Leaf gas films delay salt entry and enhance underwater photosynthesis and internal aeration of Melilotus siculus submerged in saline water

Response of underwater photosynthesis to light, CO2, temperature, and submergence time of Taxodium distichum, a flood-tolerant tree

Introduction

Partial or complete submergence of trees can occur in natural wetlands during times of high waters, but the submergence events have increased in severity and frequency over the past decades. Taxodium distichum is well-known for its waterlogging tolerance, but there are also numerous observations of this species becoming partially or complete submerged for longer periods of time. Consequently, the aims of the present study were to characterize underwater net photosynthesis (PN) and leaf anatomy of T. distichum with time of submergence.

Methods

We completely submerged 6 months old seedling of T. distichum and diagnosed underwater (PN), hydrophobicity, gas film thickness, Chlorophyll concentration and needles anatomy at discrete time points during a 30-day submergence event. We also constructed response curves of underwater PN to CO2, light and temperature.

Results

During the 30-day submergence period, no growth or formation new leaves were observed, and therefore T. distichum shows a quiescence response to submergence. The hydrophobicity of the needles declined during the submergence event resulting in complete loss of gas films. However, the Chlorophyll concentration of the needles also declined significantly, and it was there not possible to identify the main cause of the corresponding significant decline in underwater PN. Nevertheless, even after 30 days of complete submergence, the needles still retained some capacity for underwater photosynthesis under optimal light and CO2 conditions.

Discussion

However, to fully understand the stunning submergence tolerance of T. distichum, we propose that future research concentrate on unravelling the finer details in needle anatomy and biochemistry as these changes occur during submergence.

Research Status and Trends of Underwater Photosynthesis

Underwater photosynthesis is the most important metabolic activity for submerged plants since it could utilize carbon fixation to replenish lost carbohydrates and improve internal aeration by producing O2. The present study used bibliometric methods to quantify the annual number of publications related to underwater photosynthesis. CiteSpace, as a visual analytic software for the literature, was employed to analyze the distribution of the subject categories, author collaborations, institution collaborations, international (regional) collaborations, and cocitation and keyword burst. The results show the basic characteristics of the literature, the main intellectual base, and the main research powers of underwater photosynthesis. Meanwhile, this paper revealed the research hotspots and trends of this field. This study provides an objective and comprehensive analysis of underwater photosynthesis from a bibliometric perspective. It is expected to provide reference information for scholars in related fields to refine the research direction, solve specific scientific problems, and assist scholars in seeking/establishing relevant collaborations in their areas of interest.

Improving Salt Tolerance of Chickpea Using Modern Genomics Tools and Molecular Breeding

INTRODUCTION

The high protein value, essential minerals, dietary fibre and notable ability to fix atmospheric nitrogen make chickpea a highly remunerative crop, particularly in low-input food production systems. Of the variety of constraints challenging chickpea productivity worldwide, salinity remains of prime concern owing to the intrinsic sensitivity of the crop. In view of the projected expansion of chickpea into arable and salt-stressed land by 2050, increasing attention is being placed on improving the salt tolerance of this crop. Considerable effort is currently underway to address salinity stress and substantial breeding progress is being made despite the seemingly highly-complex and environment-dependent nature of the tolerance trait.

CONCLUSION

This review aims to provide a holistic view of recent advances in breeding chickpea for salt tolerance. Initially, we focus on the identification of novel genetic resources for salt tolerance via extensive germplasm screening. We then expand on the use of genome-wide and cost-effective techniques to gain new insights into the genetic control of salt tolerance, including the responsive genes/QTL(s), gene(s) networks/cross talk and intricate signalling cascades.

Leaf gas films, underwater photosynthesis and plant species distributions in a flood gradient

Traits for survival during flooding of terrestrial plants include stimulation or inhibition of shoot elongation, aerenchyma formation and efficient gas exchange. Leaf gas films form on superhydrophobic cuticles during submergence and enhance underwater gas exchange. The main hypothesis tested was that the presence of leaf gas films influences the distribution of plant species along a natural flood gradient. We conducted laboratory experiments and field observations on species distributed along a natural flood gradient. We measured presence or absence of leaf gas films and specific leaf area of 95 species. We also measured, gas film retention time during submergence and underwater net photosynthesis and dark respiration of 25 target species. The presence of a leaf gas film was inversely correlated to flood frequency and duration and reached a maximum value of 80% of the species in the rarely flooded locations. This relationship was primarily driven by grasses that all, independently of their field location along the flood gradient, possess gas films when submerged. Although the present study and earlier experiments have shown that leaf gas films enhance gas exchange of submerged plants, the ability of species to form leaf gas films did not show the hypothesized relationship with species composition along the flood gradient.

Plant Adaptation to Multiple Stresses during Submergence and Following Desubmergence

Plants require water for growth and development, but excessive water negatively affects their productivity and viability. Flash floods occasionally result in complete submergence of plants in agricultural and natural ecosystems. When immersed in water, plants encounter multiple stresses including low oxygen, low light, nutrient deficiency, and high risk of infection. As floodwaters subside, submerged plants are abruptly exposed to higher oxygen concentration and greater light intensity, which can induce post-submergence injury caused by oxidative stress, high light, and dehydration. Recent studies have emphasized the significance of multiple stress tolerance in the survival of submergence and prompt recovery following desubmergence. A mechanistic understanding of acclimation responses to submergence at molecular and physiological levels can contribute to the deciphering of the regulatory networks governing tolerance to other environmental stresses that occur simultaneously or sequentially in the natural progress of a flood event.

Flood adaptive traits and processes: an overview

Unanticipated flooding challenges plant growth and fitness in natural and agricultural ecosystems. Here we describe mechanisms of developmental plasticity and metabolic modulation that underpin adaptive traits and acclimation responses to waterlogging of root systems and submergence of aerial tissues. This includes insights into processes that enhance ventilation of submerged organs. At the intersection between metabolism and growth, submergence survival strategies have evolved involving an ethylene-driven and gibberellin-enhanced module that regulates growth of submerged organs. Opposing regulation of this pathway is facilitated by a subgroup of ethylene-response transcription factors (ERFs), which include members that require low O₂ or low nitric oxide (NO) conditions for their stabilization. These transcription factors control genes encoding enzymes required for anaerobic metabolism as well as proteins that fine-tune their function in transcription and turnover. Other mechanisms that control metabolism and growth at seed, seedling and mature stages under flooding conditions are reviewed, as well as findings demonstrating that true endurance of submergence includes an ability to restore growth following the deluge. Finally, we highlight molecular insights obtained from natural variation of domesticated and wild species that occupy different hydrological niches, emphasizing the value of understanding natural flooding survival strategies in efforts to stabilize crop yields in flood-prone environments.

Partial versus complete submergence: snorkelling aids root aeration in Rumex palustris but not in R. acetosa

The root and shoot tissues of flood-tolerant wetland plants are highly porous to enable internal gas phase diffusion of O2 during waterlogging or submergence. In the case of only partial submergence (snorkelling), the atmosphere can act as source of O2 . The aim of this study was to assess the effect of waterlogging, partial submergence and complete submergence in the dark as well as in light on O2 partial pressure (pO2 ) in roots of Rumex palustris (flood tolerant) and R. acetosa (flood intolerant). We used O2 microelectrodes to measure pO2 of adventitious roots during manipulations of the water level around the shoot. Root pO2 in both species declined significantly upon submergence but remained oxic also when shoots were completely submerged in the dark (0.8 and 4.6 kPa in R. acetosa and R. palustris, respectively). The snorkelling effect was substantial in R. palustris only. Submergence in light had less impact on root pO2 and the effect of snorkelling was also minor. Hence, the benefits of light (underwater photosynthesis) and air contact (snorkelling) upon growth and survival in submerged wetland plants can now be linked to enhanced internal aeration.

Visualisation by high resolution synchrotron X-ray phase contrast micro-tomography of gas films on submerged superhydrophobic leaves

Floods can completely submerge terrestrial plants but some wetland species can sustain O2 and CO2 exchange with the environment via gas films forming on superhydrophobic leaf surfaces. We used high resolution synchrotron X-ray phase contrast micro-tomography in a novel approach to visualise gas films on submerged leaves of common cordgrass (Spartina anglica). 3D tomograms enabled a hitherto unmatched level of detail regarding the micro-topography of leaf gas films. Gas films formed only on the superhydrophobic adaxial leaf side (water droplet contact angle, Φ=162°) but not on the abaxial side (Φ=135°). The adaxial side of the leaves of common cordgrass is plicate with a longitudinal system of parallel grooves and ridges and the vast majority of the gas film volume was found in large ∼180μm deep elongated triangular volumes in the grooves and these volumes were connected to each neighbouring groove via a fine network of gas tubules (∼1.7μm diameter) across the ridges. In addition to the gas film retained on the leaf exterior, the X-ray phase contrast micro-tomography also successfully distinguished gas spaces internally in the leaf tissues, and the tissue porosity (gas volume per unit tissue volume) ranged from 6.3% to 20.3% in tip and base leaf segments, respectively. We conclude that X-ray phase contrast micro-tomography is a powerful tool to obtain quantitative data of exterior gas features on biological samples because of the significant difference in electron density between air, biological tissues and water.