Partitioning plant hydraulic conductances is challenging with respect to organs such as roots and leaves in which water moves though both the xylem and nonvascular pathways. To date, the environmental factors and plant traits that determine the rate and direction of water flow are better understood for the xylem than for external tissues such as the leaf mesophyll or root cortex. Prime candidates for plant regulation of nonvascular hydraulic conductance are aquaporins (AQPs), protein channels in the plasma membrane and tonoplast. In leaves, AQPs are involved in turgor changes of guard cells and can affect leaf hydraulic conductance more directly, as recently indicated by light-induced increases in AQP transcripts corresponding with increases in hydraulic conductance. Other studies link leaf and root hydraulic conductance to mechanical gating of AQPs by abrupt changes in pressure, suggesting an even more direct connection between flow in the xylem and in surrounding nonvascular tissues. However, the relevance of AQP mediation to the overall movement of water into and through a root system is still in question and may vary according to environmental conditions and to plant growth habit and developmental stage.

Results/Conclusions

Hydraulic conductance for young, actively growing roots often appears to be regulated by AQPs, as shown by experiments involving gene knockouts or various binding agents and metabolic inhibitors. The evidence for AQP involvement in water uptake by older roots is more equivocal, because the absence of a response to AQP inhibitors may be due to physical barriers in the radial pathway such as suberized and lignified cell walls. Experiments involving inhibitors and partial removal or disruption of external root tissues in monocotyledonous succulents and chaparral shrubs suggest that AQPs are much more important in regulating hydraulic conductance for young roots and roots with living cortical cells than they are for woody roots. Thus, the responses of AQPs to environmental factors such as drying and rewetting can determine hydraulic adjustments made by young and actively growing roots. In contrast, the apoplastic pathway may predominate in nonvascular tissues of older roots, rendering AQPs less important as roots and root systems age.