Effects of Benzyladenine on Photosynthesis, Growth and Senescence of the Bean Plant

The net photosynthetic rate of attached primary bean leaves treated with benzyladenine (BA) decreased from 35 nano-grams carbon dioxide per square centimetre and second (ng cm^?2 s^?1) at week 2 to 17 ng cm^?2 s^?1 at week 4, and thereafter increased to 24 at week 6. By contrast the net photo-synthetic rate of the water-treated leaves decreased to 4–5 ng cm^?2 s^?1 at weeks 5 and 6. The stomatal resistance was not markedly affected by BA treatment. The BA-treated primary leaves had higher dry and specific weights, and the chlorophyll and carotenoid contents were greater than for the water controls. The pigment content of the BA-treated leaves steadily increased from week 2 to 6, whereas in the water controls it remained constant till week 4 and thereafter decreased. It is concluded that retardation of leaf senescence by BA is assoclatcd with a mnintenance of photosynthetic activity.     

Retention and allocation of <Superscript>14</Superscript>C assimilates by maintenance leaves and harvest index of tea (<Emphasis Type="Italic">Camellia sinensis</Emphasis> L.)

Partitioning of ¹⁴C-labelled photosynthates to various parts of un-pruned tea clones TV1 and TV25 was assessed in vivo by exposing maintenance leaves to ¹⁴CO₂ at monthly intervals throughout the year. The plants from shoot apex to root tip were divided into twelve components to assess the allocation and retention of ¹⁴C-photosynthates by the maintenance foliage. Out of the total photosynthates produced by the maintenance leaves, only 11.08 % was allocated to the commercially useful harvestable two and a bud shoots which is accepted as the harvest index of tea. The photosynthetically active maintenance leaves retained 19.05 % while 24.56 % was distributed to the branches. The bottom and the top parts of the trunk utilized 7.44 and 7.21 %, respectively. The thick roots at the base of the trunk, medium sized roots, pencil size roots, and feeder roots imported 7.28, 7.72, 7.65, and 8.01 % of ¹⁴C assimilates, respectively. Except retention by leaves, all the plant parts of vigorous clone TV25 required higher percentage of assimilates than TV1. The mean quantities of net photosynthates utilized by the stem and the roots were 69.37 and 30.63 %, respectively.     

Influence of Benzyladenine, Leaf Darkening, and Ringing on Movement of C-labeled Assimilates Into Expanded Leaves of Vitis vinifera L

Leaves of Vitis vinifera L., nearly fully expanded, imported only trace amounts of ¹⁴C following assimilation of ¹⁴CO₂ by a lower leaf on the same shoot, but benzyladenine (BA) application at 4.4 × 10⁻³m caused a marked increase in the movement of ¹⁴C into these leaves. Older leaves near the shoot base were less responsive; BA treatment alone had little effect on import of labeled assimilates from adjacent leaves but when the BA-treated leaves were darkened there was an increased import of labeled materials. When these 2 treatments were combined and applied to leaves on shoots with ringed bases, relatively high levels of radioactivity were detected in the BA-treated leaves but under these conditions darkening, without the application of BA, also resulted in an increased import of ¹⁴C. Accumulation of imported ¹⁴C was found to be restricted to the area of the leaf blade treated with BA. Separation of labeled compounds in ethanol extracts of treated leaves showed a lower percentage of radioactivity present in the sugar fraction from BA-treated leaves and an increased percentage present in the amino acid fraction.     

Distribution and utilization of assimilated carbon in red clover during the first year of vegetation

Summary The pattern of distribution of¹⁴C labelled assimilates and translocation with time was measured in red clover during one reproductive cycle. Measurements were made on whole plants grown outdoors in pots by exposing the aerial parts to¹⁴CO₂ during one photoperiod. Simultaneously, root respiration and N₂ fixation were recorded.At the beginning of the vegetative period, 2/3 of the assimilates remained in the leaves (basal leaves), and 1/3 were directed to the root system. Then the development of branches required as much as 40% of the C and the root allocation decreased. Reproductive structures diverted 17% of the current photosynthates. Nitrogen fixation was optimal during the maximum extension of the basal leaves and decreased during the development of branches. During this period, C allocation to the nodulated roots was high with an estimated amount of 3.2 mg of C per mg of N fixed.With time, translocation occured within the foliage, from basal leaves to the leaves of the branches and to the new basal leaves developed after senescence of the branches. Remobilization to the reproductive structures remained minimal indicating that flower and seed growth was supported by current photosynthesis.     

Translocation of Photosynthate in Curly Top Virus-infected Tomatoes

Photosynthate translocation in single leaflets of healthy and curly top virus-infected tomatoes was investigated using ¹⁴C as a marker. The amount of radioactivity found in plant parts not exposed to ¹⁴CO₂ was substantially lower in diseased than in healthy plants. The time lag for the appearance of ¹⁴C in the petiole was considerably longer in the infected plants than in the healthy. The kinetics of disappearance of ¹⁴C from the lamina during the 24-hour period following labeling showed a strong retention of recent assimilates within the diseased leaf, not accompanied by increased immobilization into insoluble forms. Sucrose was the predominant compound participating in photosynthate transport in both healthy and diseased leaves. The amount of ¹⁴CO₂ fixed was approximately 40% lower in curly top virus-infected leaves than in healthy leaves.     

Study of the senescence process in primary leaves of sunflower (Helianthus annuus L.) plants under two different light intensities

Different parameters that vary during leaf development may be affected by light intensity. To study the influence of different light intensities on primary leaf senescence, sunflower (Helianthus annuus L.) plants were grown for 50 days under two photon flux density (PFD) conditions, namely high irradiance (HI) at 350 μmol(photon) m^?2 s^?1 and low irradiance (LI) at 125 μmol(photon) m^?2 s^?1. Plants grown under HI exhibited greater specific leaf mass referred to dry mass, leaf area and soluble protein at the beginning of the leaf development. This might have resulted from the increased CO₂ fixation rate observed in HI plants, during early development of primary leaves. Chlorophyll a and b contents in HI plants were lower than in LI plants in young leaves. By contrast, the carotenoid content was significantly higher in HI plants. Glucose concentration increased with the leaf age in both treatments (HI and LI), while the starch content decreased sharply in HI plants, but only slightly in LI plants. Glucose contents were higher in HI plants than in LI plants; the differences were statistically significant (p<0.05) mainly at the beginning of the leaf senescence. On the other hand, starch contents were higher in HI plants than in LI plants, throughout the whole leaf development period. Nitrate reductase (NR) activity decreased with leaf ageing in both treatments. However, the NR activation state was higher during early leaf development and decreased more markedly in senescent leaves in plants grown under HI. GS activity also decreased during sunflower leaf ageing under both PFD conditions, but HI plants showed higher GS activities than LI plants. Aminating and deaminating activities of glutamate dehydrogenase (GDH) peaked at 50 days (senescent leaves). GDH deaminating activity increased 5-fold during the leaf development in HI plants, but only 2-fold in LI plants. The plants grown under HI exhibited considerable oxidative stress in vivo during the leaf senescence, as revealed by the substantial H₂O₂ accumulation and the sharply decrease in the antioxidant enzymes, catalase and ascorbate peroxidase, in comparison with LI plants. Probably, systemic signals triggered by a high PFD caused early senescence and diminished oxidative protection in primary leaves of sunflower plants as a result.     

Effect of assimilate utilization on photosynthetic rate in wheat

Summary Two weeks after anthesis, when the grain is filling rapidly, the rate of photosynthesis by flag leaves of wheat cv. Gabo was between 20 and 30 mg CO₂ dm^-2 leaf surface hour^-1 under the conditions used. About 45% of flag-leaf assimilates were translocated to the ear, and only about 12% to the roots and young shoots.On removing the ear, net photosynthesis by the flag leaves was reduced by about 50% within 3–15 hours, and there was a marked reduction in the outflow of ¹⁴C-labelled assimilates from the flag leaves.Subsequent darkening of all other leaves on plants without ears led to recovery of flag-leaf photosynthesis, as measured by gas analysis and ¹⁴CO₂ fixation, and to increased translocation of assimilates to the roots and young shoots. Minor changes in the rates of dark respiration accompanied these major, reversible changes in photosynthetic rate.After more than a week in continuous, high-intensity light, the rate of photosynthesis by flag leaves of intact plants had fallen considerably, but could be restored again by a period in darkness, or by inhibiting photosynthesis in the ears by spraying them with DCMU. The inhibition of ear photosynthesis increased translocation of labelled assimilates from the flag leaf to the ears, without affecting leaf sugar levels.The application of TIBA to the culm below the ear inhibited auxin movement throught the culm, but had no influence on flag-leaf photosynthesis.These results suggest that, at least in this system, photosynthesis by the flag leaf is regulated directly by the demand for assimilates from the flag leaf and not indirectly through action in the leaf of auxins produced by the sink organs.     

Growth regulator induced movement of photosynthetic products into fruits of ;black corinth' grapes

The effect of exogenous growth regulators on movement of assimilates into flowers and young fruits of `Black Corinth' grapes was studied. Clusters were treated with growth regulator and after 0.5 hr to 5 days the leaves above the clusters were exposed to ¹⁴CO₂. Control shoots received ¹⁴CO₂ but no growth regulator. At harvest, counting and radioautographic techniques were used to ascertain amount and distribution of activity in clusters. Clusters were dipped in 4-CPA (4-chlorophenoxyacetic acid), GA₃ (gibberellic acid), or BA (benzyladenine). All berries were heavier than controls within 3 days. Total counts in the fruits were increased by 4-CPA, and the distribution of radioactivity among the sugar, organic acid, and amino acid fractions was usually altered by all treatments. In a time series experiment, within 6 hr after treatment of fruits with GA₃ there was almost an 8-fold increase in total counts relative to the control. After 12 hr there was about a 9-fold and 6-fold increase in counts in tartaric and malic acids, respectively, and in γ-aminobutyric acid, pipecolic acid, and valine increases of 56, 150, and 330%. Radioactivity in fructose was increased 70% in gibberellin-treated clusters over the controls. After 96 hr there were only about 1000 cmp per g fr wt in controls, but there were about 31,000 cpm counts in treated clusters. Treatment of clusters with gibberellin attracted less assimilates into the fruits when shoots had also been sprayed with gibberellin. Dipping portions of clusters in gibberellin increased the movement of ¹⁴C assimilates into the treated portions. Hormonal control of mobilization is discussed.     

Benzyladenine-Directed Transport of Carbon-14 and Phosphorus-32 in Senescing Bean Plants

The distribution of radioactivity from applied sucrose.¹⁴C and³²P to various plant parts were studied in relation to the retardationof leaf senescence by applied benzyladenine (BA) in intact beanplants. In short-time experiments sucrose-¹⁴C was fed to theplants for 48 h through the second trifoliate leaf at weeklyintervals from the third to the eighth week after planting.In long-term experiments sucrose-¹⁴C was fed to all plants for48 h at the third week and changes in distribution examinedat weekly intervals up till the eight week. In both cases, BAapplied to the primary leaves of intact bean plants did notcause a directed mobilization of sucrose-¹⁴C. When the plantswere stripped, leaving the primary leaves and the terminal pod,and fed sucrose-¹⁴C or ³²P through the terminal leaflet of thesecond trifoliate leaf, the BA-treated leaf accumulated relativelymore radioactivity than the opposite water-treated leaf. Itwas concluded that the retardation of senescence by BA in theprimary leaves of intact bean plants does not result directlyfrom the mobilization of metabolites and nutrients from otherplant parts. It is therefore suggested that BA-induced longevityin the primary leaves of the intact plant is accomplished bymetabolic self-sustenance.     

Retardation of bean leaf senescence by benzyladenine and its influence on phosphate metabolism

The levels of glucose, sugar phosphates, and adenosine phosphates were determined in primary leaves of intact bean plants during normal senescence and compared to leaves in which senescence was delayed by application of benzyladenine (BA). In both cases there was a rise with time in the levels of glucose 1-phosphate, glucose 6-phosphate, and fructose 6-phosphate, and a decline in 2-phosphoglyceric acid, inorganic phosphate, and the adenosine phosphates (AMP, ADP, ATP). The levels of fructose 1,6-diphosphate remained fairly constant. Although the levels of hexose phosphates, adenosine phosphates, and inorganic phosphate were lower in the BA-treated leaves, the incorporation of ³²P into these compounds by 3- and 6-week-old plants was higher than in the controls. These results suggest that the retardation of leaf senescence by BA in intact bean plants is associated with increased utilization of metabolites, indicating a more rapid turnover of the adenosine phosphates. It is concluded that this effect is brought about by a regulatory coordination of metabolic processes in relation to energy production and utilization.     

Effects of DDT on carbohydrate metabolism and translocation in secale species

An inhibition of photosynthetic electron transport in susceptible rye following treatment with DDT is accompanied by an increase in dry weight of leaves contacting the pesticide due to an accumulation of fructose, glucose, and to lesser extent, sucrose. Several days after treatment over 40% of the dry weight is due to these sugars. The assimilation of ¹⁴CO₂ by leaf segments was decreased as a consequence of DDT treatment, but labelling patterns were similar to those for leaf segments from untreated plants. However, if given a prolonged period in darkness before extraction of assimilates the leaf segments from treated seedlings retained ¹⁴C in sugars and did not show the substantial decrease in extractable soluble material which was characteristic of untreated controls. In DDT-treated seedlings the translocation of metabolites from leaves to roots was severely impaired.     

Distribution of 14CO2 during ontogeny of chickpea (Cicer arietinum) grown at two moisture levels

The distribution of assimilates of ¹⁴CO₂ in ethanol soluble and insoluble fractions was measured at 20-day intervals from 45–135 days after sowing (DAS) in chickpea (Cicer arietinum) grown at two moisture levels. The contribution of pre-flowering assimilates to pods, although very low, was higher under the stress conditions. At the time of harvest, the recovery of ¹⁴C in pods was 0.4 and 0.9% of the total ¹⁴C fed 45 DAS in soluble and 2.5 and 5.1% in insoluble fractions in control and stressed plants, respectively. The %¹⁴C received by nodules continuously decreased with increasing age of plants. Stressed plants diverted more ¹⁴C to nodules, compared to control, during vegetative and flowering stages. During active seed filling, stressed plants diverted more ¹⁴C to reproductive parts and less to nodules, compared to control. Significant amounts of ¹⁴C were retgined by the stem and leaves during the seed-filling period and it appears that there is scope for the remobilisation of pre-flowering, as well as post-flowering assimilates for seed-filling of chickpea.     

Effects of cytokinin and senescence-inducing factors on expression of P ARR5 -GUS gene construct during leaf senescence in transgenic Arabidopsis thaliana plants

ARR5-gene expression was studied in the course of natural leaf senescence and detached leaf senescence in the dark using Arabidopsis thaliana plants transformed with the P _ARR5 -GUS gene construct. GUS-activity was measured as a marker of ARR5-gene expression. Chlorophyll and total protein amounts were also estimated to evaluate leaf senescence. Natural leaf senescence was accompanied by the progressive decline in the GUS-activity in leaves of the 2nd and 3rd nodes studied, and this shift of GUS-activity was more pronounced than the loss of chlorophyll content. The ability of the ARR5-gene promoter to respond to cytokinin was not eliminated during natural leaf senescence, as was demonstrated by a cytokinin-induced increase in GUS activity in leaves after their detachment and incubation on benzyladenine (BA, 5 × 10⁻⁶ M) in the dark. Leaf senescence in the dark was associated with the further decrease in the GUS-activity. The ARR5-gene promoter response to cytokinin was enhanced with the increase of the age of plants, taken as a source of leaves for cytokinin treatments. Hence, although the expression of the ARR5 gene reduces during natural and dark/detached leaf senescence, the ARR5-gene sensitivity to cytokinin was maintained in both cases and even increased with the leaf age. This data suggest that the ARR5 gene, which belongs to the type-A negative regulators of plant response to cytokinin, could be a feedback regulator able to prevent retardation by cytokinin of leaf senescence when it is important for plant life. Growth regulators either reduced ARR5 gene response to cytokinin during senescence of mature detached leaves in the dark (SA, meJA, ABA, SP) or increased it (IAA), thus modifying the resulting rate of its expression.     

Effect of Ophiobolus graminis infection on the assimilation and distribution of 14C in wheat

The uptake of ¹⁴C and movement of ¹⁴C-labelled assimilates in wheat plants inoculated with Ophiobolus graminis was examined following exposure of the second youngest leaf to ¹⁴CO₂. Autoradiographs of plants with infected seminal roots showed that assimilates were not translocated past the sites of root infection but accumulated in the undamaged portions of infected root systems, in particular the developing crown roots. There was no evidence that assimilates accumulated in the vicinity of O. graminis lesions. The net assimilation of ¹⁴CO₂ by wheat plants over a 5 h feeding period was not significantly affected by O. graminis infection. However, infection reduced the amount of ¹⁴C lost through respiration. Infection delayed the transfer of labelled assimilates from the fed leaf to the remainder of the plant but increased the proportion translocated to the roots. The latter effect was not apparent when infected plants were continuously irrigated during, and for 20 h following, the feeding period.     

Ethylene and Cytokinin in the Control of Senescence in Detached Leaves of Arabidopsis thaliana eti-5Mutant and Wild-Type Plants

We studied the effects of cytokinin benzyladenine (BA) and ethylene on the senescence in the dark of detached leaves of Arabidopsis thaliana(L.) Heynh wild-type plants and theeti-5mutant, which was described in the literature as the ethylene-insensitive one. Leaf senescence was assessed from a decrease in the chlorophyll content. The content of endogenous cytokinins (zeatin and zeatin riboside) was estimated by the ELISA technique. We demonstrated that the content of endogenous cytokinins in the leaves of the three-week-old eti-5mutants exceeded that of the wild-type leaves by an order of magnitude; in the five-week-old mutants, by several times; and in the seven-week-old plants, the difference became insignificant. Due to the excess of endogenous cytokinins in the three–five-week-old mutant leaves, their senescence in the dark was retarded and exogenous cytokinin affected these leaves to a lesser extent. The seven-week-old mutant and the wild-type leaves, which contained practically similar amounts of endogenous cytokinins, did not differ in these indices. Thus, the level of endogenous cytokinins determined the rate of senescence and the leaf response to cytokinin treatment. Ethylene accelerated the senescence of detached wild-type leaves. Ethylene action increased with increasing its concentration from 0.1 to 100 l/l. BA (10^–6M) suppressed ethylene action. Similar data were obtained for the eti-5mutant leaves. We therefore suggest that the mutant leaves comprised the pathways of the ethylene signal reception and transduction, which provided for the acceleration of their senescence.     

Effects of Induced Resistance on Disease Severity/Yield Relations in Mildewed Barley

Field experiments showed that the treatment of winter barley with microbial metabolites produced by a Bacillus subtilis strain led to increased yields in spite of a remaining mildew infection. Disease severity/yield relations obtained on a single tiller basis for either mildew infection at EC 75 or area under disease progress curve (AUDPC) were negatively correlated for untreated plants (R²= 89%, 94%) while this relation did not exist for inducer-treated ones (R²= 10%, 13%). Despite an increasing infection density. yields of main tillers of inducertreated plants were not decreased. On the other hand area under green leaf area curve (AUGLAC) showed a higher correlation with grain yield (R²= 89%) of inducer-treated plants. Possible explanations for the mitigated damaging effect of powdery mildew were expected in carbohydrate metabolism. especially carbohydrate formation and translocation. Assimilation rates of flag leaves of inducer-treated barley with similar infection densities to those of untreated plants were increased over a prolonged period and even exceeded those of non infected ones. In inducer-treated plants the export of ¹⁴CO₂ from flag leaves into ears remained unimpaired by mildew infection and the allocation of assimilates to grains was highest at late stages of grain filling. Obviously plants were stimulated by inducer treatments to compensate for the damaging effect of powdery mildew and to tolerate mildew infection without yield loss.     

Insights of carbon assimilation and allocation in young cork oak (Quercus suber L.) plants using Carbon-14

¹⁴C methods were applied to young, woody, branched and well-watered cork oak (Quercus suber L.) plants to determine carbon assimilation and its distribution among plant organs. Carbon assimilation rates by attached leaves clamped in a foliar ¹⁴CO₂ assimilation chamber containing 3.7 × 10⁴ Bq of a portable ventilated diffusion porometer were measured at different ¹⁴CO₂ pulse-labeling periods (15, 30, 45, 60 and 120 s) in summer. Allocation of recently fixed C by attached leaves within plants was evaluated 7 days after a 60-min of 5.6 MBq of ¹⁴CO₂ pulse-labeling in late winter. ¹⁴CO₂ pulse-labeling was separately induced on leaves of a lower branch, two opposite branches at the same lower level, a middle branch and a top branch. ¹⁴C activity incorporated into the plants was measured by liquid scintillation and autoradiography. Our results show the optimum ¹⁴CO₂ pulse-labeling period is between 15 and 30 s, which corresponds to 9.81 ± 0.15 and 9.16 ± 0.12 µmol m⁻² s⁻¹ C assimilation rates in summer, respectively. The investment of current assimilates ranged from 18 to 29% in leaves, 1 to 7% in lateral branches, 0 to 3% in the stem and over 65% in roots, in late winter. Roots displayed the greatest sink strength for the total ¹⁴C recovered by whole-plants. These results were expected because the trial was done in winter, when cork oak does not produce their leaves. Our results highlight the contribution of current assimilates for growth and maintenance of roots, in young woody plants under Mediterranean climate.     

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin syn‐thesis, i.e. targeted to senescing leaves and negatively auto‐regulated (P_SAG12–IPT), thus preventing developmental abnormalities. Plants were grown with growth‐limiting nitrogen supply. Compared to the wild‐type, endogenous levels of free zeatin (Z)‐ and Z riboside (ZR)‐type cytokinins were increased up to 15‐fold (total ZR up to 100‐fold) in senescing leaves, and twofold in younger leaves of P_SAG12–IPT. In these plants, the senescence‐associated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more (¹⁵N‐labelled) N than younger leaves, associated with reduced shoot N accumulation (–60%) and a partially inverted canopy N profile in P_SAG12–IPT plants. While root N accumulation was not affected, N translocation to non‐senescing leaves was progressively reduced. We discuss potential consequences of these modified sink–source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.     

Senescence Retarding Effect of Metal Ions: Pigment and Protein Contents and Photochemical Activities of Detached Primary Leaves of Wheat

Al³⁺ significantly delayed the loss of chlorophyll (Chl), protein, and carotenoids when compared to K⁺ and Mg²⁺ during dark-induced senescence of detached primary leaves of Triticum aestivum. Thylakoid membranes isolated from Al³⁺ - treated leaves showed a better retention of photosystem (PS) 2, PS1, and whole chain electron transport activities than thylakoids of K⁺- or Mg²⁺-treated leaves. These ions protected the electron transport activities and restored the DCMU-dependent fluorescence increase of thylakoid membranes in a valency-dependent manner. Al³⁺ also delayed the change of excitation energy distribution during senescence.     

Mobilization of metabolites from leaves to grains as the cause of monocarpic senescence in rice

The pattern of senescence was studied by following the changes in chlorophyll and protein in the leaves and by measuring ³²P retention and export from source to sink during development of the rice plant (Oryza sativa L. cv. Jaya) subjected to different manipulative treatments. With the advance of reproductive development, the chronological sequence of leaf senescence was changed, so that the flag and the third leaf senesced earlier than did the second leaf. In presence of the daughter shoot of defruited plants, senescence was delayed in all three leaves of the mother plant, as compared to the same leaves of intact plants. Senescence of all three leaves was further delayed when both panicle and daughter shoots were removed from the plant. The above manipulative treatments caused the initial sequential pattern of senescence of leaves to persist. Removal of both panicle and daughter shoots caused little export of ³²P between leaves. In the presence of daughter shoots of defruited plants, export of ³²P was maximum from leaves of the mother plant to the nearest daughter shoots. This led to earlier senescence of such mother plant leaves than that of plants from which both panicle and daughter shoots were removed. The pattern of senescence and export of ³²P in the flag and the second leaf of the daughter shoot was essentially the same as that of the intact plant. Based on these findings, it was concluded that mobilization of metabolites from source to sink is the primary cause of monocarpic senescence in rice.