Leaf age as a factor in anatomical and physiological acclimative responses of Taxus baccata L. needles to contrasting irradiance environments

To evaluate the acclimative ability of current-year and previous-year needles of a shade tolerant conifer Taxus baccata L. to contrasting irradiance conditions, seedlings were raised under 27% solar irradiance and at 3 years of age they were transferred to an experimental garden and grown for one season under full irradiance (HL), 18% irradiance (ML) or 5% irradiance (LL). Whereas previous year needles did not change anatomically, current year needles in HL were thicker and had a thicker palisade and spongy mesophyll, and greater leaf mass per area than ML or LL needles. LL needles had greater nitrogen concentration than HL needles irrespective of age but only previous year LL needles also had an increased N per area content, thanks to their lack of reduction in LMA. Adjustment of chlorophyll and carotenoid content occurred in both needle age classes with LL and ML needles having much higher concentrations but, in current year needles, only slightly higher per area content than HL needles. Chlorophyll a/b ratio was not affected by age or irradiance. These modifications had no significant effect on photosynthetic capacities, which did not significantly differ between the age classes in HL or LL treatment and between treatments. On the other hand, high growth irradiance resulted in a greater photochemical yield, photochemical quenching, apparent electron transport rate and inducible non-photochemical quenching in needles formed in the current season. In previous year needles, however, only inducible NPQ was enhanced by high irradiance with other parameters remaining identical among treatments. To test sensitivity to photoinhibition, at the end of the summer plants from the three irradiance levels were transferred to a HL situation and F _v/F _M was determined over the following 18 days. Sensitivity to photoinhibition was negatively related to growth irradiance and previous year needles were less photoinhibited than current year needles. Thus, differences in acclimation ability between needle age classes were most pronounced at the level of anatomy and light reactions of photosynthesis, both of which showed almost no plasticity in previous year needles but were considerably modified by irradiance in current year needles.     

Thermonastic leaf movements: a synthesis of research with Rhododendron

Thermonastic leaf movements in Rhododendron L. occur in response to freezing temperatures. These movements are composed of leaf curling and leaf angle changes that are distinct leaf movements with different responses to climatic factors. Leaf angle is controlled by the hydration of the petiole, as affected by soil water content, atmospheric vapour pressure, and air temperature. In contrast, leaf curling is a specific response to leaf temperature, and bulk leaf hydration has little effect. The physiological cause of leaf curling is not well understood, but the mechanism must lie in the physiology of the cell wall and/or regional changes in tissue hydration. Available evidence suggests that intercellular freezing is not a cause of leaf curling.
Manipulation experiments demonstrate that changes in leaf orientation in Rhododendron most likely serve to protect the leaves from membrane damage due to high irradiance and cold temperatures. In particular, the pendent leaves protect the chloroplast from photoinhibition. Leaf curling may serve to slow the rate of thaw following freezing, a common phenomenon in the Appalachian mountains of the U.S. The thermonastic leaf movements have a greater importance to plants in a dim environment because the potential impact to canopy carbon gain is greater than in high light environments.
These leaf movements have several implications for horticultural management. There seems to be a trade-off between water stress tolerance and freezing stress tolerance by leaf movements. Thermonastic leaf movements may be a major mechanism of cold stress tolerance in Rhododendron species. The actual physiological cause of leaf movement has not been elucidated and many more species need to be evaluated to verify the general importance of leaf movements to Rhododendron ecology and evolution.     

Changes in UV-B radiation screening effectiveness with leaf age in Rhododendron maximum

We examined how ultraviolet-B radiation (UV-B; 300 nm) screening effectiveness changes with leaf age in Rhododendron maximum growing in a shaded understory by measuring depth of penetration and epidermal transmittance with a fibre-optic microprobe. Depth of penetration (and epidermal transmittance) of UV-B decreased with leaf age in 1- to 4-year-old leaves, averaging 62 (32), 52 (22), 45 (16) and 48 μm (13%), respectively. Epidermal thickness increased with age in 1- to 4-year-old leaves due to a thickening of the cuticle from an average of 20 to 29μm. Ultraviolet-B-absorbing compound concentrations increased with age from 1–3 to 1–5 A₃₀₀ cm^?2 leaf area. Concentrations of UV-B-absorbing compounds (area basis) were a strong predictor of depth of penetration (r²= 0.82) and epidermal transmittance (r²= 0.95) of UV-B in mature (1–4 year-old) foliage. Chlorophyll concentrations (area basis) increased in leaves up to 3 years of age. Current-year leaves (30 d old) were exceptional in that while they were particularly effective at screening UV-B (depth of penetration and epidermal transmittance averaged 39μm and 5%, respectively) they had relatively low concentrations of UV-B-absorbing compounds (1.3 A₃₀₀ cm^?2). Our findings show that UV-B-screening effectiveness is not necessarily related to absorbing compound concentrations on a whole-leaf basis, possibly due to anatomical changes within the epidermis that occur with leaf age.     

Seasonal and diurnal monitoring of leaf polyamine content in Quercus ilex L. resprouts after fire in relation to changes in irradiation and photosynthetic parameters

Seasonal variations in free putrescine, spermidine and spermine content, gas-exchange and chlorophyll fluorescence parameters were followed during winter and summer on leaves of a similar age from undisturbed holm oak trees (control, C) and resprouts (R) originated after fire. We observed a general trend of putrescine content decrease with increasing irradiance. Putrescine content decreased markedly from winter to summer, especially in R, which were located on a site with much higher irradiation. Daily summer variations in putrescine showed a decline at midday from morning values, and they were also more accentuated in R. Measurement of gas-exchange and chlorophyll fluorescence parameters showed marked differences between C and R under their respective light conditions. R showed higher values of PSII quantum yield (Φ_PSII), photochemical quenching (qP) and intrinsic efficiency of open PSII centres () The Φ_PSII/PPFD response curve showed that under the same irradiance, Φ_PSII was enhanced in R and mainly under high light conditions. In spite of increasing irradiance from winter to summer, and especially in burned areas, the mentioned chlorophyll fluorescence parameters were maintained indicating the adaptation of the photosynthetic apparatus. Results derived from A/C _i and A/PPFD response curves showed enhanced photosynthetic capacity and lower non-stomatal limitation of photosynthesis in R during summer stress. The contribution of putrescine decline in the photoadaptation of the photosynthetic apparatus of species growing in natural forest habitats is considered.     

Seasonal movements and diurnal activity rhythms of the Grey seal (Halichoerus grypus)

The seasonal movements and diurnal activity rhythms of the Grey seals occurring along the Atlantic Coast of Nova Scotia and in the southern part of the Gulf of St Lawrence have been studied over a period of three and a half years. The seasonal movements of the adults can be divided into five periods or phases: (1) prepupping, (2) pupping and mating, (3) postmating, (4) spring concentration and (5) summer dispersal. The diurnal activity rhythm is attuned to the tides; the effects of the hours of daylight appear to be minimal. During the breeding season the cows go to sea as the tide is falling and return when it is rising. Adults of both sexes have a similar rhythm at other times of the year, except in mid-summer when the pattern is reversed.     

Seasonal patterns of leaf production in co‐occurring trees with contrasting leaf phenology: time and quantitative divergences

Neotropical savannas (‘cerrados’) of Central Brazil are characterized by the coexistence of a large diversity of tree species with divergent phenological behaviors, which reflect a great diversity in growth strategies. In the present study time behavior and quantitative aspects of shoot growth, shoot mortality, and leaf longevity and production were analyzed in 12 woody species of contrasting leaf phenology, adopting a functional group approach where 12 species were categorized into three functional groups: evergreen, decidous and brevideciduous, according to their leaf phenology. Shoot growth and leaf production were seasonal for the three functional groups, differing in their time of occurrence, but being concentrated during the last months of the dry season. Shoot growth differed between evergreens and deciduous, as well leaf production. Evergreens had higher rates of shoot growth, produced a higher number of leaves and had longer leaf longevity (around 500 days against 300 days in deciduous and brevideciduous). Leaf longevity was associated with patterns of leaf production when accounting for all phenological groups studied. It was possible to identify different patterns of aerial growth in savanna phenological groups, providing evidence of great functional variability amongst the groups studied.     

Phosphorus relations of roots and mycorrhizas of Rhododendron maximum L. in the southern Appalachians,North Carolina

Summary The mycorrhizal associations of Rhododendron maximum in the southern Appalachian mountains were studied in relation to the supply and demand of phosphate at three altitudes. A variety of mycorrhizal associations are described together with the ability of the differing mycorrhizal types to produce phosphatase enzyme, which was inversely related to the availability of inorganic phosphate determined by a root bioassay, as Ectomycorrhizal associations were shown to have a higher phosphatase production potential than other mycorrhizas. The availability of inorganic phosphate at different altitudes is related to turnover of organic matter and fixation capacity of the mineral soil. It is speculated that the ability of R. maximum to associate with a range of mycorrhizal associates is likely to improve species' fitness and enhance its competitive ability.     

Seasonal continuity of chloroplasts in white pine and Rhododendron

Summary White pine and Rhododendron leaf chloroplasts were observed with the electron microscope in late as well as in mid-winter. It was clear that even in late winter there was no indication of chloroplast disintegration, fractionation, or retrogression.Intergrana lamellae often showed close approximation as in grana lamellae, a situation now apparently considered normal. A slight space was observed between closely appressed grana lamellae where two thylakoids approached one another. This is also apparently normal in functioning chloroplasts.Vesicles were sometimes seen in pine chloroplasts, suggesting chloroplast disintegration, but this was only rarely seen, and seems to be atypical. Invagination of the inner layer of the chloroplast envelope in winter chloroplasts suggested, however, they were still in a developing stage.Fixation and embedding was much more successful in winter than in summer. Summer fixation and embedding was improved by the addition of 1 % sucrose to the KMnO₄.     

Suppression of ectomycorrhizae on canopy tree seedlings in Rhododendron maximum L. (Ericaceae) thickets in the southern Appalachians

 Thickets of Rhododendron maximum (Ericaceae) (Rm) in the southern Appalachians severely limit regeneration of hardwood and coniferous seedlings. Experimental blocks were established in and out of Rm thickets in a mature, mixed hardwood/conifer forest in Macon County, N.C. Litter and organic layer substrates were removed, composited and redistributed among plots within the blocks (except for control plots). Seedlings of northern red oak (Quercus rubra) and eastern hemlock (Tsuga canadensis) were planted in the plots and harvested at the end of the first and second growing seasons. Litter manipulation had no effect on total mycorrhizal colonization, but the distribution of Cenococcum geophilum mycorrhizae was altered. After the first year, percent mycorrhizal colonization of hemlocks not in Rm thickets (62%) was at least three times higher than in Rm thickets (19%), and the ramification index (no. of mycorrhizae cm^–1) had increased by more than a factor of four (2.83 versus 0.61). In addition, colonization of 1-year-old hemlocks by C. geophilum was significantly higher within blocks with (10.4%) than without (4.6%) Rm. Differences in mycorrhizal colonization, ramification indices and colonization by C. geophilum were absent or less pronounced on 2-year-old hemlocks and 1- and 2-year-old oak seedlings. The biomasses of first year oak roots and shoots and second year shoots were 50% less in Rm thickets. Biomasses of first year hemlock roots and second year shoots were also reduced. Mycorrhizal parameters were correlated with some growth parameters only for hemlock seedlings, but did not explain most of the variation observed. Accepted: 12 February 1999     

Variation in sclerophylly among Iberian populations of Quercus coccifera L. is associated with genetic differentiation across contrasting environments

Evergreen oaks are an emblematic element of the Mediterranean vegetation and have a leaf phenotype that seems to have remained unchanged since the Miocene. We hypothesise that variation of the sclerophyll phenotype among Iberian populations of Quercus coccifera is partly due to an ulterior process of ecotypic differentiation. We analysed the genetic structure of nine Iberian populations using ISSR fingerprints, and their leaf phenotypes using mean and intracanopy plasticity values of eight morphological (leaf angle, area, spinescence, lobation and specific area) and biochemical traits (VAZ pool, chlorophyll and β-carotene content). Climate and soil were also characterised at the population sites. Significant genetic and phenotypic differences were found among populations and between NE Iberia and the rest of the populations of the peninsula. Mean phenotypes showed a strong and independent correlation with both genetic and geographic distances. Northeastern plants were smaller, less plastic, with smaller, spinier and thicker leaves, a phenotype consistent with the stressful conditions that prevailed in the steppe environments of the refugia within this geographic area during glaciations. These genetic, phenotypic, geographic and environmental patterns are consistent with previously reported palaeoecological and common evidence. Such consistency leads us to conclude that there has been a Quaternary divergence within the sclerophyllous syndrome that was at least partially driven by ecological factors.     

ABA xylem concentrations determine maximum daily leaf conductance of field-grown Vitis vinifera L. plants

Differences in maximum leaf conductance in grapevine plants growing in soils with contrasting water availabilities during mid-summer in Portugal could be accounted for by differences in the concentration of ABA in xylem sap. This conclusion is reinforced by the observation that the relationship between leaf conductance and endogenous ABA concentration can be mimicked by the application of exogenous ABA to leaves detached from irrigated plants. During the day, leaf conductance decreased after a morning peak, even when the leaves remained in a constant environment at a moderate temperature and leaf-to-air vapour pressure difference. This decline in leaf conductance was not a consequence of an increase in the xylem ABA concentration or the rate of delivery of this compound by the transpiratory stream. The afternoon depression in leaf conductance was associated with an apparent limitation in stomatal opening potential, which persisted even when detached leaves were fed with water and rehydrated. The reason for this inhibition has still to be identified.     

Seasonal and diurnal melatonin production in exercising sled dogs

Melatonin is a hormone that is released from the pineal gland into the blood stream and is controlled by nerve impulses from the suprachiasmatic nuclei. Melatonin synthesis, which is inhibited by light on the mammalian retina, peaks in plasma concentrations during the night. Though still a subject of intense research, melatonin in mammals is known to effect the reproductive system, thyroid function, and adaptations to seasonal changes. Sled dogs in Fairbanks, Alaska (65 degrees N) can be exposed to anywhere from 21 h of daylight in the summer to 4 h in the winter. While light may be the primary factor influencing melatonin production, we hypothesized that exercise may also affect melatonin production. In the current study, sled dogs were used to study seasonal and diurnal variation in melatonin production. Sled dogs by nature are elite athletes and therefore exercise was a focus in the study. Both exercise and non exercise dogs from 2 distinct latitudes were used. The peak in melatonin production was prolonged in high latitude dogs (65 degrees N), compared with lower latitude dogs (45 degrees N). Dogs at both latitudes show a reduction in peak melatonin levels with exercise, and winter melatonin levels in both locations were higher than the summer. Surprisingly, sled dogs in Alaska had lower melatonin levels than sled dogs in New York.     

Seasonal variation and diurnal fluctuations in ephemeral desert pools

The physical variables which directly affect organisms inhabiting desert ephemeral pools were examined in four pools in southeastern Utah. During the day, pools were hyperoxic (240 torr) and hypocapnic (0.07 torr) while pH and temperature increased (7.5–9.0 & 17–35 °C respectively). Conversely, predawn pool measurements were hypoxic (40 torr) and hypercapnic (3 torr). While TA increased throughout the season (from 0.4 to 1.43 meq l^–1), due largely to increased bicarbonate concentration (from 0.5 to 1.4 mmol l^–1), water osmolarity remained relatively constant. These desert ephemeral systems represent unique environmental habitats where organisms experience both diurnal and seasonal changes in numerous physical variables over short time frames.     

Rhododendron ferrugineum L. in fitocenosi relitte dell'Appennino settentrionale

Abstract

Relict communities of Rhododendron ferrugineum L. in Northern Apennines. — On the Apennines Rhododendron ferrugineum vegetates spontaneously only on the Tuscan-Emilian part, but particularly on the Northern side, on Cima Belfiore (1810 m), on Mount Prado (2053 m), on Mount Libro Aperto (1937 m). The Rhododendron stations are to be found on the Northern slopes of the Apenninic ridges at an altitude between 1750-1937 meters (Libro Aperto). They have the aspect and sometimes the same floristic composition, of the hypsophyle Vaccinium and Empetrum heaths covering almost alla the summits of the Tuscan and Emilian Apennines. The environmental conditions which have preserved these Apenninic Rhododendron heaths could probably be the abundant snowfalls, (precipitations over 2000 mm per year), the acidity of the soil derived from the Oligocene hard sandstone and finally to a certain degree of continentality.     

Seasonal and diurnal variation in nitrate reductase activity of cowpeas

The seasonal and diurnal variations in nitrate reductase activity (NRA) of cowpea cultivars were determined. Mean activity per gram fresh weight per hour averaged 31% higher in the 20 mg N/plant treatment than the control throughout the growing period. The highest activity occurred in the seedling stage and declined towards anthesis. Diurnal variations were marked by an increase in the NRA from 0600 to 1800 hours and then declined to a minimum at 2400 hours.     

Role of pulvinar chloroplasts in light-driven leaf movements of the trifoliate leaf of bean (Phaseolus vulgaris L.)

Laminar pulvini of bean (Phaseolus vulgaris L.) contain numerouschloroplasts in cells of their motor tissue. The quantitativerelationships of the chloroplast pigments, chlorophyll a andb, ß-carotene, lutein, neoxanthin as well as the xanthophyllcycle carotenoids (violaxanthin, antheraxanthin and zeaxanthin)were similar to those of mesophyll chloroplasts from leafletlaminae. Exposure of pulvinules to light caused deepoxidationof violaxanthin to zeaxanthin, showing that the xanthophyllcycle is functioning. Chlorophyll fluorescence analysis of pulvinulesconfirmed that their chloroplasts are capable of both photosyntheticelectron transport and non-photochemical fluorescence quenching,showing that they build up a considerable transthylakoid protongradient in the light. Application of DCMU to excised pulvinulesand laminar discs, as well as to pulvinules of intact, attachedterminal leaflets blocked electron transport and fluorescencequenching. Application of the uncoupler CCCP to intact pulvinulesalso prevented non-photochemical fluorescence quenching. Therate of movement of the low-light-adapted terminal leaflet inresponse to exposure of its pulvinule to overhead red light(500 µmol m^–2 s^–1) was reduced when the pulvinulewas pretreated with DCMU. The pulvinar response to overheadblue light (50 µmol ^–2 s^–1), which is morepronounced than to red light, was not affected by similar pretreatmentwith DCMU. Pretreatment with CCCP caused a short lag in theresponse to red light, but did not affect its subsequent rate.The results suggest that the pulvinar response to red, but notto blue light, requires non-cyclic electron transport and theresulting generation of ATP Key words: Leaf movements, light, non-cyclic electron transport, Phaseolus, pulvinar chloroplasts     

Dry matter production and photosynthetic capacity in Gossypium hirsutum L. under conditions of slightly suboptimum leaf temperatures and high levels of irradiance

Summary Gossypium hirsutum L. var. Delta Pine 61 was cultivated in controlled-environment chambers at 1000–1100 mol photosynthetically active photons m^-2 s^-1 (medium photon flux density) and at 1800–2000 mol photons m^-2 s^-1 (high photon flux density), respectively. Air temperatures ranged from 20° to 34°C during 12-h light periods, whereas during dark periods temperature was 25° C in all experiments. As the leaf temperature decreased from about 33° to 27° C, marked reductions in dry matter production, leaf chlorophyll content and photosynthetic capacity occurred in plants growing under high light conditions, to values far below those in plants growing at 27° C and medium photon flux densities. The results show that slightly suboptimum temperatures, well above the so-called chilling range (0–12° C), greatly reduce dry matter production in cotton when combined with high photon flux densities equivalent to full sunlight.Abbreviations DW dry weight - F _v variable fluorescence yield - F _M maximum fluorescence yield - PFD photon flux density (400–700 nm)