Increase In Net Assimilation Rate Research Articles

Australian Journal of Plant Physiology: Editorial Report

Ten contrasting Acacia species were grown in glasshouses with normal ambient CO2 or ele-vated to 700 µL L–1. Plants were grown in sand with a complete nutrient solution, including 5 mМ nitrate. Our objective was to determine the degree to which photosynthesis, and the efficiency of nitrogen and water use, were affected by growth under elevated CO2 in contrasting plant species that differ in specific foliage area (foliage area per unit foliage dry mass). Photosynthetic characteristics were measured at several stages. Growth and measurement of gas exchange under 700 mL L–1 CO2 resulted in enhanced rates of CO2 assimilation per unit foliage area in nine of the species. The degree of enhancement was independent of specific foliage area. The exception was the slow-growing A. aneura, which had lower rates of CO2 assimilation when grown and measured at 700 µL L–1 CO2 compared to plants grown and measured at 350 µL L–1 CO2, at 50, 78 and 93 d after transplanting. Leaf conductance was reduced by growth in elevated CO2 in only six of the species. Overall, elevated CO2 improved the ratio of CO2 assimi-lation to conductance by 78% and increased CO2 assimilation per unit of foliage nitrogen by 30% at a given specific foliage area. Detailed study of A. saligna and A. aneura revealed that the effects of the CO2 treatment were similarly evident on all fully expanded phyllodes, regardless of their age. Intercellular CO2 response curves were analysed on four species and revealed no change in the ratio of electron transport to Rubisco activities. However, for A. aneura and A. melanoxylon, both electron transport and Rubisco activities were reduced per unit foliage nitrogen, by growth under elevated CO2 . For A. saligna and A. implexa, these activities per unit nitrogen, were not altered by the elevated CO2 treatment. To relate CO2 assimilation rates to net assimilation rates (dry matter increment per unit foliage area per day) derived from growth analysis, between 30 and 50% of daily photosynthesis appeared to be consumed in respiration. This proportion was not altered by CO2 treatment for seven of the Acacia species, but appeared to be reduced in the other three. The increase in CO2 assimilation rate by growth under 700 com-pared to 350 µL L–1 CO2 that was measured (26%, mean of all species from two surveys), matched the increase in net assimilation rate that had been derived from destructive sampling (30%). We conclude that the increase in CO2 assimilation rate in the selected Acacia species was independent of species, growth rate and foliage area per unit foliage dry mass.

Photosynthetic characteristics of 10 Acacia species grown under ambient and elevated atmospheric CO2

Ten contrasting Acacia species were grown in glasshouses with normal ambient CO2 or ele-vated to 700 µL L–1. Plants were grown in sand with a complete nutrient solution, including 5 mМ nitrate. Our objective was to determine the degree to which photosynthesis, and the efficiency of nitrogen and water use, were affected by growth under elevated CO2 in contrasting plant species that differ in specific foliage area (foliage area per unit foliage dry mass). Photosynthetic characteristics were measured at several stages. Growth and measurement of gas exchange under 700 mL L–1 CO2 resulted in enhanced rates of CO2 assimilation per unit foliage area in nine of the species. The degree of enhancement was independent of specific foliage area. The exception was the slow-growing A. aneura, which had lower rates of CO2 assimilation when grown and measured at 700 µL L–1 CO2 compared to plants grown and measured at 350 µL L–1 CO2, at 50, 78 and 93 d after transplanting. Leaf conductance was reduced by growth in elevated CO2 in only six of the species. Overall, elevated CO2 improved the ratio of CO2 assimi-lation to conductance by 78% and increased CO2 assimilation per unit of foliage nitrogen by 30% at a given specific foliage area. Detailed study of A. saligna and A. aneura revealed that the effects of the CO2 treatment were similarly evident on all fully expanded phyllodes, regardless of their age. Intercellular CO2 response curves were analysed on four species and revealed no change in the ratio of electron transport to Rubisco activities. However, for A. aneura and A. melanoxylon, both electron transport and Rubisco activities were reduced per unit foliage nitrogen, by growth under elevated CO2 . For A. saligna and A. implexa, these activities per unit nitrogen, were not altered by the elevated CO2 treatment. To relate CO2 assimilation rates to net assimilation rates (dry matter increment per unit foliage area per day) derived from growth analysis, between 30 and 50% of daily photosynthesis appeared to be consumed in respiration. This proportion was not altered by CO2 treatment for seven of the Acacia species, but appeared to be reduced in the other three. The increase in CO2 assimilation rate by growth under 700 com-pared to 350 µL L–1 CO2 that was measured (26%, mean of all species from two surveys), matched the increase in net assimilation rate that had been derived from destructive sampling (30%). We conclude that the increase in CO2 assimilation rate in the selected Acacia species was independent of species, growth rate and foliage area per unit foliage dry mass.

Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress: I. Carbon assimilation and biomass production.

The interactive influences of elevated carbon dioxide, water stress, and ontogeny on carbon assimilation and biomass production were investigated in northern red oak, a species having episodic shoot growth characteristics. Seedlings were grown from acorns through three shoot-growth flushes (8-11 wk) in controlled-environment chambers at 400, 530 or 700 μmol mol-1 CO2 and under well watered or water-stressed soil-moisture regimes. Increasing CO2 growth concentration from 400 to 700 μmol mol-1 resulted in a 34% increase in net assimilation rate (A), a 31% decrease in stomatal conductance to water vapour (gs ) and a 141% increase in water use efficiency (WUE) in well watered seedlings. In contrast, water-stressed seedlings grown at 700 μmol mol-1 CO2 demonstrated a 69% increase in A, a 23% decrease in gs , and a 104% increase in WUE. However, physiological responses to increased CO2 and water stress were strongly modified by ontogeny. During active third-flush shoot growth, A in first-flush and second-flush foliage of water-stressed seedlings increased relative to the quiescent phase following cessation of second-flush growth by an average of 115%; gs increased by an average of 74%. In contrast, neither A nor gs in comparable foliage of well watered seedlings changed in response to active third-flush growth. Whereas seedling growth was continuous through three flushes in well watered seedlings, growth of water-stressed seedlings was minimal following the leaf-expansion stage of the third flush. Through three growth flushes total seedling biomass and biomass allocation to root, shoot and foliage components were very similar in water-stressed seedlings grown at 700 μmol mol CO2 and well watered seedlings grown at 400 μmol mol-1 CO2 . Enhancement effects of elevated CO2 on seedling carbon (C) assimilation and biomass production may offset the negative impact of moderate water stress and are likely to be determined by ontogeny and stress impacts on carbon sink demand.

Effects of elevated CO2 and temperature on growth and allometry of five native fast-growing annual species.

Whereas much is known of the short-term growth response to elevated atmospheric CO2 concentrations, [CO2 ]elev , there is relatively little information on how the response of native species is modified by temperature, despite the fact that an increase in global mean temperature is expected to accompany the rise in [CO2 ]. In this study, five functionally related annual native species were exposed to different combinations of ambient and elevated [CO2 ] and temperatures in order to assess their response in terms of growth and allometry. Fast-growing annuals were selected for the study because their growth responses could be assessed over a major portion of the plant's life cycle and in as short a period as 8 wk. Plants were grown in eight hemi-spherical glasshouses, programmed to track outside ambient conditions and provide a replicated experimental design. Treatments comprised (i) current ambient [CO2 ] and temperature, (ii) elevated [CO2 ] (ambient+34 kPa), and ambient temperature (iii) ambient [CO2 ] and elevated temperature (ambient+3°C) and (iv) elevated [CO2 ] and elevated temperature (T°Celev ). All five species responded positively to [CO2 ]elev , although the response was statistically significant for only one, Poa annua L. Averaged over all five species, [CO2 ]elev increased total plant biomass by 25% (P=0·005) at 56 d, reflecting a proportionally greater increase in leaf and stem mass relative to root weight. Elevated [CO2 ] had no effect on leaf area, either at the individual species level or overall. Elevated T°C, by contrast, had little effect on shoot growth but increased root mass on average by 43% and leaf area by 22%. Few interactions between elevated [CO2 ] and T°C were observed, with the CO2 response generally greater at elevated than ambient T°C. Both [CO2 ]elev and T°Celev resulted in a transient increase in relative growth rate, (rgr), during the first 14 d exposure and a 3°C increase in temperature had no effect on the duration of the response. CO2 stimulation of growth operated through a sustained increase in net assimilation rate. (nar), although the potential benefit to rgr was offset by a concurrent decline in leaf area ratio (lar), as a result of a decrease in leaf area per unit leaf mass (sla). The response to T°Celev was generally opposite of that to [CO2 ]elev . For example, T°Celev increased lar through an increase in sla and this, rather than any effect on nar, was the major factor responsible for the stimulation of rgr. Allometric analysis of CO2 effects revealed that changes in allocation observed at individual harvests were due solely to changes associated with plant size. Elevated T°C, by contrast, had a direct effect on allocation patterns to leaves, with an increase in leaf area expansion relative to whole plant mass during the initial stages of growth and subsequent increased allocation of biomass away from leaves to other regions of the plant. No change in the allometric relation between roots and shoots were observed at either elevated [CO2 ] or T°C. We conclude, therefore, that allocation of biomass and morphological characteristics such as sla, are relatively insensitive to [CO2 ], at least when analysed at the whole-plant level, and where changes have been observed, these are the product of comparing plants of the same age but different size.

Simulated sea level change alters anatomy, physiology, growth, and reproduction of red mangrove (Rhizophora mangle L.).

Tropical coastal forests - mangroves - will be one of the first ecosystems to be affected by altered sea levels accompanying global climate change. Responses of mangrove forests to changing sea levels depend on reactions of individual plants, yet such responses have not been addressed experimentally. We report data from a long-term greenhouse study that assessed physiological and individual growth responses of the dominant neotropical mangrove, Rhizophora mangle, to levels of inundation expected to occur in the Caribbean within 50-100 years. In this study, we grew potted plants in tanks with simulated semidiurnal (twice daily) high tides that approximated current conditions (MW plants), a 16-cm increase in sea level (LW plants), and a 16-cm decrease in sea level (HW plants). The experiment lasted 2½ years, beginning with mangrove seedlings and terminating after plants began to reproduce. Environmental (air temperature, relative humidity, photosynthetically active radiation) and edaphic conditions (pH, redox, soil sulfide) approximated field conditions in Belize, the source locale for the seedlings. HW plants were shorter and narrower, and produced fewer branches and leaves, responses correlated with the development of acid-sulfide soils in their pots. LW plants initially grew more rapidly than MW plants. However, the growth of LW plants slowed dramatically once they reached the sapling stage, and by the end of the experiment, MW plants were 10-20% larger in all measured growth parameters. Plants did not exhibit differences in allometric growth as a function of inundation. Anatomical characteristics of leaves did not differ among treatments. Both foliar C:N and root porosity decreased from LW through MW to HW. Relative to LW and HW plants, MW plants had 1-7% fewer stomata/mm2, 6-21% greater maximum photosynthetic rates, 3-23% greater absolute relative growth rates (RGRs), and a 30% higher RGR for a given increase in net assimilation rate. Reduced growth of R. mangle under realistic conditions approximating future inundation depths likely will temper projected increased growth of this species under concomitant increases in the atmospheric concentration of CO2.

Variation in growth stimulation by elevated carbon dioxide in seedlings of some C3 crop and weed species

Seven C3 crop and three C3 weed species were grown from seed at 360 and at 700 cm3 m–3 carbon dioxide concentrations in a controlled environment chamber to compare dry mass, relative growth rate (RGR), net assimilation rate (NAR), leaf area ratio (LAR) and photosynthetic acclimation at ambient and elevated carbon dioxide. The dry mass at the final harvest at elevated carbon dioxide relative to that at ambient carbon dioxide was highly correlated with the RGR at the lower carbon dioxide concentration. This relationship could be quite common, because it does not require that species differ in the response of RGR or photosynthesis to elevated carbon dioxide, and holds even when species differ moderately in these responses. RGR was also measured for a limited period at the end of the experiment to determine relationships with leaf gas exchange measured at this time. Relative increases in RGR at elevated carbon dioxide at this time were more highly correlated with the relative increase in NAR at elevated carbon dioxide than with the response of LAR. The amount of acclimation of photosynthesis was a good predictor of the relative increase in NAR at elevated carbon dioxide, and the long‐term increase in photosynthesis in the growth environment. No differences between crops and weeds or between cool and warm climate species were found in the responses of growth or photosynthetic acclimation to elevated carbon dioxide.

Growth response of grasses to elevated CO2: a physiological plurispecific analysis

summaryThe effect of CO2 enrichment on the growth and the economy of carbon and nitrogen of 11 Mediterranean grass species was investigated in order to determine the underlying causes of the large variation observed between species in their responses to elevated CO2. Plants were grown for 26–43 d (depending on species growth rate) under productive conditions at ambient (350 μmol mol−1) and elevated (700 μmol mol−1) concentrations of CO2. Plant parameters were determined at a common biomass of 0.15 g to determine the CO2 effect independent of ontogenic effects.The effect of CO2 on RGR ranged from −6.7 to 22.5%, with a mean stimulation of 10.3%. Averaged over the 11 species, the growth enhancement resulted from an increase in net assimilation rate per unit leaf d. wt. (narw) of 10.6 %. This was the result of a large increase (18.7%) in nar per unit leaf area (NARa) associated with a 8.1% decrease in the specific leaf area (sla). This decrease in sla was due to a large increase of the non‐structural carbohydrates. The increase in shoot activity was balanced by a 70.6% increase in the specific absorption rate of nitrogen (sar). AS a result, plant nitrogen content was not modified. Leaf nitrogen productivity was significantly increased (14.9%). Shoot vs. root allocation of biomass and nitrogen was not modified.An analysis across the 11 species of the relationships between the stimulation of rgb and the alteration in rgr components showed a significant correlation only with increases in narw, sar and nitrogen productivity. The co‐ordinated increase in these three parameters constitutes a single response syndrome, whose intensity is responsible for most of the species variability.

Response of two Afromontane coniferous tree species to light and nutrient supply.

We studied the responses to light and nutrient availability of the two Afromontane conifers Juniperus procera Hochst. ex Endl., reputedly a shade-intolerant species, and Afrocarpus gracilior Polger C.N. Page, a shade-tolerant species. The species behaved similarly in response to light and nutrient availability. Both species showed positive growth rates at photon fluxes of 0.6-0.8 mol m(-2) day(-1). Maximum relative growth rates of 0.15 week(-1) were attained at an irradiance of 8 mol m(-2) day(-1) in the high-nutrient supply treatment. The growth response to high-nutrient supply was achieved by an increase in leaf area ratio rather than by an increase in net assimilation rate. In a 10-week growth experiment, both species displayed an increase in stem extension rate in response to a low red/far-red ratio (0.09) at low irradiance, and A. gracilior also responded by increasing its specific leaf area. Similiar findings were observed in a short-term experiment in which internodes were laterally irradiated.

Urea fertilization effects on nutrient uptake and growth of Platanus occidentalis during plantation establishment

The benefit of fertilizer application during establishment of a tree plantation depends on effective nutrient uptake and the utilization of the nutrients in growth. Five urea treatments (0, 50, 75, 150, and 450 kg N/ha) were applied in a completely randomized plot design to a field planted with American sycamore (Platanus occidentalis L.) seedlings to evaluate growth responses and nitrogen use efficiency during the first season of plantation establishment. The site was in the Oak Ridge Reservation in eastern Tennessee on a highly weathered soil. Harvests were conducted on 3 occasions during a 22 week experimental period, and dry weights of stems, leaves, and large and small roots were measured. Chemical analyses were conducted on plant tissues from the 0, 75, and 450 kg N/ha treatments. Plant dry weight increased with urea application and growth analysis showed that this was mainly associated with increase in leaf area and to a minor extent with increase in net assimilation rate. Root weight increased significantly with urea application. The specific absorption rate of roots for several nutrients was greater at higher urea levels for the first 2 harvest periods, but this pattern reversed during the 3rd growth period. Surprisingly, manganese uptake and the specific absorption rate for manganese were enhanced with higher urea application. The acidifying effect of urea nitrification is a likely explanation for the increased Mn availability, and nitrate leaching and/or nitrogen immobilization contributed to low uptake of urea-N by the seedlings. The proportion of the applied nitrogen incorporated into the seedlings was 1.5 and 0.6% for the 75 and 450 kg N/ha urea treatments, respectively. Broadcast fertilizer application is not an effective way of supplying nutrients to seedlings during plantation establishment.

Growth analysis of spreading euonymus subjected to root restriction

Abstract A comparative growth analysis was made of spreading euonymus (E. kiautschovica Loes. ‘Sieboldiana') plants grown in the greenhouse for 20 weeks in 0.45 L (small) or 13.5 L (large) plastic pots of Beltsville sandy loam soil [amended with composted cow manure: perlite (2:1 v/v)]. In general, plants in small vs. large pots showed a decrease in specific leaf area (SLA), leaf area ratio (LAR), and mean relative growth rate (MRGR); an increase in specific leaf mass (SLM) and leaf mass ratio (LMR); but no difference in net assimilation rate (NAR). There was an increase in SLA and LAR and a decrease in SLM and LMR from weeks 0 to 4 in both sizes of pots and an increase in NAR in small pots, from the first to the second 4‐week interval, possibly reflecting a recovery from transplanting shock. There were no differences in NAR between small and large pots during the 20‐week period. The MRGR was 23% less in small pots than in large pots during the first 4‐week interval, and 27% less during the third 4‐week i...

A growth analysis study of three Achimenes cultivars grown under three light regimes

The growth of three morphologically different cultivars of Achimenes cultivars (‘Early Arnold’, ‘Rosenelfe’ and ‘Linda’) was studied in growth chambers for 49 days at 21/18°C day/night temperature under three different light regimes of 60, 40 and 17 W m −2 photosynthetically active radiation (PAR) at the same daily light integral (300 Wh m −2). Thus day lengths were 5, 7.5 and 17.5 h, respectively. Growth characteristics were recorded and a growth analysis study was carried out. Mean values of growth parameters were compared over a dry weight (DW) interval. No interactions between cultivar and light regime were observed in any of the parameters studied except for relative growth rate (RGR). Lower light intensity at the same daily light integral favoured growth (number of leaves, plant and leaf dry weight, leaf area) and flowering. RGR increased in all three cultivars (not always significantly) due to increase in net assimilation rate (NAR) whereas leaf area ratio (LAR) remained the same at decreasing light intensity at the same daily light integral. Therefore it is suggested that a better light-use efficiency (mg DW J −1 PAR) is responsible for the increase in growth at low light intensity combined with longer days. ‘Linda’ was the shortest cultivar with the highest number of leaves and axillary shoots after 49 days of growth. ‘Rosenelfe’ produced more flowers than the other two cultivars. Plant DW, leaf DW and leaf area were lowest for ‘Linda’ at the end of the experiment. This resulted from a low RGR, caused by a low NAR (small leaves, internal shading) combined with a low LAR (thick leaves).

Effect of stress and time for recovery on the amount of compensatory growth after grazing.

We tested the hypothesis that the amount of compensatory growth after defoliation is affected by the level of stress at which plants grow when defoliated and by the length of time for recovery. Growth response to defoliation went from partial compensation when plants were growing at high relative growth rates (RGR) to overcompensation when plants were more stressed and growing at low RGR. Defoliation released plants from the limitation imposed by the accumulation of old and dead tissue and this release overrode the negative effect of biomass loss. Compensatory growth resulted from a higher RGR aboveground that was not associated with a reduction in RGR belowground. Time available for recovery had a major impact on the outcome of defoliation. With a short time for recovery, RGR was decreased by defoliation because an immediate increase in net assimilation rate was overridden by a reduction in the ratio of leaf area to plant weight. After defoliation, this ratio increased quickly due to a larger allocation to leaf growth and lower leaf specific weights, resulting in higher RGR. We conclude that the compensatory response to grazing depends on the type and level of stress limiting growth. Allocation and physiological responses to stress may positively or negatively affect the response to grazing and, simultaneously, grazing may alleviate or aggravate the effects of different types of stress.

Combined Low Temperature-High Light Effects on Gas Exchange Properties of Jojoba Leaves

Jojoba (Simmondsia chinensis [Link] Schneider) is an important crop in desert climates. A relatively high frequency of periods of chilling and high photon flux density (PFD) in this environment makes photoinhibition likely, resulting in a reduction of assimilation capacity in overwintering leaves. This could explain the low net photosynthesis found in shoots from the field (4-6 micromoles per square meter per second) when compared to greenhouse grown plants (12-15 micromoles per square meter per second). The responses of photosynthesis and stomatal conductance to changes in absorbed PFD and in substomatal partial pressure of CO(2) were measured on jojoba leaves recovering from chilling temperature (4 degrees C) in high or low PFD. No measurable gas exchange was found immediately after chilling in either high or low PFD. For leaves chilled in low PFD, the original quantum yield was restored after 24 hours. The time course of recovery from chilling in high PFD was much longer. Quantum yield recovered to 60% of its original value in 72 hours but failed to recover fully after 1 week. Measurements of PSII chlorophyll fluorescence at 77 K showed that the reduced quantum yield was caused by photoinhibition. The ratio of variable to maximal fluorescence fell from a control level of 0.82 to 0.41 after the photoinhibitory treatment and recovery was slow. We also found a large increase in net assimilation rate and little closure of stomata as CO(2) was increased from ambient partial pressure of 35 to 85 pascals. For plants grown in full light, the increase in net assimilation rate was 100%. The photosynthetic response at high CO(2) concentration may constitute an ecological advantage of jojoba as a crop in the future.

Responses of Cynoglossum officinale L. and Senecio jacobaea L. to various degrees of defoliation.

Effects of various degrees of defoliation on Cynoglossum officinale and Senecio jocobaea were studied under favourable growing conditions. Regrowth was found in both species, but did not overcompensate for defoliation. The physiological mechanisms responsible for regrowth after defoliation were found to relate either to an increase in net assimilation rate or to an increase in relative investment in aboveground parts. The morphological mechanism behind regrowth involved an increase in leaf area ratio. In almost completely defoliated plants, a combination of both mechanisms was found. The time needed to re-establish the same amount of biomass as control plants differed between the species and between levels of defoliation. These differences in recovery time after defoliation are discussed in relation to decrees of herbivory found m natural situations.

Modelling the relative growth rate as a function of plant nitrogen concentration

The relationship between the relative growth rate (RGR) and the nitrogen concentration of the whole plant (PNC) was analyzed by using experimentally determined relations (1) between the PNC and the fraction of dry matter (LWR) and nitrogen in leaves, (2) between the specific leaf area (SLA) and the leaf nitrogen concentration (LNC) and (3) between the net assimilation rate (NAR) and the LNC on an area basis. A strong dependence of RGR on nitrogen concentration resulted from the increase in NAR, LWR and SLA with increasing PNC. A curvilinear relationship between RGR and PNC gave an optimum curve for nitrogen productivity against PNC.

Weed‐crop competition studies in swedes.

SUMMARYIn each of three hand‐weeding experiments in 1980, 1981 and 1982 the biomass of field‐grown crops of swedes was unaffected by weed competition up to 6 wk after sowing but by 9 wk unweeded plots had much reduced crop biomass compared with weed‐free plots. Propachlor, applied to half the plots in 1980 only, had little effect on the severity of weed competition but caused an early check to swede growth from which the crop fully recovered by 12 wk after sowing. In 1981 there was no significant biomass reduction due to weed competition until 9 wk after sowing, yet if weeds were removed 7 or 8 wk after sowing the crop biomass at harvest was substantially lower than if weeds were removed 6 wk after sowing. In 1980 and 1981 swedes made no late compensation for early growth restriction due to weed competition, but in 1982, when weed competition was less severe, late‐weeded or unweeded swedes made partial recovery at the very end of the season.Growth analysis of the 1981 and 1982 crops showed that weed competition reduced leaf area index (LAI) more markedly than net assimilation rate (NAR), though in 1982 weed removal 9 or 12 wk after sowing was immediately followed by a short‐lived increase in NAR. The failure of swedes to recover from weed competition was shown to result from an inability to generate new leaf area rapidly when the weeds were removed. It is suggested that competition between swedes and weeds may be primarily for soil nutrients, especially nitrogen.

The physiology of forage rye (Secale cereale)

SUMMARYField and pot investigations of the vegetative and early reproductive growth of a winter wheat and a winter rye cultivar over three seasons (1978–9, 1980–2) have shown that higher and earlier forage yields of rye are due to a combination of factors, notably more rapid rates of germination, crop emergence, leaf appearance and leaf expansion coupled with higher leaf area ratios. There was no evidence, from studies of net assimilation rate, that the photosynthetic apparatus of rye plants is better adapted than wheat to the low temperature and light conditions of winter and early spring. However, the earlier initiation of rye stem extension was associated with significant increases in net assimilation rate, which compensated for reductions in the leaf area ratio. Practical aspects of these results are discussed briefly.

Interaction of Atmospheric CO2 Enrichment and Irradiance on Plant Growth1

AbstractAtmospheric carbon dioxide and irradiance are important factors affecting growth and yield of plants. Due to the wide variation in irradiance in natural plant stands and the reportedly increasing carbon dioxide concentration in the global atmosphere, it is essential to study the interacting effects of these factors on the growth and production of crop plants. Growth analysis techniques were used to study the interaction of atmospheric CO2 concentration (350 and 675 µl/l) and photosynthetic photon flux density (PPFD) (600 and 1200 µEM−2 s−1)on four species (including both seed and root crops) grown in controlled environment chambers of the Duke University Phytotron. The plants were soybean (Glycine max L. Merr.), radish (Raphanus sativus L.), sugarbeet (Beta vulgaris L.), and corn (Zea mays L.). Total dry matter production increased in all species of plants and at all growth stages with both increased CO2 concentration and PPFD levels, and the maximum dry matter was produced at the highest combined levels of CO2 and PPFD. The dry weight increase varied between the different species and between plant parts within a species. High levels of CO2 and PPFD caused a greater increase in net assimilation rate in the plants during early growth stages than in later stages because the first two or three young, rapidly growing leaves were very efficient photosynthetic organs. A high CO2 or PPFD level resulted in decreasing leaf area ratios with increasing plant age for all the species due to a rapid increase in stem and root growth later in fruit production, and to a decreasing specific leaf area.Corn, having the C4 pathway of photosynthesis, showed less response to increased CO2 and PPFD than the three C3 species. Increasing the atmospheric CO2 concentration from 350 to 675 µl/liter at low and high PPFD levels produced dry matter increases of 72.7 and 76.4%, respectively, in soybean, and 18.9 and 18.6%, respectively, in corn at 50 days after planting. None of the species tested were light saturated at levels available in the standard fluorescent and incandescent lighting as is shown by the increased growth when higher PPFD levels were obtained with a combination of multivapor and sodium lamps.

Effect of Carbon Dioxide Concentration on Growth and Dry Matter Production of Crop Plants : 1. Effects on leaf area, dry matter, tillering, dry matter distribution ratio, and transpiration

Crop plants at the 5 to 7-leaf stages possessing C3 (barley, rice, and soybean) and C4 (barnyard millet and maize) pathways of photosynthesis were cultured for 15 days in transparent growth chambers placed outdoors under 4 different carbon dioxide (CO2) concentrations from 1/2 to 10 times the normal atmosphere (ca. 350 ppm) to make clear the effects of various CO2 concentrations on growth and dry matter production. The following results were obtained: I. Increases in plant height and leaf area were promoted by high CO2 concentrations, but the degree of promotion was relatively small. At a low concentration (160 ppm), plants often elongated and had narrow drooping leaves as compared to control plants under normal CO3 concentration. 2. Increase in dry weight was promoted by high CO2 concentrations and suppressed by a low concentration. This increase proceeded up to about 3 times (sometimes 10 times) the normal concentration, and this was effected mainly through the increase in net assimilation rate. The C3- and C4-plants responded differently to CO2 concentrations, the latter giving less suppression at a low concentration and less promotion at high concentrations than the former. 3. Tillering in barley and rice was accelerated by high CO2 concentratoins and withheld by a low concentration. In rice plants, observed at the 7-leaf stage, the promotion of tillering took place at the 4th node, and the appearance of tillers was made steadier at the 2nd and 3rd nodes by high CO2 concentrations. 4. Distribution ratio of dry matter in rice and maize was in a marked contrast. In rice plant distribution to leaves was less and that to roots was increased at high CO2 concentrations, while in maize such a tendency was not observed at all. 5. Transpiration rates (dm-2 day-1) were smaller at higher CO2 concentrations. At a low concentration, the promotion of transpiration was enhanced day by day as the treatment proceeded. On fine days, the leaf temperature was about 1°C lower at a low CO2 concentration and about 1°C higher at higher concentrations as compared to control plot, but no difference was observed on cloudy days.

The growth of maize: III. The effect of date of sowing and bitumen mulch on dry-matter yields

SUMMARYExperiments are described which analyse the effects of date of sowing and of a black petroleum soil mulch on the growth of two maize hybrids. The two hybrids used were Anjou 210, which is late maturing in S.E. England, and Kelvedon 75A, which is an early hybrid.As sowing was delayed from mid-April until the end of May, so there was a greater development of vegetative tissue (mainly stem) and leaf area which reached peaks at a later stage after silking. In Anjou 210 early sowing led to higher grain yields (12%) as the production of peak vegetative weight near to the time of silking allowed dry matter produced after that stage, still at a time of high radiation, to move direct to the oars. Also, the remobilization of the secondary source of grain dry matter, namely previously stored photosynthate from the stem, occurred earlier with early sowing and to a greater extent when a longer grain-filling period was allowed.The hybrid K 75A had a lower vegetative weight, and earlier silking coincided with higher levels of radiation, and the resultant increase in net assimilation rate led to a higher proportion of dry matter being partitioned into the ear. Because this hybrid flowered and senesced early, reserves of stem carbohydrate were low and the time available to transfer carbohydrate to the ear was short, resulting in lower yields than in the later maturing Anjou 210. K 75A also responded unexpectedly to the sowing treatments. Although late sowing gave higher grain yields in both hybrids up to 156 days after sowing, in the case of K 75 A, the early senescence curtailed the continuation of grain fill that would normally give early sowing an advantage and hence the late sown K 75 A produced 10 % more grain.The application of a black petroleum mulch to the surface of the soil to raise soil temperature enhanced the benefits derived from early sowing. Plants given this treatment produced greater leaf areas and vegetative dry weights. As a result more carbohydrate was translocated from the stem during the grain-filling period, increasing grain yield by 13% following an increase in both number of grains and individual grain weight.