Nighttime evaporative demand induces plasticity in leaf and root hydraulic traits

Increasing evidence suggests that nocturnal transpiration rate (TR_N) is a non-negligible contributor to global water cycles. Short-term variation in nocturnal vapor pressure deficit (VPD_N) has been suggested to be a key environmental variable influencing TR_N. However, the long-term effects of VPD_N on plant growth and development remain unknown, despite recent evidence documenting long-term effects of daytime VPD on plant anatomy, growth and productivity. Here we hypothesized that plant anatomical and functional traits influencing leaf and root hydraulics could be influenced by long-term exposure to VPD_N. A total of 23 leaf and root traits were examined on four wheat (Triticum aestivum) genotypes, which were subjected to two long-term (30 day long) growth experiments where daytime VPD and daytime/nighttime temperature regimes were kept identical, with variation only stemming from VPD_N, imposed at two levels (0.4 and 1.4 kPa). The VPD_N treatment did not influence phenology, leaf areas, dry weights, number of tillers or their dry weights, consistently with a drought and temperature-independent treatment. In contrast, vein densities, adaxial stomata densities, TR_N and cuticular TR, were strongly increased following exposure to high VPD_N. Simultaneously, whole-root system xylem sap exudation and seminal root endodermis thickness were decreased, hypothetically indicating a change in root hydraulic properties. Overall these results suggest that plants ‘sense’ and adapt to variations in VPD_N conditions over developmental scales by optimizing both leaf and root hydraulics.