Chlorophyll Fluorescence on the Fast Timescale.

Chlorophyll fluorescence is a rapid and noninvasive tool used for probing the activity of photosynthesis that can be used in vivo and in the field. It is highly relevant to the demands of high-throughput crop phenotyping and can be automated or manually applied. In this chapter, we describe protocols and advice for making fast timescale fluorescence measurements using handheld equipment in the laboratory or in the field in the context of phenotyping. While interpretation of some measured parameters requires caution for the purpose of identifying underlying mechanisms, we demonstrate this technique is appropriate for some applications where convenience, rapidity, and sensitivity are required.

Chlorophyll Fluorescence on the Fast Timescale.

Chlorophyll fluorescence is a rapid and non-invasive tool used for probing the activity of photosynthesis that can be used in vivo and in the field. It is highly relevant to the demands of high-throughput crop phenotyping and can be automated or manually applied. Here we describe protocols and advice for making fast timescale fluorescence measurements using handheld equipment in the laboratory or in the field. While interpretation of some measured parameters requires caution, we demonstrate that this technique is appropriate for some applications where convenience, rapidity, and sensitivity are required.

Altered gene expression by sedaxane increases PSII efficiency, photosynthesis and growth and improves tolerance to drought in wheat seedlings.

Succinate dehydrogenase inhibitor (SDHI) fungicides have been shown to increase PSII efficiency and photosynthesis under drought stress in the absence of disease to enhance the biomass and yield of winter wheat. However, the molecular mechanism of improved photosynthetic efficiency observed in SDHI-treated wheat has not been previously elucidated. Here we used a combination of chlorophyll fluorescence, gas exchange and gene expression analysis, to aid our understanding of the basis of the physiological responses of wheat seedlings under drought conditions to sedaxane, a novel SDHI seed treatment. We show that sedaxane increased the efficiency of PSII photochemistry, reduced non-photochemical quenching and improved the photosynthesis and biomass in wheat correlating with systemic changes in the expression of genes involved in defense, chlorophyll synthesis and cell wall modification. We applied a coexpression network-based approach using differentially expressed genes of leaves, roots and pregerminated seeds from our wheat array datasets to identify the most important hub genes, with top ranked correlation (higher gene association value and z-score) involved in cell wall expansion and strengthening, wax and pigment biosynthesis and defense. The results indicate that sedaxane confers tolerant responses of wheat plants grown under drought conditions by redirecting metabolites from defense/stress responses towards growth and adaptive development.

Chlorophyll fluorescence parameters allow the rapid detection and differentiation of plant responses in three different wheat pathosystems.

The present study was undertaken to identify chlorophyll fluorescence (CF) parameters that can quantify changes in PSII associated with plant responses in three different wheat pathosystems of foliar, stem-base and ear diseases. The pathosystems included powdery mildew caused by Blumeria graminis, eyespot caused by Oculimacula yallundae or Oculimacula acuformis and Fusarium head blight (FHB) caused by Fusarium culmorum, F. avenaceum or F. langsethiae. Fast CF transients (OJIP) were analysed with the JIP-test to determine changes in PSII photochemistry. Measurements on asymptomatic leaves showed that electron transport related parameters (ETo/RC, ψo and ϕEo) were important to identify varietal differences in resistance to powdery mildew during early stages of infection. The same parameters also allowed differentiation between F. langsethiae and other Fusarium spp. Where infections were caused by the necrotrophic pathogens, Oculimacula spp., F. culmorum or F. avenaceum, changes related to maximum efficiency of PSII photochemistry (Fv'/Fm') as well as flux of dissipated (DIo/RC), trapped (TRo/RC), or absorbed (ABS/RC) energy per active reaction centers were significant in detecting biotic stress and the effectiveness of fungicide treatment for disease control. Our results demonstrated that Fv'/Fm' correlated significantly with visual disease and pathogen DNA of different wheat pathosystems. OJIP was shown as a sensitive technique that can be explored as diagnostic tool in future crop disease management and varietal breeding programs.

Foliar application of isopyrazam and epoxiconazole improves photosystem II efficiency, biomass and yield in winter wheat.

A range of fungicides including epoxiconazole, azoxystrobin and isopyrazam, were applied to winter wheat at GS 31/32 to determine their effect on photosystem II (PSII) efficiency, biomass and yield. Frequent, repeated measurements of chlorophyll fluorescence were carried on plants grown under different water regimes in controlled environment and in the field to establish the transiency of fluorescence changes in relation to fungicide application. Application of the succinate dehydrogenase inhibitor isopyrazam in a mixture with the triazole epoxiconazole increased PSII efficiency associated with a 28% increase in biomass in the controlled environment and 4% increase in grain yield in the field in the absence of disease pressure. Application of isopyrazam and epoxiconazole increased efficiency of PSII photochemistry (Fv'/Fm') as early as 4h following application associated with improved photosynthetic gas exchange and increased rates of electron transport. We reveal a strong, positive relationship between Fv'/Fm' and CO2 assimilation rate, stomatal conductance and transpiration rate in controlled environment and Fv'/Fm' detected just after anthesis on the flag leaf at GS 73 and grain yield in field. We conclude that application of a specific combination of fungicides with positive effects of plant physiology in the absence of disease pressure results in enhanced biomass and yield in winter wheat. Additionally, an accurate and frequent assessment of photosynthetic efficiency of winter wheat plants can be used to predict yield and biomass in the field.

The photoprotective protein PsbS exerts control over CO(2) assimilation rate in fluctuating light in rice.

A direct impact of chloroplastic protective energy dissipation (qE) on photosynthetic CO(2) assimilation has not been shown directly in plants in the absence of photoinhibition. To test this empirically we transformed rice to possess higher (overexpressors, OE) and lower (RNA interference, RNAi) levels of expression of the regulatory psbS gene and analysed CO(2) assimilation in transformants in a fluctuating measurement light regime. Western blots showed a several-fold difference in levels of PsbS protein between RNAi and OE plants with the wild type (WT) being intermediate. At a growth light intensity of 600 µmol m(-2) sec(-1) , the carboxylation capacity, electron transport capacity and dark adapted F(v)/F(m) (ratio of variable to maximum fluorescence) were inhibited in RNAi plants compared with WT and OE. The PsbS content had a significant impact on qE (measured here as non-photochemical quenching, NPQ) but the strongest effect was observed transiently, immediately following the application of light. This capacity for qE was several-fold lower in RNAi plants and significantly higher in OE plants during the first 10 min of illumination. At steady state the differences were reduced: notably at 500 µmol m(-2) sec(-1) all plants had the same NPQ values regardless of PsbS content. During a series of light-dark transitions the induction of CO(2) assimilation was inhibited in OE plants, reducing integrated photosynthesis during the light period. We conclude that the accumulation of PsbS and the resultant qE exerts control over photosynthesis in fluctuating light, showing that optimization of photoprotective processes is necessary for maximum photosynthetic productivity even in the absence of photoinhibitory stress.