The relationship between freezing tolerance and thermotropic leaf movement in five Rhododendron species

Summary Leaf movement kinetics in five species of Rhododendron were studied in response to leaf temperature, leaf freezing point, and leaf water deficit. There was a gradient in the degree of leaf curling among species in the following order from the greatest curling to the least curling: Rhododendron catawbiense, R. maximum, R. minus, R. macrophyllum, R. ponticum. Those species found to be tolerant of winter conditions had the most intense leaf movements (both curling and angle) while those species with minimal cold tolerance had limited or no leaf movements. Leaf curling occurred at leaf temperatures above the tissue freezing points in all species. Athough leaf angle was influenced by leaf turgor, general tissue desiccation was not the ultimate cause for thermotropic leaf curling in any species tested. Those species with the greatest leaf curling and angle movements had the highest osmotic potential, the lowest water deficit at the turgor loss point, and the lowest symplastic water fraction. These data suggest that there is a trade off in Rhododendron leaf physiology between cold tolerance (due to leaf movements) and water stress tolerance (due to turgor maintenance mechanisms).     

Translocation of N to and from barley roots: its dependence on local nitrate supply in split-root culture

The relationship between availability of external nitrate and N translocation between root and shoot was studied in N-limited barley ( Hordeum vulgare L. cv. Golf). Nitrate-N was added at a relative rate (i.e. N added per unit time and unit N in plant biomass) of (1.09 da-¹, and distributed between the subroots at ratios of 50:50 or 80:20. The plants were grown for 13 days under these conditions of nitrate nutrition, and for another three days with the nitrate distribution reversed from 80:20 to 20:80. The nitrale-N doses thus experienced by individual subroots ranged from 2 to 11 mg N g-¹ root dry weight day-¹ . ¹⁵N-Nitrate labellings were performed after 2 to 3 and 12 to 13 days of nitrate nutrition. and 2 to 3 days after reversal of nitrate additions. For all treatments, between 60 and 82% of the absorbed label initially left the root, and between 25 and 55% of the label recovered in roots had been supplied (cycled) via the shoot. Labelling of xylem N at the end of the 24-h labelling period ranged from to 36 to 46% indicating that a substantial fraction of the N in the xylem had been absorbed by the plant prior to labelling. It is concluded that cycling of N to roots, and cycling of N in the plant as a whole, is substantial also during N-limited growth. N allocation to roots increased with external nitrate dose. An increased utilization of non-translocated N as well as an increased translocation of N from the shoot contributed to this effect. Thus, the results indicate that increased external availability of N also increased the sink strength of the root for cycling N.