Effect of food limitation on food consumption and reproductive allocation by adult milkweed bugs, Oncopeltus fasciatus

Male/female pairs of adult milkweed bugs, Oncopeltus faciatus were offered reduced food rations (100 mg milkweed seeds/week, 50 mg seeds/week, and 25 mg seeds/week) at 232C, 14L : 10D photoperiod. The high rate of food consumption normally occurring during the teneral period (first 8 days posteclosion) and subsequent food consumption were reduced on both reduced rations. Initial fresh weight was lost by bugs on both reduced rations and their weekly fresh body weight tended to remain less than bugs on the 100 mg seeds/week ration.Normal onset of egg production (about 3 weeks posteclosion) was delayed 6 weeks and normal length of a female's life (13–14 weeks) was increased 7–8 weeks on the 25 mg seeds/week ration. Rate of egg production exhibited a linear decrease with decrease in food ration on both reduced rations due to an increased interclutch interval, while fresh egg weight, duration of egg stage, number of eggs/clutch and percentage of clutches containing fertile eggs were little affected.Gross egg production efficiency (efficiency of conversion of ingested food into eggs = dry mg egg production × 100/dry mg food ingested) was high on both the 100 and 50 mg seeds/week rations (27.9% and 21.1%, respectively) and low (7.7%) on the 25 mg seeds/week ration. These data are discussed in the context of the reciprocal interaction between nutrient intake and egg production.     

Correction of severe manganese deficiency in wheat with chemical fertilizers

Summary Manganese, N and P fertilizers were applied to wheat in field experiments on a soil so deficient in Mn that it caused the wheat to die before heading. Yields of wheat were increased linearly by soil banded Mn to 44.8 kg/ha, giving a yield of 3.03 tonnes/ha. Yields were increased to a lesser extent by foliar-applied Mn and least by soil-broadcasted Mn. Soil N and P appeared to be adequate, yet ammonium sulphate at 56 kg N/ha where applied alone caused a yield of 1.69 tonnes/ha and ammonium sulphate nitrate gave a yield of 0.98 tonnes/ha, the increases being primarily due to the release of Mn to the plants. Calcium nitrate and triple superphosphate were much less effective in releasing Mn.     

Spatial variance of nutrient limitation of periphyton in montane, headwater streams (McLeod River, Alberta, Canada)

There have been few studies that have examined the spatial variance of nutrient limitation over the scale of an entire set of headwater streams. We used nutrient diffusing substrata experiments (control, nitrogen addition, phosphorus addition, and nitrogen+phosphorus addition) to examine how nutrient limitation varied throughout the five creeks that comprise the McLeod River headwaters (Alberta, Canada). We assessed the variance of chlorophyll a accrual at spatial scales within reach, within creek, among creeks and across linear distance within the entire watershed to assess the consistency and scale of nutrient limitation. We analyzed the importance of the spatial scale using several methods. We assessed the coefficient of variation at different scales, the spatial covariance of nitrogen and phosphorus deficiency indices using a spline correlogram, and the variance through traditional analyses of variance methods. Chlorophyll a accrual responded significantly to nutrients in all creeks, though the response varied in magnitude and in the limiting nutrient among reaches and among creeks. Variance in chlorophyll a accrual was due primarily to the factor of creek (R ²=0.40) and secondarily to reach (R ²=0.07). The CV was 31.4% among creeks, 18.4% among reaches, and 17.9% within reaches. The N deficiency index showed a positive correlation at sites located <4 km apart and a negative correlation at sites greater than 6.5 km apart. The P deficiency index showed no discernible spatial correlation. Our results suggest that nutrient limitation varies on small scales and is often driven by local processes.     

Role of oxygen limitation and nitrate metabolism in the nitrate inhibition of nitrogen fixation by pea

The impact of nitrate (5–15 m M , 2 to 7 days) on nitrogenase activity and nodule-oxygen limitation was investigated in nodulated, 21-day-old plants of a near-isogenic nitrate reductase-deficient pea mutant (A3171) and its wild-type parent ( Pisum sativum L. cv. Juneau). Within 2 days, 10 or 15 m M nitrate, but not 5 m M nitrate, inhibited the apparent nitrogenase activity (measured as in situ hydrogen evolution from nodules of intact plants) of wild-type plants; none of these nitrate levels inhibited the apparent nitrogenase activity of A3171 plants. Nodule-oxygen limitation, measured as the ratio of total nitrogenase activity to potential nitrogenase activity, was increased in both wild-type and A3171 plants by all nitrate treatments. By 3 to 4 days the apparent nitrogenase activity of A3171 and wild-type plants supplied with 5 m M nitrate declined to 53 to 69% of control plants not receiving nitrate. By 6 to 7 days the apparent nitrogenase activity of A3171 plants was similar to the control value whereas that of the wild-type plants continued to decline. From 3 to 7 days, no significant differences in nodule-oxygen limitation were observed between the nitrate (5 m M ) and control treatments. The results are interpreted as evidence for separate mechanisms in the initial (O₂ limitation) and longer-term (nitrate metabolism) effects of nitrate on nitrogen fixation by effectively nodulated pea.     

Influence of plant nutrient concentration on growth rate: Use of a nutrient interruption technique to determine critical concentrations of N,P and K in young plants

A method is described for determining the way in which growth rate varies with plant nutrient concentration using a simple nutrient interruption technique incorporating only 2 treatments. The method involves measuring the changes in growth and nutrient composition of otherwise well-nourished plants after the supply of one particular nutrient has been withheld. Critical concentrations are estimated from the relationship between the growth rate (expressed as a fraction of that for control plants of the same size which remained well-nourished throughout) and the concentration of the growth-limiting nutrient in the plants as deficiency developed. Trials of the method using young lettuce plants showed that shoot growth rate was directly proportional to total N (nitrate plus organic N) concentration, and linearly or near-linearly related to K and P concentration over a wide range; the corresponding relationship for nitrate was strongly curvi-linear. Critical concentrations (corresponding to a 10% reduction in growth rate) determined from these results were similar to critical values calculated from models derived from field data, but were generally higher than published estimates of critical concentration (based on reductions in shoot weight) for plants of a similar size. Reasons for these discrepancies are discussed. Nitrate, phosphate or potassium concentrations in sap from individual leaf petioles were highly sensitive to changes in shoot growth rate as deficiency developed, with the slope of the relationships varying with leaf position, due to differences both in their initial concentration and in the rates at which they were utilized in individual leaves. Each nutrient was always depleted more quickly in younger leaves than in older ones, providing earlier evidence of deficiency for diagnostic purposes. Although the plants were capable of accumulating nitrate, phosphate and potassium well in excess of that needed for optimum dry matter production during periods of adequate supply, the rate of mobilization of these reserves was insufficient to prevent reductions in growth rate as the plants became deficient. This brings into question the validity of the conventional concept that luxury consumption provides a store of nutrients which are freely available for use in times of shortage. The implications of these results for the use of plant analysis for assessing plant nutrient status are discussed.     

Mechanisms underlying insect chill-coma

At their critical thermal minimum (CT_min) insects enter chill-coma, a reversible state where neuromuscular transmission and movement cease. The physiological mechanisms responsible for the insect CT_min remain poorly understood despite the regular use of chill-coma onset and recovery as a means to assess evolved or acquired variation in low temperature tolerance. In this review, we summarize the use of chill-coma as a metric of thermal tolerance to date, and synthesise current knowledge on the nature and plasticity of lower thermal limits to present probable physiological mechanisms of cold-induced failure. Chill-coma is likely to be driven by an inability to maintain ionic homeostasis through the effects of temperature on ion-motive ATPases, ion channel gating mechanisms, and/or the lipid membrane, leading to a loss of nerve and muscle excitability.     

The presence of co-flowering species facilitates reproductive success of Pedicularis monbeigiana (Orobanchaceae) through variation in bumble-bee foraging behaviour

Background and Aims The presence of co-flowering species can alter pollinator foraging behaviour and, in turn, positively or negatively affect the reproductive success of the focal species. Such interactions were investigated between a focal species, Pedicularis monbeigiana, and a co-flowering species, Vicia dichroantha, which was mediated by behaviour alteration of the shared bumble-bee pollinator. Methods Floral display size and floral colour change of P. monbeigiana were compared between pure (P. monbeigiana only) and mixed (P. monbeigiana and V. dichroantha) plots in two populations. Pollinator visitation rates, interspecific floral switching and successive within-plant pollinator visits were recorded. In addition, supplemental pollination at plant level was performed, and the fruit set and seed set were analysed in pure and mixed plots with different densities of P. monbeigiana. Key Results Pollinator visitation rates were dramatically higher in mixed plots than in pure plots. The higher pollinator visitation rates were recorded in both low- and high-density plots. In particular, successive flower visits within an individual plant were significantly lower in mixed plots. Supplemental pollination significantly increased fruit set and seed set of individuals in pure plots, while it only marginally increased seed set per fruit of plants in mixed plots. Conclusions The presence of V. dichroantha can facilitate pollination and increase female reproductive success of P. monbeigiana via both quantity (mitigating pollinator limitation) and quality (reducing geitonogamy) effects. This study suggests that successive pollinator movements among flowers within a plant, as well as pollinator visitation rates and interspecific flower switching, may be important determinants of the direction and mechanisms of interaction between species.