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Physiological responses to decapitation, in combination with bud removal or bud retention, were followed for 45 days in mature leaves of potted mulberry trees (single shoot with 24 to 28 leaves) held in a greenhouse. Mature leaves, whose photosynthetic activity had already attained a maximum, initially increased and subsequently maintained their rates of gas exchange after decapitation. Equivalent leaves on intact trees showed a gradual decline in photosynthesis together with other changes generally associated with early senescence viz. loss of chlorophyll, increased starch, and accumulation of one category of cytokinin‐like material presumed to be a glucose ester. Maintenance of physiological activity following decapitation, especially when combined with bud removal, was associated with greater chlorophyll content, mesophyll cell enlargement (palisade cells appeared more elongate), lower starch, and alteration in foliar levels of cytokinin‐like substances. Internal constraints on CO2 assimilation, i.e. residual resistance (rr), rather than stomatal factors, appeared to be the major influence on gas exchange. The higher photosynthetic activity of leaves on decapitated trees relative to control trees of the same age was attributed to lower r, but was also associated with higher chlorophyll content (leaf area basis) so that CO2 assimilated per unit chlorophyll was not substantially altered by treatment. [ABSTRACT FROM AUTHOR]

While citrus rootstocks differ in capacity for sodium and chloride ion exclusion, citrus scion species also vary in foliar sensitivity to NaCl salinisation. Of two common scions, 'Lisbon' lemon appears more sensitive, whereas 'Valencia' orange in less sensitive to leaf stilt. In an attempt to explain this difference, 'Valencia' orange (Citrus sinensis [L.] Osbeck) and 'Prior Lisbon' lemon (Citrus limon [L.] Burro. F.) were budded to rootstocks known to differ in their ability to exclude sodium ions viz. the strong excluder Trifoliata (Poncirus trtfidiata [L.] Raft), and thc weaker excluder Troyer citrange (C. sinensis x P. trifoliata); neither rootstock shows strong exclusion of chloride ions. Budded trees were held under a photosynthetic photon flux density of 450 μmol m[SUP-2]s[SUP-1] and watered with nutrient solution containing either 0 or 50 mol m[SUP-3] NaC1. Growth and photosynthetic responses were measured over 58d following onset of salinization; salinity effects on leaf gas exchange were studied in relation to changes in leaf water status, compatible solutes and foliar content of sodium and chloride ions, over that same period. Once root-zone salinization began to influence leaf solutes (day 30 onwards), lemon showed a steeper increase in leaf chloride than occurred for orange. Although rootstock differences were without effect on this ingress of chloride ions for either scion, sodium ions were excluded from both scions to a larger extent by Trifoliata than by Troyer citrange. Carbon dioxide assimilation of scion foliage was reduced earlier and to a much larger extent by rootzone salinization in lemon than in orange. Furthermore, comparisons of CO[SUB2] assimilation in relation to leaf tissue solutes between scions {on either rootstock) showed stronger responses for both sodium and chloride ions in lemon than in orange. Faster ingress of chloride into lemon leaves was identified as the crucial factor which predisposed towards expression of that contrast between scions. Although contrasts between scions in photosynthetic responses to salinization matched a faster ingress of chloride into lemon than into orange leaves, the sharper photosynthetic response of 'Prior Lisbon' lemon to salinity was not solely attributable to higher concentrations of chloride ions {cell sap basis). A difference between species in subcellular compartmentation of the chloride ion under saline conditions was invoked. [ABSTRACT FROM AUTHOR]

Sterile liquid suspension cultures from grape pericarp tissue incorporate label from mevalonic acid into abscisic acid (ABA), some of which is subsequently released into the surrounding medium. The addition of mannitol to the culture medium caused plasmolysis of individual cells and increased ABA generation and release. Levels in the medium increased up to 30 fold over a period of 8 h in the presence of mannitol (720 milliosmol). A dual labelling technique using 14C and 3H mevalonic acid was developed to distinguish between labelled compounds produced by turgid tissue and those specifically associated with the substantial increase in ABA synthesis following mannitol treatment. [ABSTRACT FROM AUTHOR]

Internal conductances to CO2 transfer from the stomatal cavity to sites of carboxylation (gi in hypostomatous sun-and shade-grown leaves of citrus, peach and macadamiatrees (Lloyd et al. 1992) were related to anatomical characteristics of mesophyll tissues. There was a consistent relationship between absorptance of photosynthetically active radiation and chlorophyll concentration (mmol m-2) for all leaves, including sclerophyllous Macadamia whose transmittance was high despite its relatively thick leaves. In thin peach leaves, which had high gi, the chloroplast volume and mesophyll surface area exposed to intercellular air spaces (ias) per unit leaf area were similar to those in the thicker leaves of the evergreen species. Peach leaves, however, had the lowest leaf dry weight per area (D/a),the lowest tissue density (Td) and the highest chloroplast surface area (Sc) exposed to ias. There were negative correlations between gi and leaf thickness or D/a, but positive correlations between gi and Sc or Sc/Td. We developed a one-dimensional diffusion model which partitioned gi into a gaseous diffusion conductance through the ias (gias) plus a liquid-phase conductance through mesophyll cell walls (gcw). The model accounted for a significant amount of variation (r2=0.80) in measured gi by incorporating both components. The gias component was related to the one-dimensional path-length for diffusion across the mesophyll and so was greater in thinner peach leaves than in leaves of evergreen species. The gcw component was related to tissue density and to the degree of chloroplast exposure to the ias. Thus the negative correlations between gi and leaf thickness or D/a related to gias, whereas positive correlations between gi and Sc or Sc/Td related to gcw. The gcw was consistently lower than gias, and thus represented a greater constraint on CO2 diffusion in the mesopylls of these hypostomatous species. [ABSTRACT FROM AUTHOR]

Endogenous levels of abscisic acid-like inhibitors doubled within 15 minutes of leaf excision in Vitis vinifera under laboratory conditions. At that stage water potential (ψ) had fallen to -15 bars and stomatal closure was occurring. After prolonged stress (6 days and final ψ -13 bars) inhibitor levels in intact vine leaves increased by a factor of 44 but showed a rapid decline following rewatering. Photosynthetic activity was not immediately restored, and its gradual recovery was not directly related to inhibitor level, [ABSTRACT FROM AUTHOR]

The CO2, and H2O vapour exchange of single attached orange, Citrus sinensis (L), leaves was measured under laboratory conditions using infrared gas analysis Gaseous diffusive resistances were derived from measurements at a saturating irradiance and at a leaf temperature optimum for photosynthesis. Variation in leaf resistance (within the range 1.6 to 60 s cm-1) induced by moisture status, or by cyclic oscillations in stomatal aperture, was associated with changes in both photosynthesis and transpiration. At low leaf resistance (r1 less than 10 s cm-1) the ratio of transpiration to photosynthesis declined with reduced stomatal aperture, indicating a tighter stomatal control over H2O vapour loss than over CO2 assimilation. At higher leaf resistance (r1 greater than 10 s cm-1) changes in transpiration and photosynthesis were linearly related, but leaf resistance and mesophyll resistance were also positively correlated, so that strictly stomatal control of photosynthesis became more apparent than real. This evidence, combined with direct measurements of CO2 diffusive resistances (in a -O2 gas stream) emphasised the presence of a significant mesophyll resistance, i.e., an additional and late limiting resistance to CO2 assimilation over and above that encountered by H2O vapour escaping from the leaf.

Inflorescence leaves improve fruit set on sweet orange trees. We sought an explanation for this effect in terms of carbon demand by developing fruit versus potential supply from adjacent leaves. Our assessment was based upon measurements of fruit growth, leaf photosynthesis and 14C distribution patterns in plants grown under controlled conditions. Leafy inflorescences had sufficient foliar surface (1.24 dm2) and photosynthetic capacity (CO2 10.1 mg · dm-2 · h-1) to support early development of fruits on the same shoot, and to make a substantial contribution towards subsequent growth. 14C-assimilates derived from new leaves were distributed towards adjacent fruit which showed strong competition for labelled substrate. By contrast, fruit borne on leafless inflorescences had to obtain all their assimilates from older leaves whose photo- synthetic capacity (CO2 3.5-4.6 mg · dm-2 · h-1) and individual area (0.2 dm2) were generally insufficient to wholly sustain fruit growth, so that a large number of old-leaves were needed; these fruit would be more susceptible to competition from other sinks. [ABSTRACT FROM AUTHOR]