Water relations of transplanted tree seedlings

Jonathan N. Harvey-Jones

Successful establishment of tree seedlings post-planting is largely a function of efficient delivery of water to roots, with conductivity to water at the root-soil interface being identified as a key limiting factor (Burdett, 1990). Hydrophilic compounds (hydrogels) applied to seedling roots could improve root-soil contact. A certain class of hydrogel (cross-linked polyacrylamide) is capable of low-cost delivery of water to roots at plant available tensions, while resisting drainage losses to the bulk soil. However, analysis of the literature reveals a history of conflicting evidence for the efficacy of these and other similar products in improving plant water status. It appears that just as the carbon lattice structure is able to resist tension from the soil, roots growing near the hydrated gel are unable to create a sufficient hydraulic gradient to extract water from the gel matrix. A novel method of applying hydrated masses direct to root surfaces was evolved and tested on a range of transplanted rainforest cabinet timber species. Many of the results for treated root systems were highly significant in terms of improved gas exchange, dry matter accumulation and leaf retention during the first few weeks after transplanting into the field. Observation of the nature of root growth responses to partial root systems in contact with hydrogel, led to a new hypothesis. Hydrogel water supplied to lateral roots appeared to assist the downward growth of untreated deep roots. Intra-root transfer of water from well-supplied roots to the rhizosphere of roots suffering deficit (hydraulic redistribution - HR) could be an important component of seedling establishment success. Transfer of water from roots in moist soil to distant roots in dry soil, and efflux into the rhizosphere, follows a gradient in water potential. The movement of water occurs mainly at night when stomata close and water potential gradients in the plant decline. HR has many potential benefits for all plants, including increased survival, enhanced transpiration capacity, and nutrient harvesting ability, but most work to date has been with mature shrub and tree specimens. Experiments were designed to test for the occurrence of HR in seedlings of a range of species, with differing tolerances to water deficit. Hydrogel was used as an experimental tool, delivering highly available water to a partial root system, while droughting the remaining roots. The HR process was deemed to consist of two stages, the first being water uptake from a wet soil zone, and transfer intra root to roots in dry or saline soil. When hydrogel was applied to the lateral roots of Grevillea robusta seedlings, growth of deep roots into saline sand (4dS m-1), was facilitated. The second stage of the HR process examined, was efflux of water from roots into dry soil. Both quantification of water efflux, and the pattern of efflux along root axes were addressed. Separation of wet from dry compartments in a split-root apparatus designed for a tap-rooted plant proved difficult, and reverse flow from roots was not detected. A hypothesis that the occurrence of reverse flow in a species may be related to its stress tolerance ability therefore remains to be tested. Loss of water from roots in substantial quantities goes against the logical expectation that plants should hold on to water hard-won from the soil. A review of resistance mechanisms in the normal upward water transport pathway revealed a number of insights into plant structures and processes, that could influence the pattern of water flow in the reverse pathway. No specialised structural feature was identified which might promote or prevent drainage from roots back to the soil when tension declines as stomata close. However, a new model of osmotically-driven efflux of water from roots was developed, closely following the principles of the Composite Transport model of water and solute transport across the cortex of roots (Steudle, 2000). Interestingly, a number of ways in which naturally occurring hydrophilic substances assist plant growth were identified from the literature. Mucigels exuded by roots facilitate root penetration, may influence water tension in the soil matrix, and provide a substrate for rhizosphere organisms in mutually beneficial relationships. A recently emerging line of research has described the ion mediated swelling and shrinkage of pectin-hydrogels in the pit membranes of xylem elements, acting as a variable flow restriction in the xylem. Further work is needed to ascertain whether HR is a significant factor in seedling establishment, and to develop hydrogel application techniques to assist the establishment of transplanted seedlings in a range of challenging environments.

Water relations of transplanted tree seedlings

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Water relations of transplanted tree seedlings

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