The Influence of Water Strain on the Minimum Intercellular Space Carbon dioxide Concentration {lambda} and Stomatal Movement in Wheat Leaves

Eight detached wheat leaves were each subjected to varying degreesof controlled water strain, in a specially constructed leafchamber (which is described in detail) connected to a closedsystem incorporating an infra-red gas analyser. The water strainwas varied by applying successively to their leaf sheaths twoconcentrations of mannitol solution (0·2 M. and 0·4M.) each preceded and followed by water. Conditions were maintainedconstant for light (900 f.c.), leaf temperature (25° C.)and humidity (50 per cent. RH) of the circulating air as itentered the leaf chamber. Steady-state values (gamma) of thecarbon dioxide concentration in the closed system were recordedboth for the water and mannitol treatments and the data analysedstatistically. The two mannitol treatments were found to increasegamma from 80 p.p.m. with water to 94 and 116 p.p.m. respectively,all differences being highly significant. Concomitant measurementswere made of stomata1 resistance with a porometer and of transpirationby means of wet and dry thermocouples; the stomata showed markedclosure in response to mannitol treatment (complete closurewith the higher concentration) and partial reopening with theleaf sheath in water once more, transpiration also falling andrising inversely with water strain. It is concluded that part of the closing response of wheat stomatato water strain under natural conditions must be operated byincrease in the intercellular-space carbon dioxide concentration,but a comparison with previous experimental results suggeststhat this can only be a small part unless sensitivity to carbondioxide is greatly increased by water loss. The initial response to mannitol treatment was a preliminaryopening of the stomata and a fall of about 10 per cent. in carbondioxide concentration. These results imply that under naturalconditions the preliminary opening of wheat stomata with wiltingmay be partly passive and partly active with a carbon dioxide-operatedmechanism. A subsidiary experiment with water strain varied by removingand restoring the water supply to the leaf sheath gave increasesand decreases in carbon dioxide similar to those in the mainexperiment; the results of the latter need not, therefore, beattributed to metabolism of the mannitol. It is suggested that values of gamma give a good measure ofthe efficiency of the mesophyll for net carbon dioxide absorption,with the stomatal factor eliminated, under the given conditionsand with carbon dioxide ‘limiting’ in the senseused by Maskell (1928).     

Guard Cell Pressures and Wall Properties during Stomatal Opening

Pressures required to produce stomatal apertures of differentwidths have been measured in guard cells of T. virginiana andC. communis. These pressures are lower than those that coulddevelop in guard cells at full turgor on account of their osmoticproperties. During the Spannungsphase the metabolism of guardcells seems to contribute towards a reduction of the elasticmodulus of their walls, whereas during the motorphase the volumetricmodulus of the cell as a whole, apparently increases gentlyat first and then more markedly as maximum apertures are approached.     

Seed banks cause elevated generation times and effective population sizes of Arabidopsis thaliana in northern Europe

It is commonly found that effective population sizes of natural populations are much smaller than census sizes of plants and animals. However, theoretical studies have shown that factors rarely investigated empirically, like seed banks in plants and diapause in animals, may have profound influence on effective sizes. Here we investigate whether the presence of seed banks can explain the relatively high genetic variability observed in northern European Arabidopsis thaliana populations with small census sizes. We have genotyped three above- and below- ground cohorts in 27 Norwegian populations using single nucleotide polymorphism markers. Although the populations varied extensively in levels of variability within and between cohorts, standard genetic population measures were comparable to those obtained in previous studies on above-ground cohorts using microsatellite markers. Estimated effective population sizes are larger for total populations (containing both seed bank and above-ground cohorts for 1 year) compared to each of the cohorts considered separately. Using a conservative approach, we find that the effective sizes are larger than census sizes of local populations, and that the effective generation time is higher than 1 year (3–4 years, on average), making A. thaliana a perennial semelparous plant at many northern European localities.     

Efficiency of Root Respiration of a Flood-Tolerant and a Flood-Intolerant Senecio Species as Affected by Low Oxygen Tension

A method was described to determine root growth respiration and root maintenance respiration rate of plants, grown in culture solutions of high and low oxygen concentrations, during linear growth. Root growth respiration of aerobically grown plants was three to four times lower in the flood-intolerant Senecio jacobaea L. than in the flood-tolerant species. Senecio aquaticus Hill. Root growth respiration of Senecio aquaticus Hill decreased by a factor two to three upon transplantation to a culture solution of low oxygen tension. The difference in root growth respiration between aerobically and anaerobically grown Senecio aquaticus Hill was ascribed to the presence of a highly active non-phosphorylating oxidase.     

Effects of Water Stress on the Resistance to Uptake of Carbon Dioxide in Tobacco

An attempt was made to determine the relative importance ofthe stomata in restricting CO₂ uptake under conditions of waterstress. The air-phase and liquid-phase resistances to uptakeof CO₂ were determined from measurements of the rates of assimilationand transpiration and the corresponding concentration gradientsof CO₂ and H₂O vapour. The results showed that the air-phaseresistances (stomata) could account for only half the reductionin the rate of photosynthesis accompanying water stress. Experimentsin which air was passed through the leaf confirmed that waterstress restricted CO₂ fixation within the leaf itself. The resultsand their possible explanation are discussed in relation toother work.     

Variation in multiple paternity and sperm utilization patterns in natural populations of a simultaneous hermaphrodite land snail

Mating frequency has important implications for patterns of sexual selection and sexual conflict, and hence for issues such as the maintenance of genetic diversity and speciation. We assessed the level of multiple paternity and sperm utilization patterns in four natural populations of the simultaneous hermaphrodite land snail Arianta arbustorum using four polymorphic microsatellite loci. A total of 1088 offspring from 26 wild‐caught snails were genotyped to determine the number of fathers siring each brood and paternity skew in succeeding clutches. Multiple paternity was detected in the offspring of all 26 mother snails examined with the contribution of two to six fathers. The four populations examined differed in the level of multiple paternity. Snails in the population with the highest density of adults showed the highest level of multiple paternity, whereas snails in the population with the lowest density exhibited the lowest value of multiple paternity. Highly skewed paternity patterns were found in the progeny of 15 (57.7%) of the 26 mother snails. The number and identity of fathers siring the offspring of single mothers also varied among successive clutches. Furthermore, genetic analyses indicate a low level of self‐fertilization in one of the four populations. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 350–361.