Path of water for root growth

Do roots obtain water for their growth directly from soil surrounding the growth zone or indirectly, via phloem, from water absorbed elsewhere? Wheat (Triticum aestivum L.) was studied with time-lapse imaging of seminal axile roots, growing in soil and air in a custom-made laboratory rhizotron, before and after excision. The growth data were combined with a theoretical estimate of the amount of water that could be supplied from the phloem. Roots readily extended into air, providing strong evidence that they obtain a portion of their growth-sustaining water internally. The time-lapse experiments indicated that in moist soil, internal sources provided 26-45% of the water for root growth, but the rest came externally from the soil surrounding the growth zone. From the theoretical analysis, the phloem could supply, on average, 64% of the total, accounting for all the internal sources. This indicates that phloem water could be used when root tips cannot access external water, such as in cracks or pores, or regions of dry soil. The distribution of phloem-delivered water for root growth should be considered in whole-plant modelling of root systems. Maximising phloem flux at root tips might confer more efficient use of soil water by crops.