Increase In Net Assimilation Rate Research Articles

Trait component analysis of lettuce in response to daily light integrals at two growth stages

AbstractLittle is known about the relative impact of lighting duration and light intensity on lettuce production, as well as whether daily light integrals (DLIs) play a significant role during different plant growth stages. Four DLIs were tested: 8.64, 11.52, 12.96, and 17.28 mol m−2 day−1 as supplemental lighting, which were obtained from a combination of supplemental light intensities at 200 and 300 µmol m−2 day−1 for 12 and 16 h. Sunlight was used as control. A direct correlation was observed between DLIs and yield, total fresh weight, and total dry weight. Under supplemental lighting, relative growth rate (RGR) increased due to an increase in net assimilation rate (NAR) rather than in leaf area ratio (LAR). Plants in the seedling stage were more sensitive to an elevation in DLI than in the head stage. This was confirmed by a greater increase in NAR compared to their corresponding control under the same DLI. Increasing DLI also improved lettuce quality via reducing nitrate and increasing protein content. Higher DLIs (12.96 and 17.28 mol m−2 day−1) led to a decrease in maximum quantum yield of photosystem II and an increase in performance index. In conclusion, manipulating RGR through NAR adjustments proved to be more effective than changes in LAR, and adjusting supplemental DLI at each stage was necessary to achieve a larger NAR and, consequently, a larger RGR.

Influence of Soil Amendment on the Relative Growth Rate and Net Assimilation Rate of Phaseolus vulgaris and Vigna aconitifolia

Background: Soil pH is one of the most important factors that contribute to crop growth and productivity. The present research was designed to assess the influence of soil amendment using organic manure and agricultural lime on the relative growth rate (RGR) and net assimilation rate (NAR) of Phaseolus vulgaris and Vigna aconitifolia grown on soils from different locations.
 Methodology: The three locations were: Akamkpa, Calabar Municipality and Odukpani. The pH for the three soil locations were 4.0, 7.0 and 9.0, respectively. The treatments were; control (0 g), OM1 (100 g organic manure), OM2 (200 g organic manure), AL1 (100 g agricultural lime), AL2 (200 g agricultural lime), OM1 + AL1 (50 g organic manure + 50 g agricultural lime) and OM2 +AL2 (100 g organic manure and 100 g agricultural lime).
 Results: Results obtained on the RGR of the leaf dry weight of P. vulgaris treated with OM2 was the highest (0.50 g/wk) followed by OM1 (0.41 g/wk). OM1 + AL1 had the highest RGR of the stem dry weight of P. vulgaris grown on soil from Calabar Municipality. In the RGR of the root dry weight, OM2 had the highest mean value in both plants grown on Akamkpa soil. Results obtained at 4 weeks after planting (WAP) revealed that there was significant (P<0.05) increase in NAR of plants grown on soil from Akamkpa. The highest NAR was obtained for V. aconitifolia treated with OM2 (0.0447 g/wk) followed by OM2 + AL2 (0.0057 g/wk) for both V. aconitifolia and P. vulgaris. P. vulgaris grown on Akamkpa and Odukpani soils treated with AL2 (0.0032 g/wk), OM1 + AL1 (0.0041 g/wk) and OM2+ AL2 (0.0062 g/wk) had the highest NAR at 8 WAP.
 Conclusion: The RGR and NAR of the two bean varieties were improved following treatments with organic manure and agricultural lime.

Primary productivity and physiological responses of Vitis vinifera L. cvs. under Free Air Carbon dioxide Enrichment (FACE)

Abstract Two Vitis vinifera L. cultivars, Riesling and Cabernet Sauvignon, were grown in the Geisenheim VineyardFACE (Free Air Carbon dioxide Enrichment) system under rain-fed conditions to investigate the effects of elevated CO2 on the productivity of grapevines for three consecutive years (2014–2016) following planting in 2012. The FACE system consisted of six 12 m diameter rings, with three at ambient CO2 (aCO2, 400 ppm), and three rings at elevated CO2 (eCO2, + 20% of aCO2 level). Vegetative growth, single leaf gas exchange and yield parameters were monitored for the three growing seasons. Vegetative growth parameters responded differently to CO2 treatments depending on biomass components. Trunk cross section area, as an indicator of perennial growth, showed a significant increase for Riesling under eCO2 but not for Cabernet Sauvignon. Fresh biomass as lateral leaf area and fresh weight of summer pruning were stimulated by elevated CO2 for both cultivars. Leaf gas exchange measurements for both cultivars showed a significant increase in net assimilation rate and an improved intrinsic water use efficiency for all three years under eCO2 conditions. However, contrary to expected stomatal behaviour of grapevines, transpiration rate and stomatal conductance were higher under elevated CO2 for Riesling and Cabernet Sauvignon in all three seasons. Higher values of pre-dawn leaf water potential recorded under eCO2 point towards an interaction with soil water availability and root system development. Elevated CO2 resulted in higher yield in terms of higher bunch weight, but did not affect average number of bunches per vine or sugar content of must at harvest date. Accordingly, bunch architecture was altered under elevated CO2 levels. The increase in primary productivity of grapevines under eCO2 indicates yield gains that can be expected under even modest near-future CO2 scenarios. However, higher water use, particularly if maintained as grapevines transition to maturity, may have critical implications for the future adaptation of non-irrigated viticulture to increasing temperature and periods of rainfall deficit.

English

The growth and senescence of Urochloa brizantha cv. Marandu were analyzed under the Brazilian Cerrado conditions. The seeds were sowed in November at the beginning of the rainy season. Samples of tillers were collected. The culms, inflorescences, along with green and dead leaves were removed to obtain leaf area and dry matter measurements. The maximum leaf area index (LAI = 5.80 m2 m-2) was achieved at 97 days after emergence (DAE), the maximum above ground total dry matter (TDM = 22.8 Mg ha-1) at 151 days, and the maximum crop growth rate (CGR = 0.260 Mg ha-1) at 70 days. Leaf senescence began at 55 DAE, with less than 0.001 Mg ha-1 of dead leaves dry matter (DLDM), and reached 1.753 Mg ha-1 at 151 DAE. Between 117 and 151 DAE, in the final cycle of the plants, there was lodging of the canopy by wind. This lodging had favored the sprouting of new tillers and a sudden increase in net assimilation rate (NAR). The LAI values did not show such expressive increase during the same period, suggesting that NAR may contribute to crop growth more than LAI due to increased light penetration after the occurrence of the lodging. In this paper an equation that estimates the net photosynthetic rate from the net assimilatory rate was presented. Key words: Leaf area, dry matter, growth rate, net assimilation rate, pasture grass

Functional growth analysis of ‘Sonata’ strawberry plants grown under controlled temperature and daylength conditions

In order to investigate the relationship between environmental conditions and vegetative growth and reproductive development in the strawberry, freshly rooted runner plants of the cultivar ‘Sonata’ were grown in a phytotron at temperatures of 12, 18 and 24°C and photoperiods of 10h short day (SD) and 20h long day (LD) for 31d and harvested at 10d intervals. Plant dry weight and leaf area increases were exponential versus time, giving a linear regression with the natural log (ln). This rendered the relative growth rate (RGR) constant over time at each environmental condition. Over the entire 31d growth period, the RGR increased linearly with increasing temperature across the range of temperatures with a further 10–13% enhancement by LD. A maximum RGR value of 0.077g/g/d was determined in LD at 24°C. Increases in the RGR was driven by a combined increase in net assimilation rate (NAR) and leaf area ratio (LAR) and was associated with an increased allocation of dry matter production into leaves and less into crowns and roots. Because of this, the shoot/root ratio increased consistently with increasing temperature and photoperiod, which was also associated with a significant increase in the tissue C/N concentration ratio. Low temperature promoted starch accumulation markedly in all parts of the plants, with a further enhancement by LD conditions, while the concentrations of soluble sugars were less affected by the climatic environment. Forcing of plants exposed to the various growth conditions for 31d showed that all plants at 12 and 18°C and 80% of those at 24°C had initiated flowers in SD, whereas none had initiated flowers in LD regardless of temperature conditions. All these results demonstrate an opposite environmental relationship between vegetative growth and reproductive development in the strawberry.

Growth and chlorophyll fluorescence under salinity stress in sugar beet (Beta vulgaris L.)

This study was carried out in the General Commission for Scientific Agricultural Research (GCSAR), Syria, at Der EzZour Agricultural Research Center, from 2008-2010, to examine the effect of salt conditions on some growth attributes and chlorophyll fluorescence in 10 Sugar Beet (Beta vulgaris L.) genotypes under salinity stress. Sugar beet plants were irrigated with saline water, having electrical conductivity ranged from 8.6-10 dS.m-1during first year and 8.4-10.4 dS.m-1 during second year. A randomized completely block design with three replicates was used. The results showed that all studied growth attributes, leaf area, leaf number, relative growth rate, and net assimilation rate were decreased in salinity stress conditions compared to the controlled state. The findings indicated that salinity caused a decrement of light utilizing through increased values of fluorescence origin (fo), decreased values of fluorescence maximum (fm), and maximum yield of quantum in photosystem-II (fv/fm). Genotypes differed significantly in all studied attributes except in leaf number. Under salt conditions, Brigitta (monogerm) achieved an increase in net assimilation rate, while Kawimera (multigerm) achieved the lowest decrement in quantum yield in photosystem-II. Further studies are necessary to correlate the yield with yield components under similar conditions to determine the most tolerant genotype.International Journal of Environment Vol.3(1) 2014: 1-9 DOI: http://dx.doi.org/10.3126/ije.v3i1.9937

Effect of tillage on soil and crop properties of wet-seeded flooded rice

No-tillage (NT) is an alternative cropping system for saving costs and conserving soils relative to conventional tillage (CT). However, NT effects on paddy soil and rice growth are still controversial or not fully understood. A fixed field experiment was conducted to compare soil and crop properties between NT and CT wet-seeded flooded super hybrid rice in Changsha, Hunan Province, China. After 6 years of continuous cropping, NT had higher contents of active organic carbon, NaOH hydrolysable N and NH4OAc extractable K and higher activities of invertase, urease and acid phosphatase at 0–5cm soil depth, higher bulk density at 5–10cm soil depth, and higher contents of double acid P at 5–10cm and 10–20cm soil depths. NT or associated soil compaction caused an adverse root environment for NT rice at early growth stage, which resulted in a lower capacity of photosynthetic carbon metabolism and consequent reductions in number of tillers and aboveground biomass accumulation before heading. However, no reductions were observed in total aboveground biomass and grain yield in NT rice, because the negative effects of NT or associated soil compaction on aboveground biomass production before heading were compensated for by its positive effects on aboveground biomass accumulation after heading. On one hand, the reduction in growth before heading of NT rice made its population density lower but more suitable during heading to 20 days after heading, which led to a more appropriate leaf area index, a lower leaf senescence and a consequent increase in net assimilation rate. On the other hand, N uptake was delayed in NT rice, which was another critical factor in determining its low leaf senescence. Our study suggests that the negative effects of NT or associated soil compaction on crop growth at early growth stage do not necessarily become concerns in NT wet-seeded flooded rice production.

Overexpression of the phytochrome B gene from Arabidopsis thaliana increases plant growth and yield of cotton (Gossypium hirsutum).

The phytochrome B (PHYB) gene of Arabidopsis thaliana was introduced into cotton through Agrobacterium tumefaciens. Integration and expression of PHYB gene in cotton plants were confirmed by molecular evidence. Messenger RNA (mRNA) expression in one of the transgenic lines, QCC11, was much higher than those of control and other transgenic lines. Transgenic cotton plants showed more than a two-fold increase in photosynthetic rate and more than a four-fold increase in transpiration rate and stomatal conductance. The increase in photosynthetic rate led to a 46% increase in relative growth rate and an 18% increase in net assimilation rate. Data recorded up to two generations, both in the greenhouse and in the field, revealed that overexpression of Arabidopsis thaliana PHYB gene in transgenic cotton plants resulted in an increase in the production of cotton by improving the cotton plant growth, with 35% more yield. Moreover, the presence of the Arabidopsis thaliana PHYB gene caused pleiotropic effects like semi-dwarfism, decrease in apical dominance, and increase in boll size.

Relative contributions of leaf area ratio and net assimilation rate to change in growth rate depend on growth temperature: comparative analysis of subantarctic and alpine grasses

The present study shows that the relative contributions of leaf area ratio (LAR) and net assimilation rate (NAR) to variation among species in relative growth rate (RGR) depend on growth temperature. We grew three subantarctic and three alpine Poa species at daytime temperatures of 7, 12 and 17 degrees C, and analysed interspecific and temperature-related variation in RGRs by growth analysis. Variation in NAR accounted for most of the interspecific differences in RGR at low growth temperature, whereas variation in both NAR and LAR contributed strongly to interspecific differences in RGR at high growth temperature. For most species, the increase in RGR from 7 to 12 degrees C was attributable to an increase in LAR, whereas the increase in RGR from 12 to 17 degrees C was attributable to an increase in NAR. There were no differences between native subantarctic and alpine species in the plasticity of growth responses to temperature. However, Poa annua, a species introduced to the subantarctic, showed much greater growth plasticity than other species. There was little difference among species in tolerance of high-temperature extremes.

CULTIVAR DIFFERENCES IN TEMPERATURE DEMAND OF CUT CHRYSANTHEMUM

The influence of temperature on dry matter production, growth analysis parameters, stem length, number of leaves and flower characteristics of 25 cut chrysanthemum cultivars was investigated. Plants were grown in the greenhouse at two constant temperatures setpoints, 16 and 20oC. Destructive measurements were carried out at the end of the long day period and at flowering. During the long day period relative growth rate was increased at high temperature for all cultivars due to an increase in net assimilation rate and for a few cultivars also by an increase in leaf area ratio. Significant temperature x cultivar interactions were only present for stem length, number of leaves and leaf area ratio. For all other characteristics there were clear differences between the two temperature treatments and the cultivars. Depending on the cultivar, flowering was delayed by 4 to 13 days when cultivated at low temperature. At flowering, a significant temperature x cultivar interaction was observed for all measured or calculated parameters. For example, for one cultivar both the differences in number of days till flowering and the total dry mass between 16oC and 20oC were small while for another cultivar there was a 34% higher dry mass at lower temperature, while the growth period was not much extended. Differences in dry mass at flowering between the two temperature treatments could be explained by differences in growth rate. These data show good possibilities for breeding for low temperature demand in cut chrysanthemum

Differences in the compensatory growth of two co-occurring grass species in relation to water availability

We compared the potential for compensatory growth of two grass species from the Mongolian steppe that differ in their ability to persist under grazing: the rhizomatous Leymus chinensis and the caespitose Stipa krylovii, and investigated how this ability might be affected by drought. Plants were grown in a greenhouse under wet and dry conditions and subjected to a clipping treatment (biweekly removal of 75-90% of the aerial mass). Leymus exhibited a much stronger compensatory growth after clipping than Stipa. Leymus showed a significant increase in its relative growth rate (RGR) after clipping, while for Stipa RGR was negatively affected. Clipped Leymus plants maintained leaf productivity levels that were similar to undamaged individuals, while leaf-productivity in clipped Stipa dropped to less than half of that of the controls. In Leymus, there was less compensatory growth under dry than under wet conditions, while in Stipa the compensation was increased under drought. This difference probably reflects the fact that Stipa is more drought-tolerant than Leymus. The greater compensatory growth of Leymus compared to Stipa mainly resulted from a greater stimulation of its net assimilation rate (NAR), and its greater capacity to store and reallocate carbohydrates by clipping. The greater increase in NAR was probably the result of a stronger reduction in self-shading, because Leymus shoots were much denser than those of Stipa, which resulted in a higher increase in light penetration to remaining leaves after clipping. The results of this study suggest that the greater ability of Leymus to persist under grazing is the result of its larger capacity for compensatory growth.

Interstock-induced mechanism of increased growth and salt resistance of orange (Citrus sinensis) trees.

Interstocks improve the growth and salt resistance of lemon (Citrus limon (L.) Burm. f.) trees, but their effects on orange (Citrus sinensis (L.) Osbeck) trees are unknown. We grew 'Cleopatra' mandarin (CM) seedlings, budded trees of 'Salustiano' orange (SAO) on CM, 'Valencia Late' orange (VLO) on CM (VLO/CM), and interstock trees VLO/SAO/CM in pots of sand watered with nutrient solution containing 5 (control) or 50 mM NaCl for 12 weeks. Plants were harvested on six successive occasions and the time trends in relative growth rate (RGR) and its components were estimated by fitting a Richards function regression to the harvest data. At low and high salinities, the VLO/SAO/CM combination had higher mean RGR than VLO/CM. Under control conditions, the increase in RGR caused by the interstock was the result of an increase in leaf mass fraction (LMF; leaf dry mass/plant dry mass ratio). Increases in net assimilation rate on a leaf mass basis (NARm) and LMF contributed equally to the increase in RGR in saline conditions, their growth response coefficients being 0.52 and 0.48, respectively. The structural modifications, specific leaf area (SLA) and leaf area ratio (LAR; leaf area:plant dry mass ratio), had a slight influence on the reduction in RGR by salinity. However, NARm had a large influence on RGR, except in CM. The interstock-induced mechanism increased biomass allocation to the assimilatory organs and, under saline conditions, increased Cl- and Na+ allocations to roots. Thus, the flux of ions to the leaves was either delayed or reduced or both. The dilution of imported ions by foliar growth reduced ion concentrations in leaves, resulting in higher NARm, which together with higher LMF, increased RGR.

Germination and seedling growth of five tree species from tropical dry forest in relation to water stress: impact of seed size

The impact of seed size on germination and seedling growth, as affected by water stress, was studied for five tree species from tropical dry forest of India, viz. Albizia procera, Acacia nilotica, Phyllanthus emblica, Terminalia arjuna and Terminalia chebula. Germination tests were conducted under five osmotic potential levels. Seedlings from large (LS) and small (SS) seeds were grown at four soil moisture levels. Observations were made on height, leaf area, biomass and other growth traits such as relative growth rate (RGR), net assimilation rate (NAR), specific leaf area (SLA), and root:shoot (R:S) ratio. Seeds of pioneer species and large seeds, within species, germinated earlier, and with increasing water stress, per cent germination and germination velocity declined. RGR was inversely related with drought tolerance. R:S ratio increased, RGR and SLA declined, but NAR increased with water stress. Notwithstanding successional status, the slow-growing species registered minimum reduction in biomass due to water stress. The response of LS and SS seedlings also differed for some of the growth variables. Increase in NAR could be a compensatory response to water stress, and the marked allocational plasticity could help maximize capture of the limited resource. Seedlings from smaller seeds, particularly of fast-growing species, would be able to cope with mild drought by morphogenetic and physiological plastic response in a better way than those from large seeds. However, seedlings from large seeds had greater survival than those from smaller seeds under intense water stress.

Effects of defoliation on growth of cauliflower

Three experiments were performed with defoliation of young cauliflower plants in the field. The leaves were removed 3–6 weeks after planting and about 65% of the leaf area were removed. After defoliation, several plant processes contributed to compensate for the loss of leaf area. Right after defoliation, the ratio of leaf area to plant weight was strongly decreased. However, during further growth the ratio of leaf area to plant weight was increased in defoliated plants. Together with an increase in net assimilation rate it contributed to the increase in plant relative growth rate after defoliation. Due to higher rates of leaf growth relative to stem growth, the biomass allocation pattern was re-established. Specific leaf area was increased in defoliated plants due to depletion of reserves, and contributed to the restoration of the relationship between leaf area and plant dry weight. Concentrations of non-structural carbohydrates were reduced after defoliation, showing that reserves were mobilised to support re-growth. We found that cauliflower through a number of mechanisms re-established the basic relationships in the plants after damage, and this observation and the estimated relationships can be used for modelling effects of pest damage on crop growth.

A global perspective of ground level, 'ambient' carbon dioxide for assessing the response of plants to atmospheric CO2

For most studies involving the response of plants to future concentrations of atmospheric carbon dioxide (CO2), a current concentration of 360–370 μatm is assumed, based on recent data obtained from the Mauna Loa observatory. In the present study, average seasonal diurnal values of ambient CO2 obtained at ground level from three global locations (Australia, Japan and the USA) indicated that the average CO2 (at canopy height) can vary from over 500 μatm at night to 350 μatm during the day with average 24-h values ranging from 390 to 465 μatm. At all sites sampled, ambient CO2 rose to a maximum value during the pre-dawn period (03.00–06.00 hours); at sunrise, CO2 remained elevated for several hours before declining to a steady-state concentration between 350 and 400 μatm by mid-morning (08.00–10.00 hours). Responses of plant growth to simulations of the observed variation of in situ CO2 were compared to growth at a constant CO2 concentration in controlled environment chambers. Three diurnal patterns were used (constant 370 μatm CO2, constant 370 during the day (07.00–19.00 hours), high CO2 (500 μatm) at night; or, high CO2 (500 μatm) at night and during the early morning (07.00–09.00 hours) decreasing to 370 μatm by 10.00 hours). Three plant species − soybean (Glycine max, L (Merr.), velvetleaf (Abutilon theophrasti L.) and tomato (Lycopersicon esculentum L.) − were grown in each of these environments. For soybean, high night-time CO2 resulted in a significant increase in net assimilation rate (NAR), plant growth, leaf area and biomass relative to a constant ambient value of CO2 by 29 days after sowing. Significant increases in NAR for all three species, and significant increases in leaf area, growth and total biomass for two of the three C3 species tested (velvetleaf and soybean) were also observed after 29 days post sowing for the high night/early morning diurnal pattern of CO2. Data from these experiments suggest that the ambient CO2 concentration experienced by some plants is higher than the Mauna Loa average, and that growth of some agricultural species at in situ CO2 levels can differ significantly from the constant CO2 value used as a control in many CO2 experiments. This suggests that a reassessment of control conditions used to quantify the response of plants to future, elevated CO2 may be required.

A global perspective of ground level, ‘ambient’ carbon dioxide for assessing the response of plants to atmospheric CO2

AbstractFor most studies involving the response of plants to future concentrations of atmospheric carbon dioxide (CO2), a current concentration of 360–370 μatm is assumed, based on recent data obtained from the Mauna Loa observatory. In the present study, average seasonal diurnal values of ambient CO2 obtained at ground level from three global locations (Australia, Japan and the USA) indicated that the average CO2 (at canopy height) can vary from over 500 μatm at night to 350 μatm during the day with average 24‐h values ranging from 390 to 465 μatm. At all sites sampled, ambient CO2 rose to a maximum value during the pre‐dawn period (03.00–06.00 hours); at sunrise, CO2 remained elevated for several hours before declining to a steady‐state concentration between 350 and 400 μatm by mid‐morning (08.00–10.00 hours). Responses of plant growth to simulations of the observed variation of in situ CO2 were compared to growth at a constant CO2 concentration in controlled environment chambers. Three diurnal patterns were used (constant 370 μatm CO2, constant 370 during the day (07.00–19.00 hours), high CO2 (500 μatm) at night; or, high CO2 (500 μatm) at night and during the early morning (07.00–09.00 hours) decreasing to 370 μatm by 10.00 hours). Three plant species − soybean (Glycine max, L (Merr.), velvetleaf (Abutilon theophrasti L.) and tomato (Lycopersicon esculentum L.) − were grown in each of these environments. For soybean, high night‐time CO2 resulted in a significant increase in net assimilation rate (NAR), plant growth, leaf area and biomass relative to a constant ambient value of CO2 by 29 days after sowing. Significant increases in NAR for all three species, and significant increases in leaf area, growth and total biomass for two of the three C3 species tested (velvetleaf and soybean) were also observed after 29 days post sowing for the high night/early morning diurnal pattern of CO2. Data from these experiments suggest that the ambient CO2 concentration experienced by some plants is higher than the Mauna Loa average, and that growth of some agricultural species at in situ CO2 levels can differ significantly from the constant CO2 value used as a control in many CO2 experiments. This suggests that a reassessment of control conditions used to quantify the response of plants to future, elevated CO2 may be required.

Interactive effects of increased temperature and CO2 on the growth of Quercus myrsinaefolia saplings

AbstractThe interactive effects of increased temperature and CO2 enrichment on the growth of 2‐year‐old saplings of Quercus myrsinaefolia, an evergreen broad‐leaved oak, were studied throughout an entire year in the vicinity of their northernmost distribution. Saplings were grown under different conditions in two chambers: (1) a temperature gradient chamber at ambient temperature, 3 and 5 °C warmer conditions with an ambient CO2 concentration, and (2) in a CO2 temperature gradient chamber at 3 °C warmer conditions with 1·5 times the normal CO2 concentration, and 5 °C warmer conditions with doubled CO2 concentration. The 3 and 5 °C warmer conditions enhanced the relative growth rate during almost the entire year, producing 53 and 47% increases in annual biomass production, 27 and 44% enhancement of root growth during shoot dormancy and 3 and 5 week prolongation of the shoot growing period, respectively. However, a daily mean air temperature exceeding 30 °C under the 5 °C warmer condition caused a marked reduction in net assimilation rate (NAR) from July to September. The CO2 enrichment further enhanced the positive effects of warming in spring and the resulting increases in NAR almost completely compensated for the negative effect of warming during summer. From autumn to winter, attenuation of the effects of CO2 was compensated by the increased sink strength produced by the warming. The annual biomass production was more than doubled by the combination of temperature elevation and CO2 enrichment.

Light‐dependent changes in biomass allocation and their importance for growth of rain forest tree species

Summary Sapling growth of six rain forest tree species was compared to evaluate whether species respond in a similar way to a natural light gradient. Saplings were measured non‐destructively; production and loss of leaves, stem and branches were analysed in detail. Sapling height growth was positively related to light environment and leaf area. No single descriptor of light environment explained sapling growth best. Direct or diffuse light could explain plant growth, depending on species. Seventeen percent of the saplings had negative relative biomass growth rates, although they occurred in fairly bright conditions. Negative growth rates were caused by leaf shedding and stem breakage. Sapling relative growth rate increased with irradiance, mainly because of an increase in net assimilation rate. On a shoot basis, shaded plants had a smaller leaf mass fraction (LMF) and a larger specific leaf area, resulting in similar leaf area ratios (LAR) to those of sun plants. This contrasts with the results of seedling studies under controlled conditions, where LMF and LAR increased with shade. Biomass partitioning to leaf growth decreased with irradiance and relative growth rate of the sapling. This leaf partitioning ratio was better correlated with RGR than with irradiance. Species differed in the effect of light‐dependent changes in specific leaf area (SLA) on growth. This underscores the importance of SLA in explaining differences in species performance in a forest environment. Nevertheless, the effect of SLA was not related to the shade tolerance of the species.

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.

Influence of abiotic and biotic factors on two co-occurring species of Bolboschoenus

Distribution of the emergent macrophytes Bolboschoenus medianus and Bolboschoenus caldwellii is dominated by the latter at regions higher on the elevation gradient, whereas the former is dominant further down the gradient. Monocultures and mixtures of plants were grown across a water-depth gradient in experimental ponds to determine whether distribution is due to abiotic factors, biotic factors, or a combination of both. Monocultures of each species tolerated exposure, showing little variation in relative growth rate (RGR), net assimilation rate (NAR) or leaf area ratio (LAR). Survival when initially flooded was dependent on shoot height. Plants surviving inundation responded by increasing height through reallocation of biomass. The RGR of B. medianus was maintained across the water-depth gradient by increasing NAR as LAR declined. The RGR of B. caldwellii beyond a depth of −20 cm declined because reductions in LAR were not paralleled by increases in NAR. Mixtures of species growing at 20 cm and 0 cm indicated that biotic interactions occurred and that B. caldwellii was the dominant species. Neither species dominated at −60 cm, presumably because this was beyond the depth tolerated by both species. The study suggests that the zonation of B. medianus and B. caldwellii is attributable to a combination of both abiotic and biotic factors.