Long-term drought modifies the fundamental relationships between light exposure, leaf nitrogen content and photosynthetic capacity in leaves of the lychee tree (Litchi chinensis)

Drought has dramatic negative effects on plants' growth and crop productivity. Although some of the responses and underlying mechanisms are still poorly understood, there is increasing evidence that drought may have a negative effect on photosynthetic capacity. Biochemical models of leaf photosynthesis coupled with models of radiation transfer have been widely used in ecophysiological studies, and, more recently, in global change modeling. They are based on two fundamental relationships at the scale of the leaf: (i) nitrogen content-light exposure and (ii) photosynthetic capacity-nitrogen content. Although drought is expected to increase in many places across the world, such models are not adapted to drought conditions. More specifically, the effects of drought on the two fundamental relationships are not well documented. The objective of our study was to investigate the effects of a long-term drought imposed slowly on the nitrogen content and photosynthetic capacity of leaves similarly exposed to light, from 3-year-old lychee trees cv. Kwa? Mi. Leaf nitrogen and non-structural carbohydrate concentrations were measured along with gas exchanges and the light-saturated rate of photosynthetic electron transport (J(max)) after a 5.5-month-long period of drought. Leaf nitrogen content on a mass basis remained stable, while the leaf mass-to-area ratio (LMA) increased with increasing water stress. Consequently, the leaf nitrogen content on an area basis (N(a)) increased in a non-linear fashion. The starch content decreased, while the soluble sugar content increased. Stomata closed and net assimilation decreased to zero, while J(max) and the ratio J(max)/N(a) decreased with increasing water stress. The drought-associated decrease in photosynthetic capacity can be attributed to downregulation of photosynthetic electron transport and to reallocation of leaf nitrogen content. It is concluded that modeling photosynthesis in drought conditions will require, first, the modeling of the effect of drought on LMA and J(max).     

Release of resource constraints allows greater carbon allocation to secondary metabolites and storage in winter wheat

The atmospheric CO₂ concentration ([CO₂]) is rapidly increasing, and this may have substantial impact on how plants allocate metabolic resources. A thorough understanding of allocation priorities can be achieved by modifying [CO₂] over a large gradient, including low [CO₂], thereby altering plant carbon (C) availability. Such information is of critical importance for understanding plant responses to global environmental change. We quantified the percentage of daytime whole‐plant net assimilation (A) allocated to night‐time respiration (R), structural growth (SG), nonstructural carbohydrates (NSC) and secondary metabolites (SMs) during 8 weeks of vegetative growth in winter wheat (Triticum aestivum) growing at low, ambient and elevated [CO₂] (170, 390 and 680 ppm). R/A remained relatively constant over a large gradient of [CO₂]. However, with increasing C availability, the fraction of assimilation allocated to biomass (SG + NSC + SMs), in particular NSC and SMs, increased. At low [CO₂], biomass and NSC increased in leaves but decreased in stems and roots, which may help plants achieve a functional equilibrium, that is, overcome the most severe resource limitation. These results reveal that increasing C availability from rising [CO₂] releases allocation constraints, thereby allowing greater investment into long‐term survival in the form of NSC and SMs.     

Glial Metabolism of Valine

The three essential amino acids, valine, leucine and isoleucine, constitute the group of branched-chain amino acids (BCAAs). BCAAs are rapidly taken up into the brain parenchyma, where they serve several distinct functions including that as fuel material in brain energy metabolism. As one function of astrocytes is considered the production of fuel molecules that support the energy metabolism of adjacent neural cells in brain. Astroglia-rich primary cultures (APC) were shown to rapidly dispose of the BCAAs, including valine, contained in the culture medium. While the metabolisms of leucine and isoleucine by APC have already been studied in detail, some aspects of valine metabolism remained to be determined. Therefore, in the present study an NMR analysis was performed to identify the ¹³C-labelled metabolites that are generated by APC during catabolism of [U-¹³C]valine and that are subsequently released into the incubation medium. The results presented show that APC (1) are potently disposing of the valine contained in the incubation medium; (2) are capable of degrading valine to the tricarboxylic acid (TCA) cycle member succinyl-CoA; and (3) release into the extracellular milieu valine catabolites and compounds generated from them such as [U-¹³C]2-oxoisovalerate, [U-¹³C]3-hydroxyisobutyrate, [U-¹³C]2-methylmalonate, [U-¹³C]isobutyrate, and [U-¹³C]propionate as well as several TCA cycle-dependent metabolites including lactate. This article is dedicated to Dr. George DeVries.     

水葫芦苗(Halerpestes cymbalaris)的生长特征研究

以调查统计的方法在中国科学院海北定位站研究了高寒湿地植物水葫芦苗无性系的生长特征、形态特征以及能量分配规律。结果表明：匍匐茎只有1条的水葫芦苗最多,占35．29％,匍匐茎有4条的水葫芦苗只占8．82％。同一水葫芦苗无性系中,随着匍匐茎数目的增多,分株数、间隔子数、茎总长和匍匐茎比节问重变小。分株一般在第一级最高,末级较低;第1条匍匐茎的间隔子较长。随水葫芦苗匍匐茎数目的增多,用于无性繁殖的分株干重比例逐渐增加,用于有性繁殖的花的干重比例下降。水葫芦苗无性系这种生长特征、形态特征以及能量分配规律是其生物—生态学特性和所处高寒湿地生境共同决定的。     

Central and peripheral administration of amylin induces energy expenditure in anesthetized rats

Amylin is a peptide hormone that is co-released with insulin from pancreatic β-cells following a meal. Intracerebroventricular (icv) administration of amylin (1–100 pmol), or an amylin agonist, salmon calcitonin, elicited dose-dependent thermogenic, tachycardic, and hyperthermic responses in urethane-anesthetized rats. Intravenous (iv) administration of higher doses of amylin (100 pmol–20 nmol) also induced similar responses, although the amplitudes of these responses were significantly smaller than those elicited by icv administration, suggesting the primary action of amylin to be in the brain. However, the iv administration of amylin induced the responses slightly faster than the icv injection, the former responses occurring <4 min and the latter, at 8–10 min, after the administration. The iv but not the icv injection of amylin increased the respiratory exchange ratio transiently (<20 min), though the thermogenic response lasted for a longer period after both injections, indicating a shift from mixed fuel to predominantly carbohydrate utilization in the initial phase of thermogenesis induced by the iv injection of amylin. The differences in substrate utilization and latency of the responses suggest that the actions of amylin include partly different targets when administered centrally and peripherally. Moreover, pretreatment with a β-adrenergic blocker, propranolol (5 mg kg⁻¹, iv), blocked all responses elicited by either icv or iv administration of amylin, whereas ablation of the area postrema in the hindbrain did not influence the effects of icv-administered amylin. These results suggest the involvement of amylin in postprandial energy expenditure, mediated by peripheral β-adrenoceptors.     

Effects of a nematode Meloidogyne incognita and its interaction with above‐ground herbivory on an invasive wetland plant,alligator weed (Alternanthera philoxeroides)

Invasive plants may be attacked both above ground and below ground. Few studies have, however, investigated the simultaneous effects of above‐ground and below‐ground herbivory. In the present study, we report the effects of beetle herbivory and nematode infection on alligator weed, Alternanthera philoxeroides, an invasive plant in China. We found that the root‐knot nematode Meloidogyne incognita widely occurred on the plant in south China. To examine its effect on the plant in conjunction with above‐ground herbivory, we conducted a field common garden experiment with a local insect defoliator, Cassida piperata. We also included the native congener Alternanthera sessilis in our experiments for a comparison of the response of invasive and native species. We found no significant effects on plant biomass of the nematode infection in conjunction with the above‐ground herbivory. Further chemical analysis, however, showed that the water‐soluble carbohydrate content in roots of A. philoxeroides was significantly increased in plants attacked by both the nematode and the herbivore compared with the water‐soluble carbohydrate content in plants attacked by only the nematode or herbivore alone. We found no such change in the native congener A. sessilis. Together these results may suggest that A. philoxeroides tolerates joint above‐ground and below‐ground damage by allocating more resources to below‐ground material.     

An ion-insensitive cAMP biosensor for long term quantitative ratiometric fluorescence resonance energy transfer (FRET) measurements under variable physiological conditions

Ratiometric measurements with FRET-based biosensors in living cells using a single fluorescence excitation wavelength are often affected by a significant ion sensitivity and the aggregation behavior of the FRET pair. This is an important problem for quantitative approaches. Here we report on the influence of physiological ion concentration changes on quantitative ratiometric measurements by comparing different FRET pairs for a cAMP-detecting biosensor. We exchanged the enhanced CFP/enhanced YFP FRET pair of an established Epac1-based biosensor by the fluorophores mCerulean/mCitrine. In the case of enhanced CFP/enhanced YFP, we showed that changes in proton, and (to a lesser extent) chloride ion concentrations result in incorrect ratiometric FRET signals, which may exceed the dynamic range of the biosensor. Calcium ions have no direct, but an indirect pH-driven effect by mobilizing protons. These ion dependences were greatly eliminated when mCerulean/mCitrine fluorophores were used. For such advanced FRET pairs the biosensor is less sensitive to changes in ion concentration and allows consistent cAMP concentration measurements under different physiological conditions, as occur in metabolically active cells. In addition, we verified that the described FRET pair exchange increased the dynamic range of the FRET efficiency response. The time window for stable experimental conditions was also prolonged by a faster biosensor expression rate in transfected cells and a greatly reduced tendency to aggregate, which reduces cytotoxicity. These properties were verified in functional tests in single cells co-expressing the biosensor and the 5-HT(1A) receptor.     

Microclimatic Conditions Regulate Surface Activity in Harvester Ants <Emphasis Type="Italic">Messor barbarus</Emphasis>

This paper analyses the effect of microclimatic factors (internal soil temperature, surface temperature and surface relative humidity) on surface activity of Messor barbarus harvester ants. We selected 44 colonies in an area of Mediterranean grassland near Madrid (Central Spain), which were monitored for activity between March 1998 and September 1999. Results indicate that microclimatic factors are good predictors of colony activation and intensity of activity. Colonies became active above certain critical values of internal soil temperature and relative humidity. For active colonies, surface temperature was the main regulatory factor for worker departure rate, which peaked at around 25–30°C. Worker speed was positively correlated with surface temperature, although the relationship was weaker for large-sized workers. Microclimatic factors were not enough, however, to predict task allocation outside the nest. The explanation for this aspect of ant behavior probably requires the inclusion of biotic factors in the models.