Interactive effects of elevated carbon dioxide and growth temperature on photosynthesis in cotton leaves

Cotton (Gossypium hirsutum L., cv DPL 5415) plants were grown in naturally lit environment chambers at day/night temperature regimes of 26/18 (T-26/18), 31/23 (T-31/23) and 36/28 °C (T-36/28) and CO2 concentrations of 350 (C-350), 450 (C-450) and 700 L L-1 (C-700). Net photosynthesis rates, stomatal conductance, transpiration, RuBP carboxylase activity and the foliar contents of starch and sucrose were measured during different growth stages. Net CO2 assimilation rates increased with increasing CO2 and temperature regimes. The enhancement of photosynthesis was from 24 mol CO2 m-2 s-1 (with C-350 and T-26/18) to 41 mol m-2 s-1 (with C-700 and T-36/28). Stomatal conductance decreased with increasing CO2 while it increased up to T-31/23 and then declined. The interactive effects of CO2 and temperature resulted in a 30% decrease in transpiration. Although the leaves grown in elevated CO2 had high starch and sucrose concentrations, their content decreased with increasing temperature. Increasing temperature from T-26/18 to 36/28 increased RuBP carboxylase activity in the order of 121, 172 and 190 mol mg-1 chl h-1 at C-350, C-450 and C-700 respectively. Our data suggest that leaf photosynthesis in cotton benefited more from CO_2 enrichment at warm temperatures than at low growth temperature regimes.     

Membrane-bound Adenosine Triphosphatase Activities of Oat Roots

Homogenates of oat (Avena sativa cv. Goodfield) roots contained at least five membrane-associated adenosine triphosphatase (ATPase) activities. The membrane-bound ATPases were separated on sucrose gradients and distinguished by membrane density, pH optima, sensitivity to monovalent salts, and substrate specificity.     

Episodic memory: insights from semantic dementia

Semantic dementia, also known as the temporal lobe variant of fronto-temporal dementia, results in a progressive yet relatively pure loss of semantic knowledge about words, objects and people, and is associated with asymmetric, focal atrophy of the antero-lateral temporal lobes. Semantic dementia provides a unique opportunity to study the organization of long-term memory particularly since initial observations suggested sparing of episodic memory. Recent studies reveal, however, a more complex but theoretically revealing pattern. On tests of autobiographical memory, patients with semantic dementia show a 'reverse step function' with sparing of recall of events from the most recent 2 to 5 years but impairment on more distant life periods. Anterograde recognition memory for visual materials is extremely well preserved, except in the most deteriorated cases, although performance is heavily reliant upon perceptual information about the studied stimuli, particularly for items that are no longer known by the subjects. On tests of verbal anterograde memory such as word learning, performance is typically poor even for words which are 'known' to the patients. A source discrimination experiment, designed to evaluate familiarity and recollection-based anterograde memory processes, found that patients with semantic dementia showed good item detection, although recollection of source was sometimes impaired. Semantic knowledge about studied items and measures of item detection and source discrimination were largely independent. The implications of these findings for models of long-term memory are discussed. The results support the concept that episodic memory, or at least the recall of temporally specific autobiographical experiences, draws upon a number of separable memory processes, some of which can function independently of semantic knowledge.     

Heterodimerization of the yeast MATa1 and MAT alpha 2 proteins is mediated by two leucine zipper-like coiled-coil motifs.

The yeast Saccharomyces cerevisiae has three cell types distinguished by the proteins encoded in their mating type (MAT) loci: the a and alpha haploids, which express the DNA binding proteins a1 and alpha 1 and alpha 2, respectively, and the a/alpha diploid which expresses both a1 and alpha 2 proteins. In a/alpha cells, a1-alpha 2 heterodimers repress haploid-specific genes, while alpha 2 homodimers repress a-specific genes, indicating a dual regulatory function for alpha 2 in mating type control. a1 does not form homodimers. We have identified two sequences in the alpha 2 N-terminal domain which contain the 3,4-hydrophobic heptad repeat pattern characteristic of coiled-coils. Mutational analyses show that both sequences are important to a1-alpha 2 heterodimerization. We propose that these two sequences associate in a coiled-coil-like manner with a sequence within a1 which bears two adjacent, overlapping 3,4-hydrophobic heptad repeats. This model, which describes a novel dimerization motif for homeodomain proteins, also provides a mechanism by which a1-a1 homodimerization is prevented.     

Improving the thiobarbituric acid-reactive-substances assay for estimating lipid peroxidation in plant tissues containing anthocyanin and other interfering compounds

The occurrence of malondialdehyde (MDA), a secondary end product of the oxidation of polyunsaturated fatty acids, is considered a useful index of general lipid peroxidation. A common method for measuring MDA, referred to as the thiobarbituric acid-reactive-substances (TBARS) assay, is to react it with thiobarbituric acid (TBA) and record the absorbance at 532 nm. However, many plants contain interfering compounds that also absorb at 532 nm, leading to overestimation of MDA values. Extracts of plant tissues including purple eggplant (Solanum melongena L.) fruit, carrot (Daucuscarota L.) roots, and spinach (Spinacia oleracea L.) leaves were assessed for the presence of MDA and other non-MDA compounds absorbing at 532 nm. A method described herein corrects for these interferences by subtracting the absorbance at 532 nm of a solution containing plant extract incubated without TBA from an identical solution containing TBA. The reliability and efficiency of this spectrophotometric method was assessed by altering the relative ratios of exogenous MDA additions and/or extracts of red cabbage (Brassica oleracea L.) leaves containing interfering compounds and then measuring MDA recovery. Reliability was also validated through high-performance liquid chromatography and high-performance liquid chromatography-mass spectrometry techniques. Results indicated that over 90% of exogenously added MDA could be recovered through the improved protocol. If there were no corrections for interfering compounds, MDA equivalents were overestimated by up to 96.5%. Interfering compounds were not detected in vegetables such as lettuce (Lactuca sativa L.) and spinach which had low or negligible concentrations of anthocyanidin derivatives. Comparisons between the TBARS method presented here and two currently accepted protocols indicated that the new modified method exhibits greater accuracy for quantifying TBA-MDA levels in tissues containing anthocyanins and/or other interfering compounds. This modified protocol represents a facile and rapid method for assessment of lipid peroxidation in virtually all plant species that contain interfering compounds. Received: 28 August 1998 / Accepted: 29 September 1998     

Antioxidant compound responses to chilling stress in differentially sensitive inbred maize lines

Chilling temperatures increase the amounts of potentially lethal toxic oxygen compounds present within plants. These toxic oxygen compounds can be scavenged by antioxidant compounds such as ascorbate and β-carotene. Three developmental stages (first, third and fifth leaf) of four inbred lines of maize ( Zea mays L.) exhibiting differential sensitivity to chilling were examined in order to determine if the chilling-sensitive line had lower concentrations of antioxidant compounds than did the tolerant lines. Plants were exposed to one of three treatments: (1) control (25°C constant), (2) control treatment plus a short-term chilling exposure of 11°C one day prior to harvesting, and (3) long-term (11°C constant) chilling exposure. Total ascorbate, total glutathione, β-carotene, α-tocopherol and chlorophyll contents were quantified, and ratios of dehydroascorbate/ascorbate and reduced/oxidized glutathione were determined. Lower concentrations of β-carotene were found in the chilling-sensitive relative to those in the chilling-tolerant lines for the first-leaf stage under both short- and long-term chilling treatments. Concentrations of total ascorbate and glutathione and β-carotene in the chilling-sensitive line increased as the chilling treatment progressed and as the plants developed until they ultimately became either significantly higher or no different relative to the tolerant lines. Results suggest that this sensitive line became less sensitive to chilling-induced oxidative stress with development.     

A possible mechanism for partitioning between homo- and heterodimerization of the yeast homeodomain proteins MATa1 and MATalpha2.

The yeast Saccharomyces cerevisiae has three cell types distinguished by the proteins encoded in their mating-type (MAT) loci: the a and alpha haploids, which express the DNA-binding proteins a1, and alpha1 and alpha2, respectively, and the a/alpha diploid which expresses both a1 and alpha2 proteins. In a/alpha cells, a1-alpha2 heterodimers repress haploid-specific genes and MATalpha1, whereas alpha2 homodimers repress a-specific genes, indicating dual regulatory functions for alpha2 in mating-type control. We previously demonstrated that the two leucine zipper-like coiled-coil motifs, called alpha2A and alpha2B, in the alpha2 N-terminal domain are important to a1-alpha2 heterodimerization. A unique feature of alpha2B is the occurrence of three atypical amino acid residues at a positions within the hydrophobic core. We have conducted mutational analyses of alpha2B peptides and the full-length protein. Our data suggest that these residues may play a critical role in partitioning of the alpha2 protein between heterodimerization with a1 and homodimerization with itself.     

The ovarian cycle and control of ovulation

The ovarian cycle of primates is a sequence of events reflecting follicular growth and development, the ovulation of a mature oocyte and the formation of a functional corpus luteum. A typical cycle generally consists of three phases: (1) the follicular or proliferative phase, (2) ovulation and (3) the luteal or secretory phase. Within this general pattern exists considerable species variation in terms of cycle length, timing of ovulation, presence or absence of menstruation and influence of season.
Details of the basic physiological mechanisms controlling cyclic ovarian function in primates are known for only a few species. Concentrating on information derived from studies in women and in rhesus and marmoset monkeys, this paper examines some of the hormonal mechanisms underlying the primate ovarian cycle with particular reference to the factors controlling preovulatory follicular development during the follicular phase.