Roots exposed to drying soil have been shown to generate non-hydraulic signals which can be communicated to the shoot. Such `root signals' can cause an inhibitory effect on leaf growth without causing detectable water deficits in the shoots. Plants grown in restricted root zone volumes also typically show a reduction in leaf and shoot growth. Although water stress and root restriction both impair growth, their effects on photosynthesis, leaf initiation, and C, N, and P metabolism may be quite different. Abscisic acid (ABA) has been shown to be produced in the roots after only mild dehydration and to play a major role in signal transduction from the roots to the shoots. Whether root-restricted plants are capable of generating `root signals' such as ABA or other plant hormones, which can be communicated to the shoot, remains to be determined. The application of new tools, such as gas chromatography/mass spectrography for hormone analysis, nuclear magnetic resonance imaging, and photoacoustic spectroscopy, should help to identify the nature of `root signals' generated during root restriction and clarify their regulatory role in shoot behavior.
Donald T. Krizek
Theodore W. Tibbitts and Donald T. Krizek
Donald T. Krizek and Roman M. Mirecki
Cellulose diacetate has been widely used in UV-B enhancement studies under field and controlled-environment conditions since the early 1970s to remove wavelengths below ≈290 nm, without any evidence of toxicity effects. However, while conducting UV-B exclusion studies in window boxes covered with cellulose diacetate (CA) or in Plexiglas chambers lined with CA, there was marginal chlorosis and cotyledon epinasty in `Ashley' cucumber, which is normally resistant to elevated UV-B, while seedlings exposed to open sunlight and those grown under polyester (PE) film to exclude UV-B were free of visible injury. These findings suggested that the CA filter itself may be causing toxicity. To test this hypothesis, a UV exclusion study was conducted in which CA or Teflon (T), both UV-B and UV-A transmitting films, were used to cover window boxes in the following four combinations (top/bottom): CA/CA, CA/T,T/CA, and T/T. When CA was used as the bottom filter (CA/CA and T/CA), the plants showed significantly greater leaf injury and a 2- to 3-fold reduction in growth than when T was used as the bottom filter (CA/T and T/T). These findings suggest that toxicity is caused by CA itself rather than by solar UV-B radiation, possibly as a result of outgassing of phthalates known to be used as plasticizers in the manufacture of CA. Further evidence that CA was responsible for leaf injury was provided by a companion study in which T was replaced by PE and damage was still observed, although no significant growth effects of CA position were observed.
Donald T. Krizek, Steven J. Britz and Roman M. Mirecki
The influence of ambient UV radiation on growth and flavonoid concentration of Lactuca sativa L. (`New Red Fire' lettuce) was examined. Plants were grown outdoors for 31 days from seed in window boxes covered with one of three different UV filters—UV transparent tefzel (10%T, 245 nm), UV-B-absorbing polyester (10%T, 319 nm), or UV-Aand UV-B-absorbing Llumar (10%T, 399 nm). Plants were grown in plastic pots filled with vermiculite and subirrigated with nutrient solution. Lettuce plants grown in the absence of solar UV-A and UV-B radiation showed a significant increase in leaf number and biomass of tops and roots as compared to those grown under ambient UV-A and UV-B. They also had a lower concentration of flavonoids and other UV-absorbing substances at 270, 300, and 330 nm (on both an area and on a dry-weight basis). These findings should be of interest to researchers involved in protected cultivation because the transmission of UV-B radiation is greatly attenuated by standard greenhouse glass. Our results also have implications for human nutrition, since bioflavonoids are important as antioxidants.
Donald T. Krizek, Roman M. Mirecki and Steven J. Britz
The influence of ambient UV radiation on growth, chlorosis, and flavonoid content was examined in four cultivars of cucumber (`Ashley', `Poinsett', `Marketmore', and `Salad Bush'). Plants were grown from seed in UV exclusion chambers consisting of UV transmitting plexiglass (10% T, 285 nm), lined with 3- or 5-mil Llumar (10% T, 399 or 404 nm) to exclude UV-A and UV-B, 5-mil polyester (10%T, 319 nm) to exclude UVB, or cellulose acetate (10% T, 291 nm) to transmit UV-A and UV-B. Plants were grown in 15 cm plastic pots containing vermiculite and were fertilized daily with nutrient solution. Despite their differential sensitivity to supplemental UV-B radiation, all four cultivars responded similarly to the exclusion treatments. After 19 to 21 days, plants grown under ambient UV-A and UV-B generally had less stem, leaf, and root biomass and less total height and total leaf area than those grown under conditions in which UV-A and UV-B or only UV-B was excluded. Flavonoid content, leaf number, and floral development were unaffected by UV. These findings demonstrate the extreme sensitivity of cucumber to current levels of solar UV radiation.
Donald T. Krizek, Roman M. Mirecki and Alton L. Fleming
A controlled-environment study was conducted in separate growth chambers with the wall surface covered either with white enamel paint (WEP) or polished aluminum (PA). `Williams' soybean were grown under 1500 mA cool white fluorescent lamps and internodes measured at 7, 14, and 21 days. Photosynthetic photon flux (PPF) levels in the center of each chamber were set at 320 μmol m-2 s-1 with a quantum sensor. Means ± SD for PPF levels in the WEP and PA chambers were 286 ± 28 and 307 ± 11 μmol m-2 s-1, respectively. This increase in mean PPF and decrease in variance of PPF in the PA chamber was reflected in: a) a decrease in hypocotyl, first internode, and total shoot elongation: and b) an increase in enlargement of the primary and the first trifoliolate leaves. These findings demonstrate that plants can detect small differences in irradiance within a growth chamber and suggest the advantages of using a highly polished wall surface to improve uniformity of irradiance and reduce variability in growth.
Donald T. Krizek, Abha Upadhyaya and Roman M. Mirecki
UV-B (UV) induced changes in PAL activity and UV-absorbing compounds were followed in cotton after 1 to 9 days and in cucumber after 1 to 14 days. UV increased PAL activity in the lower hypocotyl (LH) of cotton but had no effect on the upper portion. In general, PAL decreased with time, but UV treatment slowed that decline in the LH portion. Anthocyanin concentration declined with time in both portions. In cucumber cotyledons, UV had no effect on PAL. In cucumber leaves, there was no overall effect of UV; but there were significant interactions with time. In both cotyledons and in leaves, PAL decreased with time. As in LH cotton tissue, UV slowed the rate of decline of PAL in cucumber leaves. In leaves, UV absorbing compounds (at 330 nm) were increased by UV; in cotyledons, the increase in absorption was greater in controls than in UV-B irradiated seedlings. In cotton, changes in anthocyanins mirrored those in PAL, this was not the case for UV absorbing compounds in either species.
Madhoolika Agrawal, Donald T. Krizek, Shashi B. Agrawal, George F. Kramer, Edward H. Lee, Roman M. Mirecki and Randy A. Rowland
Cucumis sativus L. (cvs. Poinsett and Ashley) plants were grown from seed in a growth chamber at a +10C (28/18) or a -10C (18/28) difference (DIF) between day temperature (DT) and night temperature (NT) on a 12-hour photoperiod for 24 days prior to ozone (O3) fumigation (3 hours at 0.5 umol·mol-1). Negative DIF, compared to +DIF, reduced plant height, node count, fresh weight, dry weight, and leaf area in both cultivars. Photosynthetic rate (Pn), chlorophyll concentration, and variable chlorophyll fluorescence (Fv) were lower and O3 injury and polyamine concentrations were higher at -DIF than at +DIF. Ozone fumigation generally increased leaf concentration of polyamines and reduced Pn, stomatal conductance, and chlorophyll fluorescence. `Poinsett' generally had a higher specific leaf mass and higher concentrations of chlorophyll a and polyamines than did `Ashley', but there was no cultivar difference in O3 injury, growth response, Pn, or stomatal conductance.