Leaf Nitrogen Levels Research Articles

Herbivore Adaptations to a Low-Nutrient Food: Weed Biological Control Specialist Spodoptera pectinicornis (Lepidoptera: Noctuidae) Fed the Floating Aquatic Plant Pistia stratiotes

Performance of the specialist herbivore Spodoptera pectinicornis (Hampson) was studied when fed the floating aquatic plant waterlettuce, Pistia stratiotes L. (Araceae). Plants were either collected from 6 populations in southern Florida or from plants grown with low or high fertilizer levels. Consumption of leaves with increasing toughness resulted in increased larval mortality (>80%); most mortality occurred during the first 2 instars. Larvae compensated for low-nitrogen leaves by increasing fresh weight consumption 3–fold. Both developmental time and biomass gain were effected by the source of the plants but these performance parameters were not directly related to either leaf toughness or nitrogen levels. These results are useful in understanding the adaptations and limitations of specialist herbivores to low-nutrient foods. Additionally, they assist in the selection of suitable release sites for this weed biological control agent. Finally, they improve our mass-rearing techniques for augmentative releases of this biological control species.

Leaf level response of planted eastern white pine ( Pinus strobus L.) seven years after intensive silvicultural treatments

The present study examines the impact of intensive silvicultural treatments on environmental conditions, leaf level morphology and physiology, and growth of planted eastern white pine ( Pinus strobus L.) saplings and evaluates how silvicultural treatments and the presence of competing vegetation influence the relationships between leaf nitrogen, leaf morphology, and leaf level photosynthetic capacity of saplings. The six silvicultural treatments evaluated consisted of combinations of scarification (removal of entire humus layer), vegetation control (herbicide), and fertilization (slow release fertilizer). Competing vegetation (mainly Populus tremuloides Michx.) had negative impacts on shoot water potential ( Ψ x), leaf nitrogen, leaf mass per unit leaf area (LMA), height, and basal area of 7-year-old saplings. Net CO 2 assimilation rate at light saturation ( A; both on a mass and area basis) and stomatal conductance for water vapor ( g wv) were not significantly influenced by the presence of competing vegetation. The only significant impact of competition on gas exchange variables was to decrease water-use efficiency (both instantaneous and long-term WUE as expressed by carbon isotope discrimination or Δ). Scarification significantly increased predawn Ψ x, height, and basal area of suppressed saplings, and A area, g wv, and LMA of saplings subject to herbicide application. These positive impacts of scarification were attributed to enhanced root growth due to higher soil temperatures. No positive impact of fertilization was observed in either suppressed or open conditions. Leaf nitrogen and LMA were both driving variables for photosynthetic capacity of saplings across all conditions created by silvicultural treatments, but also within both suppressed and open conditions. This suggests that the dependency of the photosynthetic apparatus on leaf nitrogen and LMA occurs whether there is competing vegetation (and/or shade) or not. However, the gradient of light availability under suppressed conditions was accompanied by a gradient of soil temperature due to scarification. It would be pertinent to investigate more fully the interactions between light availability and soil temperature, both influencing root growth and leaf level morphology and physiology of young eastern white pine.

Effects of Elevated Co 2 , Soil Nutrient Levels, and Foliage Age on the Performance of Two Generations of Neodiprion lecontei (Hymenoptera: Diprionidae) Feeding on Loblolly Pine

We investigated how changes in loblolly pine needle phytochemistry caused by elevated CO{sub 2}, leaf age, and soil nutrient levels affected the performance of 2 individual generations of the multivoltine folivorous insect pest Neodiprion lecontei. In 2 feeding trials, mature needles produced in the previous (spring) and current (fall) year from seedlings grown in open-topped chambers under 4 CO{sub 2} and 2 soil nutrient levels were fed to 2 separate generations of red beaded pine sawfly larvae. Strong seasonal differences (i.e., spring versus fall) in leaf nutritional and defensive constituents resulted in significant between-generation differences in the growth, consumption, and growth efficiency of sawfly larvae. Enriched CO{sub 2}-grown needles bad higher levels of starch and starch/nitrogen ratios in older, overwintering spring needles, which were lower in leaf nitrogen and monoterpenes than younger, current year needles (fall). Overall, larval growth was higher and consumption lower on the fall needles, presumably because of higher levels of leaf nitrogen compared with the spring needles. The plant CO{sub 2} concentration significantly contributed to the larval consumption responses between seasons (significant CO{sub 2} X season interaction), demonstrating that the 2 sawfly generations were affected differently by CO{sub 2}-induced phytochemical alterations in spring versus fall needles.more » The data presented here suggests that when investigating multivoltine folivorous insect responses to elevated CO{sub 2}-grown tree seedlings in which multiple leaf flushes within a growing season expose insects to an array of leaf phytochemical changes, >1 insect generation should be investigated. 54 refs., 3 figs., 1 tab.« less

CO2‐mediated changes in aspen chemistry: effects on gypsy moth performance and susceptibility to virus

We investigated the effects of long‐term CO2 enrichment on foliar chemistry of quaking aspen (Populus tremuloides) and the consequences of chemical changes for performance of the gypsy moth (Lymantria dispar) and susceptibility of the gypsy moth to a nucleopolyhedrosis virus (NPV). Foliage was collected from outdoor open‐top chambers and fed to insects in a quarantine rearing facility. Under enriched CO2, levels of leaf nitrogen declined marginally, levels of starch and phenolic glycosides did not change, and levels of condensed tannins increased. Long‐term bioassays revealed reduced growth (especially females), prolonged development and increased consumption in larvae fed high‐CO2 foliage but no significant differences in final pupal weights or female fecundity. Short‐term bioassays showed weaker, and sex‐specific, effects of CO2 treatment on larval performance. Correlation analyses revealed strong, negative associations between insect performance and phenolic glycoside concentrations, independent of CO2 treatment. Larval susceptibility to NPV did not differ between CO2 treatments, suggesting that effects of this natural enemy on gypsy moths are buffered from CO2‐induced changes in foliar chemistry. Our results emphasize that the impact of enriched CO2 on plant–insect interactions will be determined not only by how concentrations of plant compounds are altered, but also by the relevance of particular compounds for insect fitness. This work also underscores the need for studies of genetic variation in plant responses to enriched CO2 and long‐term population‐level responses of insects to CO2‐induced changes in host quality.

AN ORGANIC VERSUS A CONVENTIONAL FARMING SYSTEM IN KIWIFRUIT

To determine the feasibility of growing kiwifruit organically, a kiwifruit vineyard converted to an organic farm was compared to a conventionally farmed vineyard from 1990 through 1992. January or March applications of composted chicken manure (organic system) or NH4NO3 plus CaNH 4(NO3)3 through micro sprinklers during the growing season (conventional system) were applied to give nearly equal rates of 168 kg N ha. Soil analysis showed a trend toward a higher pH and organic matter over time for the organic system. In 1992, there was a trend for the organic system to have higher NH4-N and lower NO3-N concentration in the soil. Leaf nitrogen levels from the organic system were consistently lower than those from the conventional system but were not deficient. Leaf concentrations of sodium and chloride increased over the three-year period in the organic system but not to phytotoxic levels. Leaf zinc levels were adequate and increased over time in both systems. Organically grown fruit was as firm as or firmer than conventionally grown fruit at harvest and four months after harvest. No differences were seen in percent soluble solids content. Damage from latania scale and omnivorous leaf roller was small in both systems except scale damage in the organic system in 1992. An economic analysis of the cultural practices showed that the organic system cost almost $720 per ha more then the conventional system. The grower reported fewer repack losses for organically grown fruit in 1992. We conclude that growing kiwifruit organically is feasible if an economic premium is received.

Fruit colour, leaf nitrogen level, and tree vigour in ‘Fuji’ apples

Abstract A survey of red colour in ‘Fuji’ apple (Mains domestica Borkh. ) fruit from trees in orchards situated in three regions of New Zealand has confirmed that tree vigour, tree nitrogen (N) status, and region are important factors determining the extent and intensity of red colour development. The survey was conducted in Hawkes Bay and Nelson in 1990 and Hawkes Bay and Canterbury in 1991. In both years, a multiple regression model using leaf N concentration in February, tree vigour, and region as predictors for the intensity of red colour (chroma) gave the best fit to the data (r2 = 0.65, 1990; r2 = 0.46, 1991). Interaction terms for leaf N status and region, and growth rate and region were also significant. A multiple regression model over both years for Hawkes Bay found leaf N concentration in February and growth rate to be useful predictors for chroma (r2 = 0.66). Leaf N status and tree vigour appear to act separately, and direct effects on fruit colour development seem to be implicated rather than secondary factors such as shading. Additional measurements in Hawkes Bay showed that anthocyanin concentrations were not related to changes in chroma, leaf N concentration, or fruit N concentration. Chlorophyll concentration increased with increasing fruit N concentration in individual fruit (r2 = 0.35), and this corresponded to decreased red colour intensity (chroma). The relationships explored here explained c. 50–60% of variation in the intensity of red colour. Considered against the survey method and the potential of other factors to affect red colour development, the regressions presented gave insight into ways of managing young ‘Fuji’ trees for increased red colour.

Cotton responses to simulated insect damage: Radiation-use efficiency, canopy architecture and leaf nitrogen content as affected by loss of reproductive organs

Key cotton pests feed preferentially on reproductive organs which are normally shed after injury. Loss of reproductive organs in cotton may decrease the rate of leaf nitrogen depletion associated with fruit growth and increase nitrogen uptake and reduction by extending the period of root and leaf growth compared with undamaged plants. Higher levels of leaf nitrogen resulting from more assimilation and less depletion could increase the photosynthetic capacity of damaged crops in relation to undamaged controls. To test this hypothesis, radiation-use efficiency (RUE = g dry matter per MJ of photosynthetically active radiation intercepted by the canopy) of crops in which flowerbuds and young fruits were manually removed was compared with that of undamaged controls. Removal of fruiting structures did not affect RUE when cotton was grown at low nitrogen supply and high plant density. In contrast, under high nitrogen supply and low plant density, fruit removal increased seasonal RUE by 20–27% compared to controls. Whole canopy measurements, however, failed to detect the expected variations in foliar nitrogen due to damage. Differences in RUE between damaged and undamaged canopies were in part associated with changes in plant and canopy structure (viz. internode number and length, canopy height, branch angle) that modified light distribution within the canopy. These structural responses and their influence on canopy light penetration and photosynthesis are synthetised in coefficients of light extinction ( k) that were 10 to 30% smaller in damaged crops than in controls and in a positive correlation between RUE −1 and k for crops grown under favourable conditions (i.e. high nitrogen, low density). Changes in plant structure and their effects on canopy architecture and RUE should be considered in the analysis of cotton growth after damage by insects that induce abscission of reproductive organs.

Fertilization Rate and Growth of `Hamlin' Orange Trees Related to Preplant Leaf Nitrogen Levels in the Nursery

Our objectives were to determine the effects of leaf N concentration in citrus nursery trees on subsequent growth responses to fertilization for the first 2 years after planting and the impact of N fertilizer rate on soil NO3-N concentration. `Hamlin' orange [Citrus sinensis (L.) Osb.] trees on `Swingle' citrumelo rootstock [C. paradisi Macf. × P. trifoliata (L.) Raf.] were purchased from commercial nurseries in Apr. 1992 (Expt. 1) and Jan. 1993 (Expt. 2) and were grown in the greenhouse at differing N rates. Five months later, trees for each experiment were separated into three groups (low, medium, and high) based on leaf N concentration and were planted in the field in Oct. 1992 (Expt. 1) or Apr. 1993 (Expt. 2). Trees were fertilized with granular material (8N-2.6P-6.6K-2Mg-0.2Mn-0.12Cu-0.27Zn-1.78Fe) with N at 0, 0.11, 0.17, 0.23, 0.28, or 0.34 kg/tree per year. Soil NO3-N levels were determined at 0- to 15- and 16- to 30-cm depths for the 0.11-, 0.23-, and 0.34-kg rates over the first two seasons in Expt. 2. Preplant leaf N concentration in the nursery varied from 1.4% (Expt. 1) to 4.1% (Expt. 2) but had no effect on trunk diameter, height, shoot growth and number, or dry weight in year 1 (Expt. 1) or years 1 and 2 (Expt. 2) in the field. Similarly, fertilizer rate in the field had no effect on growth during year 1 in the field. However, trunk diameter increased with increasing N rate in year 2 and reached a maximum with N at 0.17 kg/tree per year but decreased at higher rates. Shoot number during the second growth flush in year 2 was much lower for nonfertilized vs. fertilized trees at all rates, which had similar shoot numbers. Nevertheless, leaf N concentrations increased during the season for trees with initially low levels, even for trees receiving low fertilizer rates. This suggests translocation of N from other organs to leaves. Soil NO3-N levels were highest for the 0.34-kg rate and lowest at the 0.11-kg rate. Within 2 to 3 weeks of fertilizing, NO3-N levels decreased rapidly in the root zone.

Fertilization Rate and Growth of `Hamlin' Orange Trees Related to Preplant Leaf Nitrogen Levels in the Nursery

Our objectives were to determine if leaf N concentration in citrus nursery trees affected subsequent growth responses to fertilization for the first 2 years after planting and how N fertilizer rate affected soil nitrate-N concentration. `Hamlin' orange [Citrus sinensis (L.) Osb.] trees on `Swingle' citrumelo rootstock [C. paradisi Macf. × P. trifoliata (L.) Raf.] were purchased from commercial nurseries and grown in the greenhouse at differing N rates. Three to five months later trees were separated into three groups (low, medium, high) based on leaf N concentration and planted in the field in Oct. 1992 (Expt. 1) or Apr. 1993 (Expt. 2). Trees were fertilized with granular material (8N–2.6P–6.6K) with N at 0 to 0.34 kg/tree yearly. Soil nitrate-N levels were also determined in Expt. 2. Preplant leaf N concentration in the nursery varied from 1.4% to 4.1% but had no effect on trunk diameter, height, shoot growth, and number or dry weight in year 1 (Expt. 1) or years 1 and 2 (Expt. 2) in the field. Similarly, N fertilizer rate had no effect on growth during year 1 in the field. However, trunk diameter increased with increasing N rate in year 2 and reached a maximum with N at 0.17 kg/tree yearly. Shoot number during the second growth flush in year 2 was much lower for nonfertilized vs. fertilized trees. Leaf N concentrations increased during the season for trees with initially low levels even for trees receiving low fertilizer rates. Soil nitrate-N levels were highest at the 0.34-kg rate, and lowest at the 0.11-kg rate. Nitrate-N levels decreased rapidly in the root zone within 2 to 3 weeks of fertilizing.

Relationships between carbon and hydrogen isotope ratios and nitrogen levels in leaves ofClusia species and two other Clusiaceae genera at various sites and different altitudes in Venezuela

Samples of the Clusiaceae generaClusia, Oedematopus andDystovomita were collected at various sites and different altitudes in northern and south-western Venezuela. Analyses of stable isotopes of carbon and hydrogen and of leaf-nitrogen levels were performed on the dried samples. Correlations among these variables, i.e. carbon isotope discrimination (Δ), hydrogen isotope ratio (δD) and N-levels, and with altitude were assessed. In the samples, where values of Δ above 15‰ indicate predominant performance of C3 photosynthesis, there were slight tendencies of increasing Δ, δD and N-levels with increasing altitude and of increasing Δ with increasing N. Although these correlations taken separately were not statistically significant, they support each other and indicate increasing transpiration and increased leaf-nutrient supply at increasing altitude. Performance of crassulacean acid metabolism (CAM) in species ofClusia appears to be restricted to altitudes below 1500 m a.s.l. There was a significant negative correlation of Δ with altitude in the samples, where values of Δ below 10‰ indicated predominant performance of CAM. This suggests that phases II and IV of CAM are progressively suppressed towards the upper altitudinal limit of CAM inClusia in northern Venezuela. It is concluded that among the large number of environmental factors and combinations thereof, which determine the expression of CAM inClusia and trigger C3-CAM transitions in C3/CAM intermediate species, low availability of water is the most important.

Diammonium Phosphate Corrects Phosphorus Deficiency in Lowbush Blueberry

ABSTRACT Eight commercial lowbush blueberry (Vaccinium an-gustifolium Ait.) fields were fertilized with diammonium phosphate (DAP) at 0, 224 or 448 kg ha “] (0, 200, and 400 lbs per A) to determine the effect on subsequent leaf nutrient concentration. A RCB design with four replications was used. Leaf samples taken in July 1991 showed an increased concentration of phosphorus with increasing rate of DAP, but leaf nitrogen level was unaffected. Soil samples taken at the same time showed no significant increase in available phosphorus. Stems collected after leaf drop were taller and more branched with a larger number of flower buds due to application of DAP compared to the controls. There was a linear increase in yield with increasing rate of DAP.

Tritrophic Effects of Leaf Nitrogen onLiriomyza trifolii(Burgess) and an Associated ParasitoidChrysocharis oscinidis(Ashmead) on Bean

Abstract Significant differences in developmental times ofLiriomyza trifolii(Burgess) males andChrysocharis oscinidis(Ashmead) males and females were found in response to percent nitrogen levels in bean foliage. These changes represented differences in developmental times of 4.5% inL. trifoliimales and 2.7% inC. oscinidisfemales and occurred over the ranges of nitrogen examined (L. trifolii2.76–6.27% leaf N;C. oscinidis2.67–6.52% leaf N). Although pupal weights ofL. trifoliilarvae reared on leaves with various nitrogen levels increased significantly as leaf nitrogen increased,L. trifoliifecundity was not significantly affected by nitrogen in the leaves upon which the females developed. It appears that larval food acquisition of the female has less effect on fecundity than does adult food acquisition. Parasitoids were allowed to feed and oviposit onL. trifoliireared on leaves with differing nitrogen levels. Fecundity of adultC. oscinidis(averaged 69 progeny per female over a 10-day period) was significantly highest at the 4.88% leaf nitrogen level. Similar to the shortest developmental time, fecundity was enhanced at this level and adversely affected at the other levels.C. oscinidissex ratio averaged 1:1.23 (male:female); no significant differences in sex ratio were found among nitrogen treatments. Total progeny production of adultC. oscinidis,allowed to feed and oviposit onL. trifoliilarva reared on leaves with differing nitrogen levels, may increase by as much as 40%. These changes in fecundity are highly significant and could greatly affect pest/parasitoid populations in both agroecosystems and mass rearing programs.

The effects of crop microclimate and associated physiological constraints on the seasonal and diurnal distribution patterns of raspberry beetle (Byturus tomentosus) on the host plant Rubus idaeus

Abstract. The occurrence of raspberry beetles (Byturus tomentosus) on the host plant Rubus idaeus is extremely variable between seasons, between days and within days, with occupancy of the available raspberry flowers (the feeding and oviposition sites) varying from 0 to 60%. This variation could not be explained by plant chemistry or food quality (leaf nitrogen, carbon or water levels, or floral nectar reward); however, beetle distributions were in part attributable to microclimatic constraints acting via the insects’physiological constraints. Initial ascent into raspberry canes from soil emergence sites was limited by the three‐fold higher water loss rates from recently eclosed young adult beetles as compared with mature beetles. Young adults reduced their hygric stress by remaining in the humid microclimate of tightly furled primocane leaftips. Mature beetles spread upwards over the plant, but showed a preference for insolated sites (tops of canes, east or west facing according to time of day). In such sites their body temperatures could rise above the threshold for flight (requiring a Tb of 15°C in laboratory studies). Flight activity was therefore common only in the early afternoon of warm days. Later in the day, beetles moved down and sometimes off the plants, starting to return at around dawn. Thus physiological constraints, even on adult beetles (relatively well‐protected insect stadia), can be important components in predicting insect movements and locations on a host plant; they are likely to be even more crucial to less highly sclerotized plant‐feeding adult insects and many larval herbivorous pests.

Scaling carbon dioxide and water vapour exchange from leaf to canopy in a deciduous forest. I. Leaf model parametrization

ABSTRACTIn order to parametrize a leaf submodel of a canopy level gas‐exchange model, a series of photosynthesis and stomatal conductance measurements were made on leaves of white oak (Quercus alba L.) and red maple (Acer rubrum L.) in a mature deciduous forest near Oak Ridge, TN. Gas‐exchange characteristics of sun leaves growing at the top of a 30 m canopy and of shade leaves growing at a depth of 3–4 m from the top of the canopy were determined. Measured rates of net photosynthesis at a leaf temperature of 30°C and saturating photosynthetic photon flux density, expressed on a leaf area basis, were significantly lower (P = 0.01; n = 8) in shade leaves (7.9μmol m−2 s−1) than in sun leaves (11–5μmol m−2 s−1). Specific leaf area increased significantly with depth in the canopy, and when photosynthesis rates were expressed on a dry mass basis, they were not significantly different for shade and sun leaves. The percentage leaf nitrogen did not vary significantly with height in the canopy; thus, rates expressed on a per unit nitrogen basis were also not significantly different in shade and sun leaves. A widely used model integrating photosynthesis and stomatal conductance was parametrized independently for sun and shade leaves, enabling us to model successfully diurnal variations in photosynthesis and evapotranspiration of both classes of leaves. Key photosynthesis model parameters were found to scale with leaf nitrogen levels. The leaf model parametrizations were then incorporated into a canopy‐scale gas‐exchange model that is discussed and tested in a companion paper (Baldocchi & Harley 1995, Plant, Cell and Environment 18, 1157–1173).

Optimum leaf nitrogen levels in tobacco seedling

Abstract This paper establishes deficient and toxic levels of nitrogen (N) in tobacco seedlings of the Virginia type, suggesting that range of concentration in the plant required in order to avoid possible nutritional problems before transplanting. The results show that the lamina tends to contain more N than does the central midrib. The normal N content range without visual N deficiency or toxicity symptoms in leaves at the young seedling stage should be 3.5–6.9% on a dry matter basis.

Dynamic optimisation of nitrogen fertilisation of citrus and the value of information from leaf tissue analysis

The optimal long-run fertilisation and irrigation of a perennial crop is approached via two stochastic dynamic optimisation models. The first model assumes that the ex-post level of the state variable--leaf nitrogen (LN)--is known with certainty. The second model assumes that the level of leaf nitrogen is not observed, but that the farmer has a prior probability distribution on the level of leaf nitrogen, which can be updated to yield a series of posterior probability distributions conditioned on past input decisions. Comparison of the values of the objective functions of both models yields an expected economic value of complete and costless information of leaf tissue analysis. The analysis is illustrated for a mature citrus grove. The impact of improved information on nitrogen use and leaching is also examined empirically. Copyright 1995 by Oxford University Press.

Resistance of Okra-Leaf Cotton Genotypes to Twospotted Spider Mites (Acari: Tetranychidae)

Previous research showed that twospotted spider mite, Tetranychus urticae Koch, populations increase more slowly on okra-leaf than normal-leaf cotton, Gossypium Hirsutum L., genotypes. The following four differences between genotypes that may underly this plant resistance were investigated: quicker exploitation of smaller leaves, reduced nutritional quality, unfavorable canopy temperature or humidity conditions, and reduced habitat favorable for oviposition and development. Okra cotton leaves were smaller (70% of area of normal cotton leaves), but spider mite populations diverged at low levels of leaf damage (9-35%), indicating that lower population growth rates occur on okra-leaf genotypes before leaves are exploited fully. Spider mites did not discriminate between okra-leaf and normal-leaf cotton in choice tests, developmental and reproductive parameters of mites reared on excised disks did not significantly differ between leaf types, and nitrogen and tannin levels were similar between genotypes. Temperature and humidity conditions in the upper canopy, where most spider mite populations occur, were not significantly different between genotypes. In the field, mites develop colonies along the major veins, and, in leaf folds of cotton leaves and in laboratory choice tests, mites laid more eggs on leaf sections with a major vein than leaf sections without. The mechanism of resistance was defined as nonpreference caused by less suitable feeding and oviposition sites on okra-leaf genotypes

Intercropping wheat and barley with N-fixing legume species: a method for improving ground cover, N-use efficiency and productivity in low input systems

SummaryTwo cereal cropping systems are described which, through the introduction of a leguminous intercrop, increased productivity, nitrogen output and ground cover of the systems in the absence of added nitrogen fertilizer. Nitrogen-fixing legumes were cultivated between rows of wheat or barley grown at low levels of soil nitrogen, and mostly under rainfed conditions, in Mexico between 1989 and 1992. None of the legumes tested reduced yields of the cereal crop in comparison to controls where cereal yields were in the range of 1–4 t/ha, while the extra total biomass from legumes in some cases more than doubled productivity. Different legume crops were tested to demonstrate the adaptability of the system to the varying needs of farmers. The intercropped legumes achieved dry biomass yields as high as 6·5 t/ha in the case of a sequentially cropped forage crop of hairy vetch, or 1·4 t/ha of dry beans plus 3·5 t/ha of green residue in the case of Vicia faba. Total biomass in the intercropped situation gave land equivalent ratios as high as 1·54. Light measurements inside the crop canopies indicated that the intercropped systems intercepted a higher proportion of the incident solar radiation than the cereal monocrop, presumably accounting for the large differences in total biomass produced. In addition, with leaf nitrogen levels of 3·8%, it is assumed that the intercropped legumes fixed considerably more nitrogen than was removed by the wheat crop. The potential of the system to stabilize erodible soils by increasing ground cover as well as by raising inputs of soil organic matteris discussed.

A theoretical analysis of nitrogen and radiation effects on radiation use efficiency in peanut

Radiation use efficiency (RUE) of well-watered crops, measured as grams of biomass accumulated for each megajoule of intercepted total solar radiation, is affected by the level of leaf nitrogen in the canopy and has been related to the canopy specific leaf nitrogen (SLN; g N m-2 leaf area). A number of field experiments on peanut have measured RUE values greater than current theories predict on the basis of their canopy SLN levels. It is possible that these discrepancies between measured and theoretical values may be caused by non-uniform distribution of SLN in the canopy, incident radiation level, and/or the influence of diffuse radiation. In this study, we developed a theoretical framework to predict the consequences of these factors on RUE in peanut and used it to explain the causes of discrepancies between theory and practice. The framework is structured to determine photosynthesis of a layered crop canopy by distributing incident radiation among sunlit and shaded leaves in each layer. It allows for variation in incident direct and diffuse radiation associated with location (latitude), time of year, time of day, and atmospheric condition, which is expressed as the degree of transmission of extra-terrestrial radiation. It also allows for variation in photosynthetic capacity associated with average SLN of the canopy and its distribution in the canopy. Daily canopy photosynthesis, intercepted radiation, and RUE are obtained by numerical integration of instantaneous values calculated at specific times of the day. The framework predicted experimentally determined RUE values accurately and quantified the contribution of each major factor to variation in RUE. On clear days, with high canopy SLN, RUE was predicted to be 1.l g MJ-1. The major cause of previous underestimation of RUE was found to be variation in RUE associated with the level of incident radiation flux density as affected by the degree of atmospheric transmission. RUE increased by up to 0.4 g MJ-1 as atmospheric transmission decreased from 0.75 (clear sky) to 0.35 (heavy cloud). However, varying incident radiation by changing time of year or latitude did not affect RUE. Partitioning incident radiation into direct and diffuse components and consideration of canopy gradients in SLN both had significant effects on RUE, but of a lesser magnitude than effects of degree of atmospheric transmission. The former caused increases in RUE of up to 0.15 g MJ-l, while the latter caused increases of up to 0.13 g MJ-1 at low canopy SLN. Hence, by quantifying the understanding of plant physiological processes and integrating appropriately to the canopy scale, this theoretical framework has explained the causes of discrepancies between measured RUE and previous theoretical estimates.

Nitrogen nutrition of C3 plants at elevated atmospheric CO2 concentrations

The atmospheric CO2 concentration has risen from the preindustrial level of approximately 290 μl l−1 to more than 350 μl l−1 in 1993. The current rate of rise is such that concentrations of 420 μl l−1 are expected in the next 20 years. For C3 plants, higher CO2 levels favour the photosynthetic carbon reduction cycle over the photorespiratory cycle, resulting in higher rates of carbohydrate production and plant productivity. The change in balance between the two photosynthetic cycles appears to alter nitrogen and carbon metabolism in the leaf, possibly causing decreases in nitrogen concentrations in the leaf. This may result from increases in the concentration of storage carbohydrates of high molecular weight (soluble or insoluble) and/or changes in distribution of protein or other nitrogen containing compounds. Uptake of nitrogen may also be reduced at high CO2 due to lower transpiration rates. Decreases in foliar nitrogen levels have important implications for production of crops such as wheat, because fertilizer management is often based on leaf chemical analysis, using standards estimated when the CO2 levels were considerably lower. These standards will need to be re‐evaluated as the CO2 concentration continues to rise. Lower levels of leaf nitrogen will also have implications for the quality of wheat grain produced, because it is likely that less nitrogen would be retranslocated during grain filling.