Leaf Area Responses Research Articles

Thaw effects on cold-hardiness parameters in yellow birch

One-year-old, cold-hardened, container-grown yellow birch (Betula alleghaniensis Britt.) seedlings were exposed to cold treatments after being pretreated with a simulated winter thaw. Freezing injury to roots and shoots was assessed by relative electrolyte leakage and triphenyltetrazolium chloride reduction. Growth characteristics were also determined after 60 days under greenhouse conditions. Relative electrolyte leakage and triphenyltetrazolium chloride reduction measurements showed that roots became increasingly damaged with decreasing cold-treatment temperatures. However, plants pretreated with thaws showed significantly lower stem increment, shoot length, and leaf area in response to the cold temperatures than did the unthawed plants. Variation in these growth parameters was also significantly correlated with both root and shoot freezing injury parameters. Cold hardiness under different thaw pretreatments was assessed using the highest freezing temperature that caused significant injury, referred to as the critical temperature. For seedlings without the thaw pretreatment, shoot and root critical temperatures were estimated as –52.5 and 23.8°C, respectively. Following 12 days of thaw, these temperatures increased to –24.08°C for shoots and –13°C for roots. Twelve days of thaw, or growing degree-day (>4°C) accumulations greater than 66 during a thaw, could sufficiently deharden roots and shoots such that they would be susceptible to freezing damage at ambient temperatures commonly encountered in the Canadian Maritimes. We also observed that root pressure declined significantly with increasing root freezing injury. Sufficient root pressure is required for springtime refilling of xylem embolisms caused by winter cavitation of the vessels in this species. Weak root pressure caused by freezing injury would represent a risk of shoot dieback and tree decline due to the remaining embolisms reducing water flow to the developing foliage. The rapid reduction of shoot cold hardiness may also indicate the threat of late-spring frosts to this species. These induced changes are especially important under climate change scenarios that suggest increases in winter temperatures and changes in seasonality in eastern Canada.Key words: climate change, cold hardiness, electrolyte leakage, growth, root pressure, TTC reduction.

Analysis of interspecific variation in plant growth responses to nitrogen

Plant species differ greatly in their growth responses to nutrients, but little is known about the physiological and morphological factors that are responsible for this variation. To address this question, I measured the responses to added nitrogen of relative growth rate and three of its components (specific leaf area, unit leaf rate, and leaf weight ratio) for 17 C3 grass species. Plants were grown in sand culture in a greenhouse and were fertilized daily with either 5 or 0.05 mM NH4NO3. For most species, growth response to nitrogen was primarily associated with an increased mass allocation to leaves at high versus low nitrogen. Across all species, the average response at high versus low nitrogen was a 37% increase in leaf weight ratio, a 12% increase in unit leaf rate, and a 4% decrease in specific leaf area. Interspecific differences in growth response to nitrogen, however, were associated primarily with species differences in the response of unit leaf area to nitrogen supply. I determined the nitrogen response ratio of each parameter as the value of the parameter at high nitrogen divided by the value at low nitrogen. The rank-order correlation between the unit leaf area response ratio and the relative growth rate response ratio was 0.88. Reanalysis of previous experiments on plant nutrient response showed a similar pattern. In all studies, interspecific variation in the response of relative growth rate to nutrients was associated primarily with interspecific differences in the plastic response of unit leaf area.Key words: leaf weight ratio, net assimilation rate, plant growth analysis, relative growth rate, specific leaf area, unit leaf rate.

Equations for Predicting Diameter, Height, Crown Width, and Leaf Area of San Joaquin Valley Street Trees

Although the modeling of energy-use reduction, air pollution uptake, rainfall interception, and microclimate modification associated with urban trees depends on data relating diameter at breast height (dbh), crown height, crown diameter, and leaf area to tree age or dbh, scant information is available for common municipal tree species. In this study, tree height, crown width, crown height, dbh, and leaf area were measured for 12 common street tree species in the San Joaquin Valley city of Modesto, California, U.S. The randomly sampled trees were planted from 2 to 89 years ago. Using age or dbh as explanatory variables, parameters such as dbh, tree height, crown width, crown height, and leaf area responses were modeled using two equations. There was strong correlation (adjusted R2 > 0.70) for total height, crown diameter, and leaf area with dbh. Correlations for dbh with age and crown height for several species were weaker. The equations for predicting tree sizes and leaf area are presented and applied to compare size and growth for all species 15 and 30 years after planting. Tree height, crown diameter, and dbh growth rates tended to slow during the second 15 years, but the leaf area growth rate increased for most species. Comparisons of predicted sizes for three species common to Modesto and Santa Monica trees suggest that pruning has a significant impact on tree size and leaf area, potentially more than climate and soil characteristics.

Changes in competitive ability between a C4crop and a C3weed with elevated carbon dioxide

Abstract Using climate-controlled glasshouses, the growth of grain sorghum was evaluated with and without the presence of common cocklebur at current and projected future atmospheric concentrations of carbon dioxide [CO2]. Single-leaf photosynthetic rates declined for both species in competition; however, elevated CO2 reduced the percentage decline in common cocklebur and increased it in sorghum by 35 d after sowing (DAS) relative to ambient CO2. In monoculture, elevated CO2 significantly stimulated leaf photosynthetic rate, leaf area, and aboveground dry weight of common cocklebur more than that of sorghum. However, the stimulation of aboveground biomass or leaf area for monocultures of sorghum and common cocklebur at elevated CO2 did not adequately predict the CO2 response of aboveground biomass or leaf area for sorghum and common cocklebur grown in competitive mixtures. Overall, by 41 DAS, plant relative yield (PRY), in terms of aboveground biomass and leaf area, increased significantly for common cock...

Predictive Equations for Dimensions and Leaf Area of Coastal Southern California Street Trees

Tree height, crown height, crown width, diameter at breast height (dbh), and leaf area were measured for 16 species of commonly planted street trees in the coastal southern California city of Santa Monica, USA. The randomly sampled trees were planted from 1 to 44 years ago. Using number of years after planting or dbh as explanatory variables, mean values of dbh, tree height, crown width, and leaf area responses were modeled using two equations. There is strong correlation (adjusted R2 > 0.70) between dbh as a function of number of years after planting, and total height, crown diameter, and leaf area as a function of dbh. Correlation is weaker between measures of crown height and dbh. This is probably due to crown pruning increasing the variability among measurements for trees having the same or similar dbh. Equations for less-intensively pruned species displayed adjusted R2 greater than 0.70. Equations are presented for predicting dimensions and leaf area and applied to compare tree sizes and growth for all species 15 and 30 years after planting.

Effects of elevated CO2 on five plant‐aphid interactions

AbstractWe investigated interactions between five species of phloem‐feeding aphids (Homoptera: Aphididae) and their host plants at elevated CO2; Acyrthosiphon pisum (Harris) on Vicia faba L., Aphis nerii Boyer de Fonscolombe on Asclepias syriaca L., Aphis oenotherae Oestlund on Oenothera biennis L., Aulacorthum solani (Kaltenbach) on Nicotiana sylvestris Speg. & Comes and Myzus persicae (Sulzer) on Solanum dulcamara L. Host plants grown at elevated CO2 generally had greater biomass, leaf area and C:N ratios than those grown at ambient CO2, while plants with aphids had lower biomass and leaf area than those without aphids.The responses of aphid populations to elevated CO2 were species‐specific with one species increasing (M. persicae), one decreasing (A. pisum), and the other three being unaffected. CO2 treatment did not affect the proportion of alate individuals produced. In general, aphid abundance was not significantly related to foliar nitrogen concentration.We performed separate analyses to test whether either aphid presence or aphid abundance modified the response of host plants to elevated CO2. In terms of aphid presence, only three of the potential 15 interactions (five aphid species x three plant traits) were significant; A. solani slightly modified the response of the plant biomass to elevated CO2 and M. persicae affected the response of leaf area and allocation. In terms of aphid abundance, only two of the potential 15 interactions were significant with A. nerii modifying the plant response to CO2 in terms of total leaf area and allocation.We conclude that, in contrast to other insect groups such as leaf chewers, populations of most phloem‐feeders may not be negatively affected by increased CO2 concentrations in the future. The reasons for this difference include the possibility that aphids may be able to compensate for changes in host plant quality by altering feeding behaviour or by synthesizing amino acids. In addition, there is little evidence that aphid herbivory, even at high levels, will substantially modify the response of plants to elevated CO2.

Gas-exchange response and stomatal and non-stomatal limitations to carbon assimilation of sunflower under salinity

Sunflower ( Helianthus annuus) was grown in both open-field and outdoor potted conditions in Southern Italy, and irrigated with water having electrical conductivity ranging between 0.9 and 15.6 dS m −1 obtained by different NaCl concentrations. The aim of the work was to study the leaf area and photosynthetic responses of sunflower to mild salt stress. The response curve ( A/ c i) of assimilation ( A) to leaf internal CO 2 concentration ( c i) was used to determine leaf gas-exchange parameters, in order to evaluate stomatal and non-stomatal limitations to photosynthesis in relation to salt stress. In the field, a reduction of 19% in leaf area expansion occurred, while no correlation was observed between Ψ l and stomatal conductance to water vapour ( g sw) ranging between 0.76 and 1.35 mol m −2 s −1. This result was also evident at a higher salinity level reached in the pot experiment where leaf osmotic potential ( ψ s) varied from −1.35 to −2.67 MPa as compared with the field experiment, where ψ s ranged from −1.15 to −1.42 MPa. Considering the two experiments as a unique data set, the assimilation rate, the stomatal conductance to CO 2 ( g sc) and the sensitivity of A to c i variation ( g*) were not significantly influenced by salinity in the whole range of ψ s . As a consequence, the stomatal and non-stomatal limitations to photosynthesis were not affected by salt treatment, averaging around 20 and 80%, respectively. The variation in A (from 44 to 29 μmol m −2 s −1) was paralleled by the variation in g sc (from 0.47 to 0.84 mol m −2 s −1), with a remarkable constancy of both c i (200±12.5 μmol mol −1) and normalized water-use efficiency (5±0.7 μmol mmol −1 kPa), showing the optimal behaviour of the plant processes. These findings indicate that, under mild salt stress, the same as observed under water deficit, sunflower controls assimilation mainly by modulating leaf area rather than by stomatal closure, and that non-stomatal limitation of photosynthesis was not affected at all by the level of salinity reached in this study.

Specific Leaf Area Along a Nitrogen Fertilization Gradient

Specific leaf area was measured for three dominant perennial grass species in a long term nitrogen fertilization experiment in an abandoned field in Minnesota. The specific leaf area differed among the species and increased with increasing levels of nitrogen fertilization. This increase in specific leaf area was up to 82% for Agropyron repens and shows that the response of specific leaf area to nitrogen fertilization can be highly plastic. Increasing specific leaf area within species, together with a species replacement, resulted in a 202% increase in leaf area index along the nitrogen gradient, whereas the biomass increased by only 57%. One-third of this increase was attributed to species replacement and two-thirds to the change in specific leaf area within species. This plastic response of specific leaf area within plant species substantially increases the aboveground competition for light along nitrogen gradients.

BIOMASS ALLOCATION IN PLANTS: ONTOGENY OR OPTIMALITY? A TEST ALONG THREE RESOURCE GRADIENTS

We examined biomass allocation patterns throughout the entire vegetative growth phase for three species of annual plants along three separate gradients of resource availability to determine whether observed patterns of allocational plasticity are consistent with optimal partitioning theory. Individuals of the annual plant species Abutilon theophrasti, Chenopodium album, and Polygonum pensylvanicum were grown from locally field-gathered seed in controlled greenhouse conditions across gradients of light, nutrients, and water. Frequent harvests were used to determine the growth and allocation (root vs. shoot, and leaf area vs. biomass) responses of these plants over a 57-d period. Growth analysis revealed that each species displayed significant plasticity in growth rates and substantial amounts of ontogenetic drift in root:shoot biomass ratios and ratios of leaf area to biomass across each of the three resource gradients. Ontogenetically controlled comparisons of root:shoot and leaf area ratios across light ...

Physiological and morphological responses of olive plants to ozone exposure during a growing season.

We studied physiological (gas exchange and stomatal aperture) and morphological (individual leaf area and stomatal density) responses in leaves of five-year-old olive plants (Olea europaea L. cvs. Frantoio and Moraiolo) exposed to filtered air containing < 3 ppb O(3) or 100 ppb O(3) for 5 h day(-1) for 120 days in fumigation chambers. After 100 days of treatment, leaf drop and development of necrotic spots were observed in O(3)-fumigated plants of Moraiolo but not of Frantoio. Significant reductions in photosynthetic activity (57%) and stomatal conductance (69%) were detected in O(3)-fumigated plants of Frantoio compared with control plants. In O(3)-fumigated plants of Moraiolo, the decrease in photosynthetic activity (17%) was not statistically significant, but a significant reduction in stomatal conductance (40%) was observed. In both cultivars, leaves that developed after exposure to O(3) showed decreased stomatal aperture (63.6 and 54.8% with respect to the Frantoio and Moraiolo controls, respectively) and one-sided leaf area, and increased stomatal density compared with control leaves. Actual transpiring stomatal surface decreased substantially in both Frantoio (59.8%) and Moraiolo (56.3%) in response to O(3) treatment. Relative transpiring stomatal surface (RTSS) in Frantoio decreased from 0.54 (control) to 0.27% (O(3) treated) of total leaf surface. The corresponding values for Moraiolo were 0.79 and 0.42%. However, because the RTSS of Moraiolo leaves in the O(3) treatment was 0.42 versus 0.27% in Frantoio, the potential uptake of O(3) was higher for Moraiolo plants than for Frantoio plants. The large O(3)-induced reduction in transpiring stomatal surface in both cultivars could have significant effects on olive productivity in the Mediterranean area, where high O(3) concentrations persist for long periods during the year.

Seasonal responses of six Poaceae to differential levels of solar UV-B radiation

The effects of changes in solar UV-B on the growth and pigmentation of six grass species from cold-temperate grasslands (Lolium perenne, Lolium multiflorum, Festuca arundinacea, Festuca rubra, Phleum pratense and Dactylis glomerata) in spring and summer were studied. The grasses were grown in greenhouses with different foils, resulting in three treatments: no UV-B, 80% of ambient and 90% of ambient UV-BBE.(biologically effective UV-B). The results indicated important effects of ambient UV-B levels on grass, but the different species reacted in very different ways. Both morphology and biomass production were influenced by UV-B in some species. However, changes in biomass production did not necessarily occur within the same species as changes in morphology. The grasses were more sensitive in summer. Overall, only F. rubra was positively influenced by UV-B under all circumstances. The biomass of D. glomerata and L. perenne was reduced by UV-B in spring and summer. Morphological changes included reduced height and increased tillering. The sensitivity of the different species was partially explained by their ability to reduce their specific leaf area in response to UV-B. Only the more sensitive species showed increased production of protective pigments. Overall, there were important differences between the effect of a low level of UV-B, and the further increase in UV-B, indicating that several mechanisms are operating at different light levels.

Dynamics of Eucalyptus largiflorens growth and water use in response to modified watertable and flooding regimes on a saline floodplain

Reduced flooding and raised watertables have caused increased soil salinity and die-back of native forests on the floodplains of the lower River Murray of south Australia. Proposed management options include increasing flood frequency by regulating flows from upstream storages, and groundwater pumping to lower the watertable. This paper uses a soil-vegetation-atmosphere-transfer model (WAVES) to evaluate the impact of these proposals on soil salinisation processes and vegetation growth (black box, Eucalyptus largiflorens) for soils with different hydraulic properties. The changes in canopy leaf mass and plant available soil water were simulated for the period 1970–1994 using historical daily climate and river level records. The river level records were used to reconstruct the flooding and watertable history of sites where tree water use studies were conducted to calibrate the model. Then the watertable depth and/or flooding frequency was modified and the changes in the canopy leaf mass and soil water availability relative to the historical simulation were evaluated. The simulations suggest that, with the present watertable and flooding regime, very large floods (e.g. 205 days as in 1974–1976) are needed to sustain tree cover on the higher parts of the floodplain where die-back is most severe. They also indicate that soil hydraulic properties have a large influence on the magnitude and time scale of the growth response of salt stressed vegetation to floods and salt accumulation. Infrequently flooded vegetation exhibiting die-back was predicted to increase its canopy leaf area for up to 12 years following the large floods of 1974 and 1976, at sites where the soil was relatively permeable and groundwater highly saline (EC = 55 dS m −1). The changes in canopy leaf area in response to the floods was predicted to be relatively small on sites with heavier clay soils. The growth response of the vegetation to a long term lowering of watertable depth by 1 m was greater than that induced by the small potential increase in flooding frequency which is feasible given the current water storage limitations. The simulations predict that changes in the average annual soil water availability which arise from flood events and soil salinisation, drive a long term cycle in the annual average transpiration rate per unit leaf area suggesting the soil-plant-climate system is adjusting towards a hydrological equilibrium but is not in equilibrium. The proposed management options may control die-back in parts of the floodplain with more slowly salinising heavy clay soils and lower salinity groundwater but are unlikely to prevent die-back on relatively permeable soils with high salinity groundwater. However, they may assist vegetation survival between the long duration flood events which appear to be essential to sustain tree cover on the higher floodplain. The management options need to be evaluated further at the floodplain scale using the understanding from site specific conditions to test simple approaches which can be linked to a geographic information system (GIS) of the floodplain.

Leaf area index response of four obsolete and four modern cotton cultivars to two nitrogen levels1

The optimal nitrogen (N) rate for cotton (Gossypium hirsutum L.) production in the late 20th century is greater than it was in the middle of the century (112 versus 27 kg ha‐1). Part of the reason for this difference is that modern cultivars exhibit a greater harvest index than obsolete cultivars. This greater harvest index helps to allow modern cultivars to utilize greater N rates. However, factors other than harvest index, such as the development of leaf area in response to N, may also play an important role. Therefore, the objective of this study was to characterize leaf area index (LAI) of four obsolete and four modern cultivars at a low and high fertilizer N level. Cotton was grown in the field for two years (1992 and 1993) with two locations each year. The locations were a Beulah fine sandy loam and a Dubbs silt loam. Two preplant fertilizer‐N rates were used, a low (22 kg N ha‐1) and a high(112 kg N ha‐1). Leaf area index was determined at three stages in each year (early, mid, and late season). Yield was determined at maturity. Averaged across years, locations, and cultivars, late‐season LAI increased from 2.32 at 22 kg N ha‐1 to 3.15 at 112 kg N ha‐1 by late season. In 1992, modern and obsolete cultivars had similar LAI responses to N at early and mid season but by late season, LAI of modern cultivars was greater under high N than the obsolete cultivars (3.53 versus 2.95). Lint yield of the four modern cultivars was 372 kg ha‐1 greater than the four obsolete cultivars at 112 kg N ha‐1 and 289 kg lint ha‐1 greater at 22 kg N ha‐1 in 1992. The LAI response to N level of the modern cultivars was similar to that of obsolete cultivars in 1993 at all three sampling dates. In 1993, the lint yield of modern cultivars was 238 kg ha‐1 greater than obsolete cultivars under 112 kg N ha‐1 and 182 kg lint ha‐1 at 22 kg N ha‐1. In summary, our results best support the hypothesis that the higher yield of modern cultivars at high fertilizer N is unrelated to their LAI.

Leaf anatomical changes in Populus trichocarpa, Quercus rubra, Pseudotsuga menziesii and Pinus ponderosa exposed to enhanced ultraviolet‐B radiation

Leaf anatomical characteristics are important in determining the degree of injury sustained when plants are exposed to natural and enhanced levels of ultraviolet‐B (UV‐B) radiation (280–320 nm). The degree to which leaf anatomy can adapt to the increasing levels of UV‐B radiation reaching the earth's surface is poorly understood in most tree species. We examined four tree species, representing a wide range of leaf anatomical characteristics, to determine responses of leaf area, specific leaf weight, and leaf tissue parameters after exposure to ambient and enhanced levels of UV‐B radiation. Seedlings were grown in a greenhouse with photosynthetically active radiation of 39 mol m−2 day−1 and under one of three daily irradiances of biologically effective UV‐B radiation (UV‐BBE) supplied for 10 h per day: (1) approximate ambient level received at Pullman, Washington on June 21 (1 x ); two times ambient (2 x ), or three times ambient (3 x ). We hypothesized the response of each species to UV‐B radiation would be related to inherent anatomical differences. We found that the conifers responded anatomically to nearly an equal degree as the broad‐leaved trees, but that different tissues were involved. Populus trichocarpa, an indeterminate broadleaf species, showed significantly thicker palisade parenchyma in recently mature leaves at the 3 x level and in older leaves under the 2 x level. In addition, individual leaf area was generally greater with increased UV‐B irradiance. Quercus rubra, a semi‐determinate broadleaf species, exhibited significantly thicker palisade parenchyma at the 2 x and 3 x levels as compared to controls. Psuedotsuga menziesii, an evergreen coniferous species with bifacially flattened needles, and Pinus ponderosa, an evergreen coniferous species with a complete hypodermis, showed no significant change in leaf area or specific leaf weight under enhanced UV‐B radiation. Epidermal thickness was unchanged in P. menziesii. However, P. ponderosa increased the thickness and number of hypodermal layers produced, presumably decreasing penetration of UV‐B radiation into the leaf. We concluded that differences in inherent leaf anatomy of the four species examined are important in the responses to enhanced levels of UV‐B radiation.

BIOME3: An equilibrium terrestrial biosphere model based on ecophysiological constraints, resource availability, and competition among plant functional types

The equilibrium terrestrial biosphere model BIOME3 simulates vegetation distribution and biogeochemistry, and couples vegetation distribution directly to biogeochemistry. Model inputs consist of latitude, soil texture class, and monthly climate (temperature, precipitation, and sunshine) data on a 0.5° grid. Ecophysiological constraints determine which plant functional types (PFTs) may potentially occur. A coupled carbon and water flux model is then used to calculate, for each PFT, the leaf area index (LAI) that maximizes net primary production (NPP), subject to the constraint that NPP must be sufficient to maintain this LAI. Competition between PFTs is simulated by using the optimal NPP of each PFT as an index of competitiveness, with additional rules to approximate the dynamic equilibrium between natural disturbance and succession driven by light competition. Model output consists of a quantitative vegetation state description in terms of the dominant PFT, secondary PFTs present, and the total LAI and NPP for the ecosystem. Canopy conductance is treated as a function of the calculated optimal photosynthetic rate and water stress. Regional evapotranspiration is calculated as a function of canopy conductance, equilibrium evapotranspiration rate, and soil moisture using a simple planetary boundary layer parameterization. This scheme results in a two‐way coupling of the carbon and water fluxes through canopy conductance, allowing simulation of the response of photosynthesis, stomatal conductance, and leaf area to environmental factors including atmospheric CO2. Comparison with the mapped distribution of global vegetation shows that the model successfully reproduces the broad‐scale patterns in potential natural vegetation distribution. Comparison with NPP measurements, and with an FPAR (fractional absorbed photosynthetically active radiation) climatology based on remotely sensed greenness measurements, provides further checks on the model's internal logic. The model is envisaged as a tool for integrated analysis of the impacts of changes in climate and CO2 on ecosystem structure and function.

Model of Leaf Area Expansion in Field Pea Subjected to Soil Water Deficits

AbstractYield of field pea (Pisum sativum) is sensitive to soil water deficits. An important component of this sensitivity is the response of leaf area to drought. A simple, mechanistic model was developed to allow predictions of leaf area development in response to soil water deficits. The model calculates leaf areas of individual drought‐stressed leaves relative to leaves on well‐watered plants as the product of the number of cells and the size of the cells. Therefore, distinct phases of cell division and cell expansion were identified and temporally distinguished in the model. Cell division and cell expansion in the model were both sensitive to the fraction of transpirable soil water. The model proved capable of predicting the temporal development and the final area (r2 = 0.91) of individual leaves on pea plants in response to soil water deficits.

Barley response to nitrogen rate and timing in a Mediterranean environment

SUMMARYNitrogen is an important factor determining grain yield of barley (Hordeum vulgare L.). Optimal rates and application timing can vary according to the environmental conditions under which the crop is grown. For 8 years (1979–86) barley yield, tillering, and leaf-area response to N application rates and timing were studied in southern Spain. The results showed that, under our experimental conditions, the most advantageous N rate for grain yield was 60 kg/ha, either split equally between sowing and tillering, or else with the greater proportion applied at tillering. Nitrogen applied at sowing increased number of tillers per plant, and N applied at tillering favoured tiller survival for later spike formation. In addition, N applied at tillering increased the number of leaves per plant, leaf area per plant, and the leaf area index, the maximum value of which occurred at anthesis.

Soil Water Content Under Plants at High CO 2 Concentration and Interactions with the Direct CO 2 Effects: A Species Comparison

Wheat, maize and cotton, grown as spaced plants in large pots of soil, differed in the way high (2 X ambient) CO2 concentration affected the time-course of soil water use. For wheat, the tendency to conserve water owing to reduction in stomatal conductance in high CO2 was largely offset by the stimulation of leaf area development as the soil column dried. However, when the soil was maintained continuously wet, soil water conservation occurred because in the absence of water stress high CO2 did not maintain a greater leaf area. For maize, which has little or no photosynthetic response to CO2 concentrations above ambient but a strong stomatal response, water was conserved and the soil profile dried more slowly. Maize leaf area and dry matter growth increased in response to damper soil under high CO2, despite no growth response to CO2 in the absence of water stress. For cotton, which has a strong photosynthetic but weak stomatal response to CO2, the soil column dried faster under high CO2. Despite this drier soil, cotton still showed the greatest response to high CO2 of leaf area and dry matter growth of the three species compared. Under wet soil conditions, cotton exhibited a very large leaf area response to CO2 leading to much greater water use per plant. This contrasts with both wheat and maize which con- served water at high CO2 when wet. Despite these contrasting transpiration and growth responses, all three species exhibited a relatively similar increase in water use efficiency under high CO2 for both wet and dry conditions. It is concluded that the secondary effect of high CO2 on soil water content exerts a strong confounding influence on growth responses to CO2. In the longer term, the changed soil water status would influence hydrology, soil microbiology, nutrient relations and species composition. From indirect evidence it is proposed that the relative enhancement of growth owing to CO2 enrichment is greater under drought conditions than in wet soil because of the effect of water deficit on the intercel-

Corn Leaf Area Response to Early‐Season Soil Temperature under Crop Residues

AbstractResidue cover from conservation tillage modifies the soil temperature regime, compared with conventional tillage. This study was conducted to examine the influence of crop residue on the area of corn (Zea mays L.) leaves developed when the apical meristem is below the soil surface. Field studies compared the effects of residue‐covered and bare soil (Conover loam in East Lansing MI in 1988 and Brookston clay in Woodlsee, ON, in 1990), on maximum area per leaf (LAmax) at V6 and on leaf area index (LAI) after silking. LAmax for Leaves 4 to 6 was 11 to 52% higher in plants grown in residue‐covered soil than in bare soil. LAI after silking was similar in both treatments. LAmax differences could not be related to differences in volumetric water content or N concentration. Thermal effects of crop residue on LAmax and leaf appearance rates were tested in growth chamber. After emergence, the corn apical meristem was exposed to air. Nighttime air temperatures were maintained at 16°C and daytime air temperatures were controlled at 18, 21, and 25°C, simulating seed zone temperatures measured in the field. Daytime air temperatures of 21°C resulted in higher LAmax for Leaves 4 to 6 on V6 plants than did 18 or 25°C. Appearance rates increased with temperature. These results suggest that seed zone temperature under residue from emergence to V6 (≈23°C) was more favorable for LAmax than seed zone temperature under bare soil (≈26°C). Thus, for corn Leaves 4 to 6 the temperature optimum for LAmax is lower than the temperature optimum for leaf development.

Selected Components of the Shade-Avoidance Syndrome Are Displayed in a Normal Manner in Mutants of Arabidopsis thaliana and Brassica rapa Deficient in Phytochrome B.

Several growth parameters associated with the phytochrome-mediated shade avoidance syndrome have been measured in seedlings and mature plants of a wild-type and a hy3 mutant of Arabidopsis thaliana deficient in phytochrome B. Growth parameters were compared in plants grown in either white light (high red:far-red [R:FR] ratio) or white light plus added far-red (FR) light (low R:FR ratio). Wild-type Arabidopsis exhibited increased hypocotyl and petiole extension under a low, compared with a high, R:FR ratio. The hy3 mutant did not respond to low R:FR ratio by increase in hypocotyl or petiole length. Extension growth of wild-type plants was stimulated by brief end-of-day FR pulses, but similar treatment had no effect on extension growth of hy3 mutant plants. However, some responses to low R:FR ratio seen in the wild-type plants were also evident in the hy3 mutants. The number of days to bolting, the developmental stage at bolting, the leaf area, and the specific stem weight (weight per unit of length) all decreased in the wild-type and hy3 seedlings in response to low R:FR ratio. Low R:FR ratio caused a larger decrease in leaf area and specific stem weight in the mutant seedlings than in wild-type seedlings. The effects of low R:FR ratio on leaf area and specific stem weight were opposite to those of the hy3 lesion, which resulted in increased leaf area and specific stem weight in comparison with the wild type. Both leaf area and specific stem weight responses to low R:FR ratio also were unchanged in the ein mutant of Brassica rapa, known to be deficient in phytochrome B. These responses represent components of the shade-avoidance syndrome, and, consequently, the results indicate that phytochrome B cannot be solely responsible for the perception of R:FR ratio and the induction of shade-avoidance responses. The hypothesis is proposed that different phytochromes may be responsible for the regulation of extension growth and the regulation of lateral or radial expansion.