The effects of a severe water deficit on total root (L(t)) and axial (L(x)) hydraulic conductances and on the development of the hypodermis, endodermis, and xylem were studied in sorghum (Sorghum bicolor L.). Water deficit was imposed in the upper rooting zone while the lower zones were kept moist. L(t) and L(x) were based on water flow rates obtained by applying suction to proximal xylem ends of excised roots. The development of the hypodermis, endodermis, and other tissues were examined by staining with fluorescent berberine hemisulfate and phloroglucinol-HCl. The L(t) value (x 10(-8) meters per second per megapascal) for unstressed control roots was 22.0 and only 5.9 for stressed roots. The low L(t) in stressed roots was attributed, in part, to accelerated deposition of lignin and suberin in the hypodermis and endodermis. Calcofluor, an apoplastic tracer that binds to cellulose, was blocked in stressed roots at the lignified and suberized outer tangential walls of the hypodermis but readily penetrated the cortical walls of similar root regions in controls where the casparian band was not developed. L(x) per unit root length was about 100 times lower in stressed roots than in controls because of the persistence of late metaxylem cross-walls and the smaller diameter and lower number of conductive protoxylem and early metaxylem vessels.
Cerminated maize (Zea mays 1.) seedlings were enclosed in modified triaxial cells in an artificial substrate and exposed to oxygen deficiency stress (4% oxygen, hypoxia) or to mechanical resistance to elongation growth (mechanical impedance) achieved by externa1 pressure on the artificial substrate, or to both hypoxia and impedance simultaneously. Compared with controls, seedlings that received either hypoxia or mechanical impedance exhibited increased rates of ethylene evolution, greater activities of 1 -aminocyclopropane-1 -carboxylic acid (ACC) synthase, ACC oxidase, and cellulase, and more cell death and aerenchyma formation in the root cortex. Effects of hypoxia plus mechanical impedance were strongly synergistic on ethylene evolution and ACC synthase activity; cellulase activity, ACC oxidase activity, or aerenchyma formation did not exhibit this synergism. In addition, the lag between the onset of stress and increases in both ACC synthase activity and ethylene production was shortened by 2 to 3 h when mechanical impedance or impedance plus hypoxia was applied compared with hypoxia alone. l h e synergistic effects of hypoxia and mechanical impedance and the earlier responses to mechanical impedance than to hypoxia suggest that different mechanisms are involved in the promotive effects of these stresses on maize root ethylene biosynthesis.
Epicuticular wax deposition on sorghum [(Sorghum bicolor) L. Moenchl leaf blades imparts drought resist. ante. Screening of a large number of sorghum genotypes for increased leaf epicuticular wax content, utilizing the available gravimetric method, is ! .practical. A more rapid colorimetric method was developed and evaluated against the current gra metric method. The colorimetric method is based on the color change produced due to the reaction of wax with acidic K0Cr,.O, reagent. Wax content determined by the colorimetric method was highly correlated (r = 0 .984) with that determined by the grain. metric method. Leaf epicuticular wax content in 11 fieldgrown grain sorghum cultivars did not vat) between growth stage #4 (tip of flag leaf showing) and 37 (soft dough), at which stages comparable wax readings were obtained among the four top leaf blades. A CK 60 'bloom less' (bmbm) genotype had only half of the epicuticular content of the CK 60 'normal' (BmBm) genotype. Ten cultivars, all of the BmBm genotype, significantly differed in epiculticular wax content, ranging from 1 .14 to 1 .99 (SE = 0.00(i) mg/dm of leaf .
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