Seasonal and diurnal variations in soil respiration rates at a treeline ecotone and a lower distribution limit of subalpine forests

This study examined the seasonal and diurnal variations in soil respiration rates (RS) during a growing season at the treeline ecotone (2,800 m) and the lower distribution limit (1,600 m) of subalpine forests on a volcanic mountain in Japan. The aboveground biomass, the total RS during the growing season, and the RS per day during the growing season were lower at 2,800 m than those at 1,600 m. Seasonal RS variations positively correlated with those of soil and air temperatures at both elevations, and this tendency was more apparent at 1,600 m than 2,800 m. The mean volumetric soil water content (WS) during the growing season was much lower at 2,800 m than 1,600 m because of the scoria substrate at 2,800 m. The monthly mean diel cycle of RS was positively correlated with the soil temperature at each elevation every month, whereas that at 1,600 m was negatively correlated with that of the WS. The RS at 2,800 m decreased during the daytime especially in August, despite no changes in the WS. The decrease in RS after precipitation at 1,600 m was higher than that at 2,800 m. Seasonal and diurnal RS variations could be reproduced from soil and air temperatures, and WS. Estimating soil respiration rate from these variables will help understand the future carbon budget of forests due to global warming.

Diurnal changes in starch molecular structures and expression profiles of starch biosynthesis enzymes in rice developing seeds

The diurnal changes in the expression profiles of starch synthesis related enzymes (SSREs) has been previously studied in transitory starches, while its influences on storage starch molecular structures in the rice endosperm during seed development have not been elucidated. In this study, the changes in the transcript levels of starch synthesis related genes (SSRGs), the protein abundances and enzyme activities of SSREs as well as starch molecular structures in rice endosperm at 10 days after flowering (DAF) over the diurnal cycle were analyzed. It was found that the expression profiles of SSRG and the protein contents of SSREs displayed different diurnal patterns between two indica rice varieties with medium- and high-amylose content (AC), respectively. The expression levels of SSRGs were higher in the light time, and most SSREs also accumulated during this period except debranching enzymes. Amylose synthesis displayed distinct diurnal patterns in two rice varieties, which is attributed to the diurnal changes in the protein content of granule-bound starch synthase I (GBSSI), but amylopectin chain-length distributions (CLDs) remained unaltered due to its vast numbers of branches. The results provide the first step to understand the roles of each enzyme isoform involved in starch synthesis in response to diurnal regulation in rice endosperm.

Leaf water potential of field crops estimated using NDVI in ground-based remote sensing-opportunities to increase prediction precision

Remote-sensing using normalized difference vegetation index (NDVI) has the potential of rapidly detecting the effect of water stress on field crops. However, this detection has typically been accomplished only after the stress effect led to significant changes in crop green biomass, leaf area index, angle and position, and few studies have attempted to estimate the uncertainties of the regression models. These have limited the informed interpretation of NDVI data in agricultural applications. We built a ground-based sensing cart and used it to calibrate the relationships between NDVI and leaf water potential (LWP) for wheat, corn, and cotton growing under field conditions. Both the methods of ordinary least-squares (OLS) and weighted least-squares (WLS) were employed in data analysis, and measurement errors in both LWP and NDVI were considered. We also used statistical resampling to test the effect of measurement errors of LWP on the uncertainties of model coefficients. Our data showed that obtaining a high value of the coefficient of determination did not guarantee a high prediction precision in the obtained regression models. Large prediction uncertainties were estimated for all three crops, and the regressions obtained were not always significant. The best models were obtained for cotton with a prediction uncertainty of 27%. We found that considering measurement errors for both LWP and NDVI led to reduced uncertainties in model coefficients. Also, reducing the sample size of LWP measurement led to significantly increased uncertainties in the coefficients of the linear models describing the LWP-NDVI relationship. Finally, potential strategies for reducing the uncertainty relative to the range of NDVI measurement are discussed.

Factors controlling the degradation of hydrogen peroxide in river water, and the role of riverbed sand

Diurnal changes of H₂O₂ in river water during mid-summer were investigated. H₂O₂ in river water increased with the increase in intensity of solar radiation in the morning, and reached a maximum at 14:00, although solar radiation reached a maximum around 12:00. In the afternoon, a gradual decrease in H₂O₂ was observed, and H₂O₂ reached a minimum just before sunrise. Degradation rate constants determined using unfiltered river water samples were 0.081-0.161 h^-1, corresponding to a half-life of 4.3-8.5 h. We simulated diurnal changes in H₂O₂ using a simple formation, accumulation, and degradation model for static water using formation and degradation rate constants. The results of the modeling suggested that in situ degradation rate constants in rivers could be faster than those determined for unfiltered river water samples. Experiments using river sand indicated that riverbed sand could play an important role in H₂O₂ decay in rivers. We discussed the decomposition process of H₂O₂ in rivers.

Diurnal changes in CN metabolism and response of rice seedlings to UV-B radiation

Plants regulate a number of primary metabolites, including carbohydrates, organic acids, and amino acids, in response to UV-B radiation. Therefore, it is essential to understand the time-dependent response of rice plants to UV-B stress. This study focused on the response of plants to UV-B at different leaf developmental phases (emerging, growing, and maturing) in an attempt to fully comprehend the metabolic shift. We analyzed the expression levels of genes related to starch/sucrose metabolism in the leaf blades of rice seedlings (Oryza sativa L. "Saechuchenog") exposed to UV-B irradiation for short (1 day) and long terms (5 days) using quantitative real-time polymerase chain reaction. We also examined the diurnal variations in the contents of primary metabolites using an established GCTOF-MS (gas chromatography time of flight-mass spectrometry) method. The results showed that the levels of primary metabolites were largely dependent upon the diurnal rhythm and leaf developmental phase. The young leaves (sink) produced and accumulated starch rather than sucrose. The short-term (4 h, 1 day) UV-B exposure inhibited sucrose synthesis, which could be the first target of UV-B radiation. Following short- and long-term (5 days) exposure to UV-B radiation, the dynamic response of primary metabolites was evaluated. It was found that the content of carbohydrates decreased throughout the period of exposure to UV-B stress, especially in terms of sucrose concentration. However, the content of the majority of amino acids increased after an early decrease. Our data revealed that the metabolic response, as well as the gene expression, differed with the period (intensity) of exposure to UV-B radiation and with the phase of leaf development. These findings provide new insights for a better understanding of the metabolic response of a variety of plant species exposed to a wide range of UV-B radiation.

Melatonin-induced CBF/DREB1s are essential for diurnal change of disease resistance and CCA1 expression in Arabidopsis

Melatonin (N-acetyl-5-methoxytryptamine) is an important regulator of circadian rhythms and immunity in animals. However, the diurnal changes of endogenous melatonin and melatonin-mediated diurnal change of downstream responses remain unclear in Arabidopsis. Using the publicly available microarray data, we found that the transcript levels of two melatonin synthesis genes (serotonin N-acetyltransferase (SNAT) and caffeate O-methyltransferase (COMT)) and endogenous melatonin level were regulated by diurnal cycles, with different magnitudes of change. Moreover, the transcripts of C-repeat-binding factors (CBFs)/Drought response element Binding 1 factors (DREB1s) were co-regulated by exogenous melatonin and diurnal changes, indicating the possible correlation among clock, endogenous melatonin level and AtCBFs expressions. Interestingly, diurnal change of plant immunity against Pst DC3000 and CIRCADIANCLOCK ASSOCIATED 1 (CCA1) expression were largely lost in AtCBFs knockdown line-amiR-1. Taken together, this study identifies the molecular pathway underlying the diurnal changes of immunity in Arabidopsis. Notably, the diurnal changes of endogenous melatonin may regulate corresponding changes of AtCBF/DREB1s expression and their underlying diurnal cycle of plant immunity and AtCCA1.

Diurnal adjustment in ultraviolet sunscreen protection is widespread among higher plants

The accumulation of ultraviolet (UV)-absorbing compounds (flavonoids and related phenylpropanoids) in the epidermis of higher plants reduces the penetration of solar UV radiation to underlying tissues and is a primary mechanism of acclimation to changing UV conditions resulting from ozone depletion and climate change. Previously we reported that several herbaceous plant species were capable of rapid, diurnal adjustments in epidermal UV transmittance (T UV), but how widespread this phenomenon is among plants has been unknown. In the present study, we tested the generality of this response by screening 37 species of various cultivated and wild plants growing in four locations spanning a gradient of ambient solar UV and climate (Hawaii, Utah, Idaho and Louisiana). Non-destructive measurements of adaxial T UV indicated that statistically significant midday decreases in T UV occurred in 49 % of the species tested, including both herbaceous and woody growth forms, and there was substantial interspecific variation in the magnitude of these changes. In general, plants in Louisiana exhibited larger diurnal changes in T UV than those in the other locations. Moreover, across all taxa, the magnitude of these changes was positively correlated with minimum daily air temperatures but not daily UV irradiances. Results indicate that diurnal changes in UV shielding are widespread among higher plants, vary both within and among species and tend to be greatest in herbaceous plants growing in warm environments. These findings suggest that plant species differ in their UV protection "strategies" though the functional and ecological significance of this variation in UV sunscreen protection remains unclear at present.

Diurnal changes in Sorghum leaf starch molecular structure

Control of the fine structure of transitory starch synthesized during the day in leaves is required for its normal degradation during the subsequent night. In this study, the molecular structure of transitory starch from Sorghum leaves over the diurnal cycle was characterized using size-exclusion chromatography. This is the first study of diurnal changes in the chain-length distribution (CLD) of amylopectin and amylose over the entire range of chain lengths, and in the size distribution of whole starch molecules. It was found that the outer layers of leaf starch granules, which were synthesized during the daytime and degraded during the night, contained more large molecules, including amylopectin with more short chains and more branching, than those in the inner layers. The outer layers also had lower amylose content. Starch molecular sizes in leaves are much smaller than in grain starch. The starch structures observed are likely to give optimal energy control during plant growth. Lack of this control may contribute to poor plant growth.

Diurnal fluctuations in brain volume: Statistical analyses of MRI from large populations

We investigated fluctuations in brain volume throughout the day using statistical modeling of magnetic resonance imaging (MRI) from large populations. We applied fully automated image analysis software to measure the brain parenchymal fraction (BPF), defined as the ratio of the brain parenchymal volume and intracranial volume, thus accounting for variations in head size. The MRI data came from serial scans of multiple sclerosis (MS) patients in clinical trials (n=755, 3269 scans) and from subjects participating in the Alzheimer's Disease Neuroimaging Initiative (ADNI, n=834, 6114 scans). The percent change in BPF was modeled with a linear mixed effect (LME) model, and the model was applied separately to the MS and ADNI datasets. The LME model for the MS datasets included random subject effects (intercept and slope over time) and fixed effects for the time-of-day, time from the baseline scan, and trial, which accounted for trial-related effects (for example, different inclusion criteria and imaging protocol). The model for ADNI additionally included the demographics (baseline age, sex, subject type [normal, mild cognitive impairment, or Alzheimer's disease], and interaction between subject type and time from baseline). There was a statistically significant effect of time-of-day on the BPF change in MS clinical trial datasets (-0.180 per day, that is, 0.180% of intracranial volume, p=0.019) as well as the ADNI dataset (-0.438 per day, that is, 0.438% of intracranial volume, p<0.0001), showing that the brain volume is greater in the morning. Linearly correcting the BPF values with the time-of-day reduced the required sample size to detect a 25% treatment effect (80% power and 0.05 significance level) on change in brain volume from 2 time-points over a period of 1year by 2.6%. Our results have significant implications for future brain volumetric studies, suggesting that there is a potential acquisition time bias that should be randomized or statistically controlled to account for the day-to-day brain volume fluctuations.