Fine structure of the alary muscles of the American cockroach

The alary muscles of the cockroach, Periplaneta americana, are striated with an A-band of 3·0 to 3·5 μm long. Each muscle fibre was 10 to 12 μm in diameter and Z-lines appeared as small discrete units staggered throughout the sarcoplasm. Mitochondria were conspicuously located near the Z-line areas and were absent from the middle portion of the sarcomere. A transverse membrane system was present which formed dyad structures with a relatively sparse sarcoplasmic reticulum. Cockroach alary muscles were innervated by axons containing electron-dense granules of near 100 nm in diameter. These are thought to be typical of ‘neurosecretory’ axons based on their ultrastructural appearance.     

Electrokinetic properties of asparaginase sensitive or resistant mouse leukemic cells treated with L-asparaginase

The electrophoretic mobility of L5178Y cells in 0.0145 M NaCl, 4.5% sorbitol, 0.6 mM NaHCO₃, pH 7.2, at 25°C was — 1.78 μ·s^?1·V^?1·cm^?1 while that of an L-asparaginase resistant subline, L5178Y/ASN, was — 1.11 μm·s^?1·V^?1·cm^?1. Both cell lines were characterized by terminal sialic acid residues on their surfaces. Treatment of L5178Y cells for 90 min with 10 units of L-asparaginase per ml in saline decreased the electrophoretic mobility of the cells to — 1.65 μm·s^?1·V^?1·cm^?1 while treatment in Fischer's medium decreased the mobility to — 1.25 μm·s^?1·V^?1·cm^?1; neither treatment had a significant effect on the L5178Y/ASN electrophoretic mobility. The results suggest that L-asparaginase has an immediate and specific effect on synthesis of cell surface asparaginyl glycoproteins.     

The electrical activity of mechanoreceptive and neurosecretory neurons in the stick insectCarausim morosus

Summary Action potentials have been recorded from the neurosecretory cells which lie on the link nerve inCarausius morosus. The neurosecretory cells are spontaneously active in completely isolated preparations, firing with a regular but low frequency (<1 imp/s) or in small bursts (12 imp/s). The action potentials recorded extracellularly from the neurosecretory fibres are characteristically of long duration (2 to 10 ms), whereas those of motor or sensory fibres are of shorter duration (0.6 to 0.8 ms). The neurosecretory action potentials are also characterised by their slow conduction velocity (0.15 to 0.25 m/s) compared to those from motor and sensory fibres (0.54 to 0.7 m/s). The action potentials are propagated from the region of the cell body towards the terminals and have been recorded passing along all the major nerves in the periphery.Recordings from three of the non-neurosecretory cells which lie on peripheral nerves show that they respond to stretching of the nerves upon which they lie or of nerves which branch in the immediate vicinity. The action potentials are propagated away from the cell body towards the central nervous system. The neurons are termed peripheral nerve stretch receptors.We are grateful to the Science Research Council for financial support.     

Interactions of the ring gland, overies, and juvenile hormone with brain neurosecretory cells in Musca domestica.

The staining intensity (median neurosecretory cell index) of the median neurosecretory cells (MNC) in Musca domestica increased as oögenesis progressed from stages 2 to 10. The amount of neurosecretory material within the MNC was dependent upon the presence of ovaries with developing or mature follicles. Ovariectomized flies had a median neurosecretory index that was 50 per cent less than that of control flies with mature eggs. In addition, we found that ring gland removal decreased the staining frequency of three different neurosecretory cell groups; increased staining frequency in another; increased the amount of neurosecretory material within the MNC fibre tract; increased the cytoplasmic area of types A and A′ MNC. Furthermore, neither the juvenile hormone analogue nor the ring gland had a direct effect on the median neurosecretory cell index but did influence neurosecretory activity indirectly by activating the ovaries. We hypothesize that an ovarian hormone—the oöstatic hormone—regulates either the release from or synthesis of neurosecretory material within the MNC.     

Changes in oxidative metabolism and ultrastructure accompanying differentiation of the mitochondrion in Trypanosoma brucei

A method is described for obtaining optimal growth and morphological transformation of Trypanosoma brucei 792G in a monophasic blood lysate medium starting from populations of bloodstream trypanosomes containing over 90 per cent intermediate and stumpy forms. Transformation was accompanied by: (1) an increase in length of the trypanosomes from 13·8 (± 3·0) μm to 23·0 (± 2·5) μm; (2) an increase in the kinetoplastic index from 1·0 to greater than 2·0; (3) development of the mitochondrion from a single abflagellar canal to a network of subpellicular canals, with outgrowth of the post-kineto-plastic region of the mitochondrion; (4) replacement of some of the tubular mitochondrial cristae by plate-like cristae; (5) the acquisition of succinoxidase, succinate-cytochrome c reductase, and glycerophosphate-cytochrome c reductase activities; (6) a marked increase in proline oxidase and NADH-cytochrome c reductase activities; (7) loss of the surface coat of the flagellate and concomitant reduction in the smooth-membrane systems lying between the nucleus and flagellar pocket; (8) reduction in infectivity of the trypanosomes to the mammalian host. Although growth in primary culture continued up to 100 h, transformation was completed in 48 h. All the respiratory enzyme activities tested were insensitive to cyanide throughout transformation. Division of trypanosomes appeared to be taking place throughout the transformation process. Cyanide sensitivity developed only after subculture of the trypanosomes into a biphasic medium.     

Effect of particle size on the anthelmintic efficacy of mebendazole against Nippostrongylus brasiliensis in the rat.

The effect of reduction in particle size on the anthelmintic efficacy of mebendazole (methyl 5 (6)- benzoyle 1–2 benzimidazole carbamate) was studied in rats undergoing a primary infection with N. brasiliensis. A single oral treatment with fine ground mebendazole (particle size spectrum—54·95 per cent of particles less than 10·62 μ dia.; 86·06 per cent less than 21·27 μ) removed more than 98 per cent of adult worms from the intestine at a dose rate of 12·5 mg/kg body wt. On the other hand the best result achieved with coarse ground mebendazole (18·47 per cent of particles less than 10·62 μ dia; 42·26 per cent less than 21·27 μ dia) was 58 per cent of adult worms removed at a dose rate of 100 mg/kg body wt. It was also shown that fine ground mebendazole adversely affected migrating third stage larvae of N. brasiliensis.     

The neurosecretory cells of the brain and suboesophageal ganglion of Chironomus riparius

The neurosecretory cells of the supra- and suboesophageal ganglia of young, unmated, adult male midges, Chironomus riparius, have been examined by both light and electron microscopy. The 5 cell types recognized have been placed in three major categories on the basis of their ultrastructural characteristics:—α₁ cells, of which there are 8 in each medial neurosecretory cell (MNC) group and 3 in each group of ventral neurosecretory cells (VNC), contain electron-dense granules, 150 to 200 nm in diameter; α₂ cells containing irregular, electron-dense granules, 70 to 120 nm in diameter comprise the remaining 3 cells in each VNC group and the 2 or 3 cells in each outer neurosecretory cell (ONC) group; α₃ cells, of which there are 1 or 2 on each side of the midline in the ventral cortex of the sub-oesophageal ganglion (SNC₂), contain electron-lucent, spherical granules, 70 to 120 nm in diameter. The β cells contain spherical or ellipsoidal, electron-lucent granules, 80 to 100 nm in diameter, and make up the lateral neurosecretory cell (LNC) groups, each of three or four cells. The γ cells contain both spherical and flattened, electron-dense granules, 130 to 160 nm in diameter and 150 to 250 by 70 to 150 nm in size respectively, only 1 cell of this category being found in each half of the suboesophageal ganglion in the dorsal cortex (SNC₁). Axons from the MNC and VNC form the nervi corporis cardiaci I (NCCI) and those of the LNC and ONC, the nervi corporis cardiaci II (NCCII). Those of the SNC₁ appear to enter the wall of the stomodaeum but axons of the SNC₂ could not be traced.     

Metabolic Stress Responses in Drosophila Are Modulated by Brain Neurosecretory Cells That Produce Multiple Neuropeptides

In Drosophila, neurosecretory cells that release peptide hormones play a prominent role in the regulation of development, growth, metabolism, and reproduction. Several types of peptidergic neurosecretory cells have been identified in the brain of Drosophila with release sites in the corpora cardiaca and anterior aorta. We show here that in adult flies the products of three neuropeptide precursors are colocalized in five pairs of large protocerebral neurosecretory cells in two clusters (designated ipc-1 and ipc-2a): Drosophila tachykinin (DTK), short neuropeptide F (sNPF) and ion transport peptide (ITP). These peptides were detected by immunocytochemistry in combination with GFP expression driven by the enhancer trap Gal4 lines c929 and Kurs-6, both of which are expressed in ipc-1 and 2a cells. This mix of colocalized peptides with seemingly unrelated functions is intriguing and prompted us to initiate analysis of the function of the ten neurosecretory cells. We investigated the role of peptide signaling from large ipc-1 and 2a cells in stress responses by monitoring the effect of starvation and desiccation in flies with levels of DTK or sNPF diminished by RNA interference. Using the Gal4-UAS system we targeted the peptide knockdown specifically to ipc-1 and 2a cells with the c929 and Kurs-6 drivers. Flies with reduced DTK or sNPF levels in these cells displayed decreased survival time at desiccation and starvation, as well as increased water loss at desiccation. Our data suggest that homeostasis during metabolic stress requires intact peptide signaling by ipc-1 and 2a neurosecretory cells.     

鳗鲡繁殖生物学研究 Ⅱ.下海雌鳗脑垂体超显微构造的研究

神经垂体主要由神经分泌纤维、脑垂体细胞和微血管组成。神经分泌纤维主要是无髓鞘神经纤维,也有一些是有髓鞘神经纤维。神经垂体中还有一些多层体构造。神经分泌纤维有两个基本类型:A型纤维含有直径为1250—1750Å的神经分泌颗粒;B型纤维含有直径为450—1000Å的颗粒状囊泡。腺垂体的分泌细胞按其超显微构造的特点和所含的分泌颗粒大小不同可以区分为六个类型:催乳激素分泌细胞、促甲状腺激素分泌细胞,促肾上腺皮质激素分泌细胞、促生长激素分泌细胞、促性腺激素分泌细胞和后腺垂体的分泌细胞。       

Moulting and ecdysone release in response to electrical stimulation of protocerebral neurosecretory cells in Locusta migratoria

Locusta migratoria larvae were submitted to electrical stimulation of the protocerebral neurosecretory cells (median neurosecretory cells of the pars intercerebralis and lateral neurosecretory cells), during the last larval instar. The effects of the treatment were observed both on the duration of the stage and on the variations in haemolymph ecdysone levels. In untreated larvae, there was an initial ecdysone peak at the beginning of day 5, which was followed by 4 larger peaks between days 6 and 8. Stimulation of the median neurosecretory cells at the beginning of the instar resulted in the formation of one very large hormonal peak at the end of day 3: a day and a half earlier than in the control groups. Moulting was likewise accelerated. Stimulation also increased the size of the peaks, as compared with the controls. Stimulation of the lateral neurosecretory cells had a weaker ecdysiotropic effect; neither the number nor the size of the peaks were changed, though, like ecdysis, they occurred earlier. Stimulation of the deutocerebrum had no effect on either ecdysone titres or moulting. Electrical stimulation of the median neurosecretory cells at the end of day 5, that is after the occurrence of the first ecdysone peak, shortened the larval stage while having no significant effect on ecdysone levels in the haemolymph. The neuroendocrine control of ecdysis in Locusta is discussed.     

Characterization of the uptake of sucrose and glucose by isolated seed coat halves of developing pea seeds. Evidence that a sugar facilitator with diffusional kinetics is involved in seed coat unloading

Uptake of ¹⁴C-labelled sucrose and glucose by isolated seed coat halves of pea (Pisum sativum L. cv. Marzia) seeds was measured in the concentration range <0.1 μM to 100 mM. The initial influx of sucrose was strictly proportional to the external concentration, with a coefficient of proportionality (k) of 6.2 μmol·(g FW)^?1·min^?1·M^?1. Sucrose influx was not affected by 10 μM carbonylcyanide m-chlorophenylhydrazone (CCCP), but it was inhibited by 40% in the presence of 2.5 mM p-chloromercuribenzenesulfonic acid (PCMBS). Influx with diffusional kinetics was also observed for glucose (k = 4.8 μmol·(g FW)^?1·min ^?1·M ^?1) and mannitol (k = 5.1 μmol·(g FW)^?1·min^?1·M^?1). For glucose an additional saturable system was found (K_m = 0.26 mM, V _max = 4.2 nmol·(g FW)^?1·min^?1), which appeared to be completely inhibited by CCCP and partly by PCMBS. In contrast to the diffusional pathway, uptake by this saturable system was slightly pH-dependent, with an optimum at pH 5.5. The influx of sucrose appears to be by the same pathway as the efflux of endogenous sucrose, which was inhibited by 36% in the presence of 2.5 mM PCMBS (De Jong A, Wolswinkel P, 1995, Physiol Plant 94: 78–86). It is argued that passive transport may be the only mechanism for sucrose transport through the plasma membrane of seed coat parenchyma cells. The estimated permeability coefficient of the plasma membrane for sucrose (P = 3.5·10^?7 cm·s^?1) is more than 1 × 10⁶-fold higher than that reported for artificial lipid membranes. This relatively high permeability is hypothesized to result from pore-forming proteins that allow the diffusion of sucrose. Furthermore, it is shown that a sucrose gradient across the plasma membrane of the seed coat parenchyma of only 22 mM will suffice to result in the net efflux of sucrose which is required to feed the embryo.     

β-philanthotoxin,a novel glutamatergic antagonist for insect neuromuscular transmission

1. The molecular structure of β-philanthotoxin (β-PTX), a toxin from the insect paralysing venom of the wasp Philanthus triangulum, has been elucidated using NMR and mass spectrometry. β-PTX has a polyamine character: C₅H₁₁·NH·CO·(CH₂)₄·NH·(CH₂)₃·NH₂. This structure has been confirmed by synthesis.2. In the locust muscle fibre β-PTX blocks iontophoretically evoked glutamate potentials in a non-activation induced manner. β-PTX also blocks the nicotinic transmission in the insect CNS, however, at much higher toxin concentrations.3. β-PTX reduces the frequency of postsynaptic channel opening and reduces the duration of open times.4. These results suggest an effect of β-PTX on kinetics of glutamate activated ion channels, different from acting as an open channel blocker.     

In vitro correction of antigen-induced immune suppression: effects of poly(A) poly(U) and prostaglandin E

Incubation of SJL or DBA/1 mouse spleen cells with poly(lTyr, lGlu)-polylPro—polylLys, (T, G)-Pro—L in vitro reduced the immune response potential of the cells to this immunogen as tested by adoptive transfer into irradiated, syngeneic recipients, followed by immunization with (T, G)-Pro—L in complete Freund's adjuvant. This reduction in immunocompetence was antigen-specific, since incubation with another antigen (rabbit immunoglobulin G) did not result in a suppression of responsiveness of the cells to subsequent in vivo immunization with (T, G)-Pro—L. Incubation of the spleen cell-(T, G)-Pro—L mixture in the presence of either prostaglandin E₁(PGE₁) or polyadenylic-polyuridylic acid (poly(A)·poly (U)) restored the immune response potential to the normal level. Incubation of (T, G)-Pro—L with spleen cells had no effect on cyclic AMP accumulation, whereas incubation of PGE₁ with the cells stimulated cyclic AMP production, irrespective of the presence of antigens. In contrast, the level of cyclic AMP was not affected by poly(A) · poly(U). The difference in cyclic AMP accumulation suggests that PGE₁ and poly(A) · poly(A) modify immune responsiveness by different mechanisms. The above observations were verified both in SJL and DBA/1 mice, which are the respective genetic high and low responders to (T, G) -Pro—L. This implies that the modifications of responsiveness described are not related to the genetic control of immune response to this immunogen.     

Dynamic responses of photosystem II and phycobilisomes to changing light in the cyanobacterium Synechococcus sp. PCC 7942

We have examined the molecular and photosynthetic responses of a planktonic cyanobacterium to shifts in light intensity over periods up to one generation (7 h). Synechococcus sp. PCC 7942 possesses two functionally distinct forms of the D1 protein, D1∶1 and D1∶2. Photosystem II (PSII) centers containing D1∶1 are less efficient and more susceptible to photoinhibition than are centers containing D 1∶2. Under 50 μmol photons· m^?2·s^?1, PSII centers contain D1∶1, but upon shifts to higher light (200 to 1000 μmol photons·m^?2·s^?1), D1∶1 is rapidly replaced by D 1∶2, with the rate of interchange dependent on the magnitude of the light shift. This interchange is readily reversed when cells are returned to 50 μmol photons·m^?2·s^?1. If, however, incubation under 200 μmol photons·m^?2·s^?1 is extended, D1∶1 content recovers and by 3 h after the light shift D1∶1 once again predominates. Oxygen evolution and chlorophyll (Chl) fluorescence measurements spanning the light shift and D1 interchanges showed an initial inhibition of photosynthesis at 200 μmol photons·m^?2·s^?1, which correlates with a proportional loss of total D1 protein and a cessation of growth. This was followed by recovery in photosynthesis and growth as the maximum level of D 1∶2 is reached after 2 h at 200 μmol photons·m^?2·s^?1. Thereafter, photosynthesis steadily declines with the loss of D1∶2 and the return of the less-efficient D1∶1. During the D1∶1/D1∶2 interchanges, no significant change occurs in the level of phycocyanin (PC) and Chl a, nor of the phycobilisome rod linkers. Nevertheless, the initial PC/Chl a ratio strongly influences the magnitude of photo inhibition and recovery during the light shifts. In Synechococcus sp. PCC 7942, the PC/Chl a ratio responds only slowly to light intensity or quality, while the rapid but transient interchange between D1∶1 and D 1∶2 modulates PSII activity to limit damage upon exposure to excess light.     

Chick kidney microsomal cytochrome P-450 involvement in the metabolism of 25-hydroxyvitamin D3

Suspensions of 2 to 5% rat thymocytes were incubated at 35 °C in buffered balanced salt solution (pH 7.3) with lactate and β-hydroxybutyrate as fuels. The dependence of 3-O-[Me-³H]methylglucose influx on external and internal 3-O-methylglucose concentrations was studied. Entry was almost rectilinear during the first minute. From the dependence of methylglucose entry (into sugar-free cells) on external methylglucose concentration, we judged the entry K_m to be about 7.7 mm and the entry V to be about 0.64 μmol · min^?1 · (ml of packed cell volume)^?1. Methylglucose inside the cell enhanced influx, hence equilibrium exchange was faster than entry. The dependence of equilibrium exchange on methylglucose concentration (inside and outside being equal) indicated a K_m of about 25 mm and a V of about 2.1 μmol · (min)^?1 · (ml of cell volume)^?1. This effect of internal sugar indicated that entry into sugar-free cells is limited mainly by the return of empty carrier to the outside surface and that loading the carrier on the inside enhances its outward mobility. The K_m and V for influx into cells containing 21 mm methylglucose were 5.9 mm and 1.17 μmol · min^?1 · (ml of packed cells)^?1. The effect of 21 mm internal sugar on lowering the influx K_m from about 7.7 mm to about 6 mm was reproducible and contributed to the evaluation of the constants of the transport rate law. It indicated that loading of the carrier at the external surface reduces its mobility, in contrast to the effect of loading on the inside. Mechanical explanations for this behavior are discussed.     

Inorganic carbon uptake by phytoplankton in vineyard sound,Massachusetts. I. Measurements of the photosynthesis-irradiance response of winter and early-spring assemblages

The photosynthesis-irradiance dependence of natural phytoplankton assemblages from surface waters of Vineyard Sound, Massachusetts, was investigated over a several month period during late winter —early spring, 1982, when water temperatures were ? 0.5 to 8.5°C. Maximal photosynthetic rates not only were consistently observed between 7–15% I₀, but were substantially higher than previously reported rates for cold-water assemblages, averaging 10–20 μg C · μg Chl a^?1 · h^?1. At higher light intensities photo-inhibition was severe and developed within minutes to tens of minutes of the start of the experiment. Several lines of evidence, however, suggest that photoinhibition in situ may be much less that that measured in incubations of > 30 min duration; residence time of phytoplankton at surface light intensities is sufficiently short, and adaptation of photosynthetic capacity appears to have occurred to approximately the mean depth of the water column. These results further highlight the importance of establishing the time-dependent photosynthetic responses of phytoplankton and the relationship to the physical mixing regime in estimating primary productivity.     

Interplay between halogen bonds and hydrogen bonds in OH/SH···HOX···HY (X = Cl, Br; Y = F, Cl, Br) complexes

The character of the cooperativity between the HOX···OH/SH halogen bond (XB) and the Y―H···(H)OX hydrogen bond (HB) in OH/SH···HOX···HY (X = Cl, Br; Y = F, Cl, Br) complexes has been investigated by means of second-order Møller?Plesset perturbation theory (MP2) calculations and “quantum theory of atoms in molecules” (QTAIM) studies. The geometries of the complexes have been determined from the most negative electrostatic potentials (V _S,min) and the most positive electrostatic potentials (V _S,max) on the electron density contours of the individual species. The greater the V _S,max values of HY, the larger the interaction energies of halogen-bonded HOX···OH/SH in the termolecular complexes, indicating that the ability of cooperative effect of hydrogen bond on halogen bond are determined by V _S,max of HY. The interaction energies, binding distances, infrared vibrational frequencies, and electron densities ρ at the BCPs of the hydrogen bonds and halogen bonds prove that there is positive cooperativity between these bonds. The potentiation of hydrogen bonds on halogen bonds is greater than that of halogen bonds on hydrogen bonds. QTAIM studies have shown that the halogen bonds and hydrogen bonds are closed-shell noncovalent interactions, and both have greater electrostatic character in the termolecular species compared with the bimolecular species.

Effect of inhibitory amino acid antagonists on postsynaptic potentials of motoneurons of the frog Rana ridibunda

On preparations of the isolated spinal cord of the frog Rana ridibunda at intracellular recording from lumbar motoneurons, it is shown that response to the 10 mM GABA application decreased selectively by 40.7 ± 23.7% (n = 6) as a result of the spinal cord treatment with bicuculline (100–150 μM), while response to the Gly application decreased selectively by 50.7 ± 17.8% (n = 10) after the spinal cord treatment with strychnine (5–10 μM). Both strychnine and bicuculline produced potentiation of EPSP by amplitude and duration as well as paroxysmal depolarizational shifts (PDS). Strychnine produced more effectively the potentiation, while bicuculline—PDS. The inhibitory Gly effect decreased significantly the DR and RF EPSP (a decrease of amplitude and duration) as a result of the spinal cord treatment with strychnine (5–10 μM), but not with bicuculline. The inhibitory GABA effect on the DR and RF EPSP decreased as a result of the spinal cord treatment with bicuculline only in a half of the studied motoneurons and to the lesser degree than the inhibitory Gly effect on the same EPSP at the strychnine treatment. Based of the obtained data, it is suggested that the inhibitory effects on the excitatory inputs of the motoneuron in the frog are expressed weaker than in mammals and are related predominantly to the GABA and Gly effects on receptors of interneurons. It is suggested that GABA specifically acts mostly on GABA_A receptors, whereas Gly—on Gly receptors, although there is some part of cross-inhibition. Original Russian Text ? G. G. Kurchavyi, N. I. Kalinina, and N. P. Vesselkin, 2006, published in Zhurnal Evolyutsionnoi Biokhimii i Fiziologii, 2006, Vol. 42, No. 5, pp. 463–471. The present work is a continuation of the work [1].     

Magneto-electro-fusion of human erythrocytes

In inhomogeneous (static) magnetic fields close contact between ‘magnetic’ human erythrocytes was established. The cells were made magnetic by incubating them in a medium containing small Fe₃O₄-particles which adsorbed to the outer membrane surface. Fusion was induced by applying two electric field pulses (field strength: 8.5 kV · cm^?1; duration: 60 μs) to the magnetically collected cells. This procedure allowed the use of electrically conductive media (3 · 10^?1 Ω^?1 · cm^?1). Fusion of red blood cells occured very often. If cell suspensions of high density were used fusion resulted in the formation of giant red blood cells with osmotically intact membranes.     

矮嵩草光合作用与环境因素关系的比较研究

 以青海高原不同海拔地区生长的矮嵩草(Kobresia humilis)为材料,研究高山植物光合作用随海拔梯度的变化特征及对生长环境和低温胁迫的反应。随海拔升高矮嵩草叶绿素含量有降低的趋势,而叶绿素a／b值和类胡萝卜素含量则随海拔升高而增高。生长地区海拔越高矮嵩草光合速率、光补偿点、光饱和点越高;而光合表观量子产额则随海拔升高而降低。光呼吸强度有随海拔升高而降低的趋势。矮嵩草光合作用特性受生长环境因素的影响。低温胁迫导致矮嵩草光合速率、表观量子产额降低,低温下的光照加剧了光合作用抑制的程度。