Mechanisms of adaptation to drought and waterlogging in Brachiaria grasses

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Drought and waterlogging are major abiotic stresses that limit the productivity of Brachiaria forage grasses. Little attention has been given to separate productivity under drought or waterlogging, from coping mechanisms in Brachiaria forage grasses. Wide phenotypic variation exists among Brachiaria grasses to cope with these stresses. This presentation will cover : 1) the current knowledge of morpho-physiological mechanisms and functional adaptations of Brachiaria spp cultivars to cope with these stresses and 2) the use of sensors and digital image analysis for the non-destructive and automated analysis of Brachiaria growth and performance at different time scales.


<ul><li> J. A. Cardoso, J. C. Jimnez, K. Odokonyero, L.M. Pineda, Hannah Vos, Fernando Vergara, Daniela Chamorro, I. M. Rao Collaborators: CIAT: J. Polana, J. Arango, J. Nuez, Plant nutrition lab, Nutrition quality lab, Birthe Paul Beca-ILRI Hub KARI-Kenya RAB-Rwanda AgResearch-New Zealand Corpoica-Colombia INTA-Nicaragua IDIAP-Panam Mechanisms of adaptation to drought and waterlogging in Brachiaria grasses </li> <li> Brachiaria spp.: Important forage grasses in the tropics African origin Estimated 100 million hectares in Brazil alone. Breeding program started in the late 1980s. Wide range of adaptation to climatic and edaphic factors. Carbon accumulation in soil, reduction of greenhouse effect gases from soil (N20) and methane from livestock </li> <li> Why do we need to know mechanisms of abiotic stress adaptation? Identifying plant attributes that contribute to resistance/tolerance to major abiotic stresses (e.g., drought and waterlogging) Developing rapid, reliable and high throughput screening methods Phenotyping of Brachiaria genotypes developed by the breeding program </li> <li> Drought resistance (avoidance/tolerance) Assessment methods Leaf gas exchange/porometry Infrared thermometry Carbon isotope discrimination??? Chlorophyll content (SPAD) Chlorophyll fluorescence Relative water content in leaves Weighing each container on a regular basis Vertical distribution of roots in soil cylinders (120 cm height x 22 cm width; 80 cm height x 7.5 cm width) Micrographs from root cross sections High stomatal conductance Delayed leaf senescence High quantum yield High osmotic adjustment High transpiration efficiency Deep root systems Increased root length density in medium and deep soil layers Decreased resistance to water movement from soil by increasing root hair growth and xylem diameters Brachiaria hybrid cv. Cayman Drought </li> <li> Shoot growth and biomass partitioning after 5 weeks of drought Experiment 1 35 kg of soil Experiment 2 62 kg of soil </li> <li> Based on hue, chlorophyll contents (SPAD or analytical) can be estimated (r2 &gt; 0.85) Binary images are used to estimated shoot areas (difference of dark and white pixels) (r2 &gt; 0.7) Skeletonize to determine leaf apparition rates Non-destructive assessment of shoot growth over time (semi-automated) scripts at Github, juan_cardosinho 1 2 3 4 5 0 days 7 days 14 days 21 days </li> <li> Projectedshootarea(cm2) 0 days 10 days 21 days Assessment of shoot growth under progressive drought Faster growth rates for Napier grass, Rhodes grass and hybrid Cayman Greater shoot area represents greater demands for water Wilting plants </li> <li> Root growth after 5 weeks of growth </li> <li> Root growth after 5 weeks of growth </li> <li> Root growth after 5 weeks of growth </li> <li> Estimation of root length using digital images </li> <li> Photosyntheticrate(molCO2m2s-1) Leaf gas exchange and chlorophyll fluorescence Transpirationrate(mmolH2Om2s-1) Photosyntheticefficiency Preliminary results indicate that reduction of growth under drought is mainly due to restriction of leaf gas exchange Photosynthetic efficiency (Fv/Fm) is relatively insensitive to drought stress indicating stomatal limitation of photosynthesis 0. 0. 0. 0. 0. 0. 0. 0. 0. Brachiaria grasses </li> <li> 32.7 C Growth of Napier grass under drought conditions for 21 daysGrowth of Mulato II under drought conditions for 21 days Wilting plant Growing plant Mulato II shows superior resistance to terminal drought conditions than Napier grass Regulated use of water allows Mulato II to sustain growth under drought conditions. Mulato II responds to drought by early closing of stomata which reduces transpiration. Leaf and canopy temperature values are measured using infrared thermography. Thermal infrared images </li> <li> CTDdrought Napier Rhodes Cayman Marandu Toledo Llanero Mulato II Basilisk Piata Mulato Tully Napier Rhodes Cayman Marandu Toledo Mulato II Piata Mulato Tully Tupi Llanero Basilisk Canopy and leaf temperature as proxies for deep rooting Canopy temperature depression (CTD) = Air temperature - Canopy temperature A cooler canopy (higher CTD) indicates transpiring leaf and indicates access to water by roots Observable root length (m plant-1) Root depth (m plant-1) Observable root length 80-120 cm root depth (m plant-1) </li> <li> CaymanMulato II MulatoII Time course of growth and water uptake under drought conditions </li> <li> Water uptake and growth under drought conditions Currently, a simultaneous analysis of shoot and root growth and water content across the soil profile is possible using digital image analysis and TDR. 7 days 14 days Mulato II Napier Mulato II Napier </li> <li> Root traits that influence water transport XV More and greater xylem vessels (XV) conduct more water (decreased axial resistance) Increased xylem vessel (XV) under drought conditions for Napier grass Increased root diameter facilitates penetration in drying soils Greater root diameter in Napier grass Roots hairs for decreasing radial resistance Longer and denser root hairs in Mulato II Irrigated Drought NapiergrassMulatoII </li> <li> MulatoIINapiergrass Irrigated Drought (10 days) 0 h 1 h 2 h 3 h 1cm Root elongation rate Faster root elongation rates for Napier Higher inhibition of root growth under drought conditions for Mulato II (65%) than Napier (35%) </li> <li> Irrigated Drought Irrigated Drought NapiergrassMulatoIIB.humidicola Aerenchyma development AER Aerenchyma (AER) can improve the acquisition of water and nutrients by reducing the metabolic costs of soil exploration. AER t </li> <li> % Inhibition root elongation 0 5 10 15 20 25 0 50 100 %Aerrootsdrought Napier B. humidicola Mulato II Aerenchyma (AER) can improve the acquisition of water and nutrients by reducing the metabolic costs of soil exploration. We plan to quantify respiration rates of aerenchymatyous roots vs. non aerenchmatous roots using an IRGA. Aerenchyma development and root growth </li> <li> Root angles MulatoIINapiergrass Irrigated Drought (10 days) Phenotypic plasticity for root angles Straighter angles under drought conditions Evaluation for root angles under irrigated and drought conditions underway 1m </li> <li> Recovery after drought Short term drought restricts growth of Mulato II and is reflected in final biomass </li> <li> Mechanisms Drought resistance in Brachiaria Water spenders Maintaining water uptake Napier grass Cayman Water savers Reducing water loss Piat Deep roots Increased root length density at depth Increased root growth at expense of shoots Closing of stomata Leaf senescence Reduced leaf area Cayman Both mechanisms Napier grass Rhodes grass Terminal drought Intermittent drought Mulato II Basilisk Mulato Marandu Tupi Toledo Tully Llanero Cultivars Productivity </li> <li> Effects of waterlogging on Brachiaria grassesTullyToledoRuzigrass </li> <li> Drained (D) Waterlogged (W) D W D W Tully Toledo Ruzi grass Tolerant Mod. tolerant Sensitive Increased root death and smaller root system in non-tolerant genotypes Reduction in shoot growth in non-tolerant genotypes Increased leaf senescence, chlorophyll loss, stomatal closure, lower values of photosynthetic efficiency (fv/fm) in non-tolerant genotypes. 21 days of treatment Effects of waterlogging on Brachiaria grasses </li> <li> Traits associated with waterlogging tolerance in Brachiaria grasses Aerenchyma (air spaces) allows oxygen difussion from shoot to root to maintain root aerobic respiration. Brachiaria adapts to waterlogging by the development or increase or aerenchyma in root tissues. Constitutive formation of aerenchyma in B. humidicola allows immediate adaptation to waterlogging Better adapted genotypes show thicker roots, greater aerenchyma formation and smaller steles (conductive tissue) Drained Waterlogged B. humidicola Tully (tolerant) B. ruziziensis Br 44-02 (sensitive) Cross sections taken at 10 cm form the root tip Scale = 0.5mm Aerenchyma stele </li> <li> Increased suberization of the outer part of the root (OPR) B.humidicolaB.ruziziensis Drained Waterlogged Waterlogged O2 O2 B. humidicola B. ruziziensis O2 O2 O2 O2 AE AE Roots with greater aerenchyma formation and increased suberization of OPR shows deeper penetration into waterlogged soil </li> <li> LeafsheathInternodeRoot Aerenchyma (arrows) formation in shoots (internode and leaf sheath) and roots Continuum of ventilation form shoot to roots facilitates gas exchange between atmosphere and rhizosphere B. humidicola after 21 days of growth under waterlogging O2 CH4N2O? C2H4 </li> <li> Replacement rooting Roots produced after waterlogging (white) Roots produced before waterlogging (rotten or decaying) Brachiaria genotypes without constitutive formation of aerenchyma in roots depend on the formation of new roots with aerenchyma for adaptation to waterlogging </li> <li> Replacement rooting Cayman Mulato II Decaying or rotten roots Decaying or rotten roots Replacement roots Cayman </li> <li> Most of damage (e.g. leaf chlorosis and leaf senescence) occurs between 7 and 14 days after waterlogging, period where new roots with aerenchyma started to develop. After this, aerenchymatous roots confer adaptation to waterlogging. Screening for waterlogging tolerance in Brachiaria hybrids 1 day of waterlogging treatment 7 days of waterlogging treatment 14 days of waterlogging treatment 21 days of waterlogging treatment </li> <li> Recovery after drought and waterlogging B. humidicola responds to accumulated ethylene in waterlogged soil by internode elongation and hyponastic growth of leaves This plastic response allows leaves of B. humidicola to escape form water and continue photosynthesis Irrigated Drought Waterlogging Recovery period Irrigated Drought Waterlogging </li> <li> Conclusions so far.. Cayman seems to be a water spender, not a saver. It attempts to maximize carbon gain (growth) when water is available. Most Brachiaria grasses combine water saving mechanisms (by regulation of water loss by closing leaf stomata) with deep rooting ability to avoid drought stress. Digital images (e.g., RGB, Thermal IR) allows recordings of growth and responses to stresses. Leaf and canopy temperatures could be used as proxies for rooting depth in Brachiaria genotypes Aerenchyma might aid root elongation under drought conditions </li> <li> Conclusions so far. Brachiaria grasses adapt to waterlogging by increasing or developing aerenchyma in roots to sustain root aerobic respiration Plants without constitutive formation of aerenchyma in roots depend on the formation of new roots with aerenchyma for adaptation to waterlogging Maximum rooting depth can be used as an indicator of root aeration efficiency and waterlogging tolerance </li> <li> Challenges ahead Determine the role of endophytes in drought adaptation Determine the contribution of climate smart Brachiaria grasses to soil carbon accumulation and greenhouse balance Scale up for automated phenotyping in breeding populations </li> </ul>