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The influence of plant water relations on phloem loading was studied in Ricinus communis L. Phloem transport was maintained in response to bark incisions even at severe water deficits. Water stress was associated with a net increase in the solute content of the sieve tubes, which resulted in maintenance of a positive phloem turgor pressure Ψp. There was a significant increase in solute flux through the phloem with decreasing xylem water potential (Ψ). In addition, sugar uptake by leaf discs was examined in media adjusted to different water potentials with either sorbitol (a relatively impermeant solute) or ethylene glycol (a relatively permeant solute). The limitations in this experimental system are discussed. The results nevertheless indicated that sucrose uptake can be stimulated by a reduction in cell Ψp, but that it is little affected by cell Ψ or solute potential Ψs. On the basis of these data we suggest that sucrose loading is turgor-pressure dependent. This may provide the mechanism by which transport responds to changes in sink demand in the whole plant.

Phloem-sap composition was studied in plants of Ricinus communis L. grown on a waterculture medium. The sap possessed a relatively high K(+):Na(+) ratio and low levels of Ca(2+) and free H(+). Sucrose and K(+) (together with its associated anions) accounted for 75% of the phloem-sap solute potential (Ψs). In plants kept in continuous darkness, a decrease in phloem-sap sucrose levels over 24h was accompanied by an increase in K(+) levels. Measurements of phloem-sap Ψs and xylem water potential (Ψ) indicated that this resulted in a partial maintenance of phloem turgor pressure Ψp. In darkness there was also a marked decrease in the malate content of the leaf tissue, and it is possible that organic carbon from this source was mobilized for export in the phloem. The results support the concept of the phloem sap as a symplastic phase. We interpret the increase in K(+) levels in the phloem in darkness as an osmoregulatory response to conditions of restricted solute availability. This reponse can be explained on the basis of the sucrose-H(+) co-transport mechanism of phloem loading.

The osmotic characteristics of phloem-sap exudation were examined in soil-grown and watercultured plants of Ricinus communis L. Prolonged exudation occurred from bark incisions in water-cultured plants. Fresh incisions caused large alterations in solute flux, but phloem-sap solute potential Ψs changed by less than ±8% over a period of 7 h. This was associated with a constancy in the levels of sucrose and K(+), the principal solutes in the sap. Studies with foliar-applied tracers and leaf-excision experiments suggested that exudation was maintained by solute loading from mature leaves. A wide range of mass transfer values through the phloem was found, these being a function of exudation rate. We consider that the exudation process possesses essentially similar characteristics to phloem transport in the intact plant. The way in which bark incisions bring about large changes in solute flux is discussed in terms of the physical properties of the sieve-tube system.

Solute concentration gradients were studied in Ricinus communis L. stems using refractometry linked with cryoscopic measurements of osmotic pressure. Techniques were developed to study the local composition of sieve-tube sap, effectively isolating bark segments by ring-massage or massage-girdling. In intact plants concentration gradients in stems were found to be steep near leaves, as reported previously, and also near roots. However, when transport from these organs was prevented, the steep gradients disappeared. Apparently they are caused by sap influx from nearby organs into stems during sampling. Concentration gradients in stems proper were almost negligible under standard growing conditions. They became positive (more concentrated above) when photosynthesis was enhanced, and negative in darkness.During exudation, leaves could secrete abundant concentrated sap for long periods. Stems had a similar but more limited capacity to secrete and, to a still lesser extent, roots also. Secretion was triggered even from natural sinks by exudation from an incision. Release from turgor pressure seems responsible rather than dilution of the phloem sap, suggesting that in whole plants a sink induces secretion and pressure-flow from distant sources by regulating the pressure within sieve tubes. The rate of exudation decreased as sap concentration fell, indicating that a reduction in pressure corresponds to assimilate exhaustion.The hydraulic conductivity of bark was not high, but was greater when the bark was secreting solutes. A change in mass-flow conduction between sieve tubes and storage cells is proposed to explain these differences in water permeability.The common assumption that solute concentration gradients correspond to pressure gradients seems inapplicable to whole Ricinus plants. Solute transfer can exceed the water flux so that osmotic equilibrium may not be reached at a particular location. In intact plants, solute gradients probably reflect source and sink activity with the interconnecting sieve tubes behaving rather passively, like pipes with a solute exchange-capacity. Actual pressure differences in sieve tubes are best indicated by sap concentration differences between source and sink. In Ricinus plants 0.5-1 m tall these pressures can be 5-10 bar.

Exudation can be induced or greatly enhanced in Ricinus by massaging the stem periodically some time before tapping the sap by making an incision. Nevertheless temporary massage during exudation stops flow completely indicating that flow is prevented normally by a pressure sensitive sealing mechanism which becomes desensitised by regular massage.

Xylem cavitation has been studied in Ricinus plants using vibration detection to examine its induction by different factors. These observations provide considerable circumstantial evidence in justification of the new technique as already described and further developed. In general cavitation is induced only when the tissue water balance is reduced hydrostatically. Thus cavitation is promoted by intense radiation which enhances transpiration, or alternatively by the blockage of xylem conduits by suspended particles carried in the transpiration stream. In contrast a reduction in radiation, or prevention of transpiration tends to restrict cavitation. Thus cavitation can be prevented by immersing a leaf in liquid paraffin. This technique has been used to see if radioactive bombardment would trigger its induction but no detectable effect has been observed even when exposed to intense radiation.An excised leaf, losing water in air, produces a "click total". On restoration to full turgor by standing the petiole in water it recovers very slowly and subsequently its "click total" is much reduced. If however the newly wilted leaf is allowed to recover in water following gas evacuation treatment the "cavitation total" often approaches the original and the rate of recovery is extremely rapid. Apparently gas emboli develop rapidly in conduits which have cavitated, but they can be removed by vacuum injection: the conduits refill and conduction is restored.

A study, made on the phloem exudation profiles from Ricinus plants given varying degrees of pre-treatment by massage, has revealed that the profiles are governed by sealing systems of considerable complexity. One of the most interesting profiles observed involves an initial rapid escape of sap from a single cut which ceases after several minutes, then recommences and builds up prior to a second slow decline; this has been called the "rebleed" phenomenon. At least two mechanisms must be postulated to explain this profile satisfactorily.Massage pre-treatment seems to enhance potential exudation quite locally; even half an internode may be treated with little influence on adjacent tissue. The induced build up in potential exudation gradually increased over several days and after suspension of massage declined at a comparable rate. Massage during exudation can be used to stop flow but exudation may recommence after a period, resembling the "rebleed" exudation profile.

An apparatus has been constructed for the detection of vibrations generated within plant tissue. This can detect vibrations due to water cavitation within fern sporangia. Using this detector 'clicks' have been obtained from Ricinus petioles and leaves in a condition when, from previous work, water cavitation might be expected.Evidence is provided to show: 1. That 'click' production is correlated with the water status of the plant tissue. 2. There are reasons for opposing the possibility that 'clicks' result from tissue fracture alone. 3. The 'clicks' may be detected in a wide range of plants. The implications of the apparent vulnerability of water conducting systems in plants are discussed.

Acoustic detection has been used to investigate the incidence of cavitation in whole potted Ricinus plants subjected to water stress by withholding water. Cavitation proceeded rather slowly and was detectable before and during wilting. Techniques which restricted water uptake more drastically such as root cooling or overlapping cuts induced more rapid "click" production and wilting; a response already described for excised leaves. When water stress was removed by rewatering, or rewarming a cooled root system, cavitation soon ceased. This response was more sluggish of over-delayed.Cavitation in aging leaves on well watered plants has also been examined. Despite the onset of senescence over many days there was no evidence that dry patches, which often develop extensively, are a consequence of water shortage induced by xylem blockage. Leaves, falling naturally by abscission in still air, were often remarkably turgid with water potentials similar to those of healthy attached leaves. Only after losing water was cavitation apparent, as usual for excised mature leaves. Sometimes more persistent leaves did cavitate in situ, just before abscission, showing that in normal leaves xylem blockage can occasionally precede leaf fall by several hours.

Experiments are described in which the uptake of water by leaves exposed to severe water stress (80-95% R. W. C.) was found to deviate markedly from those of leaves with smaller water deficits. In its most extreme form the deviation appears as a curious increase in rate of water absorption some time after uptake has commenced. It could not be detected during the absorption of water by leaf discs suffering comparable severe water deficits.It is suggested that the differences are caused by cavitation in the xylem conducting channels during wilting. On restoring a water supply the reverse process takes place causing unusual patterns of water uptake.Since restoration takes place at 1° C it appears that metabolism is not involved; the process is physical in character.