Growth‐induced water potentials may mobilize internal water for growth

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Abstract
Wphen there is no external source of water, plants can grow by mobilizing internal water from nongrowing tissues. We investigated how this internal water moves by measuring continuously and simultaneously the water potential (ψw) of soybean (Glycine max L. Merr.) seedlings in the upper, growing stem tissues and the lower, non‐growing stem tissues. When external water was available to the roots, the stems grew rapidly and the ψw of the growing tissue was continually below that of the nongrowing tissue and the medium around the roots. This indicated that a growth‐induced gradient in ψw favoured water movement from the external source to the growing cells. When the external source was removed, the ψw of the growing tissue remained constant for a time and the ψw of the nongrowing tissue decreased somewhat. Growth took place slowly as water was withdrawn from the nongrowing tissue but ψw gradients continued to favour water transport to the growing cells. On the other hand, if this internal source was removed by excision, growth ceased abruptly. In this case, the cell walls relaxed and the ψw of the growing tissue decreased by about 0.1 MPa instead of remaining constant. The ψw of the detached nongrowing tissues remained constant instead of decreasing. This indicates not only that water mobilization required attached nongrowing or slowly growing tissues but also that mobilization affected wall relaxation. Thus, ψw differences may mobilize internal water, may explain the continued growth of plants and plant parts removed from external sources of water, and may account for discrepancies in measurements of cell wall properties in growing tissues.