Leaf Area and Absolute Leaf Water Content

Leaf area was found to be an almost linear function of absoluteleaf water content (that is fresh weight—dry weight) forseveral dicotyledonous genotypes. A closely similar relationshipheld for a given genotype in the same rooting medium despiteincreasing plant size and different treatments which resultedin leaves which were sometimes morphologically dissimilar andoften substantially different in specific leaf water contents(water/dry matter). The usefulness of the linear relationshipfor estimating leaf area in growth studies is noted.     

Studies in Stomatal Behaviour: X. AN INVESTIGATION OF RESPONSES TO LOW-INTENSITY ILLUMINATION AND TEMPERATURE IN XANTHIUM PENNSYLVANICUM

Two experiments are described in which stomatal sensitivityto low-intensity white light was studied for Xanthium pennsylvanicumWall. In the first experiment a daylength extension for 7, 9, or 15hrs. was given using 10, 40, or 160 lux to shorten a basic 16-hr.night, which was also given at its full length as a tenth treatment.Measurements were made of stomatal opening ability on the morningfollowing the different treatments. With a 15-hr. extensionthere was at all intensities a significant response, shown bya reduced rate of opening in the morning. With a 9-hr. extensionusing 40 or 160 lux, opening ability was reduced, but 9 hrs.of 10 lux was insufficinet to produce a detectable effect. The7-hr. extension was ineffective at all three intensities. In the second experiment stomatal behaviour was observed during20 hrs. of either darkness or 10 lux at four temperatures (15,22, 29, and 36°C.). During 20 hrs. of darkness there wasnight opening at all temperatures, but at lower temperaturesit began sooner and lasted longer. These responses to temperaturedid not fit a simple linear relationship, there being a significantcubic term revealed by non-linear regression analysis. Thiscould be explained if the response was considered in terms ofthe magnitude of the change in temperature (from 25°C.)at the beginning of the experiment; there appeared to be sometemperature compensation over a limited range. in 10 lux, nightopening was suppressed at 29° and 36°, but at 15°it was apparently unaffected by the light; at 22° it wasnot completely suppressed by 10 lux but the time of its occurrencewas delayed. Effects of light and temperature are discussed in relation toan endogenous rhythm in darkness which was previoulsy shownto operate in Xanthium pennsylvanicum (Part IX). It is considered that to explain effects of very low intensitylight it will be necessary to recognize a ‘low intensityresponse’ by stomata, which does not operate via changesin guard-cell carbon dioxide.     

Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech

Beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO₂ concentrations (ambient and ambient + 250 μmol mol^?1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (g_s) of well-watered plants were taken over both seasons; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population. The net leaf photosynthetic rate of well-watered plants was stimulated in elevated CO₂ by an average of 75% in beech and 33% in oak; the effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO₂ concentration and nutrient level on A or g_s in beech or oak. Stomatal conductance was reduced in elevated CO₂ by an average of 34% in oak, but in beech there were no significant reductions in g_s except under cloudy conditions (–22% in elevated CO₂). During drought, there was no effect of CO₂ concentration on g_s in beech grown with high nutrients, but for beech grown with low nutrients, g_s was significantly higher in elevated CO₂, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO₂ due to increased leaf area. It appears that beech may substantially increase whole-plant water consumption in elevated CO₂, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.     

A single-locus minisatellite discriminates chinook salmon (Oncorhynchus tshawytscha) populations

A knowledge of genetic structure in natural populations is often necessary for conservation and management purposes, especially in declining Pacific salmon populations. To test for genetic differentiation between nine populations of chinook salmon, Oncorhynchus tshawytscha, from south-western British Columbia, Canada, DNA was extracted from 603 fish and hybridized with a single-locus minisatellite probe. Multivariate statistical analyses of the resulting allele size data permitted successful overall population identification of 52% (individual population range: 24–78%; P < 0.005), indicating a high level of genetic differentiation among the nine populations. Two of the nine populations were further analysed using data from a second minisatellite locus. The discrimination success rate improved from 81.1% (one-locus analyses) to 90.0% (two-locus analyses), indicating the potential for greatly increased resolution gained by the addition of more loci. These results indicate that variation at minisatellite loci can be used for assessing population-level genetic structure, even with artificial gene flow.     

Studies in Stomatal Behaviour: VI. AN INVESTIGATION OF THE LIGHT RESPONSES OF WHEAT STOMATA WITH THE ATTEMPTED ELIMINATION OF CONTROL BY THE MESOPHYLL

As in the previous investigation (Part I), stomatal responsesin wheat were followed by means of porometer readings; betweenthese, air was forced through the stomata to sweep out the substomatalcavities. Instead of forcing the whole of a relatively slowair-flow in through the stomata, however, a fast flow was sweptacross the leaf surface and a portion of this passed into theintercellular space system. This helped to maintain the desiredcarbon dioxide content close to the stomatal openings and thestomata could thus be treated with different carbon dioxideconcentrations. A factorial experiment was carried out in which6 carbon dioxide concentrations from zero to 0084 per cent,were combined with 3 light intensities from 90 to 800 f.c, the18 treatments being replicated 6 times in three ‘66 latinsquares’. A supplementary experiment in which the same6 carbon dioxide treatments were given in darkness was carriedout in duplicate. The results confirmed previous findings inthat the stomata showed no appreciable response to variationof external carbon dioxide supply between zero and 001 percent, concentration and that a large and highly significantlight response was found at each of these two carbon dioxidelevels. This is held to indicate the existence of a light effect(direct or indirect) not operating by reduction of the internalcarbon dioxide content of the guard cells. On the assumptionthat the rates of air flow through the stomata were adequate,the combined results of the two experiments are interpretedmainly in terms of two direct effects of light upon the stomata,one depending on photosynthetic reduction of carbon dioxidein the guard cells and the other, mentioned above, not so operating.To these must be added the indirect effect responsible for the‘transmission’ described in Part 1, which mightpossibly account for the whole of the second direct effect postulated.Finally, if the rates of flow were in fact grossly inadequate,the trends attributed to the first direct effect mentioned abovemight have been due to indirect control by the mesophyll viathe intercellular space atmosphere. Such control must accountfor part of the light responses which occur under natural conditions.The natural light response is thus made up of at least two andmore probably four components—two direct and two indirect,in each case one operating by the internal carbon dioxide andthe other not. A highly significant diurnal rhythm of stomatalmovement under constant illumination and temperature is shownto occur in wheat. The measure of stomatal resistance used (logresistance per 10,000 stomata) is discussed, and it is concludedthat although it is empirical a much more satisfactory measureis not at present available.     

The pulmonary circulation of the Tibetan snow pig (Marmota himalayana)

Studies are presented on the pulmonary circulation of three Himalayan marmots ( Marmota himalayana (Hodgson)) captured in the mountains north of Lhasa, Tibet. The pulmonary arterial pressure and resistance were low, the right ventricle was not hypertrophied and the arterioles and terminal portions of the muscular pulmonary arteries had thin walls. The observations suggest that this species has adapted through natural selection to high altitude by losing the capacity for hypoxic pulmonary vasoconstriction.     

Carnivores and carotenoids are associated with adaptive behavioural divergence in a radiation of gall midges

1. Adaptive divergence in sympatry is supposed to be inhibited by the homogenizing role of gene flow. However, studies continue to uncover examples of sympatric divergence. In this study, two divergent phenotypes in a complex of four syntopic gall midge morphotypes [nominally Asteromyia carbonifera Osten Saken, Diptera: Cecidomyiidae: Alycaulini] are characterised. The first is a behavioural phenotype governing within‐host tissue preference and the second is a trait governing accessory‐gland carotenoid quality and quantity. 2. One gall morphotype (crescents) lay most of their eggs on mature tissue whereas the other three gall morphotypes oviposit only on young emerging leaves. Ecological maintenance of this divergent trait appears to be driven by enemy‐reduced space. That is, nearly 40% of the crescent morphotype galls that develop high on the plant are attacked by the egg parasitoid Platygaster solidaginis Ashmed, whereas those low on the plant are relatively protected. 3. All morphotypes contain carotenoids in their accessory glands, but the quality and quantity of these pigments differs significantly between the morphotypes and is positively associated with the probability of parasitism by P. solidaginis. 4. Larval salivary glands also contain carotenoids and the plant hormone abscisic acid, which in plants is synthesized from carotenoid precursors and is involved in regulating plant defences. This hormone may facilitate gall development and influence gall morphology. 5. Ecological fitness trade‐offs between carotenoids, parasitoid attack, and plant resistance may be fostering adaptive divergence in ovipositional phenotypes and sympatric speciation in this complex of gall midge morphotypes.     

Development of pea bacterial blight caused by Pseudomonas syringae pv. pisi in winter and spring cultivars of combining peas (Pisum sativum) with different sowing dates

Pea bacterial blight occurred by natural infection in a field trial on peas in 1995. Disease development in the winter cultivars Rafale, Frilene and Froidure was compared with that in the spring cultivars Baccara, Conquest and Bohatyr, each sown on six dates in October, November, December, mid-March, late March and April. Disease incidence had reached 100% plants affected in all treatments by mid-July. Disease severity was greater in winter-sown (October, November or December) than in spring-sown peas of each cultivar at each assessment. Significant (P < 0.05) differences in disease severity occurred between cultivars in the winter-sown plots in May and June and the spring cultivars were affected more severely than the winter cultivars. Comparison of areas under the disease progress curves for both disease incidence and severity also showed that the winter-sown peas were more affected by disease than spring-sown peas and that spring cultivars were more severely affected than winter cultivars. Yield was strongly correlated with disease severity. A linear regression model suggested that, for peas sown in October, November or December, a yield loss of 0.5 tha^-1 occurred for each 10% increase in canopy area affected by pea bacterial blight.