A reinterpretation of stomatal responses to humidity

The stomatal conductance (g) for single leaves and the equivalent canopy conductance for stands of vegetation are often represented in models as empirical functions of saturation vapour pressure deficit or relative humidity. The mechanistic basis of this dependence is very weak. A reanalysis of 52 sets of measurements on 16 species supports the conclusion of Mott & Parkhurst (1991, Plant, Cell and Environment 14, 509–515) that stomata respond to the rate of transpiration (E) rather than to humidity per se. In general, ?g/?E is negative and constant so that the relation between g and E can be defined by two parameters: a maximum conductance g_m obtained by extrapolation to zero transpiration, and a maximum rate of transpiration E_m obtained by extrapolation to zero conductance. Both parameters are shown to be functions of temperature, CO₂ concentration, and soil water content. Exceptionally, transpiration rate and conductance may decrease together in very dry air, possibly because of patchy closure of stomata.     

Long‐term variation in the littoral food web of an acidified mountain lake

Evidence of chemical recovery from acidification in European freshwaters has emerged in recent years, with many previously damaged systems responding to decades of reduced acidifying emissions. Biological recovery, however, has often lagged behind, and this has been ascribed to several possible mechanisms, including inertia in the food web. We examined two decades of change in hindcasted food webs for Lochnagar, a Scottish mountain lake, to make inferences about the potential dynamical stability of the system and to assess the prospects for future biological recovery. Although community composition tracked temporal changes in acidity, this was neither sustained nor directional, and mainly manifested as shifts in relative abundances rather than the establishment of more acid‐sensitive species. The food web was highly interconnected and reticulate, especially in years when species richness was low, and subsidized by external inputs of detritus. Among the primary consumers, generalist herbivore–detritivores maintained feeding links with the scant algal resources, which appeared insufficient to support viable populations of specialist grazers. Together, these characteristics, which are shared with many other acidified freshwaters, are likely to make the community dynamically stable and resistant to invasions of potential new colonists, thereby slowing the pace of future biological recovery.     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): II. ALTERNATING TEMPERATURE

Methods of analysing the response of germination to constanttemperature are extended to alternating temperatures. The analysisis illustrated for seeds of pearl millet (Pennisetum typhoidesS. & H.) germinated on a thermal gradient plate in pairsof alternating temperatures ranging from 15?C to 47?C. Alternatingtemperatures had a small but systematic effect on germinationrate such that below 42 ?C, alternations in temperature hadno effect on the maximum fraction of seeds which germinatedin the population, but increases in temperature amplitude fromzero to ?8 ?C caused a small but systematic increase in therate of germination.     

Daylength change and leaf appearance in winter wheat

Abstract In the field successive leaves of winter wheat appear at a rate which varies because it depends strongly upon temperature. When plotted against ‘thermal time’, however (temperature accumulated above a fixed base of 0°C), leaf appearance was a strictly linear function of temperature. The mean rate of leaf appearance in thermal time, R′, was faster for a spring sowing than for an autumn sowing. The variation in R′ between sowings was better correlated with the rate at which daylength was changing when the plants emerged than with the mean daylength while leaves were appearing.     

Response to Temperature in a Stand of Pearl Millet: VI. LIGHT INTERCEPTION AND DRY MATTER PRODUCTION

Stands of pearl millet were grown in glasshouses in which meanair temperature was controlled to 19, 22, 25, 28 and 31 ?C withan amplitude of ?5 ?C. During the main growth period, leaf areaindex increased at a constant rate which was proportional tomean temperature above a base of 10 ?C. The warmest stand, therefore,intercepted more radiation before anthesis but the transmissioncoefficient was independent of temperature (K 0.3). Based ondry weight at final harvest, the efficiency of conversion forintercepted radiation ranged from about 2.1–2.4 g MJ^–1consistent with field experience. Combining this informationwith figures for the duration of growth in relation to temperaturesuggests that growth rate should be maximal at 25–27 ?Cand total dry weight at 20–22 ?C. Key words: Temperature, Pearl millet, Growth rate, Light     

Evaluation of Techniques for Categorizing Group Membership of White-Tailed Deer

Abstract: We studied sexual segregation, particularly patterns of group membership for white-tailed deer (Odocoileus virginianus), in Lincoln County, Minnesota, USA, to evaluate current techniques used to categorize animals when studying sexual segregation. We categorized group membership according to Hirth (1977) and grouped individuals using our solitary categorization method. Our solitary method was most sensitive to changes in behavior and physiology exhibited by reproductively active females and their association with other deer during sexual segregation.     

The recent palaeolimnology of a remote Scottish loch with special reference to the relative impacts of regional warming and atmospheric contamination

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Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): I. CONSTANT TEMPERATURE

The germination of pearl millet (Pennisetum typhoides S. &H.) seeds was investigated at constant temperatures between12 ?C and 47 ?C on a thermal gradient plate. The rate of germination increased linearly with temperaturefrom a base T_b to a sharply defined optimum T_o beyond whichthe rate decreased linearly with temperature, reaching zeroat T_m. The linearity of the response both above and below T_oallowed time and temperature to be combined in a thermal timeat which a specified fraction of the seeds germinated. Withinthe population T_b and T_m were constant.     

Time, Temperature and Germination of Pearl Millet (Pennisetum typhoides S. & H.): III. INHIBITION OF GERMINATION BY SHORT EXPOSURE TO HIGH TEMPERATURE

Seeds of pearl millet were germinated on wet filter paper attemperatures up to 50 ?C. In one experiment, the temperaturewas held at 50 ?C during imbibition and was then lowered to32 ?C or 25 ?C. Germination rate and the maximum fraction ofseeds germinating (G_m) both decreased as the time of exposureto 50 ?C increased. In contrast, exposure to 50 ?C after imbibitionfor 8 h slowed germination but did not significantly reduceG_m. When the ‘high’ temperature imposed after imbibitionwas reduced from 50 ?C to 45 ?C, there was a small reductionin the rate of germination but not in G_m. The responses haveimplications for the optimum time of sowing in the tropics whenmaximum daytime soil temperature at the depth of sowing is inthe range of 45–50 ?C. Key words: Pennisetum typhoides, Temperature, Germination