LED and HPS Supplementary Light Differentially Affect Gas Exchange in Tomato Leaves.

Using light emitting diodes (LED) instead of conventionally used high pressure sodium (HPS) lamps as a supplemental light source in greenhouses results in a higher efficacy (µmol light per J electricity) and makes it possible to customize the light spectrum. To explore the effects of LED and HPS on gas exchange, thermal relations, photosynthesis, and water status of young tomato plants, seven genotypes were grown in a greenhouse under LED (95% red, 5% blue) or HPS lamps in four experiments differing in the fraction of lamp light over natural light. HPS lights emit a broader spectrum of red (40%), green-yellow (50%), blue (5%), and far-red (5%) and a substantial amount of infrared radiation (heat). Young tomato plants grown under LED showed lower leaf temperature and higher stomatal density, stomatal conductance (gs) and transpiration rate (E) than plants grown under HPS; this may be due to the different supplemental light spectrum. The young plants grown under LED tended to have increased photosynthetic capacity. Furthermore, the water stress indices CWSI and IG, which were obtained using thermal imaging, were positively correlated with gas exchange-derived gs and E, putting forward the use of thermal imaging for the phenotyping of transpiration. Under LED light, photosynthetic gas exchange was generally increased, which agreed with the water stress indices. The extent of this increase was genotype-dependent. All differences between LED and HPS were smaller in the experiments where the fraction of lamp light over natural light was smaller.

Natural variation of photosynthetic efficiency in Arabidopsis thaliana accessions under low temperature conditions.

Low, but non-freezing, temperatures have negative effects on plant growth and development. Despite some molecular signalling pathways being known, the mechanisms causing different responses among genotypes are still poorly understood. Photosynthesis is one of the processes that are affected by low temperatures. Using an automated phenotyping platform for chlorophyll fluorescence imaging the steady state quantum yield of photosystem II (PSII) electron transport (ΦPSII ) was measured and used to quantify the effect of moderately low temperature on a population of Arabidopsis thaliana natural accessions. Observations were made over the course of several weeks in standard and low temperature conditions and a strong decrease in ΦPSII upon the cold treatment was found. A genome wide association study identified several quantitative trait loci (QTLs) that are associated with changes in ΦPSII in low temperature. One candidate for a cold specific QTL was validated with a mutant analysis to be one of the genes that is likely involved in the PSII response to the cold treatment. The gene encodes the PSII associated protein PSB27 which has already been implicated in the adaptation to fluctuating light.

Chlorophyll fluorescence imaging reveals genetic variation and loci for a photosynthetic trait in diploid potato.

Physiology and genetics are tightly interrelated. Understanding the genetic basis of a physiological trait such as the quantum yield of the photosystem II, or photosynthetic responses to environmental changes will benefit the understanding of these processes. By means of chlorophyll fluorescence (CF) imaging, the quantum yield of photosystem II can be determined rapidly, precisely and non-invasively. In this article, the genetic control and variation in the steady-state quantum yield of PSII (ΦPSII ) is analyzed for diploid potato plants. Current progress in potato research and breeding is slow due to high levels of heterozygosity and complexity of tetraploid genetics. Diploid potatoes offer the possibility of overcoming this problem and advance research for one of the globally most important staple foods. With the help of a diploid genetic mapping population two genetic loci that were strongly associated with differences in ΦPSII were identified. This is a proof of principle that genetic analysis for ΦPSII can be done on potato. The effects of three different stress conditions that are important in potato cultivation were also tested: salt stress, low temperature and deficiency in the macronutrient phosphate. For the last two stresses, significant decreases in photosynthetic activity could be shown, revealing potential for stress detection with CF based tools. In general, our findings show the potential of high-throughput phenotyping for physiological research and breeding in potato.

Metabolic and diffusional limitations of photosynthesis in fluctuating irradiance in Arabidopsis thaliana.

A better understanding of the metabolic and diffusional limitations of photosynthesis in fluctuating irradiance can help identify targets for improving crop yields. We used different genotypes of Arabidopsis thaliana to characterise the importance of Rubisco activase (Rca), stomatal conductance (gs), non-photochemical quenching of chlorophyll fluorescence (NPQ) and sucrose phosphate synthase (SPS) on photosynthesis in fluctuating irradiance. Leaf gas exchange and chlorophyll fluorescence were measured in leaves exposed to stepwise increases and decreases in irradiance. rwt43, which has a constitutively active Rubisco enzyme in different irradiance intensities (except in darkness), showed faster increases than the wildtype, Colombia-0, in photosynthesis rates after step increases in irradiance. rca-2, having decreased Rca concentration, showed lower rates of increase. In aba2-1, high gs increased the rate of change after stepwise irradiance increases, while in C24, low gs tended to decrease it. Differences in rates of change between Colombia-0 and plants with low levels of NPQ (npq1-2, npq4-1) or SPS (spsa1) were negligible. In Colombia-0, the regulation of Rubisco activation and of gs were therefore limiting for photosynthesis in fluctuating irradiance, while levels of NPQ or SPS were not. This suggests Rca and gs as targets for improvement of photosynthesis of plants in fluctuating irradiance.

High throughput screening with chlorophyll fluorescence imaging and its use in crop improvement.

Marker assisted plant breeding is a powerful technique for targeted crop improvement in horticulture and agriculture. It depends upon the correlation of desirable phenotypic characteristics with specific genetic markers. This can be determined by statistical models that relate the variation in the value of genetic markers to variation in phenotypic traits. It therefore depends upon the convergence of three technologies; the creation of genetically characterised (and thus marked) populations, high throughput screening procedures, and statistical procedures. While a large number of high throughput screening technologies are available, real-time screening techniques are usually based on some kind of imaging technologies, such as chlorophyll fluorescence imaging, that offers physiological data that are eminently suitable as a quantitative trait for genetic mapping.

Relationships between growth, growth response to nutrient supply, and ion content using a recombinant inbred line population in Arabidopsis.

Growth is an integrative trait that responds to environmental factors and is crucial for plant fitness. A major environmental factor influencing plant growth is nutrient supply. In order to explore this relationship further, we quantified growth-related traits, ion content, and other biochemical traits (protein, hexose, and chlorophyll contents) of a recombinant inbred line population of Arabidopsis (Arabidopsis thaliana) grown on different levels of potassium and phosphate. Performing an all subsets multiple regression analyses revealed a link between growth-related traits and mineral nutrient content. Based on our results, up to 85% of growth variation can be explained by variation in ion content, highlighting the importance of ionomics for a broader understanding of plant growth. In addition, quantitative trait loci (QTLs) were detected for growth-related traits, ion content, further biochemical traits, and their responses to reduced supplies of potassium or phosphate. Colocalization of these QTLs is explored, and candidate genes are discussed. A QTL for rosette weight response to reduced potassium supply was identified on the bottom of chromosome 5, and its effects were validated using selected near isogenic lines. These lines retained over 20% more rosette weight in reduced potassium supply, accompanied by an increase in potassium content in their leaves.