Sarita Keski-Saari

Impacts of increasing ozone on Indian plants

Increasing anthropogenic and biogenic emissions of precursor compounds have led to high tropospheric ozone concentrations in India particularly in Indo-Gangetic Plains, which is the most fertile and cultivated area of this rapidly developing country. Current ozone risk models, based on European and North American data, provide inaccurate estimations for crop losses in India. During the past decade, several ozone experiments have been conducted with the most important Indian crop species (e.g. wheat, rice, mustard, mung bean). Experimental work started in natural field conditions around Varanasi area in early 2000s, and the use of open top chambers and EDU (ethylene diurea) applications has now facilitated more advanced studies e.g. for intra-species sensitivity screening and mechanisms of tolerance. In this review, we identify and discuss the most important gaps of knowledge and future needs of action, e.g. more systematic nationwide monitoring for precursor and ozone formation over Indian region.

Searching for common responsive parameters for ozone tolerance in 18 rice cultivars in India: Results from ethylenediurea studies

Eighteen rice (Oryza sativa) cultivars were screened for ozone (O3) tolerance and for the most responsive parameters with ethylenediurea (EDU) treatments at two experimental sites experiencing high ambient O3 conditions in the Indo-Gangetic Plains (IGP) of India. EDU was applied at 15 day intervals until the final harvest phase as a foliar spray at 300 ppm in order to protect the plants from the adverse effects of O3. Antioxidant activity, malondialdehyde content (MDA), chlorophyll content, gas exchange, and chlorophyll fluorescence (Fv/Fm) at the vegetative and flowering phases and harvest-related parameters were studied, for a total of 24 parameters. Seven of the studied cultivars had higher than average grainweightplant(-1) in all site and treatment combinations and can be recommended for cultivation in areas suffering from high O3 concentrations. The most responsive parameters with EDU treatment in high O3 across all cultivars were superoxide dismutase (SOD) and catalase (CAT) activities, the contents of oxidised (GSSG) and reduced (GSH) glutathione and MDA, and shoot weight plant(-1). These results indicated that the O3 scavenging activity of EDU is mediated through an antioxidant defence system rather than a direct effect on physiological parameters, such as photosynthesis and stomatal conductance.

Ozone affects ascorbate and glutathione biosynthesis as well as amino acid contents in three Euramerican poplar genotypes

Ozone is an air pollutant that causes oxidative stress by generation of reactive oxygen species (ROS) within the leaf. The capacity to detoxify ROS and repair ROS-induced damage may contribute to ozone tolerance. Ascorbate and glutathione are known to be key players in detoxification. Ozone effects on their biosynthesis and on amino acid metabolism were investigated in three Euramerican poplar genotypes (Populus deltoides Bartr. × Populus nigra L.) differing in ozone sensitivity. Total ascorbate and glutathione contents were increased in response to ozone in all genotypes, with the most resistant genotype (Carpaccio) showing an increase of up to 70%. Reduced ascorbate (ASA) concentration at least doubled in the two most resistant genotypes (Carpaccio and Cima), whereas the most sensitive genotype (Robusta) seemed unable to regenerate ASA from oxidized ascorbate (DHA), leading to an increase of 80% of the oxidized form. Increased ascorbate (ASA + DHA) content correlated with the increase in gene expression in its biosynthetic pathway, especially the putative gene of GDP-l-galactose phosphorylase VTC2. Increased cysteine availability combined with increased expression of γ-glutamylcysteine synthetase (GSH1) and glutathione synthetase (GSH2) genes allows higher glutathione biosynthesis in response to ozone, particularly in Carpaccio. In addition, ozone caused a remobilization of amino acids with a decreased pool of total amino acids and an increase of Cys and putrescine, especially in Carpaccio. In addition, the expression of genes encoding threonine aldolase was strongly induced only in the most tolerant genotype, Carpaccio. Reduced ascorbate levels could partly explain the sensitivity to ozone for Robusta but not for Cima. Reduced ascorbate level alone is not sufficient to account for ozone tolerance in poplar, and it is necessary to consider several other factors including glutathione content.

Effects of Overproduction of Condensed Tannins and Elevated Temperature on Chemical and Ecological Traits of Genetically Modified Hybrid Aspens (Populus tremula × P. tremuloides)

Gene transfer techniques offer new possibilities to study regulation of phenolic pathways and the defensive role of phenolics. Hybrid aspen lines (Populus tremula × tremuloides) that overexpress the PtMYB134 transcription factor were used to study the effects of condensed tannin production on plant physiology and plant defenses. The MYB134 protein activates all the known genes of the biosynthetic pathway for condensed tannins (CTs), so overexpression of MYB134 was expected to increase CT concentration in all tissues of the plants. Two out of three MYB134 overexpression lines (46 and 54) accumulated high levels of CTs and (+)-catechin, with a concomitant decrease in the levels of salicylates, but one transgenic line, MYB 61, failed to overproduce CTs. The concentrations of phenolic compounds generally were lower in the aspen leaves grown under elevated temperature than in those grown under ambient temperature. A specialist leaf beetle, Phratora vitellinae (Coleoptera: Chrysomelidae), was chosen to examine how over-expression of MYB134 and elevated temperature affect the food choice of a beetle adapted to feed on leaves rich in salicylates but containing little CT. Specialist beetles preferred the leaves grown at ambient temperatures possibly because these leaves had higher concentrations of salicylates, which are feeding stimulants. Beetles also preferred MYB line 61, which contained a normal level of CT but a slightly elevated level of salicylates. Our results show that transgenic plants are powerful tools, but that enhancing one secondary pathway may lead to unexpected effects on other pathways, and thus impact characteristics such as plant resistance against herbivores, especially under changing climatic conditions.

Boron nutrition affects the carbon metabolism of silver birch seedlings

Boron (B) is an essential micronutrient whose deficiency is common both in agriculture and in silviculture. Boron deficiency impairs the growth of plants and affects many metabolic processes like carbohydrate metabolism. Boron deficiency and also excess B may decrease the sink demand by decreasing the growth and sugar transport which may lead to the accumulation of carbohydrates and down-regulation of photosynthesis. In this study, we investigated the effects of B nutrition on the soluble and storage carbohydrate concentrations of summer leaves and autumn buds in a deciduous tree species, Betula pendula Roth. In addition, we investigated the changes in the pools of condensed tannins between summer and autumn harvests. One-year-old birch seedlings were fertilized with a complete nutrient solution containing three different levels of B: 0, 30 and 100% of the standard level for complete nutrient solution. Half of the seedlings were harvested after summer period and another half when leaves abscised. The highest B fertilization level (B100) caused an accumulation of starch and a decrease in the concentrations of hexoses (glucose and fructose) in summer leaves, whereas in the B0 seedlings, hexoses (mainly glucose) accumulated and starch decreased. These changes in carbohydrate concentrations might be related to the changes in the sink demand since the autumn growth was the smallest for the B100 seedlings and largest for the B30 seedlings that did not accumulate carbohydrates. The autumn buds of B30 seedlings contained the lowest levels of glucose, glycerol, raffinose and total polyols, which was probably due to the dilution effect of the deposition of other substances like phenols. Condensed tannins accumulated in high amounts in the birch stems during the hardening of seedlings and the largest accumulation was detected in the B30 treatment. Our results suggest that B nutrition of birch seedlings affects the carbohydrate and phenol metabolism and may play an important role in the hardening process of the seedlings.

Restoration of secondary metabolism in birch seedlings relieved from PAL-inhibitor

Phenylalanine ammonia lyase (PAL) plays a key role in phenylpropanoid metabolism, catalyzing the deamination of phenylalanine (Phe) to form trans-cinnamic acid. Inhibitors of PAL have been used to study the physiological role of the different compounds derived from trans-cinnamic acid, and to test theories about a trade-off between growth and defence in plants. In a previous study with birch (Betula pubescens Ehrh.) seedlings, the PAL inhibitor 2-aminoindane-2-phosphonic acid monohydrate (AIP) caused an accumulation of Phe and a strong decrease in the quantity of simple phenolics, soluble condensed tannins and growth, whereas flavonol glycosides were generally not affected. The present study demonstrates restoration of secondary metabolism in the previously AIP treated birch seedlings. Our results indicate that Phe accumulated during PAL inhibition could be partly used to increase the content of the phenolic acids, flavan-3-ols and to some extent the soluble condensed tannins. Seedling growth also increased when the supply of PAL inhibitor ceased. We thereby show that the inhibition of PAL by AIP in vivo is reversible, at least for moderate AIP concentrations and the rate of restoration is dependent on the inhibitor concentration.

Thermal and hyperspectral imaging for Norway spruce (Picea abies) seeds screening

Hyperspectral and thermal lifetime imaging were used to assess spruce seed quality.Viable, empty and infested seeds were resolved with high accuracy with both methods.400-1000nm data was not as informative as 1000-2500nm and thermal decay data.Classification of 93% accuracy was obtained using three wavelengths in SWIR range.The results suggest that high-throughput spruce seed quality testing is possible. The quality of seeds used in agriculture and forestry is tightly linked to the plant productivity. Thus, the development of high-throughput nondestructive methods to classify the seeds is of prime interest. Visible and near infrared (VNIR, 400-1000nm range) and short-wave infrared (SWIR, 1000-2500nm range) hyperspectral imaging techniques were compared to an infrared lifetime imaging technique to evaluate Norway spruce (Picea abies (L.) Karst.) seed quality. Hyperspectral image and thermal data from 1606 seeds were used to identify viable seeds, empty seeds and seeds infested by Megastigmus sp. larvae. The spectra of seeds obtained from hyperspectral imaging, especially in SWIR range and the thermal signal decay of seeds following an exposure to a short light pulse were characteristic of the seed status. Classification of the seeds to three classes was performed with a Support Vector Machine (nu-SVM) and sparse logistic regression based feature selection. Leave-One-Out classification resulted to 99% accuracy using either thermal or spectral measurements compared to radiography classification. In spectral imaging case, all important features were located in the SWIR range. Furthermore, the classification results showed that accurate (93.8%) seed sorting can be achieved with a simpler method based on information from only three hyperspectral bands at 1310nm, 1710nm and 1985nm locations, suggesting a possibility to build an inexpensive screening device. The results indicate that combined classification methods with hyperspectral imaging technique and infrared lifetime imaging technique constitute practically high performance fast and non-destructive techniques for high-throughput seed screening.

Low vapour pressure deficit affects nitrogen nutrition and foliar metabolites in silver birch

Highlight Changing leaf to air vapour pressure deficit (VPD) modifies primary metabolism of plants as demonstrated by excess starch synthesis and reduced amino acid, chlorophyll, and nitrogen concentrations in leaves under low VPD.

Low vapor pressure deficit reduces glandular trichome density and modifies the chemical composition of cuticular waxes in silver birch leaves

Cuticular wax layer is the first barrier against the outside environment and the first defense encountered by herbivores and pathogens. The effects of environmental factors on cuticular chemistry, and on the formation of glandular trichomes that account for the storage and secretion of lipophilic compounds to the leaf surface are poorly understood. Low vapor pressure deficit (VPD) has shown to reduce the nitrogen (N) status of plants. Thus, we studied the effects of elevated air humidity, indicated as VPD, and the effect of N fertilization on cuticular waxes and glandular trichome density in silver birch (Betula pendula Roth). Experiments were carried out in growth chambers with juvenile plants and in a long-term field experiment with older trees. Low VPD reduced the glandular trichome density in both experiments, in chamber and in field. The contents of the major triterpenoid and flavonoid aglycones correlated positively with glandular trichome density, which supports the role of trichomes in the exudation of secondary compounds to the leaf surface. A closer examination of the cuticular wax chemistry in the chamber experiment revealed that low VPD and N supply affected the composition of cuticular waxes, but not the total wax content. The deposition of different wax compounds followed a co-ordinated pattern in birch leaves, but different compound groups varied in their responses to N fertilization and low VPD. Low VPD reduced the hydrophobicity of cuticular waxes, as demonstrated by lower alkane content and less hydrophobic flavonoid profile in low VPD than in high VPD. Reduced hydrophobicity of the wax layer is presumed to increase leaf wettability. Together with reduced trichome density in low VPD it may enhance the susceptibility of trees to fungal pathogens and herbivores. High N supply under low VPD reduced the effect of low VPD on the cuticular wax composition. Total fatty acid content and the expression of β-amyrin synthase were lower under high N supply than under moderate N supply irrespective of VPD treatment. Nitrogen availability and decreasing VPD will modify leaf surface properties in silver birch and thereby affect tree defence against abiotic and biotic stress factors that emerge under climate change.

Intrapopulation Genotypic Variation of Foliar Secondary Chemistry during Leaf Senescence and Litter Decomposition in Silver Birch (Betula pendula)

Abundant secondary metabolites, such as condensed tannins, and their interpopulation genotypic variation can remain through plant leaf senescence and affect litter decomposition. Whether the intrapopulation genotypic variation of a more diverse assortment of secondary metabolites equally persists through leaf senescence and litter decomposition is not well understood. We analyzed concentrations of intracellular phenolics, epicuticular flavonoid aglycones, epicuticular triterpenoids, condensed tannins, and lignin in green leaves, senescent leaves and partly decomposed litter of silver birch, Betula pendula. Broad-sense heritability (H2) and coefficient of genotypic variation (CVG) were estimated for metabolites in senescent leaves and litter using 19 genotypes selected from a B. pendula population in southern Finland. We found that most of the secondary metabolites remained through senescence and decomposition and that their persistence was related to their chemical properties. Intrapopulation H2 and CVG for intracellular phenolics, epicuticular flavonoid aglycones and condensed tannins were high and remarkably, increased from senescent leaves to decomposed litter. The rank of genotypes in metabolite concentrations was persistent through litter decomposition. Lignin was an exception, however, with a diminishing genotypic variation during decomposition, and the concentrations of lignin and condensed tannins had a negative genotypic correlation in the senescent leaves. Our results show that secondary metabolites and their intrapopulation genotypic variation can for the most part remain through leaf senescence and early decomposition, which is a prerequisite for initial litter quality to predict variation in litter decomposition rates. Persistent genotypic variation also opens an avenue for selection to impact litter decomposition in B. pendula populations through acting on their green foliage secondary chemistry. The negative genotypic correlations and diminishing heritability of lignin concentrations may, however, counteract this process.