No abstract

The cytokinin content of root exudate of sunflowers increases during the exponential growth phase of the plants. The concentration of cytokinins drops, however, by a factor of ten when the plants have reached their final size. The reduced supply of cytokinins from the root to the shoot is regarded as one of the factors bringing about shoot senescence.

Sumnniary. The capacity of tobacco (Nicotiatna rutstica) leaf discs to incorporate L-leuicine "-C into proteins was measured. Leaf discs were obtained from plants which experienced soil water depletion, or which were exposed to a saline or osmotic stress in the root nmediuim. The stresses were brief of relatively short duration and water potential did not decrease below 4 bars in the root media. Leaf discs were sampled 2 hours a fter stress removal, achieved by reirrigation, or replacement of saline and osmotic solutions with normal nutrient solution. Plants were always tutrgid when leaves were sampled.All stressed tissues showed redutced capacity to incorporate L-leucine 14C into protein. The reduction was about 50 % and could not be attributed either to reduced uLptake into the discs, or to possible isotopic diluition. Incorporation decreased progressively with leaf age in control discs as well as in stressed leaf discs. At all ages tested, incorporation in stressed discs was lower than that of the control. Full recovery of incorporation capacity in stressed discs was obtained when discs were sampled 72 hours after stress removal but not earlier.Kinetin pretreatment prior to incuibation with labelled letucine partially restored incorporation in stressed discs. The differences in response to kinetin of stressed and control discs suggest a lower endogenouis level of cytokinins in the stressed discs. The results were qualitatively similar regardless of the kind of stress given to the plants dturing pretreatment and oni recovery of these rates in the presence of kinetin. Materials and MethodsSingle tobacco plants (Nicotiana rustica) were grown for about 10 weeks in a cooled greenhoulse (20°-30°) in pots containing either 3 kg of a soil-manutre mixture (3: 2 v/v)

Water stress applied to the plant shoot or Dowex 50W x 4 50-100 mesh). The column was rinsed with 1 liter of 70%to (v/v) ethanol and with H20. The active substance was eluted with 1 liter of 4 N NH40H. The eluate was concentrated to 300 ml under vacuum at 45 C, and the pH was adjusted to 8.0 and extracted twice on a shaker for 20 hr with 200 ml of n-butanol. The extracts were then combined and dried under vacuum. The residue was dissolved in 5 ml of water at pH 8. This solution was divided into equal portions and bioassayed by means of soybean tissue culture (9).The incorporation of '4C-L-leucine into trichloroacetic acidinsoluble leaf disc extracts was measured according to BenZioni et al. (2). Kinetin treatment was applied prior to incubation in "4C-L-leucine, by placing leaf discs for 45 min between Whatman No. 3 papers wet with kinetin solution.Leaf discs were exposed to two extreme relative humidities by placing them for 30 min in desiccators which contained either wet filter paper or CaCl2. Fresh weight change was recorded.Experiments with labeled kinetin were done as follows. The petiole of a fully expanded leaf was placed in a vial containing 2 ml of 0.5 4c kinetin-8-14C solution (Cal Atomic, Los Angeles, specific radioactivity 24.6 Ac/Mmole). The 2 ml were taken up by the leaf in 60 to 120 min. The leaf was cut in half, and the middle rib was removed. One half was treated as indicated in each experiment, and the other was put in a humid chamber for the duration of the treatment. On termination of treatment the halfleaf was frozen in liquid air and homogenized with 2 ml of cold ethanol. The homogenate was filtered off, and the residue was washed with 2 ml of cold water. To the combined filtrate 1 ml of chloroform was added. The nonpolar fraction contained no label. The polar fraction was separated and applied to Whatman No. 3 chromatography paper. The chromatogram was developed for 30 cm in an ascending-descending fashion in one of the two solvent mixtures: H20-acetic acid-n-butanol (1:1:4) or ethanolchloroform (9:1). The chromatogram was then dried and cut into 10 equal portions which were placed into vials containing dioxane-ethanol-toluene scintillation solution. The vials were then placed into a liquid scintillation counter (Packard TriCarb 3308) and assayed for "4C. RESULTSPlants were exposed for 30 min to an airstream which caused slight wilting of the leaves. The airstream was turned off and the shoots of both the treated and the control plants were sprinkled with water. After 10 min, leaves recovered their turgor, all the plants were topped, and the mature leaves were frozen in liquid air. A rubber hose was mounted on the stump, and exudate was collected for 6 hr. The amount of exudate did not differ between treatments. The data in Table I demonstrate a reduction in the activity of cytokinin in the leaves and in exudate when the stress was applied to the intact plant. This experiment was repeated three times. The range of the reduction was 39 to 60% in exudate and 22 to 59% in leaves.

Heat treatments of two minutes at 46 to 47°C to root systems of Nicotiana rustica and Phaseolus vulgaris affected roots and shoots. Xylem exudate of Phaseolus was collected, and it was found that heat treatment reduced cytokinin levels and increased abscisic acid levels in the exudate. Shoot and root growth of both species was reduced. Root membrane integrity of Nicotiana was measured and was found to be impaired. It is suggested that the changes in hormone activity due to heat treatment regulate the reduction in shoot growth. Cell wall metabolism and glucosyl transferases making β‐glucans were investigated in Phaseolus leaves. Incorporation of 14C from 14CO2 into wall constituents was slightly inhibited but neither photosynthesis nor extracted β‐glucan synthetases were affected during the first 12 hours after treatment.