Plant level response of seven herbaceous perennials to diurnal temperature cycling and sub-diurnal temperature variation

Climate warming has not affected day and night symmetrically: daily temperature range (DTR) is narrowing, as daily average temperature (DAT) increases. The potential impacts of combined DAT and DTR variation on plants are unknown. Four experiments were conducted to assess such impacts. In the first two, plants of seven herbaceous perennial taxa were exposed to 18 combinations of day and night temperature for 50+ days. Effects on total plant dry weight (DW) were analyzed with the aid of a thermodynamic model, modified to include two temperature dimensions. Results showed that the effects of temperature on DW cannot be accounted for by variation in either DAT or DTR alone, and that their magnitude is equivalent. The greatest effect of variation in DTR on DW was on plants growing closest to their optimal DAT. Time-to-event data analysis methods were used to determine the relative effects of DAT and DTR on anthesis and death in two taxa. Effects of day and night temperature could be separated for both events, but were almost entirely subsumed into DAT. Although effects of DTR were significant, they only became meaningful at the extremes. Results suggest changes in DTR are a lesser concern for anthesis and survival than for gross productivity. Two experiments were then conducted to assess the effect on DW of temperature variation within each of the two phases of the diurnal cycle. Plants of Delphinium x cultorum and Rudbeckia fulgida var. sullivantii L. were grown for 40 days under 28 temperature regimes all resulting in cycles of 12-h days/12-h nights at average either 15/15°C, or 25/15°C. There were 14 regimes for each combination, during which temperature varied between 5 and 35°C every 4 hours. Temperature varied during the light period in the first experiment, and the dark period in the second. Few significant differences in DW were found among regimes for either taxon, and they did not produce a consistent pattern. Results indicate that over season-long durations, effects of sub-diurnal variation on gross productivity are far less consequent than those of diurnal variation.