Helianthus nighttime conductance and transpiration respond to soil water but not nutrient availability.

We investigated the response of Helianthus species nighttime conductance (g(night)) and transpiration (E(night)) to soil nutrient and water limitations in nine greenhouse studies. The studies primarily used wild Helianthus annuus, but also included a commercial and early domesticate of H. annuus and three additional wild species (Helianthus petiolaris Nutt., Helianthus deserticola Heiser, and Helianthus anomalus Blake). Well-watered plants of all species showed substantial g(night) (0.023-0.225 mol m(-2) s(-1)) and E(night) (0.29-2.46 mmol m(-2) s(-1)) measured as instantaneous gas exchange. Based on the potential for transpiration to increase mass flow of mobile nutrients to roots, we hypothesized that g(night) and E(night) would increase under limiting soil nutrients but found no evidence of responses in all six studies testing this. Based on known daytime responses to water limitation, we hypothesized that g(night) and E(night) would decrease when soil water availability was limited, and results from all four studies testing this supported our hypothesis. We also established that stomatal conductance at night was on average 5 times greater than cuticular conductance. Additionally, g(night) and E(night) varied nocturnally and across plant reproductive stages while remaining relatively constant as leaves aged. Our results further the ability to predict conditions under which nighttime water loss will be biologically significant and demonstrate that for Helianthus, g(night) can be regulated.