Diurnal and seasonal variation in root xylem embolism in neotropical savanna woody species: impact on stomatal control of plant water status

Blackwell Publishing
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Journal Article
Plant Cell and Environment, 2006, 29 (1), pp. 26 - 35
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ulnerability to water-stress-induced embolism and variation in the degree of native embolism were measured in lateral roots of four co-occurring neotropical savanna tree species. Root embolism varied diurnally and seasonally. Late in the dry season, loss of root xylem conductivity reached 80% in the afternoon when root water potential (?root) was about -2.6 MPa, and recovered to 2540% loss of conductivity in the morning when ?root was about -1.0 MPa. Daily variation in ?root decreased, and root xylem vulnerability and capacitance increased with rooting depth. However, all species experienced seasonal minimum ?root close to complete hydraulic failure independent of their rooting depth or resistance to embolism. Predawn ?root was lower than ?soil when ?soil was relatively high (>-0.7 MPa) but became less negative than ?soil later in the dry season, consistent with a transition from a disequilibrium between plant and soil ? induced by nocturnal transpiration to one induced by hydraulic redistribution of water from deeper soil layers. Shallow longitudinal root incisions external to the xylem prevented reversal of embolism overnight, suggesting that root mechanical integrity was necessary for recovery, consistent with the hypothesis that if embolism is a function of tension, refilling may be a function of internal pressure imbalances. All species shared a common relationship in which maximum daily stomatal conductance declined linearly with increasing afternoon loss of root conductivity over the course of the dry season. Daily embolism and refilling in roots is a common occurrence and thus may be an inherent component of a hydraulic signaling mechanism enabling stomata to maintain the integrity of the hydraulic pipeline in long-lived structures such as stems.
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