Leaf discs from expanding leaves of Rumex patientia L. were exposed to 7 hours of visible plus different levels of ultraviolet radiation in the 290 to 315 nm waveband (UV-B) and then placed in darkness. Leaf disc expansion was reduced and anthocyanin production was increased in discs exposed to moderate or high levels of UV-B radiation when compared to control discs. The possibility that the inhibition of leaf expansion by UV-B radiation might be at least partially phytochrome-mediated was examined by giving discs brief red or far red irradiation folowing exposure to UV-B radiation. Brief red radiation (R) following treatment with moderate or high UV-B radiation did not alter the pattern of growth or anthocyanin production compared to discs placed in darkness following UV-B treatment. However, a posttreatment with far red radiation (FR) reduced the growth of discs subjected previously to either moderate UV-B or no UV-B irradiation to the level of growth of discs given high UV-B. FR posttreatment also decreased anthocyanin production in discs in moderate and high UV-B treatments. Effects of FR and UV-B radiation apparently do not involve the same mechanism. This was demonstrated by experiments in which FR following the UV-B treatments was in turn followed by R, which reversed the effects of the FR but did not alter the growth inhibition or increased anthocyanin production induced by moderate or high levels of UV-B radiation.Recent work has shown that enhanced UV-B (280-320 nm) radiation corresponding to global solar UV irradiance which would occur with a substantial atmospheric ozone reduction resulted in inhibition of photosynthesis and leaf growth (23). The effect on photosynthesis was cumulative, but the inhibition of leaf expansion was most pronounced during the first days of leaf expansion. Fully expanded leaf dimensions were also less than those of control plants. Even UV-B irradiation corresponding to present-day levels of solar radiation was sufficient to effect some inhibition of leaf growth (24).The mechanism by which UV-B radiation inhibits leaf expansion is not known. It appears that inhibition of expansion is not entirely due to reduction of photosynthetic rates (23). It is important to elucidate the physiological mechanisms involved in UV-B radiation inhibition of leaf expansion and how these may interact with mechanisms known to control leaf expansion. One mechanism operating in the control of leaf expansion is the low irradiance phytochrome system. Brief R2 promotes expansion (18), and 'Research supported by the National Aeronautics and Space Administration.2 Abbreviations: R: red radiation; FR: far red radiation; UV-BBE: biologically effective UV-B radiation; HIR: high irradiance reaction. this can be reversed by brief FR (14), which can be given 8 to 48 hr after the R, showing persistent photoreversibility in control of leafgrowth (4, 13). Growth, as measured by fresh weight increases, of bean seedlings grown in light can be depressed by exposure to brief FR (13).Butler et al. (2) show...

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Soybean, a monocarpic plant, has been found to undergo rapid senescence as its fruits mature. In soybeans (Glycine max[L.J Merrill) cv. Anoka, foliar senescence begins during the period of most rapid pod-fill (seed growth), and it can be eliminated by surgical removal of the seeds at an early stage of their growth. Experiments in which fruits are removed from some regions of the plant but aBowed to remain on other regions have established that the transmission of the senescence signal is limited; it affects mainly those leaves nearest to the nodes bearing the fruits. The implications of this localized signal movement are discussed.The phenomenon of monocarpy (a single fruiting phase followed by death) has been recognized for some time, and various theories have been offered to explain the mechanism by which flowering/fruiting induces the death of the entire plant (8, 9). In Anoka soybeans, senescence begins when most of the fruits on the plant are undergoing rapid pod-fill (5), and we and others (4) have found that the fruits cause the rest of the soybean plant to begin senescence which leads to death. In this paper, we will use the term "senescence signal" to refer to this, as yet, uncharacterized correlative influence of the fruits on foliar senescence without attempting to limit the notion of a signal to any one of the possible mechanisms for monocarpic senescence. Information on the behavior of this killing signal, such as degree of movement within the plant, will be important in analyzing the mechanism by which the fruits induce death. The results discussed here have been presented at meetings (10, 11 In other surgical experiments, the seeds were excised by slitting the fruit pods with a razor blade next to each seed and parallel to the main longitudinal vein of the pod but without touching the seed (see Fig. 1). The seeds were then forced out through the slits and lanolin was applied to the cuts. In shamoperated plants, the pods were treated in the same way except the seeds were not forced out.Measurement of Senescence and Fruit Development. Generally, each treatment group consisted of five plants (an average value was used in the figs.), and each experiment was repeated at least once.Senescence was measured as the percentage of total leaves on the plant greater than '/2 yellow (scored visually). This visual method of scoring leaf chlorosis has been described in detail and has been correlated with methods which use leaf absorbance of light at 660 nm or which measure amounts of extractable Chl (5).In order to describe fruit maturation quantitatively, we developed a measure of the average fruit maturity based on the proportion of total fruits in each of five fruit development stages given below.Stage 1: pod length greater than 1 cm but pod not full width and the characteristic bulges of the seed cavity not yet developed.Stage 2: pod full width but the largest seed less than 1 cm long (can be estimated by examining the fruit in front of a bright light).Stage 3: seed length greater than or equal to 1 c...

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To determine if increased abscisic acid (ABA) levels are associated with ultraviolet‐B (UV‐B) radiation (288–320 nm) stress, Rumex patientia L. plants were exposed to visible light with high or low (control) UV‐B irradiance and then assayed for ABA by gas chromatography. Though leaf growth was inhibited by the UV‐B irradiation, no differences in levels of free ABA were found between leaves from the two treatments after 1, 3 or 5 days of exposure. Unlike the situation in most environmental stresses, increased ABA levels are not associated with UV‐B radiation stress. Further, photolysis or isomerization are not likely explanations for the absence of ABA increase. Calculations indicate that if ABA in the leaf were fully exposed to solar UV‐B radiation only about 6.7% would be photolyzed by existing daily radiation at 40°N latitude, and 11.3% by radiation resulting from a 40% decrease in atmospheric ozone. The quantum yield for isomerization of cistrans‐ABA by radiation in the 288–312 nm waveband was determined. Existing daily solar UV‐B irradiation is capable of isomerizing ABA in solution to an equilibrium mixture of 50% cisjrans and 50%irans, trans‐ABA., However, because of radiation attenuation in the leaf, it is estimated that only a small fraction of the cistrans‐A.BA. would be isomerized.