高温胁迫对桃和抽细胞膜透性和光合色素的影响

研究了桃和柚离体叶片在高温胁迫下细胞膜透性（电解质渗漏和Ｐｉ渗漏）和光合色素的变化及其与抗热性的关系。结果表明。（１）高温处理下,无论是黑暗条件下浸水２４ｈ,还是光照条件下浸水２４ｈ,两种果树的电解质和Ｐｉ渗漏都显著增加,光照有利于膜的修复;（２）高温引起Ｃｈｌ含量下降,Ｃｈｌａ／ｂ比值上升;（３）上述这些生理反应与果树的抗热性有关,可用于果树的抗热性鉴定。     

低温胁迫下不同光照条件对锦熟黄杨抗氧化酶活性的影响

研究了锦熟黄杨（Buxus sempervirens L.）在低温胁迫下不同光照条件（12 h光照/12 h黑暗、24 h全光、24 h全黑）对其抗氧化酶活性的影响,结果表明：低温不同光照条件下细胞膜透性和丙二醛含量均高于对照（20℃ 12 h光照/12 h黑暗）,细胞膜透性和丙二醛含量在5℃ 24 h光照条件下最大;低温胁迫下SOD活性高于对照,并在5℃ 24 h光照条件下达到最高值;CAT的活性仍维持较高水平,5℃ 12 h光照/12 h黑暗显著高于对照及其它处理;在低温有光照条件下,POD活性升高,黑暗条件下POD活性低于对照。低温胁迫下POD、SOD和CAT的活性均呈上升趋势,可能是植株具有较强抗性的原因。     

玉米幼苗叶片保护酶活性的昼夜变化

人为控制环境下,对玉米幼苗叶片中电解质外渗率、MDA含量以及保护酶活性的昼夜变化同步进行了测定。结果表明：叶片电解质外渗率的昼夜变化呈现双峰曲线,分别在光照4 h和光照后黑暗4 h达到峰值;叶片中MDA含量表现为单峰的波形曲线,黑暗开始的4 h内最高;随着光照时间的延长,SOD酶、POD酶和CAT酶活性均表现出不同程度的降低,但均在16：00（光照8 h）至20：00（光照12 h）之间达到一昼夜内的最低点。随着光照后黑暗时间的延长,各保护酶活性均增加,但增加的速率不同,至次日8：00所有保护酶活性均接近光照前水平。光照和黑暗条件下,各保护酶活性差异极显著。保护酶活性的昼夜变化与叶片中的可溶性蛋白质含量并无显著的相关关系。     

光周期对穗花狐尾藻生长、开花与种子形成的影响

通过人工补光和遮光处理,初步研究了光周期对狐尾藻生长和开花的影响。研究发现:8h短日照条件不利于狐尾藻的生长和花序形成,导致狐尾藻不能正常开花和结实;与自然日照长度(平均日照14h)条件下相比,16h长光照条件可以增加植株的高度和分枝数,形成更大的生物量;但24h全光照条件对生长有抑制作用。长光照条件下,狐尾藻花序形成时间和开花时间均比自然条件下延迟,形成花序的数目也显著较自然条件下的少,但长光照条件下形成的种子比自然条件下形成的种子具有更高的萌发率。     

pH、食物和光周期对福寿螺生长发育和繁殖的影响

以7.5 g左右的成年福寿螺Pomacea canaliculata为对象,研究p H值(2.5—11.5)、光周期(24 h光照、12 h光照和无光照)和食物(喂食和不喂食)三因子的联合作用下对福寿螺生长发育和繁殖的影响。结果表明,无光照和饥饿条件下,福寿螺在酸性p H值4.5和p H值3.5试验的第24天均达到50%致死率,在碱性p H值10.5试验的第21天死亡率达到53.3%;试验第27天,无光照和饥饿条件下,p H值2.5和p H值11.5的死亡率则分别达到最大为66.7%和80%,产卵量均仅为20粒,孵化率分别低至6.06%和5.97%;24 h光照和喂食条件下,p H值7.5试验的第27天福寿螺体重增长率最高,为17.78%(而12 h光照和无光照条件下则分别仅为7.03%和5.47%),但此时产卵量只有180粒,孵化率仅为8.23%,延长孵化历时到16.6 d,而此酸度下12 h光照(最接近于自然节律),最终产卵量达到1600粒,且卵的孵化率高达84.66%,孵化历时为13.53 d。以上结果说明,福寿螺具有较强的耐酸碱和抗饥饿能力,但过酸(p H值≤3.5)和过碱(p H值≥10.5)的环境,也会严重抑制福寿螺的存活、生长发育和繁殖,特别是在福寿螺缺乏足够食物条件下,抑制作用更强;光周期对福寿螺的生长发育和繁殖影响较大,连续24 h光照会极大地刺激福寿螺的体重增长,但同时会干扰福寿螺繁殖,减少产卵量和卵的孵化率,延长孵化历时;而12 h光照在3种光照条件下最有利于福寿螺的生长发育和繁殖;完全黑暗则会严重抑制福寿螺的生长发育及降低其繁殖能力。     

山楂叶螨滞育的初步研究

本文采用海绵水盘法研究了山楂叶螨Tetranychus viennensis Zacher的滞育与温度、光照和寄主营养等环境因子的关系。结果表明21℃下,临界光周期为12 h 32 min,光敏感螨态为幼螨至第二若螨之间的连续两个螨态,单独一个发育阶段对短光照不敏感或仅有微弱的感应。在相同寄主（梨树）叶片上,病叶和老叶诱导的滞育率比嫩叶高,分别为69.7%、55.3%和20%。相同叶片,短光照(12.5 h/d)条件下随螨密度的增大滞育率升高。相同短光照下,低温促进滞育,在15℃、18℃和21℃下滞育率分别为100%、93.1%和66.7%;高温（24℃）抑制滞育,滞育率仅为13.3%;温度为27℃时不滞育。     

5种植物生长抑制剂对香果树种质离体保存的影响

以香果树带芽茎段为外植体,采用单因子实验法研究了5种植物生长抑制剂在不同温度、不同光照条件下对香果树试管苗离体保存的影响。结果表明,在25℃条件下,香果树种质离体保存的最佳CCC、MH、B9、S3307和ABA浓度分别为4、4、2、1和4mg／L;在9℃条件下,香果树种质离体保存的最佳CCC、MH、B9、S3307和ABA浓度分别为2、1、1、0．5和2mg／L。在光照和黑暗条件下,香果树种质离体保存的最佳CCC、MH、B9、S3307和ABA浓度分别为4、4、2、1和4mg／L。与常温条件相比,低温条件下5种植物生长抑制剂的成活率更高,最佳浓度普遍降低。与光照培养比较,在黑暗条件下5种植物生长抑制剂的成活率也更高,但处理的最佳浓度相同。离体保存后的试管苗没有发生遗传变异。     

蓝细菌病毒A-4L在鱼腥藻(Anabaena variabilis)藻苔中形成同心圆噬斑的成因

摘要:【目的】分析蓝细菌病毒A-4L在鱼腥藻(Anabaena sp.) PCC 7120藻苔中形成同心圆噬斑的原因,阐明A-4L的一个重要生物学特征,为分离、鉴定和筛选新的水华蓝藻病毒提供借鉴。【方法】用初始滴度为2.8×1010PFU/mL的A-4L悬液感染鱼腥藻,在不同时间点收集裂解液绘制一步生长曲线,获得A-4L对鱼腥藻的潜伏期和释放量。将适量A-4L悬液感染不同培养时间的藻苔,逐日观察和记录藻苔病变情况。培养藻苔并接种适量A-4L悬液,分别置于完全持续光照(Light:Dark=24 h:0 h,L:D=24 h:0 h)条件;完全周期光照(L:D=14 h:10h)条件;或前3 d周期光照转后3 d持续光照的条件下,比较不同光照条件对同心圆噬斑形成的影响。然后挑取单个噬斑进行扩大培养,纯化后,负染电镜观察A-4L的超微形态。【结果】A-4L的潜伏期为0.5－2h,释放量约为247 IU/cell (Infectious Units)。在周期光照条件下,藻苔接种A-4L 3－4 d后,出现同心圆噬斑,且同心圆数量与攻毒后的天数(n)有相关性,为“n－1”;同心圆间距约为3 mm。与周期光照条件相比,在持续光照条件下未形成同心圆噬斑。而在周期光照条件转持续光照条件下,由先周期光照时所形成的同心圆在转持续光照后逐渐消失,证实同心圆噬斑的形成依赖于周期光照。负染电镜观察显示A-4L具有一个近似球形的头部,直径约为50 nm以及长度约为10 nm的尾部,形态与短尾蓝细菌病毒相似。【结论】A-4L是一株能形成同心圆噬斑的蓝细菌病毒,并揭示其同心圆噬斑形成的关键条件是周期光照。     

不同高温胁迫对白桦幼苗几个生理生化指标和电阻抗图谱参数的影响

以白桦1年生实生苗为试材,分别用37℃与42℃高温处理2、4、8、12、24 h,以常温处理(白天25℃、12 h/夜间15℃、12 h)苗木为对照,对其叶片叶绿素含量、根系活力、可溶性糖含量、超氧化物歧化酶(SOD)活性、超氧阴离子(O·-2)含量、丙二醛(MDA)含量、蛋白质含量和电阻抗进行测定,研究其抗热性.结果表明:37℃处理的叶片中叶绿素含量低于对照,SOD活性和MDA含量高于对照;42℃处理的叶片中叶绿素含量、根系活力、可溶性蛋白含量、MDA含量和弛豫时间τ低于对照,SOD含量和胞外电阻(re)高于对照;并且42℃处理的叶片中叶绿素、可溶性糖、SOD活性高于37℃处理,而可溶性蛋白、(O·-2)、MDA含量和根系活力较37℃处理低.37℃下,胞内电阻(ri)与MDA含量呈显著正相关(r=0.68),β系数与可溶性糖含量呈显著负相关(r=-0.75),胞内电阻(ri)与可溶性蛋白含量呈显著正相关(r=0.75);42℃下,胞外电阻(re)与可溶性糖含量成显著正相关(r=0.91),胞内电阻(ri)与SOD活性成显著负相关(r=-0.51).37℃与42℃处理下的电阻抗图谱参数差异显著,胞外电阻(re)、胞内电阻(ri)、弛豫时间τ和β系数变化规律不一致.研究发现,42℃是高温胁迫下诱导白桦幼苗抗热性的更理想条件;用电阻抗图谱法研究白桦幼苗的抗热性是可行的.     

蓝细菌病毒A-4L在鱼腥藻(Anabaena variabilis)藻苔中形成同心圆噬斑的成因

【目的】分析蓝细菌病毒A-4L在鱼腥藻(Anabaena sp.)PCC 7120藻苔中形成同心圆噬斑的原因,阐明A-4L的一个重要生物学特征,为分离、鉴定和筛选新的水华蓝藻病毒提供借鉴。【方法】用初始滴度为2.8×1010PFU/mL的A-4L悬液感染鱼腥藻,在不同时间点收集裂解液绘制一步生长曲线,获得A-4L对鱼腥藻的潜伏期和释放量。将适量A-4L悬液感染不同培养时间的藻苔,逐日观察和记录藻苔病变情况。培养藻苔并接种适量A-4L悬液,分别置于完全持续光照(Light∶Dark=24 h∶0 h,L∶D=24 h∶0 h)条件;完全周期光照(L∶D=14 h∶10 h)条件;或前3 d周期光照转后3 d持续光照的条件下,比较不同光照条件对同心圆噬斑形成的影响。然后挑取单个噬斑进行扩大培养,纯化后,负染电镜观察A-4L的超微形态。【结果】A-4L的潜伏期为0.5-2 h,释放量约为247 IU/cell(Infectious Units)。在周期光照条件下,藻苔接种A-4L 3-4 d后,出现同心圆噬斑,且同心圆数量与攻毒后的天数(n)有相关性,为"n-1";同心圆间距约为3 mm。与周期光照条件相比,在持续光照条件下未形成同心圆噬斑。而在周期光照条件转持续光照条件下,由先周期光照时所形成的同心圆在转持续光照后逐渐消失,证实同心圆噬斑的形成依赖于周期光照。负染电镜观察显示A-4L具有一个近似球形的头部,直径约为50 nm以及长度约为10 nm的尾部,形态与短尾蓝细菌病毒相似。【结论】A-4L是一株能形成同心圆噬斑的蓝细菌病毒,并揭示其同心圆噬斑形成的关键条件是周期光照。     

Nitric oxide increases tolerance responses to moderate water deficit in leaves of Phaseolus vulgaris and Vigna unguiculata bean species

Drought stress is one of the most intensively studied and widespread constraints, and nitric oxide (NO) is a key signaling molecule involved in the mediation of abiotic stresses in plants. We demonstrated that a sprayed solution of NO from donor sodium nitroprusside increased drought stress tolerance responses in both sensitive (Phaseolus vulgaris) and tolerant (Vigna unguiculata) beans. In intact plants subjected to halting irrigation, NO increased the leaf relative water content and stomatal conductance in both species. After cutting leaf discs and washing them, NO induced increased electrolyte leakage, which was more evident in the tolerant species. These leaf discs were then subjected to different water deficits, simulating moderate and severe drought stress conditions through polyethylene glycol solutions. NO supplied at moderate drought stress revealed a reduced membrane injury index in sensitive species. In hydrated discs and at this level of water deficit, NO increased the electron transport rate in both species, and a reduction of these rates was observed at severe stress levels. Taken together, it can be shown that NO has an effective role in ameliorating drought stress effects, activating tolerance responses at moderate water deficit levels and in both bean species which present differential drought tolerance.     

The effects of freezing on membrane electric potential in winter oilseed rape leaves

Extracellular ice formation in winter oilseed rape leaf discs (Brassica napus L. var. oleifera L. cv. Jantar) at different temperatures resulted in a transient membrane depolarization, which was followed by a decrease in membrane electric potential. In discs which underwent supercooling (no extracellular ice was formed), no membrane depolarization was observed. The inhibitors of calcium ion channels, gadolinium and lanthanum, decreased to some extend the amplitude of the frost-induced (−6 °C) depolarization and completely eliminated the decrease in membrane potential. Changes in membrane potential were associated with the increased electrolyte efflux, measured after thawing of the discs. No efflux from supercooled discs was observed. Application of calcium channel blockers decreased the level of the efflux induced by freezing at −6°C. It is suggested that membrane depolarization is one of the primary events induced by ice formation at a leaf surface. The possible reasons for changes in the membrane electric potential and their physiological consequences are discussed.     

Effect of cutting on solute uptake by plasma membrane vesicles from sugar beet (Beta vulgaris L.) leaves.

The uptake of sucrose, 3-O-methylglucose (3-O-MeG), and valine were studied in discs and in purified plasma membrane vesicles (PMV) prepared from sugar beet (Beta vulgaris L.) exporting leaves. The uptake capacities of freshly excised leaf discs were compared with the uptake in discs that had been floated for 12 h on a simple medium (aging) and with discs excised from leaves that had been cut from the plant 12 h before the experiments (cutting). After cutting, sucrose uptake amounted to twice the uptake measured in fresh discs, whereas the uptake of 3-O-MeG and valine remained unaffected. In aged leaf discs, there was a general stimulation of uptake, which represented 400, 300, and 400% of the uptake measured in fresh discs for sucrose, 3-O-MeG, and valine, respectively. Sucrose uptake in fresh discs was sensitive to N-ethylmaleimide (NEM), to p-chloromercuribenzenesulfonic acid (PCMBS), and to mersalyl acid (MA). Although cutting induced the appearance of a sucrose uptake system that is poorly sensitive to NEM but sensitive to PCMBS and MA, aging induced the development of an uptake system that is sensitive to NEM but poorly sensitive to PCMBS and MA. Autoradiographs of discs fed with [14C]sucrose show that cutting resulted in an increase of vein labeling with little effect in the mesophyll, whereas aging induced an increase of labeling located mainly in the mesophyll. The data show that cutting is sufficient to induce dramatic and selective changes in the uptake properties of leaf tissues and that the effects of cutting and aging on the uptake of organic solutes are clearly different. Parallel experiments were run with purified PMV prepared from fresh and cut leaves. The uptake of sugars and amino acids was studied after imposition of an artificial proton motive force (pmf). Comparison of the uptake properties of PMV and of leaf tissues indicate that the recovery of the sucrose uptake system in PMV is better than the recovery of the hexose and of the valine uptake systems. As observed with the leaf discs, cutting induced a 2-fold increase of the initial rate of sucrose uptake in PMV but did not affect the uptake of valine and 3-O-MeG. Cutting induced an increase of both Vmax and Km of the sucrose transport system in PMV. Measurements of the pmf imposed on the vesicles indicated that the increase of sucrose uptake induced by cutting was not due to a better integrity of the vesicles. Hexoses did not compete with sucrose for uptake in PMV from fresh and cut leaves, and maltose was a stronger inhibitor of sucrose uptake in PMV from cut leaves than in PMV from fresh leaves. The sensitivity of sucrose uptake to NEM, PCMBS, and MA in PMV from fresh and cut leaves paralleled that described above for the corresponding leaf discs. These data show that (a) the changes induced by cutting on sucrose uptake by leaf discs are due to membrane phenomena and not to the metabolism of sucrose; (b) the study of sucrose uptake with PMB gives a good account of the physiological situation; and (c) the specific effects induced by cutting on the sucrose uptake system are not lost during the preparation of the PMV.     

Effects of lead and kinetin on the growth, and some physiological components of safflower

Pollution of the root environment with excess of Pb retarded shoot growth, decreased chlorophyll (Chl) content and reduced Chl stability (CSI) to heat. Plants growing in Pb polluted soil accumulated much more free amino acids and less soluble sugars than the control plants. Stability of leaf membranes to heat (51 °C) or dehydration stresses (40% polyethylene glycol, 6000) decreased in response to Pb pollution where the membranes of leaf discs excised from Pb-treated plants were damaged more than those taken from plants growing in Pb free soil. Supplying kinetin ameliorated the deleterious effects of Pb pollution on the parameters tested. Kinetin-treated plants had higher Chl, soluble sugars content and produced more biomass in their shoots. Also, kinetin increased leaf membrane stability especially in Pb-treated plants, effectively protected chlorophyll degradation by heat and increased Chl a and b stability index; the most effective concentration was 10 mg L^–1. The effects of Pb and kinetin as well as their interaction (Pb × Kin) on the parameters tested were statistically significant. Applied kinetin had a dominant role (as indicated by ²) in affecting shoot growth, soluble sugars, Chl a and b contents, stability of leaf membranes to dehydration stress as well as the Chl stability index. Pb had a dominant role on total free amino acids (TAA) and leaf relative water content (RWC). The interaction between Kin × Pb influenced the stability of leaf membranes to heat stress in a major way.     

Gender-specific responses of Piper betle L. to low temperature stress: changes in chlorophyllase activity

Gender based differences in response to low temperature stress in leaf chlorophyll (Chl), and carotenoids (Car) contents and chlorophyllase (Chlase) activity were monitored in male (Kapoori Vellaikodi and Madras Pan Kapoori) and female (Bangla Mahoba, Desi Bangla and Kaker) betel vine landraces. Although female plants contained nearly two fold more Chl than male counterparts, the low temperature induced Chl loss was comparable, however, male plants showed higher Chl a/b ratio than females. Chlase activity increased due to cold stress in all the landraces. Male plants always showed higher activities of Chlase, which may be one of the reasons for the rather low Chl contents in male plants.     

The apoplastic antioxidant system in Prunus: response to long-term plum pox virus infection

This work describes, for the first time, the changes taking place in the antioxidative system of the leaf apoplast in response to plum pox virus (PPV) in different Prunus species showing different susceptibilities to PPV. The presence of p-hydroxymercuribenzoic acid (pHMB)-sensitive ascorbate peroxidase (APX) (class I APX) and pHMB-insensitive APX (class III APX), superoxide dismutase (SOD), peroxidase (POX), NADH-POX, and polyphenoloxidase (PPO) was described in the apoplast from both peach and apricot leaves. PPV infection produced different changes in the antioxidant system of the leaf apoplast from the Prunus species, depending on their susceptibility to the virus. In leaves of the very susceptible peach cultivar GF305, PPV brought about an increase in class I APX, POX, NADH-POX, and PPO activities. In the susceptible apricot cultivar Real Fino, PPV infection produced a decrease in apoplastic POX and SOD activities, whereas a strong increase in PPO was observed. However, in the resistant apricot cultivar Stark Early Orange, a rise in class I APX as well as a strong increase in POX and SOD activities was noticed in the apoplastic compartment. Long-term PPV infection produced an oxidative stress in the apoplastic space from apricot and peach plants, as observed by the increase in H2O2 contents in this compartment. However, this increase was much higher in the PPV-susceptible plants than in the resistant apricot cultivar. Only in the PPV-susceptible apricot and peach plants was the increase in apoplastic H2O2 levels accompanied by an increase in electrolyte leakage. No changes in the electrolyte leakage were observed in the PPV-inoculated resistant apricot leaves, although a 42% increase in the apoplastic H2O2 levels was produced. Two-dimensional electrophoresis analyses revealed that the majority of the polypeptides in the apoplastic fluid had isoelectric points in the range of pI 4-6. The identification of proteins using MALDI-TOF (matrix-assisted laser desorption/ionization-time of flight) and peptide mass fingerprinting analyses showed the induction of a thaumatin-like protein as well as the decrease of mandelonitrile lyase in peach apoplast due to PPV infection. However, most of the selected polypeptides showed no homology with known proteins. This fact emphasizes that, at least in Prunus, most of the functions of the apoplastic space remain unknown. It is concluded that long-term PPV infection produced an oxidative stress in the leaf apoplast, contributing to the deleterious effects produced by PPV infection in leaves of inoculated, susceptible Prunus plants.     

Proline accumulation in canola leaf discs subjected to osmotic stress is related to the loss of chlorophylls and to the decrease of mitochondrial activity

Many stress studies use simplified experimental models like leaf discs, but the validity of such approaches remains a matter of debate, especially when attempts are made to obtain suitable biotests. We report here the analysis of the resistance of canola leaf discs to osmotic stress, as related to the accumulation of proline. For that purpose, the explanted tissues were incubated for 24 h under continuous light and at 28°C on polyethylene glycol solutions of external potentials (Π_ext) ranging from −0.1 to −8.0 MPa. Proline content, water deficit and chlorophyll content were quantified. The cell membrane stability, which is a structural component of viability, was assessed according to electrolyte leakage. Biomembrane oxidative damage was measured through the production of malondialdehyde and the mitochondrial activity was quantified by assessing the ability of the tissues to reduce 2,3,5-triphenyltetrazolium chloride. Although the water deficit of the tissues reached high values (i.e. up to 70%), the cell membrane stability remained high. Furthermore, the oxidative damage to biomembranes was quite low, even in highly dehydrated tissues. In contrast, osmotic stress induced a significant decrease in the chlorophyll content and a strong accumulation of proline. These two processes each reached a maximum at a Π_ext of −3.0 MPa, although the tissues appeared to be viable at even lower Π_ext, suggesting they are not responsible for primary resistance. The mitochondrial activity was strongly decreased in the stressed leaf discs, although this activity was at least partially restored in tissues incubated for 24 h on a non-stressing medium. It appears that the cell components of the osmotic stress resistance assessed observed in this study do not reflect the modes of resistance of an intact plant.     

Inductive responses of some organic metabolites for osmotic homeostasis in peanut (Arachis hypogaea L.) seedlings during salt stress

The salt tolerance of peanut (Arachis hypogaea L.) seedlings was evaluated by analyzing growth, nutrient uptake, electrolyte leakage, lipid peroxidation and alterations in levels of some organic metabolites under NaCl stress. The plant height, leaf area and plant biomass decreased significantly in salt-treated seedlings as compared with control. The relative water content (RWC %) of leaf decreased by 16 % at high concentrations of NaCl. There was an increase in the lipid peroxidation level and decrease in the electrolyte leakage at high concentrations of NaCl. The total free amino acid and proline contents of leaf increased by 5.5- and 43-folds, respectively in 150 mM NaCl-treated plants as compared with control. Total sugar and starch content increased significantly at high concentrations of NaCl. Chl a, Chl b, total chlorophyll and carotenoid contents decreased significantly at high salinity. Na⁺ contents of leaf, stem and root increased in dose-dependent manner. K⁺ content remained unaffected in leaf and root and decreased in stem by salinity. The results from present study reveal that the peanut plants have an efficient adaptive mechanism to tolerate high salinity by maintaining adequate leaf water status associated with growth restriction. In order to circumvent the stress resulting from high salinity, the levels of some organic metabolites such as total free amino acids, proline, total sugars and starch were elevated. The elevated levels of the organic metabolites may possibly have some role in maintenance of osmotic homeostasis, nutrient uptake and adequate tissue water status in peanut seedlings under high-salinity conditions.     

Polyamines and senescence of maintenance foliage of tea,Camellia sinensis L.

In leaf discs of maintenance foliage of tea (Camellia sinensis) polyamines (PA_s) and kinetin retarded chlorophyll (Chl) loss, whereas inhibitors of PA biosynthesis [difluoromethyl arginine, difluoromethyl ornithine, methylglyoxal-bis(guanylhydrazone)] and abscisic acid (ABA) promoted senescence. The contents of RNA and protein were significantly higher in PA and kinetin treated leaf discs as compared to those treated with inhibitors and ABA. The contents of total and reducing saccharides declined with the progressive loss of Chl, and the concentration of starch increased in all the PA treated leaf discs. Free amino acid content also increased under all the treatments, but the increase was comparatively larger in case of inhibitors application. The authors thank Director, CSIR Complex, Palampur for providing necessary facilities and Merrel Dow Research Institute (Cincinnati, OH) for generous gift of DFMO and DFMA.     

Water stress-sensitized photoinhibition in senescing cotyledons of clusterbean: Changes in thylakoid structures and inactivation of photosystem 2

Seedlings of Cyamopsis tetragonoloba were grown on Petri dishes either in water or water plus 3 % PEG-6000 to induce water stress. The senescing cotyledons experiencing the stress exhibited loss in contents of leaf proteins and chlorophyll (Chl) and a decline in oxygen evolution. The effect of PEG treatment was more pronounced at moderate (MI) than low (LI) irradiance. The stress-induced loss in the activity of superoxide dismutase and increase in the thylakoid lipid peroxidation accompanied a change in the physical status of the bilayer membrane as demonstrated by an enhancement of room temperature Chl a fluorescence polarization and decrease in energy transfer efficiency in pigment assembly. This resulted in a sustained decrease in photosystem 2 activity blocking channels of energy utilization. The absorbed quanta, thus unutilized, were excess even at MI, leading to photoinhibitory response.