Maximum Net Assimilation Research Articles

Growth Analysis of Biomass Production in Sole‐Crop and Double‐Crop Corn Systems

ABSTRACTIncreased biomass productivity could be achieved through double‐cropping if extended growth duration could be realized with minimal reductions in growth efficiency relative to sole‐cropping. To test this hypothesis, functional growth analysis was used to assess the relative importance of photosynthetic duration and efficiency in determining biomass production by sole‐crop corn (Zea mays L.; SC) and double‐crop triticale (×Triticosecale Wittmack)–double‐crop corn (DT–DC). Aboveground dry matter and leaf area were measured weekly, and net dry matter production was assessed for each crop at harvest. Over 2 yr, average harvested dry matter was 25% greater for DT–DC (22.7 Mg ha−1) than for SC (18.2 Mg ha−1), despite greater maximum leaf area index and greater maximum crop growth rate for SC relative to DT–DC. Leaf area duration was increased by 23% for DT–DC compared with SC, while maximum net assimilation rate and seasonal net assimilation rate did not differ between cropping systems. Across systems, variation in yield was positively related to maximum crop growth rate, maximum leaf area index, and leaf area duration but was not associated with maximum or seasonal net‐assimilation rate. Therefore, leaf duration was more important than leaf efficiency in determining productivity in both cropping systems, and greater biomass yield for DT–DC was the outcome of photosynthesis occurring over an extended duration. These results suggest that potential exists to increase biomass productivity by expanding the seasonal interval of photosynthesis, and that in the case of double‐cropping, expansion of leaf duration is not necessarily associated with reductions in leaf efficiency.

Precipitation as driver of carbon fluxes in 11 African ecosystems

Abstract. This study reports carbon and water fluxes between the land surface and atmosphere in eleven different ecosystems types in Sub-Saharan Africa, as measured using eddy covariance (EC) technology in the first two years of the CarboAfrica network operation. The ecosystems for which data were available ranged in mean annual rainfall from 320 mm (Sudan) to 1150 mm (Republic of Congo) and include a spectrum of vegetation types (or land cover) (open savannas, woodlands, croplands and grasslands). Given the shortness of the record, the EC data were analysed across the network rather than longitudinally at sites, in order to understand the driving factors for ecosystem respiration and carbon assimilation, and to reveal the different water use strategies in these highly seasonal environments. Values for maximum net carbon assimilation rates (photosynthesis) ranged from −12.5 μmol CO2 m−2 s−1 in a dry, open Millet cropland (C4-plants) up to −48 μmol CO2 m−2 s−1 for a tropical moist grassland. Maximum carbon assimilation rates were highly correlated with mean annual rainfall (r2=0.74). Maximum photosynthetic uptake rates (Fpmax) were positively related to satellite-derived fAPAR. Ecosystem respiration was dependent on temperature at all sites, and was additionally dependent on soil water content at sites receiving less than 1000 mm of rain per year. All included ecosystems dominated by C3-plants, showed a strong decrease in 30-min assimilation rates with increasing water vapour pressure deficit above 2.0 kPa.

Midday depression of leaf CO2 exchange within the crown of Dipterocarpus sublamellatus in a lowland dipterocarp forest in Peninsular Malaysia

We observed diurnal and seasonal patterns of leaf-scale gas exchange within the crown of a Dipterocarpus sublamellatus Foxw. tree growing in a lowland dipterocarp forest at Pasoh, Peninsular Malaysia. Observations were carried out nine times over 6 years, from September 2002 to December 2007. Observation periods included both wet and mild-dry periods, and natural and saturated photosynthetic photon flux density (PPFD) light conditions. In situ measurements of the diurnal change in net photosynthetic rate and in stomatal conductance were carried out on canopy leaves of a 40-m-tall D. sublamellatus tree, which was accessed from a canopy corridor. A diurnal change in electron transport rate was observed under saturated PPFD conditions. The maximum net assimilation rate was approximately 10 micromol m(-2) s(-1). There was a clear inhibition of the net assimilation rate coupled with stomatal closure after late morning and this inhibition occurred year-round. Although the electron transport rate decreased alongside this inhibition, it sometimes followed on. Numerical analysis showed that the main factor in the inhibition of the net assimilation rate was patchy bimodal stomatal closure, which occurred in both mild-dry and wet periods. The midday depression occurred year-round, though there are fluctuations in soil moisture during the mild-dry and wet periods. The magnitude of the inhibition was not related to soil water content but was related to vapor pressure deficit (VPD): that is, whether the days were sunny and hot or cloudy and cool. On cloudy, cool days in the wet period, the net photosynthesis was only moderately inhibited, but it still decreased in the afternoon and was coupled with patchy stomatal closure, even in quite moderate VPD, leaf temperature and PPFD conditions. Our results suggest that patchy stomatal closure signaled by the increase in VPD, in transpiration and by circadian rhythms, was the key factor in constraining midday leaf gas exchange of the D. sublamellatus canopy leaves.

Growth form and seasonal variation in leaf gas exchange of Colophospermum mopane savanna trees in northwest Botswana

We investigated differences in physiological and morphological traits between the tall and short forms of mopane (Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard) trees growing near Maun, Botswana on a Kalahari sandveld overlying an impermeable calcrete duricrust. We sought to determine if differences between the two physiognomic types are attributable to the way they exploit available soil water. The tall form, which was located on deeper soil than the short form (5.5 versus 1.6 m), had a lower leaf:fine root biomass ratio (1:20 versus 1:6), but a similar leaf area index (0.9-1.0). Leaf nitrogen concentrations varied between 18 and 27 mg g(-1) and were about 20% higher in the tall form than in the short form. Maximum net assimilation rates (A sat) occurred during the rainy seasons (March-April 2000 and January-February 2001) and were similar in the tall and short forms (15-22 micromol m(-2) s(-1)) before declining to less than 10 micromol m(-2) s(-1) at the end of the rainy season in late April. As the dry season progressed, A sat, soil water content, predawn leaf water potential (Psi pd) and leaf nitrogen concentration declined rapidly. Before leaf abscission, Psi pd was more negative in the short form (-3.4 MPa) than in the tall form (-2.7 MPa) despite the greater availability of soil water beneath the short form trees. This difference appeared attributable to differences in root depth and density between the physiognomic types. Stomatal regulation of water use and carbon assimilation differed between years, with the tall form having a consistently more conservative water-use strategy as the dry season progressed than the short form.

Biomass allocation, morphology and photosynthesis of invasive and noninvasive exotic species grown at four irradiance levels

Wetestedthehypothesesthatinvasivespecieshadhigherirradianceplasticity,captureability and efficiency than noninvasive species using two invasive aliens ‐ Ageratina adenophora and Chromolaena odorata, and one noninvasive alien ‐ Gynura sp. The three aliens were grown at 4.5%,12.5%,36%,50%and100%irradiancesfor64daysbeforeharvesting.Theplasticresponse of specific leaf area (SLA) contributed to improved light interception at low irradiance, carbon gain and water balance at high irradiance. It was a good predictor for intraspecific irradiance responses of leaf area ratio (LAR), leaf area:root mass ratio, maximum photosynthetic rate (Pmax) and net assimilation rate (NAR). Biomass allocation-related traits were species specific and their plasticity to irradiance was low. The high root mass fraction, leaf mass fraction and LAR distinguished the two invaders from Gynura. However, other resource capture-related

Photosynthetic gas exchange characteristics in three different almond species during drought stress and subsequent recovery

Three different drought stress levels (water potential of the nutrient solution, Ψ s = −0.6, −1.2 and −1.8 MPa, respectively), and a control treatment ( Ψ s = −0.1 MPa), were applied during 2 weeks to three almond species, followed by 3 weeks of recovery. The selected test species were Prunus dulcis (Miller) D.Webb (bitter almond) and two wild almond species, P. lycioides (Spach) C.K. Schneider and P. scoparia (Spach) C.K. Schneider. All three are species native to Iran, and can be used as rootstock, but only P. dulcis is actually used for commercial almond production. In the absence of drought stress, maximum net assimilation rate ( A max) is highest for P. scoparia and lowest for P. dulcis. For all species A max was above 16 μmol CO 2 m −2 s −1. A similar relationship between A max and dark respiration rate ( R d), was observed for all species. This relationship suggests that optimisation of the carbon budget is independent of species. The three investigated species seem to have a different reaction to a similar stress, indicating different drought stress coping strategies. P. scoparia lost all its leaves during the experiment, while P. lycioides only kept some leaves, however, the remaining leaves were almost totally wilted and did not allow for any photosynthesis measurement. P. scoparia did not recover during the experiment, as no new leaves were developed once Ψ s was restored to pre-drought stress levels. However, this species has green stems, indicating that stem photosynthesis might play an important role in the plants’ overall carbon balance. This species is an opportunistic one ( sensu [Higgins, S.S., Larsen, F.E., Bendel, R.B., Radamaker, G.K., Bassman, J.H., Bidlake, W.R., Alwir, A., 1992. Comparative gas-exchange characteristics of potted, glasshouse-grown almond, apple, fig, grape, olive, peach and Asian pear. Sci. Hortic. 52 (4), 313–329]), where assimilation is seriously limited by non-stomatal processes as evidenced by measurements of intercellular CO 2 concentration, eventually resulting in total leaf loss. All P. lycioides leaves almost completely wilted during the experiment, but this species recovered rather quickly. Leaves, newly formed at the end of the experiment, obtained maximal assimilation rates under control Ψ s levels, equivalent to those measured in the control treatment. Finally, P. dulcis did keep at least part of its leaves during drought stress. However, assimilation rates after 2 weeks of drought treatment and 3 weeks of recovery were only about half of those measured in the control treatment. Of the three investigated species, non-stomatal limitation of assimilation seems to be the least important in P. dulcis. Intrinsic water use efficiency, defined as the ratio of assimilation rate over stomatal conductance, increased for P. dulcis with increasing drought stress, while a different pattern was observed for P. lycioides and P. scoparia, indicating non-stomatal processes prevail over stomatal limitations of the assimilation process. It was concluded that P. dulcis is the species most tolerant to drought. P. scoparia tries to avoid drought, whereas P. lycioides has an intermediate behaviour. Besides P. dulcis, also P. lycioides seems to have some potential for use as rootstock for commercial almond production.

Size dependency of photosynthetic water- and nitrogen-use efficiency and hydraulic limitation in Acer mono.

We examined open-grown Acer mono Maxim. trees of different sizes to test the hypotheses that (1) hydraulic limitation increases with tree size, thereby reducing photosynthesis, and (2) photosynthetic water- and nitrogen-use efficiencies change with tree size. Maximum net assimilation rate per unit dry mass was significantly lower in large trees than in small trees, whereas leaf nitrogen concentration increased with tree size. As a consequence, photosynthetic nitrogen-use efficiency decreased with increase in tree size. Photosynthetic water-use efficiency, however, increased with tree size, partly as a result of reduced stomatal conductance. Neither root-to-leaf hydraulic conductance nor minimum leaf water potential changed with tree size.

Bottlenecks to water transport in Quercus robur L.: the abscission zone and its physiological consequences

Abstract Cladoptosis, the abscission of twigs, is the main mechanism of changes in crown structure in senescing pedunculate oak (Quercus robur L.). We tested the hypotheses that abscission zones in nodes of old pedunculate oak trees reduce leaf-specific hydraulic conductance of shoots and thereby limit the stomatal conductance and assimilation. Hydraulic conductance and leaf-specific hydraulic conductance, measured with a high pressure flowmeter in 0.5–1.5 m long shoots, were significantly lower in shoots of low vigour compared to vigorous growing shoots in a 165-years-old stand in the southeast of Germany. Two types of bottlenecks to water transport could be identified in shoots of old oak trees, namely nodes and abscission zones. In young twigs, vessel diameter and vessel density in nodes with abscission zones were significantly reduced compared with internodes. In nodes without abscission zones, vessel density was significantly reduced. The reduction of hydraulic conductance was especially severe in the smallest and youngest shoots with diameters less than 2 mm. Internodes of 1–5 mm sapwood diameter had an average hydraulic conductance of 7.13×10 −6 ±0.2×10 −6 kg s −1 m −1 MPa−1, compared to 4.54×10 −6 ±0.3×10 −6 kg s −1 m −1 MPa−1 in those with nodes. Maximum stomatal conductance and maximum net assimilation rate increased significantly with hydraulic conductance and leaf-specific hydraulic conductance. Maximum rate of net photosynthesis Amax of the most vigorous shoots (VC0) ( 7.34±0.55 μmol m −2 s −1 ) was significantly higher (P 5.97±0.28 μmol m −2 s −1 ). Our data support the hypothesis that the changes in shoot and consequently crown architecture that are observed in ageing and declining trees can limit photosynthesis by reducing shoot hydraulic conductance. Abscission zones increase the hydraulic disadvantage of less vigorous compared to vigorously growing twigs. Cladoptosis might serve as a mechanism of selection between twigs of different efficiency.

Reduced photosynthesis in old oak (Quercus robur): the impact of crown and hydraulic architecture.

We tested the hypothesis that changes in crown architecture of old pedunculate oak trees (Quercus robur L. ssp. robur Kl. et Kr. et Rol.) reduce leaf specific hydraulic conductance of shoots, thereby limiting stomatal conductance and assimilation of affected shoots. At the end of summer 1999, hydraulic conductance and leaf specific hydraulic conductance, measured with a high-pressure flow meter in 0.5- to 1.5-m long shoots, were 27 and 39% lower, respectively, in shoots of low vigor compared with vigorously growing shoots in a 165-year- old stand in southeastern Germany. Two types of bottlenecks to water transport can be identified in shoots of old oak trees, namely nodes and abscission zones. The reduction in hydraulic conductance was especially severe in shoots with diameters of less than 2 mm. Maximum stomatal conductance and maximum net assimilation rate increased significantly with hydraulic conductance and leaf specific hydraulic conductance. Our data support the hypothesis that changes in shoot and consequently crown architecture observed in aging trees can limit photosynthesis by reducing shoot hydraulic conductance. Thus, in addition to increasing pathway length and lower conductivity of xylem in old trees, structural changes in shoot and crown architecture need to be considered when analyzing water relations and photosynthesis in mature and declining trees.

A comparison of growth responses between two species of Potamogeton with contrasting canopy architecture

This study examines the response of two species of Potamogeton (Family: Potamogetonaceae), with differing canopy architectures, to an artificial light gradient. Potamogeton ochreatus Raoul and P. tricarinatus F. Meull. and A. Bennett were grown in water with an attenuation coefficient of 8.8 m −1 at various depths (10–81 cm) to give initial instantaneous irradiances between 0.4 and 460 μmol m −2 s −1. The average daily water column irradiances ( I ̄ ave ) between the planting depth and the water surface, over 15 daylight hours, ranged from 3.8 to 18.4 mol m −2. After about 80 days all P. tricarinatus plantings, except those at 81 cm, formed dense surface canopies which could access atmospheric CO 2 and had a maximum relative growth rate (70±4 mg g −1 per day ) and net assimilation rates (0.1–0.9 mg cm −2 day −1) significantly above those of P. ochreatus ( 57±3 mg g −1 day −1 and 0.1–0.5 mg cm −2 day −1 , respectively). P. ochreatus, which had a more diffuse and fully submersed habit, had a lower specific absorption coefficient (0.1 m −2 g −1) and average daily light compensation point (37 μmol m −2 s −1) than P. tricarinatus ( 0.9–1.2 m −2 g −1 and 57 μmol m −2 s −1 , respectively), but had a relative growth rate of approximately 25 mg g −1 per day even at an initial instantaneous irradiance of 0.4 μmol m −2 s −1. In addition, P. ochreatus allocated about 80% of its biomass to leaves and stems irrespective of the light climate, whereas only small P. tricarinatus plants preferentially allocated biomass above ground. As energy levels increased, P. tricarinatus allocated a greater proportion of biomass to tissues capturing the limiting resource, light. As the light climate became more favourable, P. tricarinatus allocated more biomass to the rhizome. However, when compared to a wider range of submerged macrophytes, the two species optimised their respective growth rates by reacting to varying I ̄ ave in a similar way. Both responded to lower than optimal I ̄ ave by increasing photosynthetic area and to above optimal values of I ̄ ave by decreasing photosynthetic area.

Structural and physiological adaptation to light environments in neotropical Heliconia (Heliconiaceae)

Influence of habitat on physiological and structural characteristics was investigated for broad-leaved tropical monocotyledons in the genus Heliconia (Heliconiaceae). Seven species were selected from three different light regimes, enabling an analysis of the extent to which this genus has adapted its photosynthetic strategies and morphological characteristics to different daily photon flux densities (PFD). Predictably, light response curves showed a clear gradient with respect to light saturation and rates of maximum net assimilation ( A max ). Heliconia latispatha , an open site species, showed saturation at higher PFD (1400 μmol m −2 s −1 ) and higher A max (14–16 μmol m −2 s −1 ) than H. mathiasiae , a forest edge species (PFD 1000 μmol m −2 s −1 ; A max 7.5–8.5 μmol m −2 s −1 ) and H. irrasa of deep-shade forest understorey (PFD 250 μmol m −2 s −1 ; A max 3.5 mol m −2 s −1 ). Leaf blade areas were largest in open sites, and leaf specific mass was also significantly higher, but leaf support efficiency was highest in understorey species. Species in open sites had thicker leaves with more chlorenchyma, whereas deep-shade species had very thin leaves and low stomatal densities. These rapidly growing herbaceous perennials appear to allocate much of their above-ground biomass to leaf tissues and have a relatively low investment in support tissues. This contrasts with understorey palms, in which leaf form and structural investment has been interpreted as a trade-off between economy and protection against tissue loss from falling branches. Presence of below-ground rhizomes in Heliconia may provide the key to this difference.

The ecology of the climbing fern Dicranopteris linearis on windward Mauna Loa, Hawaii

Summary Dicranopteris linearis (Gleicheniaceae), a native fern common throughout the Old World tropics and Polynesia, forms dense thickets > 3m deep over large areas of open‐canopy, oligotrophic, wet Hawaiian rainforests. Our objectives were to identify leaf‐ and whole plant‐level traits that are key to its success and to determine its community‐and ecosystem‐level consequences in primary successional sites. Along an elevational gradient from 90 to 1660m, mean maximum net assimilation rates of Dicranopteris ranged from 2.9 to 5.0 μmol m−2 s−1, compared with 3.6–9.5 μmol m−1 s−1 in the codominant tree Metrosideros polymorpha. Gas‐exchange characteristics did not explain Dicranopteris' success, nor its trends in production. However, indeterminate, clonal growth form, shallow rhizomes, marcescent leaves with low decomposability, and a mat‐forming capacity enabled Dicranopteris to colonize sites and to maintain dominance via high effective leaf area, despte its low biomass. Phosphorus use efficiency, which reached 24 kg g−1, was exceptionally high, allowing colonization of phosphorus‐poor sites. Dicranopteris contributed up to 74% of above‐ground net primary productivity in a site where it contained only 14% of live biomass. It accounted for up to 57% and 47% of total nitrogen and phosphorus uptake by plants, respectively, where it contained only 24% and 30% of plant nitrogen and phosphorus. Dicranopteris leaves are short‐lived and slow to decompose; thus, fixed carbon is transferred quickly to soil detrital pools where it contributes to aggrading soil organic matter pools and may exacerbate oligotrophic conditions, thereby strongly influencing soil genesis and ecosystem development. The fern therefore influences forest‐floor light regimes and directs later community development. An exclusion experiment demonstrated that Dicranopteris competed with Metrosideros, but lack of revegetation in 40% of the exclusion area after 39 months showed that Dicranopteris also colonized microenvironments unavailable to its endemic codominants. Dicranopteris may play an important role in resisting invasions of exotic species into Hawaiian rainforests.

Gas-exchange responses of rose plants to CO2enrichment and light

SummaryThis paper describes the response of gas exchange rates and water use efficiency of rose plants, by means of the characterization in situ and the analysis of the response of photosynthesis, transpiration and water use efficiency of whole plants to CO2 enrichment under the irradiance conditions prevailing in greenhouses of southern France. Net CO2 assimilation (An) and transpiration (E) of whole rose plants (Rosa hybrida, cv. Sonia) were measured during winter and spring periods. The response of An to light and CO2 were fitted to a double hyperbola function (r2 = 0.84). Maximum net assimilation rate (Anmax), light and CO2 utilization efficiencies (α1, αc) as well as light and CO2 compensation points (Γ1 , Γc) were calculated for the whole plant and compared with leaf and canopy data in the literature. The whole-plant characteristics generally had values intermediate between those related to leaf and canopy. Light saturation at sub-ambient air CO2 concentration (Ca) was reached for relatively low PFF...

Photosynthesis and dark respiration in three alpine snow tussocks (Chionochloaspp.) under controlled environments

Abstract Rates of carbon dioxide exchange for individual tillers of three alpine species of snow tussock, Chionochloa rigida, C. macra, and C. oreophila, were measured under controlled conditions to study influences of several environmental factors. Although maximum net assimilation rates are uniformly very low (range 2.7–4.6 mg CO2 g−1 hr−1), the differences between species in optimum temperatures, responses to low and high temperatures, and adaptation to increasing soil moisture stress all reflect what is known of these species in their natural habitats. Tolerance of actively growing plants to sub-freezing temperatures and their maintenance of a positive carbon balance under such conditions, ranks these snow tussocks with several Northern Hemisphere alpine species in their ecophysiological adaptations to cold environments. The ability of at least one snow tussock species to achieve maximum net assimilation rates during a brief period above freezing is a phenomenon apparently so for unreported. It is suggested that the low-temperature tolerance of these species may be facilitated by a permanent internal moisture stress, as has been reported in a few other cold region species. The physiological strategy involving slow assimilation and growth rates, revealed for the ecologically successful alpine snow tussocks, supports the view that the evergreen habit coupled with low energy demands are an effective adaptation to cold windy environments.

CO2 assimilation by Dryas integrifolia on Devon Island, Northwest Territories

In situ measurements of CO2 assimilation by Dryas integrifolia at different stages of development and under different environmental conditions were made on Devon Island, Northwest Territories. Dryas can fix CO2 in excess of respiration over a 24-h period under conditions of clear nights and cloudy days. The maximum net assimilation rate measured was 4.2 mg g−1 dry weight h−1. The maximum amount of CO2 fixed in 24 h was 61.54 mg g−1 dry weight. Maximum net assimilation occurred at 8 to 10 °C leaf temperatures. Positive net assimilation occurred at 1 °C leaf temperature. Light compensation was shown to be less than 0.04 langley min−1. Leaf temperatures were always greater than ambient. The maximum leaf temperature measured was 39 °C. Net assimilation rates appear to decrease as the season progresses.

Effects of Controlled Temperature and Light Intensity on Growth and Carbohydrate Levels of ‘Thompson Seedless’ Grapevines1

Abstract Nonfruiting ‘Thompson Seedless’ grapevine rootings were grown for 30-day periods in phytotron rooms at variable day temp (6:00 AM to 6:00 PM) from 20 to 40°C, in combination with high light (HL,>2,000 ft-c) and low light (LL, <400 ft-c). Night temp in all treatments was 15°C. Maximum total vine growth and net assimilation rate occurred at 30°C under HL and at 20°C under LL. Gain in dry matter per vine was 3- to 18-fold greater under HL than under LL. The level of total carbohydrates (sugars plus starch) in roots of plants grown under HL was also maximal at 30°C. The concn of total carbohydrates in trunk tissues however, varied little with temp from 20 to 35°C, but was significantly (P<0.01) reduced at 40°C. Low light intensity markedly reduced (P<0.01) the levels of sugars and starch in root and trunk tissues compared to the levels in vines grown at HL, regardless of temp.

The relationship between moisture content and net assimilation rate of lichen thalli and its ecological significance

The relationship between net assimilation rates and percentage thallus saturation has been investigated in 12 widely occurring lichen species in southern Ontario. Optimum net assimilation rates occur between 35 and 70% of thallus saturation levels for different species. A close relationship has been shown to exist between their ecology and the percentage saturation at which maximum assimilation occurs. Optimum net assimilation rate values range from 0.03 mg CO2/g dry weight in Umbilicaria mammulata up to 0.11 mg CO2/g dry weight in Peltigera praetextata. Five of the species also exhibit a plastic physiology in that maximum net assimilation occurs at different levels of thallus saturation in replicates collected from different microhabitats. The implications of this variability are briefly discussed.

Estimation of Climatic Productivity by Phytometers (1)

A Clement's phytometer, combined with growth analysis technique, was applied to the estimation of climatic productivity, which was tentatively defined as the maximum net assimilation rate per unit photo-synthetic system (leaf area, land area, etc.) of a plant in a given climatic condition.In this experiment, isolated young rice plants, grown up to a definite leaf stage in full nutrition, were used as test plants. The main measure of climatic productivity adopted was the dry matter production rate per dry weight of leaf blade, EB, which was found to be a sensitive and reproducible indicator over a wide range of climatic conditions.As shown in Fig. 3, the plot of EB in g·g-1·day-1 versus daily solar radiation in ly·day-1, I, gave saturation curves, showing linear ascending part approximated byEB=0.00177(I-20) (A)A marked dependence of EB on temperature was observed under sufficient radiation, showing the high Q10 value of 6 at lower temperasures (see Fig. 4). This fact indicates the risk of the calculation of dry matter production from the short term measusrements of photosynthesis and respiration.There seems to be a limit of photosynthetic activity imposed by the capacity of roots. Young rice plants ceased to increase EB under favourable climates when the dry matter production rate per root weight reached to 1.3-1.4g·g-1·day-1 (see Figs. 3, 4, and 5). Rise in air temperature or decrease in solar radiation was accompanied by relative reduction of root system (see Tables 1, 2, 3, and Fig. 2).In plants with full developed roots, the relationship between daily maximum air temperature, Tmax, and EB under sufficient radiation, EB·sat, was approximated byEB·sat=-0.000891 T2max+0, 0811Tmax-1.137 (B)Combining this with equation (A), we obtainIsat=-0.503(Tmax-45.5)2+421 (C)where Isat means the daily solar radiation necessary for saturation of EB, If I is larger than Isat, the equation (B) is adopted, while in the opposite case, equation (A) can be used. Thus the seaasonal change of climatic productivity can be expressed as functions of solar radiation and air temperature.The lower limit of air temperature for net assimilation of young rice plants was inferred to be 17°C in daily maximum or 7°C in daily minimum (see Figs. 4 and 5).

Effects of shading and time of year on net assimilation rates of young glasshouse tomato plants

SUMMARYMinimum and maximum net assimilation rates were determined from calculated curves of closest fit to data for 2 years. The minimum (14 g./m.2/week) occurred in mid‐December, immediately before the shortest day, and the maximum (78 g./m.2/week) between 27 June and 8 July, just after the longest day. The northern limit at which the tomato cannot maintain vegetative growth under glass in mid‐winter is probably latitude 59–60° and is determined by a limiting daylength of approximately 6 hr. The proportional reduction in net assimilation rate by shading was independent of the time of the year. The photosynthetic system of glasshouse tomato plants is not light‐saturated in midsummer in southern England.