The Limits to Xylem Embolism Recovery in Pinus sylvestris L.

In this study we test the hypothesis that, when water supplyis under tension, reversal of cavitation can occur as long aswater continuity is maintained in the vicinity of tracheids.The experiments were conducted on young branches, 7–8mm diameter, of Pinus sylvestris L., freshly collected and allowedto lose water on the bench after being debarked. During dehydration,the volumetric fractions of water (V_w) and gas (V_s) changedsteadily as relative water content () declined. Meanwhile, ultrasonicemissions (UAE) started after a threshold = 90% was reachedand were maximal at = 75%. Before and after dehydration, branchsegments were connected to water-filled tubing and placed from0.2 to 3.6 m above a water source and water inflow and outflowwere recorded. These distances provided a source of water ata potential of –2.0 to –36kPa. We considered thatthe segment water potential would be a function of the surfacetension across the water meniscii at the ends of the embolizedtracheids. Thus, water potentials calculated from tracheid dimensionswould be as low as –43 kPa. Water inflow to segments declinedwhen the distance from the source was increased or the segmentswere very dehydrated. Increasing the distance above the watersource would be expected to increase the water potential differencebut to reduce water uptake. The most dehydrated segments absorbedwater faster at the beginning of the refilling period (2h),but at the end of 16h, was lower and V_g larger than in lessembolized tissue. Recovery of water flow followed a similartrend, and was lowest when embolisms increased. For a narrowrange of , hydraulic conductance was reduced sharply, indicatingthat wide tracheids were still gas-filled. Thus, the numberof tracheids remaining embolized increased when the source waterpotential was low and there was severe embolism. We concludethat embolism can be reversed in P. sylvestris at a rate dependingon the water potential of the source, severity of embolism andhydraulic conductivity. Key words: Pinus sylvestris L., cavitation induction-recovery, embolized tracheids, water content and embolism, matrix potential, capillary, hydraulic conductance     

Changes in Cell Number and Cell Division Activity during Endosperm Development in Allohexaploid Wheat, Triticum aestivum L.

Nuclear or cell number, and the mitotic index, were recordedin endosperms of Triticum aestivum cv. Mardler to test if aparticular stage of endosperm development was critical in determiningthe final grain weight. The basal four florets of emasculatedspikelets (controls), and the third and fourth florets of spikeletswhere the two basal ovaries were removed (ovary-removed), weresampled at various times up to 360 h after hand-pollination.The coenocytic phase in endosperms ended about 84 h after pollinationregardless of both grain position and the treatment. The onsetof the cellular stage was characterized by the final large fluctuationsin the mitotic index reflecting the culmination of the synchronousnuclear division of the coenocytic stage. Thereafter, the mitoticindex fluctuated with smaller amplitudes and, by 216 h afterpollination, was < 1%. Neither floret position in the spikeletnor the treatment affected the pattern of alteration to themitotic index. However, ovary removal from first and secondflorets resulted in significantly heavier grains and higherendosperm cell number in the 3rd and 4th florets compared withthe controls. In all florets, mean endosperm cell number peakedat 280 h but decreased by 360 h after pollination. At this time,the mean cell numbers in endosperms of the 3rd and 4th floretsof ovary-removed spikelets were significantly higher than inthe corresponding endosperms in the controls. Thus, a key contributoryfactor in determining the final endosperm cell number may bethe number of cells which are lost during the late period ofthe cellular stage of endosperm development. Key words: Endosperm cell number, florets, grain weight, mitotic index, Triticum aestivum     

Effect of predatory fish presence on habitat use and diel movement of the stream amphipod, Gammarus minus

SUMMARY.

• 1 We examined how the presence of predatory fish affected macro—and microhabitat use and movement patterns of the amphipod, Gammarus minus Say, in a second order stream.
• 2 Among macrohabitats, amphipods were consistently more abundant in runs than in pools. Densities in pools were low regardless of fish presence, whereas low densities in runs were correlated with fish presence. Siltation and the lack of coarse substrate particles probably accounted for the scarcity of G. minus in pools. Among microhabitats, G. minus was more abundant in leaf litter and gravel than in silt/sand substrates. Fish presence did not affect microhabitat use in the field. Laboratory manipulations showed, however, that predation rates by green sunfish (Lepomis cyanellus Rafinesque) were significantly lower on amphipods in leaf litter than in other microhabitats.
• 3 Most movement by G. minus in runs occurred at night and was directed downstream. In the laboratory, amphipods significantly reduced swimming activity at night in response to water conditioned with green sunfish. In the field, however, amphipods were only slightly less active in runs at night when exposed to chemical cues of confined green sunfish. The distribution of amphipods in streams is influenced by the presence of fish, but chemical cues alone may not be important in triggering predator avoidance behaviours in nature.

     

Three Centuries of Protozoology: A Brief Tribute to its Founding Father,A. van Leeuwenhoek of Delft*

SYNOPSIS. It was exactly 300 years ago this month (August 1974) that the 17th century modest draper from Delft, Holland—Antony van Leeuwenhoek—discovered protozoa. Describing them, often with amazing accuracy considering the optical equipment he was using (simply a home-made “glorified”hand lens), in letters to the Royal Society of London, he established himself, certainly, as the founding father of protozoology. It is particularly appropriate for an assemblage of protozoologists to pay homage to this intrepid “philosopher in little things,”a man with an insatiable curiosity about his wee animalcules, on the tricentenary of his discovery of them, since it was an event of such long-lasting significance.     

Root Density and Water Potential Gradients near the Plant Root

The models of Gardner (1960) and Cowan (1965) for water transferto the plant root are used to estimate the differences in waterpotential between the root and the bulk soil for a wide rangeof root densities and water extraction rates at a series ofmatric potentials for a Yolo light clay. For root densities and extraction rates reported both in theliterature and in this paper there is good evidence to suggestthat the large potential gradients originally predicted by Gardnerand Cowan are restricted to situations involving very low rootdensities and high extraction rates in relatively dry soil.     

Changes in Urease Activity in Apple Trees as Related to Urea Applications

Changes in urease (E.C.3.5.1.5.) were followed during the growth of 1-year-old MM 106 and 9-year-old Golden Delicious apple trees (Malus pumila Rehd.). Urease was found in leaves, roots, and bark with actively growing tissues containing more activity than senescing tissues. The urease activity in the leaves declined steadily during leaf senescence but abscised leaves still contained about half of their initial urease activity. In the bark the urease activity changed only slightly. Urease activities in the leaves and bark of apple trees were always greater in those trees which had received an application of urea. In senescing apple leaves, urea induced a rapid increase in urease activity. The changes in total activity and specific activity of urease were parallel and suggests that urease was synthesized de novo. After urease activity reached a maximum, a rapid decline occurred. Urease was inhibited by low concentrations of ammonia and this decline may be due to product inhibition.     

Remarks on the Composition of the Large Ciliate Class Kinetofragmophora de Puytorac et al., 1974, and Recognition of Several New Taxa Therein,with Emphasis on the Primitive Order Primociliatida N. Ord.*

With the realization that new data (especially ultrastructural) and new ideas are making necessary a major revision of the scheme of classification of the Ciliophora, several groups of ciliatologists are preparing treatises on the subject. The present paper is concerned with the composition of the large new class of ciliates, Kinetofragmophora de Puytorac et al., 1974, established very recently by the French group. Several new taxa, at ordinal and subordinal levels, are proposed for inclusion in that class, with special emphasis on the new order to contain the most primitive of extant species. Actions taken here are incorporated in a major review and revisory work of the author which is being published elsewhere. The class Kinetofragmophora, by far the largest of the 3 classes now recognized as comprising the whole phylum Ciliophora, is itself considered to contain 4 sizeable subclasses and to embrace a total of 13 orders and 14 suborders. Two orders and 6 suborders are named and described here as new, enumerated and briefly identified as follows: Order Primociliatida n. ord., for the most “primitive” of gymnostomes, with three new suborders— Homokaryotina n. subord., for the homokaryotic genus Stephanopogon; Karyorelictina n. subord., for a number of mostly interstitial ciliates which, though heterokaryotic, possess nondividing, diploid macronuclei (e.g. Trachelocerca, Trachelonema, and Tracheloraphis); and Prorodontina n. subord., for a group of relatively specialized formerly “rhabdophorine” gymnostomes such as Coleps, Placus, and Prorodon and order Haptorida n. ord., for rapacious carnivorous forms, formerly lumped with the preceding groups as “rhabdophorines,” many with oral toxicysts and well developed thigmotactic ciliature (e.g. Actinobolina, Didinium, Dileptus, Enchelys, Spathidium, and Trachelius). All foregoing taxa are members of the 1st kinetofragmophoran subclass, the Gymnostomata. In the taxonomic conclusions drawn, new significance is placed on ultrastructural data, on macronuclear differences of evolutionary importance, and on habitat and behavior. A brief review of the literature on psammophilous ciliates is presented. In the subclass Vestibulifera is now located the order Entodiniomorphida Reichenow, a group formerly considered to be a spirotrich taxon. A suborder, Blepharocorythina n. subord., is proposed to contain the old “trichostome” family Blepharocorythidae, species commensalistic in horses and ruminants and now—with their syncilia, etc.—considered ancestral to the ophryoscolecids and relatives. In the subclass Hypostomata, order Nassulida, the suborder Paranassulina n. subord. is established to contain nassulids which appear more highly evolved than Nassula itself (e.g. Paranassula and Enneameron) in perioral ciliature, mode of stomatogenesis, etc. In the enigmatic and still vexatious order Rhynchodida, the suborder Aneistrocomina n. subord. is erected to embrace rhynchodid genera with an anteriorly located sucking tentacle (and other unique characteristics)—for example, Ancistrocoma, Crebricoma, Holocoma, and Sphenophrya. With the banishment of the bulk of the old “thigmotrichs” to the oligohymenophoran order Scuticociliatida, the ancistrocomines are left with the family Hypocomidae (and relatives) in the order Rhynchodida. It is not yet clear, however, how closely related the 2 suborders of rhynchodids should be considered. Special nomenclatural problems are also involved.