Ultrasound emission after cycles of water stress in Picea abies

The relationships among rate of ultrasound acoustic emission (AE), xylem water potential and transpiration rate were investigated in 5-year-old potted saplings of Picea abies Karst. after cycles of water stress. Water-stressed plants displayed minimum xylem water potentials of -3.9 MPa, near-zero transpiration rates and up to 45 AE counts per minute. After rewatering, water-stressed plants no longer produced AEs. Well-watered control plants produced only a small number of ultrasonic AEs. After three cycles of water stress (lasting 24 days in total), it was estimated that about two-thirds of the functional tracheids were embolized. The concomitant reduction in hydraulic conductance was about 70%.     

Seasonal variation in the photosynthetic capacity of a willow (Salix cv. Aquatica gigantea) canopy. 1 Changes in the activity and amount of ribulose 1,5-bisphosphate carboxylase-oxygenase and the content of nitrogen and chlorophyll at different levels in the canopy

The role of ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) in the regulation of photosynthesis in the field was assessed by following changes in the amount and activity of Rubisco and in the amounts of chlorophyll and total nitrogen in a willow (Salix cv. Aquatica gigantea) canopy during two growing seasons. An additional aim of this study was to determine how nitrogen is partitioned in the canopy and whether Rubisco and chlorophyll serve as sources of nitrogen. During both growing seasons, the total activity and the amount of Rubisco were high in young, sun-exposed leaves and decreased as the leaves aged and became shaded. In 1985, the specific activity was low (on average 0.8 micromol mm(-1) mg(-1) Rubisco protein) compared with the values obtained in 1986 (on average 1.8 micromol min(-1) mg(-1) Rubisco protein). These differences in the specific activity of Rubisco between the two years might be associated with changes in partitioning of nitrogen. During the cool summer of 1985, about 20% of the total nitrogen was partitioned to Rubisco compared with about 13% during the warm summer of 1986. Rubisco comprised a high proportion (about 60%) of the total soluble protein throughout the 1985 growing season, whereas during the 1986 growing season, the proportion of Rubisco in the total soluble protein fraction was lower and decreased markedly as the leaves became shaded. Chlorophyll did not serve as a source of nitrogen in the canopy. On the contrary, the proportion of total nitrogen in chlorophyll increased with time, which indicates acclimation of the thylakoid membranes to decreasing irradiance. The degree of activation of Rubisco in samples collected in 1986 was high in young, sun-exposed leaves, which suggests that in these leaves, Rubisco may limit the rate of CO(2) uptake. In the shaded leaves of the lower canopy, the degree of activation of Rubisco was low, which indicates that in these leaves, photosynthesis is limited by factors other than Rubisco.     

U.s. Transit subsidy policy: in need of reform

Public transit in the United States has depended increasingly on public subsidies since the inception ofthe federal mass transit assistance program in the early 1960s. The subsidies are associated with declining efficiency and labor productivity, as urban transit systems have overcapitalized, simplified fare structures, and extended service into sparse suburban markets. Despite these subsidies, transit has not proved successful in countering the effects on its market of increased automobile ownership and use and of decentralizaton of residences and places of employment.