Solute Accumulation in Plant Cells V. An Aspect of Nutrition and Development

The need to re-evaluate concepts of salt and solute accumulationin the light of evidence derived from cells at all stages oftheir growth and development is recognized. The problem is seenin terms of the nutrition of flowering plants, the growing cellsof which are essentially heterotrophic, and the solutes of whichare progressively acquired and redistributed during ontogeny.This is traced from the zygote in the embryo sac to an establishedplant body with its evident ‘source-sink’ relationshipsand physiological ‘division of labour’ between organs.The evidence accrued from aseptic cultures which were manipulatedto reveal the range of solutes in cells which simulated thenormal course of development in situ as they multiplied, vacuolated,enlarged, and eventually matured. The regulatory control exercisedby cells in these developmental stages over the total osmoticvalue and the relative composition of their solutes (organicand inorganic) is both described and interpreted. The reversiblechanges that may occur (within a regulated osmotic value) inthe solutes of established cells as they replace sugars by saltsof organic acids, by organic nitrogen compounds, or by alkalihalides are both described and related to events that occurin the developed plant body. Particular significance is attachedto the consequences of the normal need of land plants to acquirenitrogen from nitrate and of the intervention of reduced nitrogenunder circumstances in which the need for non-metabolizableions (e.g. alkali halides) is, thereby, drastically curtailed.Cells in multiplication require energy to create new structureand do not emphasize the accumulation of solutes in bulk; however,when they enlarge, energy is obligated to the storage of solutes(organic and inorganic) to support their cytoplasm which isbeing ‘spread out thin’. These events involve morethan the properties of membranes, or their relations to individualions or molecules, for they require an understanding of cellsas compartmented, metabolic, and osmotic machines, and of theirvariously obligated energy relationships. Moreover, the subjectnow needs to be seen as an aspect of the over-all nutritionof cells, organs, and organisms as they grow and develop.     

Growth and Development of the Banana Plant: II. The Transition from the Vegetative to the Floral Shoot in Musa acuminata cv. Gros Michel

The changes that occur in the shoot apex of the banana, as itpasses from the vegetative to the flowering stage, are described.The crucial events occur well before floral primordia are evident,and they require a redistribution of activity in the variousgrowing regions. The vegetative shoot apex is in a central depressionin the rhizome; there is virtually no internodal growth in theaxis, the most active growth is in the leaf bases; vegetativebuds do not form in the leaf axils but only appear adventitiouslyfar from the tip of the shoot. With the onset of flowering thisis changed; growth in the axis itself, previously suppressed,occurs and flower buds arise as primordia in the axils of subtendingbracts. The bracts do not show the market growth in their baseswhich is so characteristic of leaves. Thus, the shoot apex risesto the level of the rhizome and then above it; as it does so,its tip changes in shape from a broad flattened some to a pointedcone. At the transitional stage, more activity occurs in thecells of the mantle, or tunica, which now consists of 3 to 4layers over the central dome. Below, in the central or mothercell zone of the corpus, which was quiescent in the vegetativeshoot, the cells spring into greater activity, becoming moreprotoplasmic and stain more deeply. Directly below this regionin the rib meristem, cells show transverse divisions. Bractprimordia occur high on the flanks of the apex, and, thoughthey originate in the manner of leaves, their subsequent growthis different. Flower primordia occur even in the axils of bractsclose to the shoot tip. Thus, the problem now is to designatethe source, nature, and mode of action of the stimuli whichinitiate and control this quite different distribution of growthin the floral, as contrasted with the vegetative, shoot. Thesignificance of the previously more quiescent central, or mothercell zone, of the apex as the source of such stimuli, is stressed.Thus, flowering first requires that the limiting controls whichapply to the vegetative shoot be released, and, secondly, thatthe apex of the shoot, rather than the leaf base, becomes themain centre of growth and development.     

Predation by wasps on lepidopteran larvae in an Ozark forest canopy

ABSTRACT.

• 1 Predation by birds, crawling arthropods (ants, harvestmen, spiders), and social wasps (Vespula) spp. on introduced stocks of Heliothis virescens (Fabricius) larvae was investigated in a oak-hickory forest canopy in northwestern Arkansas (U.S.A.).
• 2 Wasps, Vespula maculifrons (Buysson) and V.squamosa (Drury), removed over 90% of the larvae. Repeated visits to a feeding site by the same marked wasps accounted for removal of either a single larva or all larvae. Larvae pinned (punctured) to artificial leaves were selected over 70% of the time by wasps when compared to attachments that did not puncture larvae; however, unpunctured larvae were taken.
• 3 Crawling arthropods accounted for low levels of predation, and birds did not appear to prey on larvae. Apparently wasps removed larvae rapidly and efficiently, thereby depleting the feeding sites before other predators discovered the larvae.
• 4 Although attaching larvae to artifical hickory leaves provided an easy method for placing larvae into the forest canopy, a lower percentage of larvae were removed from these leaves compared to natural hickory leaves. Moving feeding sites did not influence the number of larvae taken.

     

The Effect of Various Media on the Growth Responses of Different Clones of Carrot Explants

The induction of growth in otherwise quiescent tissue explantedfrom carrot root has been investigated with reference to theeffects of different kinds of growth-promoting substances addedas supplements to a basal medium, singly and in combination.The effects of these media upon different clones of carrot explantsare described. The idiosyncrasies of different clones of explantswere detected by their responses measured by the incidence ofcell division, the extent of cell enlargement, and by theirnucleic acid content. The basal medium which contains salts,sugar, and vitamins supported only a minimal amount of growth;the basal medium supplemented with casein hydrolysate and coconutmilk (10 per cent by volume) supported the highest level ofgrowth obtained in any of the treatments tried. The active componentsof the coconut milk (AF_cm) when refined required the furtherparticipation of either indole-3yl-acetic acid (IAA) or inositol,and were further stimulated by casein hydrolysate (CH). Thusthe over-all stimulus of the coconut milk comprised two parts—nowrecognized as growth-promoting systems I and II, respectively.The effects of System I were mediated by appropriate combinationsof inositol and the corresponding active growth-promoting factors(AF₁) which were, in turn, represented by a purified factorpreviously isolated from Aesculus (AF₂). System I induced bothcell division and cell enlargement in balance, whereas SystemII stimulated internal cell division more than cell enlargement.The effects of System II were mediated by appropriate combinationsof IAA and active growth-promoting factors (AF₂), which wererepresented by the substance zeatin. The maximum growth of anygiven clone of carrot explants isolated from a given carrotroot was only supported by exogenous requirements, over andabove a basal nutrient medium, which meet its specific endogenouslimitations. The paper shows how these limitations may be diagnosed,and discusses the over-all growth stimulus due to coconut milkin terms of the partial responses elicited by the known componentsof Systems I and II.     

Solute Accumulation in Plant Cells: I. Reciprocal Relations Between Electrolytes and Non-Electrolytes1

This paper presents the concepts, the analytical methods, andthe experimental devices used in a reappraisal of the problemsof solute and water uptake which utilizes both quiescent andactively growing cells. The tissue used is drawn from the secondaryphloem of the carrot root and, in all experiments, it is underconditions of aseptic culture which permit both inorganic andorganic solutes to be studied for relatively long periods. The range of responses of the explanted carrot tissue has beenobserved in different media. These include simple inorganicsalt solutions (CaCl₂, KC1, NaCl, etc.), a full organic andinorganic nutrient medium and also the latter supplemented bystimuli that unleash the full ability of the otherwise restingcells to grow. The effects on both growth and composition of the cells havebeen observed with time. The high osmotic value of the maturenon-growing cells may be made up, non-specifically, by salts(KC1, NaCl) or organic solutes (sugars) which are accumulated;when growth is not primarily involved these solutes may thenbehave reciprocally in accordance with supply, in the media,and demand, in the cells. Rapidly dividing cells, on the other hand, creating vacuoles,have lower osmotic value, greater specificity for potassium,and the solutes they store are under more endogenous than exogenouscontrol. Between these extremes the solutes which are accumulated dependupon the levels of growth induced which in turn are responsesto the nutrients and stimuli furnished. These observations and their interpretation set a trend forthe papers that are to follow.     

Solute Accumulation in Plant Cells: III. Treatments which Arrest and Restore the Course of Absorption and Growth in Cultured Carrot Explants1

Prior papers have dealt with the range of the growth responsesof carrot explants to the composition of the ambient media andhow these affected the solute concentration and compositionof the tissue. They have also dealt with the sequential eventsalong the time course of growth of the tissue explant. Thispaper presents results and conclusions derived from experimentswhich exploit changes in the growing explants when their normalcourse of growth and solute uptake is interrupted by exposingthem sequentially to different ambient media. After explants were induced to grow during an initial 6 days,they were placed in a minimal nutrient medium which lacked growthstimuli and salts, notably potassium ions. Thus the internalsalt concentrations of the cells declined as they continuedto divide and enlarge at a reduced rate and their osmotic valuewas maintained by storage of organic solutes (sugar). The subsequentresponses of these ‘low salt’ cells to differentnutrient regimes were studied. When salts were resupplied, growth was stimulated somewhat andthe osmotic value of the cells increased as salts were accumulated;with a renewed full nutrient medium and a full complement ofgrowth substances, the cultures re-embark upon their growthand attain the average cell size and composition as if theyhad not been reversibly arrested. Thus reversible trends insolute composition of cells may be superimposed upon their normaldevelopmental course by alternately withholding and restoringthe stimuli to their growth and by changing the balance betweenorganic and inorganic solutes supplied in the medium. The emphasis is on the control of osmotic value in the cellsas they enlarge and mature and this over-rides the changes inthe solutes they receive (salts or sugars) via the ambient medium.The effects here observed were induced by sequential changesin the culture medium, but these obviously relate to similarresponses of cells in the intact plant body as their growthand solute supply is modified through interactions between organsas the plant grows.     

Solute Accumulation in Plant Cells: II. The Progressive Uptake of Non-electrolytes and Ions in Carrot Explants as They Grow

An earlier paper established the range of solute compositionthat may obtain in aseptically cultured carrot explants growingin different media which regulate either cell enlargement orcell division in the explants. This paper concentrates uponthe most rapidly growing cultures, containing cells which individuallypass through their cycle of division and enlargement and collectivelytrace a sigmoidal curve of growth for the explant as a whole.The time course of growth is interpreted in terms of the numberand average size of the cells and, in different phases, in termsof the changes in solute uptake and content of the cells. Thesedata are correlated with the concomitant metabolic characteristicsof the tissue, notably its protein synthesis. In the early exponential growth phase of the explant, when theemphasis is on cells in division, the organic solutes whichare absorbed are used to create form and complex substance;concomitantly, the cells develop a specific requirement forpotassium, selectively preferred to sodium, and balanced byorganic anions rather than halide. These relationships changeas cells develop; the emphasis is then upon the maintenanceof osmotic value in cells with enlarging vacuoles. The developingvacuoles preferentially store organic solutes but, later, thesesolutes may be replaced by natural salts (KC1, NaCl) when organicsupplies are depleted. This latter accumulation of salts doesnot display as markedly the disparity between potassium, sodium,and chloride which was so evident in the cell multiplicationphase of growth. Superimposed upon these relationships are certain clonal differenceswhich are interpreted compatibly with the above concepts. Thedata obtained on many clones reinforce the view that the changesin ion relations of cells with growth, as noted above, are compensatedby accumulation of organic solutes as cells build osmoticallyactive concentrations of solutes in a system that primarilycontrols the internal activity of their water.