SummaryIn intact apple plants, sorbitol, rather than sucrose, is the main carbohydrate involved in phloem transport. The behaviour of excised phloem, either freshly excised (fresh) or washed for 20 hr after excising (aged), towards sucrose, glucose, and sorbitol was studied. All three carbohydrates were accumulated rapidly, rates being higher when more concentrated solutions were supplied and when aged tissues were used. Both effects were more marked for sorbitol than for sucrose or glucose. When sucrose or glucose was accumulated by fresh or aged phloem, sucrose was the main product and no sorbitol was formed. When sorbitol was accumulated it was the main product, though in aged phloem sucrose was also formed. In comparison with sucrose sorbitol is readily accumulated from the more concentrated solutions, but is only slowly metabolized by phloem tissues. It is suggested that, in the intact plant, sorbitol, which is present in the leaf in high concentration, is preferentially accumu· lated into the phloem, but once there is metabolically rather inert and so is not altered until it reaches its destination. The tissue there, like aged phloem, can readily utilize sorbitol. Thus, sorbitol is well suited to translocation in apple.

Very young apricot leaves behave like the young leaves of most plants; that is, [14C]sucrose is formed as the main product of 14CO2 photosynthesis, and also when the leaves are supplied with [14C]glucose. [14C]sorbitol is not produced, and is poorly metabolized when fed to the leaf. Expanding leaves behave differently: [14C]sorbitol and [14C]sucrose are formed in similar amounts from both 14CO2 and [14C]glucose; and when [14C]sorbitol is supplied, it is readily metabolized and utilized for growth. Mature leaves are different again. They form [14C]sorbitol as the main product from 14CO2 and from [14C]glucose, and they do not metabolize [14C]sorbitol at all. Thus during development, apricot leaves gain but then lose the ability to utilize sorbitol. They also gain and keep the ability to synthesize sorbitol. This suggests that different biochemical paths exist for sorbitol formation and utilization, and that these paths are differently developed in the various stages of leaf development. Although the very young leaves did not synthesize sorbitol from CO2 or glucose, they contained it as their major sugar. Translocation behaviour was therefore studied. Neither the very young leaves nor the expanding leaves export any photosynthate, but the mature leaf rapidly translocates carbohydrate, mainly in the form of sorbitol, to the younger leaves as well as the rest of the plant. [14C]sorbitol supplied to the mature leaf can be recovered in that form from the very young leaf on the same shoot. This further establishes the role of sorbitol in apricot as a specific transport carbohydrate.

Sugar uptake by slices of sugar-cane storage tissue took place in two stages. The initial uptake reached an equilibrium within 1 hr, the level being proportional to the external sugar concentration, independent of the sugar, and unaffected by anaerobic conditions. This sugar diffused out rapidly when the tissue was placed in water. It was thus contained in the apparent free space, 10-20 per cent. of the tissue volume. The secondary uptake continued up to 60 hr at a slow, constant rate, 1-5 mgjgjday, independent of sugar concentration above 2� 0 per cent., dependent on the sugar, and inhibited by anaerobic conditions. This sugar did not diffuse out when the tissue was placed in water. It was concluded that the secondary uptake was an active accumulation process.

Ab8tractPhosphorus deficiency (P deficiency) caused a 10-20·fold increase in phosphatase activity of S. oligorrhiza homogenates. Specific staining showed that phosphatase in control plants was located primarily in and around the vascular strands, and in P·deficient plants it was located in the epidermis of the root and undersurface of the frond. Dissection experiments showed that roots of P·deficient plants contained a disproportionately high phosphatase activity. When plant and root homogenates were fractionated, phosphatase activity was recovered in the supernatant rather than in any cell particle fraction. However, intact plants released very little phosphatase to the external medium.Intact plants were placed in solutions containing phosphatase substrates. Control plants hydrolysed the substrates at 4-8% of the rates of comparable homogenates. P·deficient plants hydrolysed substrate at 30-40% of the homogenate rate. When glucose 1.[32P]phosphate was supplied to intact plants, more 32p; appeared in the medium than in the tissue. The pattern of appearance was con· sistent with hydrolysis of glucose 1.[32P]phosphate in the external medium followed by accumulation of 32P! into the tissue; and was not consistent with hydrolysis of glucose 1-[32P]phosphate in the tissue followed by loss of 32p; to the medium. We conclude that a large part of the phosphatase which arises in the plant S. oligorrhiza under the stress of P deficiency is located in either the cell wall or the external membrane, where its function is to utilize phosphate esters released to the medium by dying plants. Although a new isoenzyme appears during P deficiency it does not appear to function specifically as the externally active phosphatase.

Although sorbitol is the major soluble carbohydrate in plants of the woody Rosaceae, the floral nectars contained virtually no sorbitol. Glucose and fructose occurred in approximately equal proportions, with sucrose making up the balance. Nectar secretion was studied using excised flowers. Extensive conversion of sorbitol to other sugars occurred within the flower, at least partly within the nectary itself. Cutting the nectary tissue into slices greatly reduced its ability to transform sorbitol to other sugars. It is suggested that during nectar secretion sorbitol conversion occurs mainly during phloem unloading. Changes in sugar composition of nectars were studied. There was no evidence that sucrose was secreted then subsequently hydrolysed by invertase or by a microbial flora. There was bidirectional movement of sugars: radioactive sucrose and other sugars added to the nectar entered the nectary tissue, were metabolized there to other sugars, and were secreted again to the nectar. Composition of the nectar was therefore a product of both secretion and uptake processes.