Photosynthesis and predisposition of plants to infection with certain viruses

The effects on susceptibility to infection with certain viruses of subjecting plants to various periods of darkness or reduced illumination before and after inoculation were tested. The viruses and hosts used were a tobacco necrosis virus in French bean and tobacco; tomato aucuba mosaic virus in tobacco; and tobacco mosaic and tomato bushy stunt viruses in Nicotiana glutinosa . All the virus-host combinations give necrotic local lesions, and susceptibility was measured by local lesion counts. Susceptibility was consistently increased by pre-inoculation treatments of host plants, whereas post-inoculation treatments had relatively little effect, but most often decreased susceptibility.
Short periods in the dark produced similar responses to longer periods in shade, but the different plants varied in their response to, and tolerance of, darkness. The maximum number of lesions was usually obtained with bean plants kept for only 24 hr. in the dark before inoculation, but with tobacco plants susceptibility increased with increasing time in the dark up to 5 days.
It is suggested that the successful establishment of infection occurs in two stages, the first of which is affected by. the accumulation of photosynthetic products. Whether these products confer resistance by increasing cell turgor or by reacting specifically with virus particles is unknown, but sap from plants in the light possesses no greater virus-inhibiting power than sap from plants kept in the dark.     

Effects of temperature on infection of French bean leaves (Phaseolus vulgaris L.) by lucerne mosaic virus

The effect of temperature on the number of lesions and the time of their appearance was studied by inoculating French bean leaves (Phaseolus vulgaris L. cv. Perli?ka) with lucerne mosaic virus either 24 or 48 h before or, 24 or 48 h after they were exposed to various temperatures. The temperatures tested were 23, 25, 27, 30, 33 and 36° C. Before and after such exposures the plants were kept in a constant temperature of 25° C. By increasing the temperature before inoculation the number of lesions increased in comparison with the control. The optimal temperature for the maximum number of lesions is between 27° and 30° C. There is no significant difference between those experiments when the exposure time was 24 h or 48 h before inoculation. The same temperatures applied for 24 or 48 h after inoculation have a decreasing effect upon the number of lesions formed by LMV on French bean leaves. The decrease is 30 to 75%. In this case the first necrotic local lesions appeared 42 h after inoculation when exposed to higher temperatures above 27° C for 24 h, and 60 h after inoculation when exposed to these temperatures for 48 h. The shape of lesions varied a little in both cases as the pictures show.     

Effect of Temperature on Susceptibility of the Primary Leaves ofPhaseolus vulgaris L. to Red Clover Mottle Virus

Red clover mottle virus isolated in Czechoslovakia was studied in relation to its reaction to varying temperature on primary French bean leaves (Phaseolus vulgaris L.) on which it forms local necrotic lesions. The plants were kept 24 or 48 h before, or 24 or 48 h after inoculation at the temperatures 23, 25, 27, 30, 33 and 36°C. After such exposures the French beans were kept at a constant temperature of 25°C. The lesions were counted at various intervals. In the experiment the optimal temperature for the maximum number of lesions seems to be 36°C 48 h before inoculation. The temperature above 25°C applied 24 h after inoculation seems to have a decreasing effect upon the number of lesions formed by RCMV on primary leaves of French beans and the lesions appeared several hours later, especially at 30, 33 and 36°C. The temperatures 27, 30 and 33°C applied 48 h after inoculation have a further decreasing effect on the number of lesions. The temperature of 36°C applied 48 h after inoculation has an inactivating effect upon RCMV inoculated on French bean leaves and no lesions appeared 5 days after inoculation.     

Rhizobium infection and nodule development in soybean are affected by exposure of the cotyledons to light

The initiation of Rhizobium infections and the development of nodules on the primary root of soybean Glycine max L. Merr cv Williams seedlings are strongly affected by exposure of the cotyledons/hypocotyls to light. Seedlings in plastic growth pouches were inoculated with R. japonicum in dim light and the position of the root tip of each seedling was marked on the face of the pouch. The pouches were covered and kept in the dark for various times before exposing the upper portions of the plants (cotyledons and hypocotyls) to light. Maximum nodulation occurred if the plants were kept in the dark until 1 day after inoculation. The exposure of plants to light 2 days before inoculation reduced the number of nodules by 50% while the number of nodules was reduced by 70% if the plants were kept in the dark until 7 days after inoculation. Anatomical studies revealed that exposure to light prior to inoculation reduced both the number of infection centers with visible infection threads and the number of infections which developed nodule meristems. Plants kept in the dark for 7 days after inoculation formed a normal number of infection threads above the root tip mark, but very few of these infections developed a nodule meristem. It appears that light stimulates soybean to produce substances which can both inhibit the formation of infection threads and enhance the development of nodules from established infection threads. The effects of light on nodulation appear to be expressed independently of the Rhizobium-induced suppression of nodule formation in younger regions of the root.     

THE EFFECT OF TEMPERATURE ON INFECTION WITH STRAINS OF CUCUMBER MOSAIC VIRUS

Cucumber mosaic virus strains differed in their ability to multiply in plants at 37° C. Some strains multiplied in inoculated leaves and produced systemic symptoms in plants at this temperature; plants systemically infected with one such strain remained infected after prolonged treatment at 37° C. Other strains did not appear to multiply in inoculated leaves at 37° C. and heat treatment was successful in freeing plants from infection with these. Tests with one strain of each type showed both to be rapidly inactivated in expressed sap at 37° C.
Strains of cucumber mosaic virus forming small necrotic local lesions in leaves of french bean var. Canadian Wonder, produced many fewer lesions in plants kept after inoculation at 25° C. for 24 hr. and then at 15° C. than in plants kept continuously at the lower temperature.     

SOME EFFECTS OF HIGH TEMPERATURE ON THE SUSCEPTIBILITY OF PLANTS TO INFECTION WITH VIRUSES

When plants were kept at 36°C. for some time before inoculation, their susceptibility to infection by five mechanically transmissible viruses was greatly increased. When kept at 36° after inoculation, fewer local lesions were produced than at lower temperatures, but the effects of the post-inoculation treatment differed with different viruses. Tomato spotted wilt and tobacco mosaic viruses multiply in plants at 36°, and the post-inoculation treatment reduced the local lesions they caused to numbers that varied between 10 and 90% of the control; these two viruses also have large thermal coefficients of heat inactivation. By contrast, tobacco necrosis, tomato bushy stunt and cucumber mosaic viruses, were much affected by post-inoculation treatment, lesion formation being completely prevented by exposure to 36° for a day or more. These three viruses appear not to multiply in plants at 36°, and although they have high thermal inactivation points, they have small temperature coefficients of thermal inactivation.
The extent to which lesion formation was affected by pre- or post-inoculation exposure of plants to 36° depended not only on the length of the treatment, but also on the physiological condition of the plants.
The symptoms of infected plants changed considerably if kept at 36°. At 36° Nicotiana glutinosa , inoculated with tobacco mosaic virus, gave chlorotic local lesions instead of necrotic ones, and became systemically infected. When systemically infected plants were brought to ordinary glasshouse temperature, the infected tissues all collapsed and died in a day.     

EFFECTS OF DARKNESS ON THE CONSTITUTION OF TOBACCO LEAVES AND SUSCEPTIBILITY TO VIRUS INFECTION

An attempt was made to find the causes of increased susceptibility to virus infection when tobacco plants are kept in the dark before inoculation. The changes in certain nitrogen fractions, viz. insoluble-N, amino-N, amide-N, ammonia-N and nitrate-N, and in dry matter and water content were followed in tobacco plants subjected to a period of darkness before inoculation with tobacco aucuba mosaic virus. Only nitrate-N was strongly correlated with the susceptibility to infection, but the evidence suggests that the correlation is indirect and not causal.
Dry matter and water content, determined either as dry matter percentage of fresh weight or measured separately on a leaf-disk basis Ivere found to vary directly with variation in susceptibility.     

The Effects of Temperature on the Susceptibility of Potato Plants to Infection and Accumulation of Potato Leafroll Virus

Temperature both before and after aphid inoculation with potato leafroll virus (PLRV) greatly influenced the susceptibility of potato plants to infection and virus accumulation, as evaluated with ELISA using cultivars with different ratings for the resistance to PLRV. Pre-incubation at 15 compared to 27 °C increased the susceptibility of plants to infection and a subsequent PLRV accumulation. The virus was detected by ELISA in a greater proportion of plants and reached a higher concentration, when the plants were kept at 27 than at 15 °C after inoculation. The mean ELISA values obtained with PLRV-infected plants in the 15/27 combination of the pre-/post-inoculation temperatures over the period 1—6 wks after inoculation were significantly higher than those in the 27/27, 15/15 and 27/15 treatments, and the values obtained in the 27/27 treatment were significantly higher than those in the 15/15 and 27/15 ones. A hypersensitive-like intolerance reaction to PLRV occurred in the resistant cv. Irga only in the plants kept at 27 °C after inoculation.     

PHOTOSYNTHESIS AND RESPIRATION RATES OF LEAVES OF NICOTIANA GLUTINOSA INFECTED WITH TOBACCO MOSAIC VIRUS AND OF N. TABACUM INFECTED WITH POTATO VIRUS X

The rates of respiration and of photosynthesis of tobacco leaves infected with potato virus X were not affected until the leaves showed symptoms; the respiration rate was then increased by more than 30% and the photosynthesis rate decreased by 20%. When local lesions appeared on the leaves of Nicotiana glutinosa infected with tobacco mosaic virus, but not before, the respiration rate was increased by an amount, up to 30%, that varied with the number of lesions. The photosynthesis rate was decreased by 20%, but there was no effect on photosynthesis or respiration until symptoms appeared. These results differ from those previously reported for tobacco leaves infected with tobacco mosaic virus, in which both respiration and photosynthesis were affected within 1 hr. of inoculation. The validity of extrapolating arguments based on the results obtained with other combinations to this commonly used combination and vice-versa is questioned.     

Non-photosynthetic effects of red and far-red light on root-nodule formation by leguminous plants

Summary Nodulation of pea and broad bean plants grown in the light was found to be reduced when the roots were exposed to far-red light for 5–15 minutes daily during 5 consecutive days following inoculation with nodule bacteria. Similar results were obtained following a single exposure to far-red light during a period of 15 minutes at the 3rd or 4th day after inoculation. When the roots were exposed to far-red light either before inoculation or during the first two days afterwards there were either no effects or only slight effects on nodulation The inhibitory effect of far-red light on nodulation was partly reduced by subsequent exposure to red light, provided that the same part of the plant was exposed to both red and far-red light,viz either the root or the shoot. When different parts of the plant were exposed to red and far-red light respectively, there was no interaction between the two kinds of light on nodulation. Plants whose roots were exposed to far-red light did not subsequently show stem elongation.Nodules were found to develop on the roots of pea plants grown in the dark, provided that the plants were kept at or below 22°C. At 25°C nodulation was almost absent. Nodulation was decreased by addition of kinetin and IAA. In contrast to plants grown in the light pea plants grown in the dark, inoculated with either an effective or ineffective strain of Rhizobium, developed equal numbers of nodules. Exposure to red light slightly increased the percentage of nodulated plants but decreased the number of nodules per plant. Exposure to far-red light slightly decreased both the percentage of nodulated plants and the number of nodules per plant. The effect of far-red light was counteracted by red light andvice versa.     

THE RESPIRATION OF TOBACCO LEAVES IN THE 20-HOUR PERIOD FOLLOWING INOCULATION WITH TOBACCO MOSAIC VIRUS

The effect of infection with tobacco mosaic virus on the respiration rates of detached tobacco leaves in the period immediately after inoculation differed in plants grown at different times of the year. During winter, infection increased respiration rates, and in summer decreased them. In winter-grown plants, increasing the light intensity during the period before inoculation decreased respiration rates after infection. Extending the day length for winter-grown plants did not alter the effect of infection on respiration. Respiration rates began to change in less than 1 hr. after inoculation and are unlikely to be associated with the formation of new virus.     

Some effects of supplying benzyladenine to leaves and plants inoculated with viruses

With tomato spotted wilt virus in petunia leaf strips, N-6 benzyladenine (BA) was as effective as kinetin in decreasing numbers of local lesions, a result which could not be attributed to an effect on the virus per se. Benzimidazole, adenine and ammonium nitrate were without effect. Benzyladenine was more effective than kinetin when supplied through the petioles of excised whole leaves. Local lesions and infectivity of TSWV in detached leaves of Nicotiana rustica were decreased by supplying BA before and after inoculation. Lesions and infectivity were also decreased in attached leaves when BA was applied 9 days before inoculation. BA supplied to attached leaves after inoculation increased infectivity. Supplying BA to the lower leaves of tomato plants before inoculating with TSWV decreased infectivity of unsprayed, systemically infected tip leaves taken as inoculum; BA supplied after inoculation increased infectivity. Local lesions caused by lucerne mosaic virus in excised leaves of Phaseolus vulgaris were decreased in number by supplying BA. The effects of pre- and post-inoculation sprays of BA are considered in relation to cell metabolism. Since pretreating leaves with kinins did not prevent infection, it is suggested that those which move freely through plants without adverse effects on normal growth may prove of value in increasing the tolerance of plants to virus infection.     

THE EFFECTS OF VARYING THE WATER SUPPLY OF PLANTS ON THEIR SUSCEPTIBILITY TO INFECTION WITH VIRUSES

Increasing the amount of water supplied to plants before they were inoculated with viruses greatly increased their susceptibility to infection; plants that received unlimited water produced 10 or more times as many local lesions as plants that received only enough to prevent wilting. Susceptibility was increased throughout the year, but the full response occurred in 2 weeks in winter and 4 weeks in summer. Plants that received unlimited water for the 2 weeks immediately preceding inoculation were no more susceptible than those that received it during the previous 2 weeks, although the external appearance of the plants differed at the time of inoculation. Varying water supply after inoculation did not affect the numbers of lesions.
The differences in susceptibility to infection produced by differential watering were decreased, but not abolished, by growing plants under shade or by incorporating a diatomaceous earth in the inoculum.
Increasing water produced plants with larger and more succulent leaves; the cuticular layer was thinner, and the palisade tissue was less regularly arranged than in the plants kept dry. The increased susceptibility caused by an abundant water supply may be at least partly due to these structural differences, which allow the leaf to be damaged more easily when rubbed with inocula and so present more entry points for virus particles.     

EXPERIMENTS ON THE PHOTOPERIODIC RESPONSE IN PINEAPPLE

Gowing , Donald P. (Pineapple Research Institute of Hawaii, Honolulu.) Experiments on the photoperiodic response in pineapple. Amer. Jour. Bot. 48(1): 16–21. 1961.—The initiation of flowering of ‘Smooth Cayenne’ pineapple plants is neither strictly a response to photoperiod (day lengths of 10 hr. 51 min.–13 hr. 24 min.) nor to a minimum temperature (minima from 50° to 72°F. in different areas) under natural Hawaiian conditions. Depending on the kind of planting material used and the time of planting, natural initiation of flowering may take place any month of the year. Slips planted in the fall generally initiate flowering in December of the following year. However, exposure of an 8-mo.-old slip-planting to a day length of 8 hours for 40 days starting Sept. 8 induced flowering irrespective of night temperatures from about 60 to 80°F. Interruption of the dark period by illumination at 30 ft.-c. from midnight to 1 a.m. suppressed the inductive effect. Lowering the night temperature to 60°F. was, of itself, non-inductive. Field-grown, 11-mo.-old plants treated in place responded similarly, in that 25 periods of 8-hr. day length starting Sept. 5 induced 60% of the plants to flower, and the night illumination suppressed the inductive effect as before. Daily application of 0.12 mg. of the major native pineapple auxin (indole-3-acetic acid) at the beginning of the dark period had no detectable effect on the short-day treatment, and similar application of an antiauxin (4-chlorophenoxyisobutyric acid) did not affect the suppression of flowering by the light-break. Supplemental illumination of field-grown 12-mo. plants to provide a photoperiod of more than 15 hr. daily from Nov. 4 to Jan. 30 did not suppress the natural initiation of flowering which occurred in early December (day length about 10 hr. 50 min.). ‘Smooth Cayenne’ pineapple is therefore a quantitative, but not an obligate, short-day plant.     

THE EFFECT OF DARKENING ON THE SUSCEPTIBILITY OF PLANTS TO INFECTION WITH VIRUSES

The susceptibility of French bean plants to infection by the Rothamsted strain of tobacco necrosis virus as measured by the local-lesion method is increased by a rise in temperature and usually by darkening the plant before inoculation. If part only of a leaf is darkened, that part becomes more susceptible. Plants in full light also become more susceptible if carbon dioxide is removed from the air, whereas the susceptibility of plants in the dark is not altered.
Darkening leaves decreases their content of malic, fumaric, succinic and glycolic acids and increases the content of citric acid; the content of oxalic and malonic acids remains constant. These changes occurred in winter and summer and whether or not darkening increased susceptibility.
The effect on susceptibility of individual acids infiltrated into the leaf was measured in leaves kept in the light or in the dark before inoculation. None of the acids used produced any large change in susceptibility.     

Oscillations in stomatal conductance of hybrid poplar leaves in the light and dark

Cycling of stomatal conductance in three hybrid poplar ( Populus sp.) cultivars was observed under a variety of conditions. Illumination of plants kept previously in the dark induced very large oscillations with a period of about 40 min and large oscillations with a shorter period (< 10 min) were superimposed on the longer cycles. During these oscillations, large changes in conductance could occur very rapidly (1.0 cm s⁻¹ in 3 min). Plants in constant light also displayed both long and short term cycles in conductance, but these were smaller in amplitude than those induced by sudden illumination. Stomatal oscillations were also observed in darkness and after darkening of previously illuminated plants. These oscillations had shorter (< 30 min) and less regular periods than those observed in the light. Such cycling in the dark is rare. Cycling of the two leaf surfaces was sometimes in synchrony in the light, and more so after a perturbation. Little synchrony between the two surfaces was observed in the dark. Stomatal movements of different leaves on a plant were usually relatively independent. Transient stomatal opening occurred following leaf excision in the light or dark, and often after sudden darkening of intact leaves. Also, stomata of intact leaves sometimes transiently closed following illumination.     

SOME EFFECTS OF TEMPERATURE AND OF VIRUS INHIBITORS ON INFECTION OF FRENCH-BEAN LEAVES BY RED CLOVER MOTTLE VIRUS

Keeping French-bean plants before inoculation at 36, 32 or 28°C. for 1–2 days increased their susceptibility to infection with red clover mottle virus, but longer exposures to 36 and 32°C. decreased susceptibility. Susceptibility increased most rapidly at 36°C. The number of infections was unaffected by changes in post-inoculation temperatures between 12 and 24°C., but decreased above 24°C. The rate virus multiplied increased with increase of temperature up to 28°C., but the maximum virus concentrations reached at 18, 24 and 28°C. were very similar and above the maximum reached at 30°C.
Thiouracil inhibited infection slightly but neither it nor azaguanine affected the multiplication of red clover mottle virus in French bean. Trichothecin inhibited infection and interfered with virus accumulation. Inhibition of infection was associated with macroscopic injury to the leaves, and washing leaves up to 1 hr. after inoculation prevented both inhibition and leaf damage. Virus multiplication was not resumed when leaves were transferred from trichothecin solutions to water.     

Nitrate Reductase Activity in Soybeans (Glycine max [L.] Merr.): I. Effects of Light and Temperature

The optimum in vivo nitrate reductase (NR) assay medium for soybean (Glycine max [L.] Merr.) leaves was 50 mm KNO(3), 1% (v/v) 1- propanol, and 100 mm potassium phosphate buffer (pH 7.5).Loss of in vivo NR activity from leaves of soybeans exposed to dark was fastest at 40 C and slowest at 20 C. However, by the end of a 16-hr dark period, even those plants exposed to the lowest (20 C) temperature had lost 95% of the initial activity. Upon re-exposure to light, following a 16 hr-30 C dark period, in vivo NR activity increased rapidly to maximum levels after 4 hr light. The rate of increase was proportional to light intensity (6, 16, and 45 klux) and independent of temperature (20, 30, and 40 C).Studies with field-grown soybeans indicated that mighttime temperature (16-27 C) had no effect on the subsequent in vivo NR activity in sunlight at ambient temperature. There was a marked decrease in in vivo NR activity in late afternoon with the field-grown plants. This decrease continued throughout the night with elevated temperature (27 C) while NR activity increased when a cooler (16 C) night temperature was imposed.The changes in in vivo NR activity in response to light and dark treatments were quite rapid and thought to be related to energy limitations as well as enzyme level.     

Effect of red and far-red illuminations at the end of short days and their interaction with night-break illuminations, on flowering of Chrysanthemum morifolium plants

Flowering of chrysanthemum plants grown for 8 hr in sunlightwas delayed by 20 min of FR given prior to 16 hr dark periods.These FR illuminations increased the inhibition caused by thresholdamounts of night-break illuminations. On the other hand, 20min of R at the start of the dark period alleviated some ofthe inhibitory effects of the threshold of night-break illuminations. ¹Contribution from The Volcani Center, Agricultural ResearchOrganization, Bet Dagan, Israel, 1972 Series No. 2184 E. (Received August 26, 1972; )     

Effects of short-term cold exposure on pineal biosynthetic function in rats

In light of recent studies demonstrating stress-induced changes in pineal indoleamine metabolism, we tested the effect of acute cold stress on pineal biosynthetic function. Adult male rats were subjected to 30, 60, or 120 min of cold exposure (Ta = 2 degrees C) during either the light or dark phase of the daily photoperiodic cycle. Controls were kept at room temperature (22 +/- 2 degrees C). Animals were killed by decapitation and pineals were analyzed by radioimmunoassay for melatonin content and by radioenzymeassay for the activity of N-acetyltransferase (NAT). Cold exposure during the day elicited no significant changes in pineal indoleamine metabolism. Exposure to cold for 1 hr during the second hour after lights off slightly increased pineal melatonin content, without a concomitant change in NAT activity. Rats exposed to 2 hr of cold beginning 2 hr after lights off, however, displayed a 50% reduction in NAT activity, whereas pineal melatonin content remained unchanged. The paradoxical response of pineal NAT activity and melatonin content are not uncommon when rats are exposed to adverse stimuli.