesis, internal aeration, sugar status and growth (Pedersen et al., 2009). For submerged plants, gas films provide an immediate benefit in contrast with semi-aquatic leaves that are fully formed only a few days after flooding occurs.
The study by Winkel et al. was performed on lowland ‘paddy’ rice (Oryza sativa). This rain-fed lowland rice is typically cultivated in paddies with a shallow layer of standing water, and root aeration is sustained by the diffusion of O2 from the atmosphere, via the shoot to the root tips. However, > 16% of paddy fields are vulnerable to flash floods which can result in submergence of the entire plant (Singh et al., 2011). The beneficial effects of gas films for underwater photosynthesis and internal aeration in completely submerged rice was recently demonstrated by the same authors (Pedersen et al., 2009). The present study, using unique measurements in the field, confirmed these previous findings performed under controlled laboratory conditions, and demonstrated the impact of environmental variables (light and dissolved CO2 and O2 in the floodwater) on plant O2 status in situ, also adding to earlier controlled experiments (Waters et al., 1989) on rice.
The particular rice line used here is the introgression line Swarna-Sub1. Swarna-Sub1 rice plants are introgressed with the submergence tolerance locus SUB1A. This chromosome fragment contains three genes that belong to the group VII Ethylene Response Factor family of transcription factors (Fukao et al., 2006). One of them, SUB1A, confers submergence tolerance via conservation of carbohydrates via reduced underwater elongation (Perata & Voesenek, 2007). This trait is associated with plant species that survive submergence by means of a quiescence strategy, during which expensive processes, including growth, are down regulated to conserve carbohydrates and energy (Fukao et al., 2006).
The field study of Winkel et al. was performed on rice plants growing in outdoor ponds in the Philippines, under natural environmental conditions (light, temperature, humidity). While O2 measurements inside the plants were performed with extremely sensitive, thin (25 μm) microelectrodes, similar measurements in the flood water used a water quality monitoring combined probe system that also recorded floodwater pH and temperature. The measurements were carried out on 4-wk-old plants with an average height of almost 30 cm. Complete submergence was imposed for 48 h followed by a period of de-submergence when the pond was drained. The O2 concentration in the floodwater followed a diurnal pattern with the highest values (19 kPa) just before sunset and the lowest concentrations (5.2 kPa) just before sunrise. The calculated CO2 concentration followed an opposite diurnal pattern. The dynamics of O2 and CO2 in the floodwater are explained by the balance between total ecosystem photosynthesis and respiration. It is important to realize that net photosynthesis underwater by the rice plants occurred at a very slow rate, that was only 2.3% of the rate in air. Despite this extremely slow rate, the O2 produced and/or the carbohydrates delivered contribute substantially to the survival of submerged plants (Mommer et al., 2006), even in relatively flood intolerant plants such as Arabidopsis thaliana (Vashisht et al., 2011). Winkel et al. demonstrate that under field conditions, gas films enhance the levels of endogenous root O2 in the light period as well as in darkness. In the light this is caused by strongly enhanced rates of underwater photosynthesis especially at lower CO2 concentrations. Interestingly, 70% of the variation in root O2 levels could be explained by the light level (photosynthetically generated O2 moved via aerenchyma from shoots into the roots) and only 16% was explained by O2 that diffuses from the floodwater into the plant. In plants in which the gas films were artificially removed, only 44% of the variation in root O2 concentration could be explained by underwater photosynthesis, due to the high resistance for CO2 uptake in these plants. During the night, 73% of the variation in root O2 could be explained by the O2 concentration of the floodwater, indicating that inward diffusion of O2 from the water column into the plant, facilitated by gas films, is important for maintaining some oxygen in the roots throughout the night. The absolute O2 concentration in the roots is not only determined by the O2 concentration of the water column, but also by the respiration rate of the root tissue (temperature dependent), loss of O2 from the root to the environment (radial oxygen loss) and the distance from the O2 source and thus the total internal diffusion resistance.
The study by Winkel et al. provides a valuable inventory of floodwater variables that strongly affect plant growth and survival under submerged field conditions. Not only do these data help correlate plant performance with environmental conditions in the present study, but such environmental data will also provide a guideline for the manipulation of these variables in future studies under controlled laboratory conditions. In addition, Winkel et al. demonstrate that underwater photosynthesis, persistent gas films on leaves and aerenchyma in shoot and root tissues, collectively form a suite of traits that strongly contribute to survival of rice when completely submerged. These traits typically belong to the so-called escape strategy exploited by many flood tolerant plants to avoid low O2 stress (Bailey-Serres & Voesenek, 2008). Interestingly, SUB1A also induces reduced chlorophyll degradation thus allowing continued underwater photosynthesis (Fukao et al., 2006, 2012). This observation, together with the findings in Winkel et al. demonstrating the importance of underwater photosynthesis, suggest that genes like SUB1A control traits involved in both escape (chlorophyll maintenance) and quiescence (carbohydrate conservation) strategies.
Flash floods that result in stagnant flooding or complete submergence (Singh et al., 2011) of paddy fields can cause yield losses from 10% to 100% depending on water depth, duration of the flooding event, turbidity, temperature and the developmental stage of the plant (Das et al., 2009). The increase in the frequency and intensity of flash floods due to changing global weather patterns (Singh et al., 2011), makes the development of more flood tolerant rice varieties critical. This challenge in turn requires not only an understanding of the genetic control of tolerance traits but also a more in-depth and accurate knowledge of environmental variables and the consequences for submerged plants during flooding in the field.