Variability in intrinsic photosysnthetic thermotolerance (PT) among evergreen citrus genotypes and effects of short-term warming on PT and photoprotection capacity
Guha A, Vharachumu T, Khalid M F, Kelly M, Avenson T and Vincent C [2019-2020] With rise in global temperature, terrestrial vegetation is increasingly witnessing unusual warming events. Our knowledge on genotypic variability and plasticity in photosynthetic thermotolerance (PT) is surprisingly limited. We don’t have enough empirical data quantifying PT response to short-term rise in air temperature (Tair). Citrus provides a useful model to study acclimation and variation in PT because its evergreen habit and leaf thermal behavior make PT under a variety of environmental conditions crucial for leaf survival. We conducted mesocosm experiments in the subtropical climatic conditions of central Florida. In experiment 1, 22 genotypes were studied for quantifying variability in intrinsic PT as measured by different chlorophyll fluorescence indices (Fo, Fm and Fv/Fm) of photosystem-II (PSII) performance. In experiment 2, we simulated progressively increasing growth temperature over 3 weeks (weekly day/night Tair: 26/20°C< 33/30 °C<36/32 °C) followed by a recovery period (26/20°C). By the end of each week, PT was examined to identify potential shifts in PT. Further, photochemical and non-photochemical quenching responses were studied over increasing light intensity to understand how elevated Tair affected PSII efficiency and photoprotection capacity. Supported by IFAS Citrus Initiative Grant |
Shade amelioration of citrus huanglongbing symptoms: Photosynthetic and hydraulic functioning, host-pathosystem interactions, canopy growth, metabolite profile, fruit yield and quality
Guha A, Killiny N, Wang Yu, and Vincent C [2019-2022] This project centered around understanding and management of citrus greening disease (usually called HLB), which is not well understood, but has devastated Florida's citrus industry. Generally, HLB causes reduced phloem functionality, which leads to carbohydrate accumulation in leaves and starvation of fruits and roots. On the photosynthetic side, this leads to down-regulation of carbon fixation and to increased photoinhibition. On the sink side, this leads to reduced water and nutrient uptake due to reduced root functionality. The disease is highly variable in terms of symptom severity, and we think that environmental variables are a big part of this. One of our hypotheses was that we can manipulate the growth environment in ways that mitigates disease severity. This can contribute to management as well as understanding the disease. Under low to high shade conditions, we studied photosynthetic physiology, plant water relations, fruiting, tree growth, vector load, and disease progression. We also looked at the whole-leaf metabolome profiles and volatile compounds in the HLB affected trees growing under various light regimes. Supported by IFAS Citrus Initiative Grant |
Can natural under-story environment promote photosynthetic efficiency and mitigates severity of HLB-infection in feral grapefruit population?
Vincent C, Guha A, Nabil Killiny N and Diepenbrock L [2019-2020] Understory life can shape species interactions with the abiotic and biotic stressors in surroundings and can be advantageous when compared to life in the open. Anecdotally, feral citrus trees growing under forest overstory were observed to be healthy and to show no symptoms of huanglongbing disease (HLB; citrus greening disease), even when growing in close proximity to HLB-infected open commercial plantings. We undertook an experiment in a natural ecosystem area in central Florida with abundant understory feral grapefruit (Citrus x paradisi Macfad.) trees located near a HLB-infected commercial citrus farm to understand how natural understory conditions affect distribution of HLB causal agent, bacterium Candidatus Liberibacter asiaticus, or its vector, Asian citrus psyllid Diaphorina citri Kuwayama. Citrus trees in the tropics and subtropics are also known to undergo dynamic photoinhibition, and loss in maximum quantum efficiency of photosystem II (PSII). As a typical shade species, citrus has a low light-saturation photosynthesis and therefore we hypothesized that the understory environment can reduce overexcitation of PSII, minimize photodamage and will improve leaf photochemical functions. We aimed to find out whether the naturally shaded understory environment can inhibit the HLB pathosystem by deterring the arrival of vector (D. citri) on hosts and can simultaneously improve the photochemical performance, making the shaded environment more advantageous for the feral grapefruit population. Supported by IFAS Citrus Initiative Grant |
How leaf and stem vascular anatomy will respond to increasing elevated growth temperatures? A study on Populus deltoides
Guha A, McLennan DA, and Warren JM [2017-2018] Global temperatures are rising, and for many plant species, their physiological response to elevated growth temperature has not been well characterized. Available data shows positive, negative as well as neutral growth effects in various tree species so far investigated. Unlike labile photosynthetic machinery, changes in vascular tissue anatomy has received less attention in the context of elevated global temperature. In particular, plant functional anatomy has been examined experimentally in only few species under warming conditions. To address this, we created three different growth mesocosms for well-irrigated Populus deltoides saplings using the greenhouse (24/18°C), growth chamber (32/26°C) and extreme growth chamber (40/34°C) facilities. P. deltoides (eastern cottonwood poplar) is native to North America, growing throughout the eastern, central, and southwestern United States, the southernmost part of eastern Canada and northeastern Mexico. Our objectives were to investigate how temperature increase affects 1) leaf functional anatomy and 2) stem wood density and stem xylem and phloem anatomy and 3) plant growth and water use. Supported by ORNL LDRD (Laboratory Directed Research and Development) |
Photosystem-II responses in boreal peatland tree and shrub species exposed to elevated temperature and CO2
Guha A, McLennan DA, and Warren JM [2018-2019] Thermal stress can induce irreversible photodamage with longer consequences for plant metabolism. We focused on photosystem II (PSII) behaviour to understand how photosynthetic processes respond in various co-occuring boreal trees and shrubs when exposed to future warming and eCO2 atmosphere. In a manipulative FACE setup, we monitored PSII performance and tracked both transient and chronic PSII damages using both continuous and pulse-amplitude modulated fluorimeters. Chlorophyll a fluorescence signals were used to simulate PSII bioenergetics. The light (Fv´/Fm´) and dark-adapted (Fv/Fm) fluorescence traits including PS-II quantum efficiency, fast induction kinetics (OJIP) and quenching capacities were significantly affected by the treatments. Loss in PSII efficiency was more apparent in certain plant functional types whereas some species maintained relatively better PSII functioning. PSII down-regulation could be one of dominant factors for the loss in operational photosynthesis during warming atmosphere. Both light and dark-adapted fluorescence characteristics showed loss in photo-regulatory functions and photodamage. Some species showed rapid recovery from transient PSII damage, whereas fingerprints of chronic PSII damage were observed in others. Supported by the U. S. Department of Energy, Office of Science, Biological and Environmental Research Program |
Leaf level detection of solar induced chlorophyll (SIF) fluorescence during heat wave events using advanced spectral fitting methods
Wang F, Guha A, Han J, McLennan DA, Gu Li and Warren JM [2017] The intensity and number of extreme heat events, such as short-term heat waves (hw), has increased, and current projections forecast the global land area experiencing heat waves will double by 2020. Sun-induced chlorophyll fluorescence (SIF) has been a new probe to detect plant photosynthesis and their response to extreme climate event at regional- and global- scale. However, understanding the SIF response of different tree species to short-term hw is still limited due to the lack of knowledge of the detailed physiological mechanism for specific tree species. In this study, we explored the SIF response of four temperate tree species, i.e. Southern red oak (Quercus falcata), Shumard oak (Quercus shumardii), and Eastern white pine (Pinus echinata) and Yellow poplar (Liriodendron tulipifera) to the simulated short-term hw in a plant growth chamber by a novel SIF observing system - the Fluorescence Auto Measurement Equipment (FAME). We developed an advanced spectral fitting method (SFM) to retrieve the SIF of four tree species. The fluorescence yield of Southern red oak and Shumard oak observed by FAME is less 53.3% than Yellow poplar and Eastern white pine during hw cycle. Our results show that tree species with lower SIF, i.e. Southern red oak and Shumard oak have higher heat tolerance than ones with higher SIF, i.e. Yellow poplar and Eastern white pine. Furthermore, this study implied that satellite SIF has great potential to detect heat stress conditions for tree species in a timely manner on a large scale. Supported by ORNL LDRD (Laboratory Directed Research and Development) |
Growth and physiological responses of poplar (Populus deltoides) under elevated temperature and VPD regimes
Guha A, McLennan DA, Bellarie S, Child J and Warren JM [2017-2018] Photosynthetic acclimation and trade-offs in biomass allocation have not been widely incorporated into physiological and ecosystem models, suggesting that models may over- or underestimate how species might respond to air temperature (Ta) increases and how ecosystem productivity could be altered under global warming. Eastern cottonwood (Populus deltoides) is a fast growing, widely distributed North American tree species used for bioenergy and bioproducts. Since future warming is likely to have significant direct effects on poplar carbon allocation, ecophysiology and productivity, we designed studies to examine their responses and test mechanistic representation in a terrestrial biosphere model. We grew well-irrigated P. deltoides saplings at three different Ta cycles including 24/18 °C (day/night), 32/26 °C and 40/34 °C for five months. We quantified changes in net photosynthesis (Anet), dark respiration (Rd), water use, growth and biomass allocation. Subsequent thermotolerance was tested by exposing plants to short-term heat waves (reaching Tmax 47, 48 and 51°C over a period of 3 days) followed by photosynthetic measurements. Supported by ORNL LDRD (Laboratory Directed Research and Development) |
Experimental and modelling studies to understand how temperate forest tree species of southeastern United States will respond to warming and short heat wave events
Guha A, McLennan DA, Warren JM, Ricciuto D, Gu L, Cumming C, Han J, Bellarie S and Child J [2016-2018] Earth system models (ESMs) include mechanistic, process-level photosynthetic biochemistry such as the maximum rates of carboxylation or electron transport, which depend on, and respond to changes in resources or environmental conditions as dependent on generalized plant functional types (PFTs) that group many species. However, there are currently no damage thresholds in response to acute heat wave (hw) events, which may be species-specific. This supports the need to move away from generic PFTs towards trait-based modelling, where additional mechanistic thresholds might be introduced. In this study, we selected several temperate tree species of southeast United States. Regional distribution of these species vary across microsites, some tending toward dry upland sites, some tolerating a range of sites, from moist bottomlands to dry ridges, some growing best in fertile moist soil, and some distributed along the full moisture gradient from wet bogs and moist stream bottoms to xeric sand plains and rocky ridges. Our study investigated the effects of extreme hw on key ecophysiological processes in these forest species. Supported by ORNL LDRD (Laboratory Directed Research and Development) |
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Resilience of dry tropical forest trees to drought: photosynthesis, stem wood and leaf traits
Guha A, Jezeera A, Mohan MK, Mohanbabu N, Sunny R, Upadhyay H and Barua D [2015-2016] In the tropical dry deciduous forest of Western Ghats, India, several evergreen and deciduous tree species co-occur. It has been suggested that these two plant functional types (PFTs) differ in carbon gain and water use strategies. However, there is no agreement over the differences in stem hydraulic traits between these two groups co-occurring in the same forest ecosystem. In this study, we investigated whether stem wood anatomy and hydraulic architecture contribute to plant growth and photosynthetic capacity. In a controlled greenhouse experiment, we maintained healthy 6-8 months old seedlings of 6 deciduous and 6 evergreen tree species for 4 months under well-watered conditions. We characterized stem xylem traits, potential hydraulic conductivity, leaf functional traits, and plant growth characteristics and looked at whether stem hydraulic traits correlate with leaf structural and functional traits and plant growth rate. Also, we investigate the relative drought tolerance responses of these two PFTs. opting a progressive dry-down treatment. Species’ drought resilience capacity and physiological performance were correlated to stem wood density, LMA and xylem traits. Supported by the intramural funding of Indian Institute of Science Education and Research (IISER) Pune, India |
Leaf thermotolerance in dry tropical forest tree species: relationships with leaf traits and effects of drought
Sastry A, Guha A and Barua D [2015-2016] In this study, we quantified leaf thermotolerance in saplings of 12 seasonally dry tropical forest tree species that were grown in a common environment. We measured the temperature response of dark-adapted chlorophyll a fluorescence (Fv /Fm), and estimated the temperature that results in a 50% loss of function (T50). The temperature response of chlorophyll a fluorescence is a physiological measure of the integrity of the thylakoid membrane, is highly sensitive to high temperature, represents a good indicator of photosynthetic and organismal thermotolerance and has been used extensively in determining plant sensitivity to extreme temperatures. Specifically, we examined the relationships between thermotolerance and leaf functional traits—LMA, LDMC, leaf size and photosynthetic rates. Additionally, we examined how exposure to experimental drought affects photosystem II (PSII) function at high temperature, and tested whether this was positively related to drought tolerance. Performance under drought conditions was quantified by measuring leaf wilting, leaf relative water content (RWC) and the decrease in photosynthesis as compared to well-watered plants. Supported by the intramural funding of Indian Institute of Science Education and Research (IISER) Pune, India |
Growth and physiological performance of young seedlings of tropical deciduous and evergreen tree species under optimal light and water conditions
Guha A, Jezeera A, Mohan MK, Mohanbabu N, and Barua D [2014-2015] Seedlings represent a particularly vulnerable life stage of a tree and generally exhibit high rates of mortality due to both abiotic and biotic factors. In many ecosystems, seedling survival and establishment are major bottlenecks in species recruitment and seedling abundance alone may not guarantee the regeneration of a species. This study was aimed to investigate various ecophysiological traits in 6 species, at their six-month old development stage in order to understand the reasons behind their predominance in different forest habitats forests of the WG. We hypothesize that various traits expressed in the seedling stage of first year of growth might explain such habitat preference evident in those woody species, could explain seedling establishment and might improve our ability to predict future species distributions, especially as climate change is predicted to alter the dynamics, composition and diversity of many plant ecosystems. Supported by the intramural funding of Indian Institute of Science Education and Research (IISER) Pune, India |
Drought responses in four Sorghum bicolor genotypes varying in plant water use and whole-plant hydraulic anatomy
Guha A, Chhajed SS, Choudhury S, Sunny R and Barua D [2014-2015] Genotypic variability and plasticity in hydraulic anatomy and their impact on ecohydrology are not well-understood in herbaceous monocots. In this study, we used Sorghum bicolor, a monocotyledonous, tropical grass model, to understand whether differential plant water use is associated with xylem anatomy and if whole-plant xylem anatomy responds to water stress, justifying differential genotypic sensitivity to drought. We studied four sorghum genotypes that are known to genetically differ in growth and exhibit differential sensitivity to drought. Under well-watered scenario, transpiration variability and plant growth traits correlated with xylem anatomical traits at both the leaf and stem level, including xylem area and predicted xylem-specific hydraulic conductivity. High water use genotypes had inherently higher hydraulic capacity, but under drought, their transpiration declined at higher fractions of transpirable soil water (FTSW) and they showed greater plasticity in hydraulic anatomy. However, lower FTSW thresholds and modest anatomical changes were identified in the low water use genotypes with inherently lower hydraulic conductivity. Drought, induced modular phenotypic plasticity in hydraulic anatomy, whereby plasticity in leaf xylem traits was remarkably higher than stem xylem, while root xylem showed a reverse nature of vascular modification. Xylem traits were in agreement with phloem anatomy, irrespective of water regime. Our study indicates that hydraulic anatomy can be critical for herbaceous monocots in determining limits to plant water use and genotypic response to drought with implications on whole-plant functions and habitat ecology. Supported by the intramural funding of Indian Institute of Science Education and Research (IISER) Pune, India |
Photosynthesis, growth and productivity responses to elevated CO2 in a fast-growing tropical deciduous tree species, Gmelina arborea
Rasineni K G, Guha A and Reddy AR [2007-2011] The photosynthetic response of trees to rising CO2 concentrations largely depends on source-sink relations, in addition to differences in responsiveness by species, genotype, and functional group. In this study, Gmelina arborea, a fast-growing tropical deciduous tree species, was selected to determine the photosynthetic efficiency, growth response and overall source–sink relations under near elevated atmospheric CO2 concentration (460 micro mol mol−1). Net photosynthetic rate of Gmelina was ∼30% higher in plants grown in elevated CO2 compared with ambient CO2-grown plants. The elevated CO2 concentration also had significant effect on photochemical and biochemical capacities evidenced by changes in FV/FM, ABS/CSm, ET0/CSm and RuBPcase activity. The study also revealed that elevated CO2 conditions significantly increased absolute growth rate, above ground biomass and carbon sequestration potential in Gmelina which sequestered ∼2100 g tree−1 carbon after 120 days of treatment when compared to ambient CO2-grown plants. Our data indicate that young Gmelina could accumulate significant biomass and escape acclimatory down-regulation of photosynthesis due to high source-sink capacity even with an increase of 100 micro mol mol−1 CO2. Supported by DBT fund BT/PR6402/BCE/08/416/2005. |
Leaf functional traits and stem-wood characteristics influencing biomass productivity of mulberry (Morus spp. L.) genotypes grown in short rotation coppice system
Guha A and Reddy AR [2009-2012] This study was undertaken to obtain insights into the productivity determinant traits of mulberry (Morus spp. L.), a potential bioenergy tree crop. Our objectives were to identify leaf functional traits and stem wood characteristics that are correlated to biomass yield of mulberry. Based on the growth performance, six mulberry genotypes from different performance groups including high, average and poor performers were selected for the study, along with a reference high-yielding genotype. All trees were cultivated in a short-rotation coppice system under well-irrigated optimum farming conditions. Data were collected on biomass yield along with several leaf-level physiobiochemical characteristics and wood quality parameters. Significant genetic variation was recorded amongst the genotypes in most of the studied parameters. Fifteen out of a total of 22 traits, used in computing correlation coefficient matrix, were found to correlate with aboveground biomass yield. Light-saturated rate of photosynthesis, performance index, leaf nitrogen content, minimum leaf water potential and leaf-specific hydraulic conductance showed strong positive correlation with biomass productivity. Wood density, wood cross-sectional area and fibre cell density exhibited tight correlation with woody biomass yield. In conclusion, the identified 15 characteristics could be useful in the selection of suitable mulberry genotypes for higher biomass yield. Supported by DST funded research project (SP/SO/PS-27/05) |
Ecophysiological and molecular approaches to understand drought tolerance in different mulberry genotypes
Guha A, Rasineni G, Sengupta D and Reddy AR [2007-2012] Our understanding of functional flexibility of photosynthetic system to environmental cues is predominantly confined to unicellular model organisms and few higher systems like arabidopsis, barley, pea, tobacco etc. Unfortunately, trees species have received less attention and little information is available on how rapidly they can cope with energy imbalance in order to protect photosynthetic apparatus under drought-stressed situations. Thus, exploring the dynamics of photosynthesis in trees during drought stress conditions still remain as highly desirable goals. Because long-term acclimation of photosynthesis is largely regulated at post-translational level, integration of proteomic studies with chl a fluorescence analysis can improve our understanding on overall photosynthetic acclimation processes in trees. In general, most photosynthetic genes are down-regulated in a multitude of stress conditions including drought elucidating that down-regulation is part of the general stress response. Nevertheless, hunting drought-induced over expressed proteins that may have a photoprotective function can be helpful in identifying environmental biomarkers and in developing strategies for genetic improvement of higher plants including trees for enhanced drought tolerance. In this study our objectives were (i) to track drought-induced modulations in photosynthetic processes and (ii) to identify the photoacclimation strategies that are preferred by drought tolerant mulberry genotype for sustaining net CO2 fixation during water-limited conditions. Supported by DST funded research project (SP/SO/PS-27/05) |
Oxidative stress responses to water deficit in different mulberry cultivars: Roles of non-enzymatic antioxidants
Guha A, Rasineni G, Sengupta D and Reddy AR [2007-2009] Low-molecular-weight metabolites which include tocopherols, ascorbic acid and glutathione; photo-protective pigments like carotenoids; and cellular osmoprotectants like proline are some of the important non-enzymatic antioxidative defence constituents counteracting negative effects of ROS. It is generally assumed that drought-tolerant genotypes have higher levels of nonenzymatic antioxidants than those of drought-susceptible ones. So far, not many perennial tree crops have been investigated for their non-enzymatic antioxidative protection under water-limited conditions. Based on this lack of comprehensive information, the aim of the present study was to provide a detailed understanding of photosynthetic processes and regulation of non-enzymatic antioxidants across different water stress regimes (with increasing stress intensity) which might explain the mechanisms employed by mulberry to combat oxidative damage and better maintenance of photosynthetic functions under water stress conditions. We hypothesized that oxidative stress linked to drought could differentially alter the levels of non-enzymatic antioxidants in mulberry leaves with increasing stress intensity. Further, we predicted that a relatively tolerant genotype could exhibit higher levels of certain non-enzymatic antioxidative metabolites that could significantly contribute protection against water stress, help to maintain better photosynthetic CO2 fixation and might be used as markers for drought tolerance. Supported by DST funded research project (SP/SO/PS-27/05) |
Morpho-anatomical, and biochemical responses of three mulberry varieties under two light regimes
Guha A, Babu AM, and Kumar JS [2004-2005] Plants respond to variation in light intensity through morphological, anatomical and physiological adjustments that help them to cope with such variations. Mulberry being a heliophylious plant, its growth is very much affected when grown under shaded condition. A shade tolerant variety will be the one which gives higher yield than others when all the genotypes are exposed to the same level of shade stress. Better performance of a variety under shade could be a result of its efficient structural and functional adaptations for maintaining the plant’s metabolic activities under reduced light. Hence, this study has been proposed to understand and compare the morphological, micro morphological, biochemical and anatomical responses of mulberry varieties with known differences in adaptation to shade. The study also examined the plasticity of each attribute under shade in all the three varieties. Supported by the intramural funding of Central Sericultural Research and Training Institute, Mysore |