First evidence for hydraulic fragmentation in an herbaceous aridland perennial: Cryptantha flava

Background/Question/Methods

Hydraulic fragmentation, the preferential transport of resources from particular roots to a specific sector of the shoot, is thought to confer several advantages: (i) lowering whole-plant mortality risk by localizing stress, avoiding disease spread , and runaway embolisms (ii) increasing the overall plant efficiency of resource uptake, and (iii) ensuring costly vital functions such as reproduction, by preventing the sharing of resources between efficient and inefficient foraging units.

Although common in arid ecosystems, hydraulic fragmentation has only been recognized in woody species.  Here, evidence is presented for hydraulic fragmentation in the herbaceous aridland perennial Cryptantha flava (Boraginaceae).  Individuals consists of 1 to >100 leaf rosettes that connect via a caudex to a single taproot.  Lateral roots branch from the taproot at 10-30 cm depth.

Hydraulic pathways were tracked using dyes (acid fuchsin, FCF fast green, toluidine blue). An individual lateral root and/or the main taproot was excavated in situ, cut under water and the cut end immediately placed in a 25 mL container with a dye solution (0.5% v/w).  Additionally, a root injury experiment was implemented in which either one lateral root or the taproot was severed in order to examine effects on shoot wilting (12 juveniles, 24 adults).

Results/Conclusions

When dye was applied to a lateral root, the distribution of dye within the shoot supported hydraulic fragmentation.  Each dye traveled from a particular lateral root to a spatially aggregated group of leaf rosettes.  However, when dye was administered to the taproot, the entire individual wilted very rapidly, and no dye was observed in the shoot, probably due to induced cavitation.  Thus lateral roots seem to supply particular portions of the shoot while the taproot seems to supply the entire shoot.  Additionally, there is an ontogenetic effect in the degree of fragmentation.  Juveniles did not wilt when one lateral root was injured, suggesting compensating hydraulic integration, while in adults a portion of the shoot wilted and never recovered from a similar injury.  In order to assure that dye was not simply following the least hydraulically resistance pathway even though other pathways were possible, transpiration was impeded by saturating the surrounding environment of the dyed rosettes with water; then the plant was examined for the redistribution of dye.  After 24-h dye showed up in other rosettes in juveniles, and nowhere else in large adults.  These results, as well, support hydraulic fragmentation but demonstrate ontogenetic differences in the degree of fragmentation.