Flow velocity in xylem and phloem during the daytime course
The applied technique allows us to quantify simultaneously the mass flow of water in the xylem as well as in the phloem in the hypocotyl over a daily time course with good time resolution. Due to the demands of the NMR equipment and the limited space therein, the plants used in this technique must be small. On the other hand, the spatial resolution of NMR imaging is low in comparison with light microscopy and the size of the single image pixel (for imaging, nominal ‘in-plane’ resolution = 31 μm × 31 μm, or for flow measurements, a minimum 47 μm × 188 μm in-plane) is in the range of thickness of the phloem.
Sap was transported in both phloem and xylem systems, throughout the day. However, there was only a pronounced effect of light in the xylem. Significantly higher velocities were measured in xylem in the light than in the dark. Additionally, there was a strong cross-correlation between the light regime and the xylem velocity or flow-time series. In the light, velocity was increased by a factor of about 1·6 and correspondingly, the volume flow was increased. These changes corresponded to changes in transpiration – not surprising as larger loss of water from leaves must be compensated by a larger supply via the xylem. Huber (1956), using an early heat-tracing method, found larger water-flow velocities during the day than during the night. In a comparison of modern heat-tracer methods, similar results were obtained with differences in absolute values depending upon the system of measurement (Köstner et al. 1996). In the present investigations there were consistent discrepancies between relative flow in xylem and transpiration. Transpiration was more than doubled in the light compared with an increase of 1·6 for xylem parameters.
We determined no clear trend in transport parameters for the phloem with variation in the daily light regime. Flow velocities were more or less constant and only in experiments 1 and 3 were there significant cross-correlation between light and phloem velocity. These correlations had a large delay (73–76 min) but a low numerical factor. The apparent volume flow was increased in the dark in four of the six experiments but the reverse effect was observed in the remaining two experiments. Poor spatial resolution may account for these variations. Time-series analysis suggested no significant cross-correlation and it is concluded that there is – surprisingly – no effect of light on the transport velocity and relative flow in phloem over a daily light/dark cycle.
The primary literature and a number of botanical compendia list a wide range of velocities of flow in xylem and phloem. Strasburger (1983) suggested that xylem flows of about 0·30 mm s−1 in conifers and 17 mm s−1 in deciduous trees might be less than those found in vines (maximum at noon) of 40 mm s−1. For the phloem, values are typically less and between 0·14 mm s−1 and 0·28 mm s−1. Mohr & Schopfer (1985) and Marschner (1995) both mentioned a range of 0·05–280 mm s−1 for both xylem and phloem. Thus while it is widely accepted that the velocity in the phloem is less than in the xylem, there is considerable variation. Köckenberger et al. (1997) applied an NMR technique to very young Ricinus seedlings (6 d old) and found that the average velocity in the phloem (0·58 mm s−1) was somewhat higher than that (0·47 mm s−1) in the xylem. However, these experiments with plants in the early stage of development are of limited value as the plants had no transpiring leaves and endosperm was still the major source of carbohydrates and nutrients for growth. This may explain the high rates of flow in the phloem.
The present data for the phloem demonstrate that transport in the phloem is possible without concomitant photosynthetic activity. Sharkey & Pate (1976) found earlier that phloem bleeding varied little over a daily cycle. According to the pressure flow theory of Münch (1930), the difference in osmotic potential between source and sink tissues generates flow in the phloem that is sustained by loading and unloading of the phloem; processes which must also function in the dark (Eschrich & Eschrich 1989). During the light period, photo-assimilates were stored in the form of starch, whereas the concentration of soluble carbohydrates remained more or less constant (see Fig. 7). In the dark, starch was continuously degraded resulting in a reduced, but still steady, concentration of soluble carbohydrate in the leaves. This process presumably maintained the loading of the phloem in the dark. Interestingly, concentrations of soluble sugars in the leaves corresponded to those in the phloem. The sugar export from leaves may be directly related to the current concentration of sugars in leaves (Sharkey & Pate 1976). On the basis of recent experiments the following three compounds represent the major solutes in the phloem sap of Ricinus: sucrose 73%, potassium 13%, and amino acids 9%. Organic acids, are present in the form of malate 2% and others are present only in traces. In the present experiment potassium was not affected, but sucrose and amino acids decreased in the night. Therefore, it is very likely that the osmopotential of the phloem decreased in the dark. Additionally the availability of water for phloem loading was higher at night, as the transpiration was reduced. These results support the idea that leaves grow at night (Prantl 1874; Schmundt et al. 1998). In conclusion the lower concentration of solutes in the phloem must be compensated for by higher availability of water for phloem loading in the dark to maintain the same flow in the phloem over the whole day.
The lack of variation in rates of phloem transport was surprising in view of the major role of photosynthesis in producing soluble sugars. This transport behaviour is possibly related to the structure of the phloem. Constant transport will maintain pressure within the phloem, a condition required to maintain phloem structure as there is no supporting tissue. A further putative argument for night-time transport is that the phloem transport system may be unable to cope with the amounts of carbohydrate synthesized during the day.
The major feature of xylem transport was the delay in response to changes in light intensity. Wegner & Zimmermann (1998) and Thürmer et al. (1999) showed that changes in xylem pressure were only observed several minutes after changing illumination but we know of few other studies that may shed light on our observations.
Effects of changes in mass flow due to the light regime on solute transport in xylem and phloem
On the basis of relative volume flow and the concentration of solutes in xylem and phloem, a rough estimation of relative transport over a daily time course can be made. Our results suggest considerable daily fluctuations in concentrations of solutes in phloem and xylem. These results are in general agreement with earlier investigations for phloem (in Lupinus, Sharkey & Pate 1976; Hocking et al. 1978), xylem (Schurr & Schulze 1995), as well as for the chemical composition of the leaves (Sharkey & Pate 1976).
During the night, changes in relative volume flow in the phloem (increased by a factor of 1·14) were compensated for by lower solute concentrations (reduced by a factor of 0·80). The combination of both observations result in the outcome that solute transport in the phloem is more or less constant over the whole day. In contrast the relative flow in the xylem was reduced in the dark (by about 30%) and generally the solute concentrations increased. These effects will again help to ‘balance’ solute transport over a 24 h period. In general, and in both transport systems, changes in relative flow were compensated for by changes in solute concentrations. Nonetheless, there were major variations in composition of the solute load. For example, nitrate (and potassium as a counter-ion) transport in the xylem was far slower during the night than during the day. We can conclude that loading of solutes into transport systems has a larger impact on fluctuations in daily transport than the effects of flow. This conclusion must be viewed in the context of the hypothesis that transport and distribution of elements are independent of transpiration (Shaner & Boyer 1976; Tanner & Beevers 1990).
The macro-element composition of leaves was not affected by xylem import or phloem export. Only the concentrations of starch and soluble carbohydrates were correlated to daily transport.