Hugh J. Earl

Bio: Hugh J. Earl is an academic researcher from University of Guelph. The author has contributed to research in topics: Chlorophyll fluorescence & Soil water. The author has an hindex of 18, co-authored 40 publications receiving 1310 citations. Previous affiliations of Hugh J. Earl include University of Georgia & Natural Sciences and Engineering Research Council.

Effect of Drought Stress on Leaf and Whole Canopy Radiation Use Efficiency and Yield of Maize

Abstract: Drought stress reduces yield of maize (Zea mays L.) and other grain crops by (i) reducing canopy absorption of incident photosynthetically active radiation (PAR), (ii) reducing radiation use efficiency (RUE), and (iii) reducing harvest index (HI). The primary objective of this work was to quantify yield losses attributable to each of these components for maize exposed to drought stress in a 2-yr field study. A second objective was to examine the relationship between RUE at the single leaf level (estimated using chlorophyll fluorescence techniques) and RUE at the whole crop level. Two levels of soil water defidt and a control treatment were established using drip tape irrigation, and dry matter harvests were taken at midseason and at physiological maturity. Mild and severe water stress treatments reduced final grain yield by 63 and 85%, respectively, in 2000, and by 13 and 26%, respectively, in 2001. Reduction of intercepted PAR (IPAR) was generally a very minor yield loss component. Yield losses attributable to reduced RUE and reduced HI were of similar magnitude. Weekly chlorophyll fluorescence measurements were used to estimate the average quantum efficiency of photosystem II at a photosynthetic photon flux density of 1200 μmol m -2 s -1 (Φ II1200 ) for each plot. Crop dry matter accumulation was not linearly related to IPAR, due to decreased RUE in the water stress treatments. However, the linear relationship was restored when daily IPAR was multiplied by the current estimate of Φ II1200 , suggesting that Φ II1200 can be used as an indicator of whole-crop RUE.

Physiological Limitations to Photosynthetic Carbon Assimilation in Cotton under Water Stress

Abstract: Water stress may reduce leaf net photosynthetic carbon assimilation (AN) through both stomatal effects, which reduce the leaf internal CO 2 concentration (C 1 ), and nonstomatal effects, which result in reduced AN at a given level of C 1 . However, the leaf gas exchange techniques used to calculate C 1 are susceptible to important artifacts when applied to water-stressed leaves, making such C 1 estimates unreliable. As an alternative to C 1 , the CO 2 concentration in the chloroplast (C C ) can be calculated from simultaneous measurements of AN from gas exchange measurements, and the thylakoid electron flux from chlorophyll fluorometry. This permits diffusional effects (stomatal plus mesophyll limitations to CO 2 diffusion) to be differentiated from chloroplast-level effects. We used this method to investigate physiological restrictions to photosynthesis in leaves of water stressed cotton (Gossypium hirsutum L.) plants in a series of greenhouse experiments. A null-balance lysimeter was used to slowly induce four distinct levels of water stress. Combined leaf gas exchange/chlorophyll fluorescence measurements differentiated the treatments more effectively than gas exchange measurements alone. All treatments reduced C C , but only the two most severe stress treatments significantly increased nondiffusional restrictions, detectable as a reduction in the slope of AN on C C . In a second experiment, recovery of leaf photosynthesis was determined 24 and 48 h after relief of a severe stress by rewatering. Recovery of the A N /C C relationship was substantial but incomplete after 24 h and did not recover further by 48 h after rewatering, indicating lasting chloroplast-level injury as a result of the stress. Similar experiments should be conducted under field conditions to determine if water stress results in irreversible chloroplast-level injury in field-grown cotton.

Genotypic Variation for Three Physiological Traits Affecting Drought Tolerance in Soybean

Abstract: Three physiological traits that may affect performance of soybean [Glycine max (L.) Merr.] when soil water availability is limiting are (i) water use efficiency (WUE), (ii) regulation of whole plant water use in response to soil water content, and (iii) leaf epidermal conductance (ge) when stomata are closed. Six soybean plant introductions (PIs), eight breeding lines derived from them, and nine cultivars were compared for variability in these three traits during vegetative growth in two greenhouse studies. In the first experiment, whole plant water use, normalized both to plant size and evaporative demand (the normalized transpiration ratio, NTR), was monitored during a 10-d cycle of gradually increasing drought stress and then for an additional 2 d following rewatering. The critical soil water content at which each plant began to reduce its water use (FTSWC), was determined. The WUE was estimated as the ratio of total plant dry weight to total water used. In the second experiment, ge was determined for these same 23 genotypes by measuring leaf water vapor exchange after a 36-h dark adaptation. Substantial variation was found among genotypes for WUE, FTSWC, ge, and also the extent to which NTR recovered on rewatering. Generally, adapted cultivars had greater WUE and lower ge than did PIs. However, PI 471938 and its progeny N98-7264 were clear exceptions to this trend. An unexpected finding was that WUE was significantly negatively correlated with ge across genotypes.

Maize Leaf Absorptance of Photosynthetically Active Radiation and Its Estimation Using a Chlorophyll Meter

Abstract: Recent evidence suggests that chlorophyll fluorescence techniques can be used to determine photosynthetic rates of maize (Zea mays L.) leaves much more rapidly than is possible with conventional gas exchange methods. However, the accuracy of such measurements depends directly on the accuracy with which leaf absorptance of incident photons in the PAR (photosynthetically active radiation) region can be estimated. Our objectives were (i) to monitor changes in leaf absorptance in a typical maize crop over the growing season, and (ii) to determine if absorptance of maize leaves could be accurately estimated with a SPAD 502 hand-held chlorophyll meter (Minolta Corporation, Ramsey, NJ). Leaves from six leaf positions were harvested on eleven dates between 55 and 134 d after planting. An integrating sphere and field-portable spectroradiometer were used to measure reflectance and transmittance of leaf tissue samples at 5-nm intervals between 400 and 700 nm, and a SPAD value was also determined for each sample. Greening of young leaves was associated with decreases in both reflectance and transmittance in the middle wavelengths of the PAR region, while leaf senescence was associated with increases in both reflectance and transmittance, primarily in the longer wavelengths of the PAR region. Leaf absorptance of incident photons in the PAR region by healthy, fully expanded leaves ranged from approximately 0.88 to 0.91; much lower values were observed for young, chlorotic, or senescing leaves. Regression analysis revealed a strong relationship (r 2 = 0.98) between leaf absorptance and SPAD value, suggesting that the SPAD meter could be used to provide a rapid estimate of leaf absorptance in the field.

A Physio-Morphological Trait-Based Approach for Breeding Drought Tolerant Wheat.

Abstract: In the past, there have been drought events in different parts of the world, which have negatively influenced the productivity and production of various crops including wheat (Triticum aestivum L.), one of the world's three important cereal crops. Breeding new high yielding drought-tolerant wheat varieties is a research priority specifically in regions where climate change is predicted to result in more drought conditions. Commonly in breeding for drought tolerance, grain yield is the basis for selection, but it is a complex, late-stage trait, affected by many factors aside from drought. A strategy that evaluates genotypes for physiological responses to drought at earlier growth stages may be more targeted to drought and time efficient. Such an approach may be enabled by recent advances in high-throughput phenotyping platforms (HTPPs). In addition, the success of new genomic and molecular approaches rely on the quality of phenotypic data which is utilized to dissect the genetics of complex traits such as drought tolerance. Therefore, the first objective of this review is to describe the growth-stage based physio-morphological traits that could be targeted by breeders to develop drought-tolerant wheat genotypes. The second objective is to describe recent advances in high throughput phenotyping of drought tolerance related physio-morphological traits primarily under field conditions. We discuss how these strategies can be integrated into a comprehensive breeding program to mitigate the impacts of climate change. The review concludes that there is a need for comprehensive high throughput phenotyping of physio-morphological traits that is growth stage-based to improve the efficiency of breeding drought-tolerant wheat.

Plant drought stress: effects, mechanisms and management

Abstract: Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in crop yield probably exceeds losses from all other causes, since both the severity and duration of the stress are critical. Here, we have reviewed the effects of drought stress on the growth, phenology, water and nutrient relations, photosynthesis, assimilate partitioning, and respiration in plants. This article also describes the mechanism of drought resistance in plants on a morphological, physiological and molecular basis. Various management strategies have been proposed to cope with drought stress. Drought stress reduces leaf size, stem extension and root proliferation, disturbs plant water relations and reduces water-use efficiency. Plants display a variety of physiological and biochemical responses at cellular and whole-organism levels towards prevailing drought stress, thus making it a complex phenomenon. CO2 assimilation by leaves is reduced mainly by stomatal closure, membrane damage and disturbed activity of various enzymes, especially those of CO2 fixation and adenosine triphosphate synthesis. Enhanced metabolite flux through the photorespiratory pathway increases the oxidative load on the tissues as both processes generate reactive oxygen species. Injury caused by reactive oxygen species to biological macromolecules under drought stress is among the major deterrents to growth. Plants display a range of mechanisms to withstand drought stress. The major mechanisms include curtailed water loss by increased diffusive resistance, enhanced water uptake with prolific and deep root systems and its efficient use, and smaller and succulent leaves to reduce the transpirational loss. Among the nutrients, potassium ions help in osmotic adjustment; silicon increases root endodermal silicification and improves the cell water balance. Low-molecular-weight osmolytes, including glycinebetaine, proline and other amino acids, organic acids, and polyols, are crucial to sustain cellular functions under drought. Plant growth substances such as salicylic acid, auxins, gibberrellins, cytokinin and abscisic acid modulate the plant responses towards drought. Polyamines, citrulline and several enzymes act as antioxidants and reduce the adverse effects of water deficit. At molecular levels several drought-responsive genes and transcription factors have been identified, such as the dehydration-responsive element-binding gene, aquaporin, late embryogenesis abundant proteins and dehydrins. Plant drought tolerance can be managed by adopting strategies such as mass screening and breeding, marker-assisted selection and exogenous application of hormones and osmoprotectants to seed or growing plants, as well as engineering for drought resistance.

Improving Photosynthetic Efficiency for Greater Yield

Abstract: Increasing the yield potential of the major food grain crops has contributed very significantly to a rising food supply over the past 50 years, which has until recently more than kept pace with rising global demand. Whereas improved photosynthetic efficiency has played only a minor role in the remarkable increases in productivity achieved in the last half century, further increases in yield potential will rely in large part on improved photosynthesis. Here we examine inefficiencies in photosynthetic energy transduction in crops from light interception to carbohydrate synthesis, and how classical breeding, systems biology, and synthetic biology are providing new opportunities to develop more productive germplasm. Near-term opportunities include improving the display of leaves in crop canopies to avoid light saturation of individual leaves and further investigation of a photorespiratory bypass that has already improved the productivity of model species. Longer-term opportunities include engineering into plants carboxylases that are better adapted to current and forthcoming CO2 concentrations, and the use of modeling to guide molecular optimization of resource investment among the components of the photosynthetic apparatus, to maximize carbon gain without increasing crop inputs. Collectively, these changes have the potential to more than double the yield potential of our major crops.

Crop Production under Drought and Heat Stress: Plant Responses and Management Options

Abstract: Abiotic stresses are one of the major constraints to crop production and food security worldwide. The situation has aggravated due to the drastic and rapid changes in global climate. Heat and drought stress are undoubtedly the two most important stresses having huge impact on growth and productivity of the crops. It is very important to understand the physiological, biochemical and ecological interventions related to these stresses for better management. A wide range of plant responses to these stresses could be generalized into morphological, physiological and biochemical responses. Interestingly, this review provides a detailed account of plant responses to heat and drought stresses with special focus on highlighting the commonalities and differences. Crop growth and yields are negatively affected by sub-optimal water supply and abnormal temperatures due to physical damages, physiological disruptions and biochemical changes. Both these stresses have multi-lateral impacts and therefore, complex in mechanistic action. A better understanding of plant responses to these stresses has pragmatic implication for remedies and management. A comprehensive account of conventional as well as modern approaches to deal with heat and drought stresses have also been presented here. A side-by-side critical discussion on salient responses and management strategies for these two important abiotic stresses provides a unique insight into the phenomena. A holistic approach taking into account the different management options to deal with heat and drought stress simultaneously could be a win-win approach in future.

Mesophyll conductance to CO2: current knowledge and future prospects

Abstract: During photosynthesis, CO2 moves from the atmosphere (Ca) surrounding the leaf to the sub-stomatal internal cavi- ties (Ci) through stomata, and from there to the site of carboxylation inside the chloroplast stroma (Cc) through the leaf mesophyll. The latter CO2 diffusion component is called mesophyll conductance (gm), and can be divided in at least three components, that is, conductance through intercellular air spaces (gias), through cell wall (gw) and through the liquid phase inside cells (gliq). A large body of evidence has accumulated in the past two decades indicat- ing that gm is sufficiently small as to significantly decrease Cc relative to Ci, therefore limiting photosynthesis. More- over, gm is not constant, and it changes among species and in response to environmental factors. In addition, there is now evidence that gliq and, in some cases, gw, are the main determinants of gm. Mesophyll conductance is very dynamic, changing in response to environmental variables as rapid or even faster than stomatal conductance (i.e. within seconds to minutes). A revision of current knowl- edge on gm is presented. Firstly, a historical perspective is given, highlighting the founding works and methods, fol- lowed by a re-examination of the range of variation of gm among plant species and functional groups, and a revision of the responses of gm to different external (biotic and abiotic) and internal (developmental, structural and meta- bolic) factors. The possible physiological bases for gm, including aquaporins and carbonic anhydrases, are dis- cussed. Possible ecological implications for variable gm are indicated, and the errors induced by neglecting gm when interpreting photosynthesis and carbon isotope discrimi- nation models are highlighted. Finally, a series of research priorities for the near future are proposed.