Water use, both at the level of a single leaf and the whole plant, was studied for 1- to 4-year-old almond trees (Prunus dulcis) under arid conditions in the Negev Desert (Israel). By planting the trees into lysimeters of different volumes (7, 14 and 21 m), the amount of water available to the plants was experimentally controlled. Each year, at the beginning of the growing season, the lysimeters, which had been filled with local homogenized loess, were watered to field capacity. The trees received different relative amounts of water in relation to their leaf area on the one hand and lysimeter volume on the other, which caused different rates of soil drying throughout the season. The following hypotheses were tested. (1) The amount of CO assimilated per transpiration and (2) biomass production per unit of water used increases with (a) decreasing amount of soil water applied and (b) increasing leaf area, which should enhance growth in spring during periods of low evaporative demand. At the leaf level, the ratio of daily CO assimilation (A) to daily transpiration (E) was independent of lysimeter size, leaf area and pre-dawn water potential, but decreased with increasing leaf-to-air vapour pressure difference (D ). Consequently, during the course of the season, A/E decreased from spring to summer in accordance with rising D. However, when measured at a constant D , the seasonal course in A/E disappeared. At the whole plant level, the ratio of total lifetime biomass production (B) to the amount of water transpired (W) increased with leaf area (i.e. demand for water), the increase being stronger with increasing water supply. We conclude that almond trees did not adapt physiologically to a limited water supply, but maximized their carbon gain for a given amount of water available by phenological processes such as high growth rate during periods of low evaporative demand of the atmosphere.

This is the first in a series of papers on the growth, photosynthetic rate, water and nutrient relations, root distribution and mycorrhizal frequency of two Norway spruce forests at different stages of decline. One of the stands was composed of green trees only while the other included trees ranging in appearance from full green crowns to thin crowns with yellow needles. In this paper we compare the growth and carbohydrate relations of the two stands and examine relationships among growth variables in ten plots. The declining stand produced 65 percent of the wood per ground area compared with the stand in which all trees were green because its foliage produced less wood at any level of leaf area index. The difference in foliage efficiency between the sites could not be explained by differeneces in climate, competition or stand structure. The declining stand appeared to have lower carbon gain as indicated by a smaller increase in reserve carbohydrates before bud break, and weaker sinks for carbohydrates as indicated by less use of the stored carbohydrates than the healthy stand. Thus, growth reduction was probably related to factors which affect both photosynthesis and, even more, the sinks for carbohydrate.

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Chamaegigas intrepidus Dinter is a poikilohydric aquatic plant that lives in rock pools on granite outcrops in central Namibia. The pools are filled with water only intermittently during the wet season, and the plants may pass through up to 20 rehydration/dehydration cycles during the summer rains. The potential nitrogen sources for the rehydrated plants are ammonium, which is only present at 10-20 µM, amino acids, particularly glycine, and urea, which is generally present at 20-30 µM. We show that urea can be utilised by plants in the field through the presence of urease in the sediments of the rock pools. Urease activity is higher in non-submerged than in submerged sediments, and it can survive 6 months of complete dryness at temperatures up to 60°C. Experiments with [C]urea under laboratory conditions show that the roots of C. intrepidus are unable to take up urea; while N-nuclear magnetic resonance experiments show that [N]urea is only metabolised to labelled glutamine and glutamate after ammonium has been released by the action of urease. Thus urease plays a vital role in allowing urea to be utilised as a major N source in this nutrient-limited aquatic ecosystem.

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