Lower Light Saturation Points Research Articles

Physiological efficiency of grapevine canopies having varying geometries: Seasonal and diurnal whole canopy gas exchange assessment under well-watered and water deficit conditions

Grapevine training systems should be thoroughly examined to contribute to vineyards more resilient to climate change. This study was conducted on 18 three-year-old fruiting Sangiovese vines grown in pots outdoors with canopies trained to a vertical shoot positioned (VSP) spur-pruned cordon, a single high wire (SHW), and a Pergola (P) training system. Whole-canopy net CO2 exchange rate (NCER) and transpiration (T) were monitored by an enclosure system on a 24/7 basis from DOY 179–249. All vines were kept well-watered (WW) from DOY 179 to DOY 199, and the water supply was first reduced to 50% (DOY200–204) and then to 35% (DOY 205–214) before final rewatering (DOY 215–217). Vegetative growth, yield components, and grape composition were also assessed. Seasonal gas exchange on a per vine basis reflected vine size given as total leaf area (LA) varying from 2.5 to 3.1 m2; however, when diurnal trends of NCER/LA and T/LA within the WW and rewatering periods were considered, the good correlations found with the modeled total canopy light interception (TCLI) in SHW and P, were not confirmed in VSP. In all canopy geometries, the best predictor of NCER/LA was the total direct light intensity, whereas T/LA was linearly and closely correlated with air VPD. In terms of NCER/LA during the WW period, SHW had 24% higher rates than VSP and P. Under progressive water stress, SHW again exhibited higher drought resilience, while NCER and T were severely reduced in VSP and P. Severe stress also caused lower light saturation points and quantum yield in VSP and P. Under similar yield levels, P reached the best fruit maturity. In terms of photosynthetic efficiency and adaptive response to increasing water stress, the SHW performed best. Results extrapolated at the field level indicate the P system as the most rewarding.

Functional trait divergence associated with heteromorphic leaves in a climbing fig.

Plants that display heteroblasty possess conspicuous variations in leaf morphology between their juvenile and adult phases, with certain species retaining juvenile-like leaves even in adulthood. Nevertheless, the ecological advantages of maintaining two or more distinct leaf types in heteroblastic plants at the adult stage remain unclear. The aim of this study is to examine the adaptive significance of heteroblastic leaves sampled from branches with divergent functions (sterile and fertile branches) of mature Ficus pumila individuals by comparing their morphological, anatomical, and physiological characteristics. Leaves on sterile branches (LSs) exhibited a significantly larger specific leaf area, thinner palisade and spongy tissues, lower chlorophyll contents, and lower light saturation points than leaves on fertile branches (LFs). These results demonstrate that LSs are better adapted to low light environments, while LFs are well equipped to take advantages of high light conditions. However, both LFs and LSs have a low light compensation point with no significant difference between them, indicating that they start to accumulate photosynthetic products under similar light conditions. Interestingly, significant higher net photosynthetic rate was detected in LFs, showing they have higher photosynthetic capacity. Furthermore, LFs produced significant more nutrients compared to LSs, which may associate to their ability of accumulating more photosynthetic products under full light conditions and higher photosynthetic capacity. Overall, we observed a pattern of divergence in morphological features of leaves on two functional branches. Anatomical and physiological features indicate that LFs have an advantage in varied light conditions, providing amounts of photosynthetic products to support the sexual reproduction, while LSs adapt to low light environments. Our findings provide evidence that heteroblasty facilitates F. pumila to utilize varying light environments, likely associated with its growth form as a climbing plant. This strategy allows the plant to allocate resources more effectively and optimize its overall fitness.

Determining the ecophysiological limits of a narrow niche tropical conifer tree (Podocarpus trinitensis).

Many tropical species live close to their thermal limits within a narrow niche. Here, we investigate the ecophysiological limits of the tropical tree Podocarpus trinitensis, which is endemic to Trinidad and Tobago where most populations exist as isolated stands on hilltops. Five wild stands from a range of elevations were compared in the field with measurements of leaf temperature, canopy cover, stomatal conductance (gs), chlorophyll content and several chlorophyll fluorescence parameters. A parallel greenhouse experiment was used to acclimate seedlings to 'CONTROL' and 'HEAT' treatments (with mid-day air temperatures of 34.5 and 37°C respectively), after which the above parameters were measured along with photosynthetic light and temperature response curves, leaf morphology and in vitro Fv/Fm thermostability. There was a positive association between improved physiological performance and elevation. In the high elevation sites, leaf temperatures were significantly lower while most of the physiological parameters were higher (gs, chlorophyll content, ɸ PSII, ETRmax and Isat90). In the greenhouse, HEAT and CONTROL plants were similar for most parameters, except leaf temperature (which was coupled with air temperature) and leaf mass per unit area (which was higher in HEAT plants). Temperature response curves showed an optimum temperature for photosynthesis of 30±0.5°C (TOpt) and in vitro Fv/Fm indicated a critical temperature of 47.4±0.38°C for HEAT and 48.2±0.24°C for CONTROL (T50), with no indication of heat acclimation. Podocarpus trinitensis was found to be shade tolerant. In the field, seedlings established under a close canopy (>95% canopy cover) and had a low light saturation point (LCP). In the greenhouse, where more light was available, seedlings retained a low light compensation point, light saturation point (LSP) and maximum photosynthetic rate (Amax). The results suggest that P. trinitensis is moderately heat tolerant with the higher elevation sites being more habitable, but stands are also able to survive near sea level under a closed canopy. The narrow niche, along with the 30±0.5°C optimum temperature for photosynthesis and the lack of thermal plasticity in critical temperature, suggests that P. trinitensis has little room to acclimate to temperatures higher than those currently experienced.

Effect on the Growth and Nutritional Components in Two Red Lettuces (Lactuca sativa L.) Cultivated Under UV Light in a Mini Plant Factory

Lettuce is one of the most suitable plant species to grow indoor or in a Plant Factory with Artificial lighting (PFAL) system, due to its short height and the relatively low light saturation point. Red lettuces are highly cultivated in PFALs, highly appreciated for its unique pigmentation and antioxidant activity, especially, the anthocyanin content that gives it the red color. However, red lettuces cultivated with PFAL in controlled environments face the challenge of having low anthocyanin content due of the lack of ultraviolet (UV) light that is necessary for its production. In this study, two red-lettuce cultivars ‘Annapolis’ and ‘Salanova’ were grown in a plant factory with LEDs (light-emitting diode) and UV light treatments to compare their growth trajectory and nutritional characteristics. Other characteristics such as Fresh Weight (FW) and total leaf area were measured to evaluate the influence of UV light in the lettuce growth. The nutritional quality of lettuces was assessed by measuring anthocyanins and polyphenols contents using a microplate reader and their ascorbic acid content measured via a reflectometer. Fresh Weight (FW), leaf area and leaf thickness under the conditions of this study, showed no effect (p>0.05) on the plants growth with the incorporation of UV radiation. However, the incorporation of UV light increased the functional components in ‘Annapolis’ and ‘Salanova’ lettuces. The polyphenol and anthocyanins contents in both lettuce cultivars showed significant differences (p<0.05). For ascorbic acid content, a significant difference was found only for ‘Annapolis’ lettuce. The incorporation of UV light can increase the functional ingredients such as polyphenols and anthocyanins without growth suppression for ‘Salanova’ cultivar. UV light increased polyphenol and anthocyanins contents in both cultivars, making them suitable cultivars for PFAL under the growth conditions used in this experiment. These results taken together can be used to improve the accumulation of functional ingredients in red lettuces without growth suppression incorporating UV light and that could be suitable for production in PFALs.

Reassimilation of Leaf Internal CO2 Contributes to Isoprene Emission in the Neotropical Species Inga edulis Mart.

Isoprene (C5H8) is a hydrocarbon gas emitted by many tree species and has been shown to protect photosynthesis under abiotic stress. Under optimal conditions for photosynthesis, ~70%–90% of carbon used for isoprene biosynthesis is produced from recently assimilated atmospheric CO2. While the contribution of alternative carbon sources that increase with leaf temperature and other stresses have been demonstrated, uncertainties remain regarding the biochemical source(s) of isoprene carbon. In this study, we investigated leaf isoprene emissions (Is) from neotropical species Inga edulis Mart. as a function of light and temperature under ambient (450 µmol m−2 s−1) and CO2-free (0 µmol m−2 s−1) atmosphere. Is under CO2-free atmosphere showed light-dependent emission patterns similar to those observed under ambient CO2, but with lower light saturation point. Leaves treated with the photosynthesis inhibitor DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) failed to produce detectable Is in normal light under a CO2-free atmosphere. While strong temperature-dependent Is were observed under CO2-free atmosphere in the light, dark conditions failed to produce detectable Is even at the highest temperatures studied (40 °C). Treatment of leaves with 13C-labeled sodium bicarbonate under CO2-free atmosphere resulted in Is with over 50% containing at least one 13C atom. Is under CO2-free atmosphere and standard conditions of light and leaf temperature represented 19% ± 7% of emissions under ambient CO2. The results show that the reassimilation of leaf internal CO2 contributes to Is in the neotropical species I. edulis. Through the consumption of excess photosynthetic energy, our results support a role of isoprene biosynthesis, together with photorespiration, as a key tolerance mechanism against high temperature and high light in the tropics.

Leaf gas exchanges responses of atemoya scion grafted onto Annona rootstocks

We examined the hypothesis that leaf gas exchange of scion is affected by different rootstocks in wood plants. We investigated daily courses of gas exchange and photosynthetic potential using the CO2 assimilation rates as a function of photosynthetic photon flux density, and then assessed CO2 response curves in atemoya scion (Annona × atemoya Mabb.) grafted onto araticum-de-terra-fria [A. emarginata (Schltdl.) H. Rainer var. terra-fria]: ATF, araticum-mirim [A. emarginata (Schltdl.) H. Rainer var. mirim]: ATM, biriba [A. mucosa (Bail.) H. Rainer]: ATB, atemoya (autograft): ATA, and in ungrafted atemoya plants: CTR. Throughout the entire evaluation period, the net assimilation rate (Anet) and stomatal conductance (gs) of CTR plants remained practically constant, being lower than those of grafted plants between 08:00 a.m. and 12:00 a.m., regardless of the rootstock used. Moreover, ATM plants proved to be more efficient in keeping the stomata open, even during the hottest hours of the day, improving Anet and carboxylation use efficiency. However, this occurred at the lowest maximum carboxylation rate of ribulose-1,5-bisphosphate (Vcmax). Overall, ATF plants presented a low light saturation point and photosynthetic electron transport rates, though increased maximum quantum yield of photosynthesis was observed. Thus, we accept our hypothesis and conclude that grafting might affect the photosynthetic metabolism of the atemoya hybrid, regardless of the combination used, which promotes enhanced Anet and low Vcmax and light saturation points.

Interactive effects of temperature and light intensity on photosynthesis and antioxidant enzyme activity in Zizania latifolia Turcz. plants

We compared the interactive effects of temperature and light intensity on growth, photosynthetic performance, and antioxidant enzyme activity in Zizania latifolia Turcz. plants in this study. Plants were grown under field (average air temperature 9.6–25°C and average light intensity 177–375 W m−2) or greenhouse (20–32°C and 106–225 W m−2) conditions from the spring to the early summer. The results indicated that greenhouse-grown plants (GGP) had significantly higher plant height, leaf length, and leaf width, but lower leaf thickness and total shoot mass per cluster compared with field-grown plants (FGP). Tiller emergence was almost completely suppressed in GGP. Significantly higher chlorophyll (Chl) content and lower Chl a/b ratio were observed in GGP than in FGP. From 4 to 8 weeks after treatment (WAT), net photosynthetic rate (PN) was significantly lower in FGP than in GGP. However, from 9 to 12 WAT, PN was lower in GGP, accompanied by a decrease in stomatal conductance (gs) and electron transport rate (ETR) compared with FGP. Suppressed PN in GGP under high temperature combined with low light was also indicated by photosynthetic photon flux density (PPFD) response curve and its diurnal fluctuation 10 WAT. Meanwhile, ETR in GGP was also lower than in FGP according to the ETR — photosynthetically active radiation (PAR) curve. The results also revealed that GGP had a lower light saturation point (LSP) and a higher light compensation point (LCP). From 4 to 8 WAT, effective quantum yield of PSII photochemistry (ΦPSII), photochemical quenching (qP), and ETR were slightly lower in FGP than in GGP. The activities of ascorbate peroxidase (APX), guaiacol peroxidase (POD), glutathione reductase (GR), superoxide dismutase (SOD), and malondialdehyde (MDA) content were significantly higher from 4 to 8 WAT, but lower from 10 to 12 WAT in FGP. However, catalase (CAT) activity was significantly lower in FGP from 4 to 8 WAT. Our results indicated that the growth and photosynthetic performance of Z. latifolia plants were substantially influenced by temperature, as well as light intensity. This is helpful to understand the physiological basis for a protected cultivation of this crop.

Comparison of the photosynthetic characteristics of four Lycoris species with leaf appearing in autumn under field conditions.

The diurnal trends of gas exchange and chlorophyll fluorescence parameters in four Lycoris species (L. houdyshelii, L. aurea, L. radiata var. pumila and L. albiflora) were determined and compared with a portable photosynthesis analysis system. Our study revealed that L. houdyshelii had the lowest light compensation point (LCP), while the other three species had higher LCP (12.37–14.99 μmol m−2 s−1); L. aurea had the highest light saturation point (LSP) (1,189 μmol m−2 s−1), and L. houdyshelii and L. albiflora had lower LSP with the values being 322 and 345 μmol m−2 s−1, respectively, and L. radiata var. pumila showed the intermediate LSP. Both the species L. houdyshelii and L. albiflora exhibited a typical and obvious decline in net photosynthetic rate (P N) during midday, which was not observed in L. aurea. This indicated a possible photoinhibition in L. houdyshelii and L. albiflora as the ratio of variable to maximum fluorescence (Fv/Fm) values were higher in these two species. The minimal fluorescence (F0) values were lower in L. aurea and L. radiata var. pumila. The diurnal changes of transpiration rate (E) in all four species presented only one peak, appearing between 11:00 h or 13:00 h. By using simple correlation analyses, it was observed that the environmental factors affecting P N were different among four species and the main factors were photosynthetic photon flux density (PPFD) and relative humidity especially for L. aurea and L. radiata. The results of studying indicated that the four species could be divided into two groups. The species L. radiata var. pumila and L. aurea were more adapted to a relatively high irradiance, and L. houdyshelii and L. albiflora could be grown in moderate-shade environment in order to scale up their growth and productivity.

Neither elevated nor reduced CO 2 affects the photophysiological performance of the marine Antarctic diatom Chaetoceros brevis

Enhanced or reduced pCO 2 (partial pressure of CO 2) may affect the photosynthetic performance of marine microalgae since changes in pCO 2 can influence the activity of carbon concentrating mechanisms, modulate cellular RuBisCO levels or alter carbon uptake efficiency. In the present study we compared the photophysiology of the Antarctic diatom Chaetoceros brevis at two pCO 2 extremes: 750 ppmv (2× ambient) and 190 ppmv (0.5× ambient) CO 2. Cultures were acclimated to four irradiance regimes: two regimes simulating deep or shallow vertical mixing, and two regimes mimicking limiting and saturating stable water column conditions. Then, growth rate, pigmentation, RuBisCO large subunit expression, RuBisCO activity, photosynthesis vs irradiance curves, effective quantum yield of PSII (F v/F m), and POC were measured. The four irradiance regimes induced a suite of photophysiological responses, ranging from low light acclimation to efficient photoprotection. Growth was reduced under the low constant and the deep mixing regime, compared to the shallow mixing and the stable saturating regime. Low stable irradiance resulted in higher light harvesting pigment concentrations, lower RuBisCO activity and a lower light saturation point (E k) compared to the other irradiance regimes. Highest RuBisCO activity as well as P max levels was measured in the shallow mixing regime, which received the highest total daily light dose. Photoprotection by xanthophyll cycling was observed under all irradiance regimes except the low stable irradiance regime, and xanthophyll cycle pool sizes were higher under the dynamic irradiance regimes. For the fluctuating irradiance regimes, F v/F m was hardly affected by previous excess irradiance exposure, suggesting minimal PSII damage. No significant differences between the two pCO 2 levels were found, with respect to growth, pigment content and composition, photosynthesis, photoprotection and RuBisCO activity, for all four irradiance regimes. Thus, within the range tested, pCO 2 does not significantly affect the photophysiological performance of C. brevis.

Light acclimation of Chlamydomonas acidophila accumulating in the hypolimnion of an acidic lake (pH 2.6)

Summary1. The unicellular green alga Chlamydomonas acidophila accumulates in a thin phytoplankton layer in the hypolimnion (deep chlorophyll maximum, DCM) of an extremely acidic lake (Lake 111, pH 2.6, Lusatia, Germany), in which the underwater light spectrum is distorted and red‐shifted.2. Chlamydomonas acidophila exhibited a significantly higher absorption efficiency and a higher cellular chlorophyll b content when incubated in the red shifted underwater light of Lake 111 than in a typical, blue‐green dominated, light spectrum.3. Chlamydomonas acidophila has excellent low light acclimation properties (increased chlorophyll b content, increased oxygen yield and a low light saturation point for photosynthesis) that support survival of the species in the low light climate of the DCM.4. In situ acclimation to the DCM under low light and temperature decreased maximum photosynthetic rate in autotrophic C. acidophila cultures, whereas the presence of glucose under these conditions enhanced photosynthetic efficiency and capacity.5. The adaptive abilities of C. acidophila to light and temperature shown in this study, in combination with the absence of potent competitors because of low lake pH, most probably enable the unusual dominance of the green alga in the DCM of Lake 111.

Physiological and Growth Tolerance in Wheat to Russian Wheat Aphid (Homoptera: Aphididae) Injury

Plant tolerance to injury from insect herbivores has several advantages as a pest management approach. However, its use is limited because mechanisms conferring plant tolerance are not well understood. We hypothesize that plant physiological responses, specifically photosynthesis, substantially contribute to plant tolerance to arthropod injury. This hypothesis was tested on 3 wheat ( Triticum eastivum L.) lines that differed in their mode of resistance to the Russian wheat aphid, Diuraphis noxia (Mordvilko). The lines were ‘Arapahoe’ (a susceptible line), PI 137739 (an antibiotic line), and PI 262660 (a tolerant line). These lines were grown in a greenhouse, aphids were maintained on plants for 1 wk, and physiological responses of these lines were determined. Light curve and fluorescence data indicated that the primary mechanism for photosynthetic rate reduction in aphid-injured leaves is via interference of the photochemical efficiency at the initial stage of photosynthesis. Aphid-injured seedlings had lower light-saturation points, which suggested less efficient use of light energy compared with control seedlings. Immediately after aphid removal, aphid injury reduced chlorophyll fluorescence and photosynthetic rates in all lines, but PI 137739 (with antibiosis) had significantly greater photosynthetic rate reduction. Photosynthetic rates of the tolerant line, PI 262660, began recovering 3 d after aphid removal with complete photosynthetic recovery 7 d after aphid removal. This gradual photosynthetic compensation did not occur in the other 2 lines. PI 262660 also had greater leaf area and more dry matter when compared with the other cultivars. This study demonstrates that photosynthetic adjustments can significantly contribute to plant tolerance resulting from arthropod injury. Moreover, evidence here indicates that an active plant defense through antibiosis comes at the cost of reduced capacity for physiological tolerance and compensation.

Use of pulse amplitude modulated (PAM) fluorometry for in situ measurements of photosynthesis in two Red Sea faviid corals

Measurements of the photosynthetic activity of symbiotic zooxanthellae in corals under natural growth conditions has been limited until recently, and this is one of the first reports on utilising a newly developed underwater pulse amplitude modulated (PAM) fluorometer (the Diving-PAM, Walz Gmbh, Germany) for such studies in situ. Photosynthetic responses to irradiance (photosynthetic photon flux, PPF) of the two faviid corals Favia favus (Forskal) and Platygyra lamellina (Ehrenberg) were measured while snorkelling or SCUBA diving (in August 1997), and we report here the results in terms of effective quantum yields of photosystem II (Y ) and estimated rates of photosynthetic electron transport (ETR, calculated as Y × 0.5 × PPF × FA, where FA is the estimated fraction of light absorbed by the photosymbiont-containing tissue). Both species showed a reduction in Y with increasing actinic irradiances produced by the instrument above 500 μmol photons m−2 s−1, and the corresponding ETR values yielded apparently typical photosynthesis versus irradiance (P-I ) curves, which saturated between 1500 and 2000 μmol photons m−2 s−1. It was found that 30 s irradiation at each PPF level was sufficient to give optimal ETR values and, therefore, each P-I curve could be obtained within a few minutes. In situ point measurements from various areas of colonies under ambient light showed average ETR values within the range expected from the P-I curves. In order to test the Diving-PAM in an eco-physiologically relevant experiment, photosynthetic ETR versus PPF was measured for three sections of a large P. lamellina, each section of which received different natural irradiance levels. The results clearly demonstrated adaptations to the ambient light field in that vertical and downward-facing portions of the colony showed gradually lower maximal ETRs, steeper initial slopes of the P-I curves and, accordingly, lower light saturation points than upward-facing areas receiving higher light levels. Based on these trials, some evaluations are given as to the applicability of the Diving-PAM for photosynthetic measurements when monitoring similar corals.

Regulation and role of photosynthesis in the colonial symbiotic ciliate Ophrydium versatile

Symbiotic photosynthesis of the colonial ciliate Ophrydium versatile from transparent temperate lakes was studied with O2 microelectrodes and measurements of O2 and C exchange in laboratory and field incubations. Oxygen gradients were steep just above the colony surface and shallow toward the center of the colony, reflecting that O2 metabolism is high in the peripheral layer containing the ciliates and low in the jelly. Photosynthesis responses resembled those of unicellular and thin multicellular algae by having relatively low light compensation point (25 µmol photon m−2 s−1), low light saturation point (190 µmol photon m−2 s−1), and a sharp transition from light‐limited to light‐saturated photosynthesis. Inorganic C supply to Ophrydium supported photosynthetic rates close to maximum in air‐saturated alkaline lake water. Photosynthesis patterns were similar in short‐time laboratory experiments and diel outdoor experiments and showed no indication of photoinhibition and rhythmicity in the relationship of photosynthesis to irradiance during the day. The endosymbiotic zoochlorellae dominated the O2 metabolism of Ophrydium ciliates, resulting in much steeper O2 gradients at the colony surface in the light than in the dark and a large diel surplus of gross photosynthesis relative to respiration. Symbiotic photosynthesis produced sufficient carbon to support the measured growth rate, the respiration, and the substantial carbon content of the colony jelly, whereas particle filtration of ciliates presumably supplied the nutrients for net growth of the assemblage. The location of the ciliates at low density in the periphery of the colony and the water movements generated by the ciliates are probably important for the efficient plant‐like use of light and inorganic C by Ophrydium and the ability to form high densities of zoochlorellae.

The effects of ultraviolet-B radiation on photosynthesis of different aged needles in field-grown loblolly pine.

We examined the effect of supplemental UV-B radiation (290-320 nm) on photosynthetic characteristics of different aged needles of 3-year-old, field-grown loblolly pine (Pinus taeda L.). Needles in four age classes were examined: I, most recently fully expanded, year 3; II, first flush, year 3; III, final flush, year 2; and IV, oldest needles still present, year 2. Enhanced UV-B radiation caused a statistically significant decrease (6%) in the ratio of variable to maximum fluorescence (F(v)/F(m)) following dark adaptation only in needles from the youngest age class, suggesting transient damage to photosynthesis. However, no effects of enhanced UV-B radiation on other instantaneous measures of photosynthesis, including maximum photosynthesis, apparent quantum yield and dark respiration, were seen for needles of any age. Foliar nitrogen concentration was unaffected by UV-B treatment. However, the (13)C/(12)C carbon isotope ratios (delta(13)C-a time integrated measure of photosynthetic function) of needles in age classes II and IV were 3% (P < 0.01) and 2% (P < 0.05) more negative, respectively, in treated plants than in control plants. Exposure to enhanced UV-B radiation caused a 20% decrease in total biomass and a 4% (P < 0.05), 25% (P < 0.01), and 9% (P < 0.01) decrease in needle length of needles in age classes I, II, and IV, respectively. The observed decreases in delta(13)C, and F(v)/F(m) of the needles in the youngest needle age class suggest subtle damage to photosynthesis, although overall growth reductions were probably a result of decreased total leaf surface rather than decreased photosynthetic capacity. Needles of age class IV had lower light- and CO(2)-saturated maximum photosynthetic rates (39%), lower dark respiration (34%), lower light saturation points (37%), lower foliar nitrogen concentration (28%), and lower delta(13)C (14%) values than needles of age class I. Apparent quantum yield and F(v)/F(m) did not change with needle age. The observed changes in photosynthesis and foliage chemical composition with needle age are consistent with previous studies of coniferous trees and may represent adaptations of older needles to shaded conditions within the canopy.

Comparative analysis of dry matter production and photosynthesis between mungbean (Vigna radiata (L.) and black gram (V. mungo (L.) Hepper) grown in different light intensities

Both pulse crops, mungbean (Vinga radiata (L.) Wilczek) and black gram (V. mungo (L.) Hepper), are widely grown and consumed as main protein foods in Bangladesh. In this research light-responses of growth, dry matter production, photosynthesis and leaf or mesophyll conductance were compared between mungbean cultivar chinese and black gram cultivar MAK-1 grown in different light intensities. Dry matter production and leaf area of black gram decreased with shade, while those of mungbean became the highest under the moderate shade condition (55% in relative light intensity). The light-photosynthetic response of shade-grown mungbean presented a typical shade-leaf pattern characterized by photosynthetic reduction and low light-saturation point. While in black gram photosynthetic difference was not apparent between control and shade-grown plants, both of which had no light saturation within a light intensity range up to 1,800 mu-mol m(-2)s(-1), Mesophyll conductance in shade-grown black gram was considerably higher than that of control plant at a light intensity below 1,000 mu-mol m(-2)s(-1), which may suggest the enzymatic activity in CO2 fixation system was enhanced under the shade conditions.

Photosynthesis in flashing light in soybean leaves grown in different conditions. I. Photosynthetic induction state and regulation of ribulose‐1,5‐bisphosphate carboxylase activity

ABSTRACTThe photosynthetic induction state under conditions of different lightfleck frequencies or durations, or different shade periods was studied in soybean leaves in order to examine how it might limit utilization of sunflecks in leaf canopies. Induction following an increase in photon flux density (PFD) from strongly limiting to saturating PFDs exhibited two phases; a fast‐inducing one, requiring about 1 min and a slow one, requiring up to 60 min for completion. Transfer of fully induced leaves to low light resulted in a rapid decrease in the fast‐inducing component, a slower decrease in the slow‐inducing component and an even slower decrease in stomatal conductance. Therefore, the decreases in extent of induction appeared to be due to biochemical factors and not to stomatal closure. Under flashing light regimes consisting of 1‐s lightflecks given at different frequencies for long periods, a constant induction state was achieved, the measure of induction state increased with the frequency of the lightflecks. This constant induction state also depended on the growth conditions, with shade leaves having a higher value than those grown at high light at any particular lightfleck frequency. The measure of induction state was mostly lower in flashing light as compared to constant light of the same mean PFD, particularly in leaves with a low light saturation point and in short lightflecks. Initial activities of ribulose‐1,5‐bisphosphate carboxylase (rubisco) were also higher in continuous light and were highly correlated with the measure of induction state. The rapid decrease in extent of induction of soybean leaves during shade periods is an important limitation to the ability of the leaves to respond to light increases similar to those occurring with sunflecks. At least part of the limitation on carbon assimilation during sunflecks due to photosynthetic induction is based on regulation of rubisco activity.

VINE PHOTOSYNTHESIS AND RELATIONSHIPS TO CLIMBING MECHANICS IN A FOREST UNDERSTORY

Photosynthesis in a deciduous forest understory was studied for three exotic vine species (Pueraria lobata, Lonicera japonica, and Hedem helix) and five common native species (Rhus radicans, Clematis virginiana, Smilax rotundifolia, Vitis vulpina, and Parthenocissus quinquefolid) possessing a variety of climbing mechanisms. The adventitious‐root climbers (H. helix and R. radicans) had the lowest maximum photosynthetic rates of all species (5.5 and 6.4 μmol m–2 s–1, respectively). The twining vine P. lobata was the most poorly adapted to the understory with a high light‐compensation point (43 μmol m–2 s–1), low photosynthesis under low light (0.5 μmol m2 s–1 at 50 μmol m–2 s–1), and the highest light requirement for obtaining 90% of maximum photosynthesis (860 μmol m–2 s–1). Lonicera japonica, another twining vine, was better‐adapted to low light conditions, but vines with tendril climbing mechanics were physiologically the best adapted to low light. The adhesive‐tendril climber P. quinquefolia was the most highly adapted to shade, with a low light compensation point (20 μmol m–2 s–1), a high photosynthetic rate under low light (3.5 μmol m–2 s–1), and a low light saturation point (160 μmol m–2 s–1). Results suggest that physiological adaptability of vines to low‐light environments may be related to climbing mechanics.

Vine Photosynthesis and Relationships to Climbing Mechanics in a Forest Understory

Photosynthesis in a deciduous forest understory was studied for three exotic vine species (Pueraria lobata, Lonicera japonica, and Hedem helix) and five common native species (Rhus radicans, Clematis virginiana, Smilax rotundifolia, Vitis vulpina, and Parthenocissus quinquefolid) possessing a variety of climbing mechanisms. The adventitious-root climbers (H. helix and R. radicans) had the lowest maximum photosynthetic rates of all species (5.5 and 6.4 μmol m–2 s–1, respectively). The twining vine P. lobata was the most poorly adapted to the understory with a high light-compensation point (43 μmol m–2 s–1), low photosynthesis under low light (0.5 μmol m2 s–1 at 50 μmol m–2 s–1), and the highest light requirement for obtaining 90% of maximum photosynthesis (860 μmol m–2 s–1). Lonicera japonica, another twining vine, was better-adapted to low light conditions, but vines with tendril climbing mechanics were physiologically the best adapted to low light. The adhesive-tendril climber P. quinquefolia was the most highly adapted to shade, with a low light compensation point (20 μmol m–2 s–1), a high photosynthetic rate under low light (3.5 μmol m–2 s–1), and a low light saturation point (160 μmol m–2 s–1). Results suggest that physiological adaptability of vines to low-light environments may be related to climbing mechanics.

Environmental tolerance of the estuarine diatom Melosiranummuloides (Dillw.) Ag

Growth rate in culture of three isolates of Melosira nummuloides (Dillw.) Ag. from the Clyde Estuary was uniform over the salinity range 5–34%. although there was no growth in freshwater medium (0.5%.). Plants maintained at 0.5%. for up to 8 days suspended growth and it was resumed on transfer back to a higher salinity. The low light saturation point for growth (37 μE·m −2·s −1) would enable growth to occur at the low light intensity found in a turbid estuary. Large salinity fluctuations (5–23%.) had little effect on the net rate of photosynthesis. These features are adaptive to the estuarine environment but cannot alone explain the unusual abundance of this species in the Clyde Estuary. For this, previous reports concerning heterotrophic growth and effects of grazing must also be considered.