Spatial and temporal variations in phloem sap composition of plantation-grown Eucalyptus globulus

Abstract

Spontaneous bleeding of sugar-rich sap from cambial-deep incisions in the bark of trunks was demonstrated for Eucalyptus globulus and other eucalypts across a range of localities and seasonal conditions in south-west Australia. High levels of sucrose and raffinose (up to 31% w/v total sugars) were present in the exudates, and upward and downward gradients in exudate sugar concentrations were recorded between samples obtained at different heights up trunks of E. globulus. The data indicated a phloem origin for the exudates, with source:sink pressure gradients driving translocation. Concentration ratios of sugars to amino acids were consistently lower in exudate from upper (distal) than basal regions of trunks, suggesting preferential partitioning of nitrogen upwards towards the trunk apex. A comparison of phloem and xylem sap composition from one plantation over a season showed nitrate in xylem but not phloem and substantial amounts of sodium, and high concentrations of chloride and sulphate relative to phosphate in xylem and phloem. Phloem sap sampled across a range of 29 contrasting plantations of E. globulus at peak stress (autumn) showed great inter-site variability in concentrations of amino acids, sulphur, sodium and certain trace elements and in C:N and Na:K ratios of sap. Carbon isotope ratios (δC) were strongly correlated with sugar concentrations of the sap samples from these and other plantations. Use of sap compositional attributes of phloem and δC values of translocated carbon is suggested for assessing the current nutritional condition and water status of E. globulus plantings.

Water Relations Link Carbon and Oxygen Isotope Discrimination to Phloem Sap Sugar Concentration inEucalyptus globulus

“…The strong relationship between phloem sap sugar ⌬ 13 C and phloem sap sugar concentration previously observed by Pate et al (1998) featured prominently in the present data set ( Fig. 2A).…”

“…Pate et al (1998) Pate et al (1998) data were derived from bulked phloem sap samples collected from 37 plantations distributed across southwestern Australia, such that each datum represented one plantation. Therefore, the present data set serves to confirm on an individual tree basis what was found previously on a plantation basis.…”

“…This pattern was also reflected in the bleeding behavior of the trees, with trees that had less negative water potentials bleeding more profusely than those with more negative water potentials. In their earlier E. globulus sampling efforts, Pate et al (1998) remarked, "Failure to bleed was rare but encountered occasionally when severely water stressed plantations were sampled during very hot afternoons of late summer and autumn. Even then, the same trees produced sap when sampled after recovery of water stress the following evening.…”

“…In a companion paper, Pate et al (1998) reported a strong relationship between phloem sap sugar ⌬ 13 C and phloem sap sugar concentration in E. globulus. According to the pressure flow hypothesis of phloem translocation (Mü nch, 1930), photosynthate is distributed from source to sink regions within a plant via gradients in turgor within sieve tubes generated by the loading and unloading of sugars.…”

“…growing in southwestern Australia (Pate and Arthur, 1998); variation occurred between rain-fed plantations experiencing drought stress and irrigated plantations, and seasonally within rain-fed plantations in correspondence with seasonal rainfall patterns. Based on the data provided by Pate and Arthur (1998), phloem sap sugar ⌬ 13 C appeared to integrate drought stress more directly, and over more physiologically relevant timescales, than did whole-tissue ⌬ 13 C. An additional advantage was the relative ease of analyzing phloem sap, which was so dominated by photosynthetic sugars that it did not require further extraction, as would be the case in the analysis of leaf soluble sugars or starch.…”