Gibberellin (GA) is believed to be involved in thermoperiodic stem elongation. With this in mind, we studied the correlation between gibberellin A1 (GA1) levels and stem elongation affected by alternating day (DT) and night temperature (NT) in 5 genotypes of Pisum sativum differing in their degree of dwarfism. The endogenous GA content in the tissue of two of the genotypes was determined by combined gas chromatography and mass spectrometry. The wild genotype developed 40 to 50% shorter stems and internodes under a low DT and high NT combination (negative difference [DIF] between DT and NT, DT/NT 15.5/21.5 or 14/24°C) than under the opposite regime of high DT and low NT (positive DIF, DT/NT 22.5/16.5 or 24/14°C). The GA biosynthetic mutants ls and le, and the auxin and brassinosteroid mutant lkb responded in a similar way, but not as strongly as the wild type. The stem length of the GA‐insensitive slender mutant (la crys) was reduced by only 8% under negative compared to positive DIF. In the wild type endogenous GA levels decreased by 60% from positive to negative DIF in the upper part of the stem. Further, there was a corresponding decrease in the levels of precursors to GA1, i.e. GA53, GA44, GA19 and GA20, while 2β‐hydroxylated GA20 and GA1, GA29 and GA8, respectively, were unaffected by DIF. A similar increase in the ratios of GA29 to GA20 and GA8 to GA1 from positive to negative DIF was seen in the stem tissue of the le mutant as in the wild type. The temperature regimes affected the levels of GA1 and its precursors in combined leaf and petiole samples and in the shoot tip in a similar manner as in the stem tissue. However, the different temperature regimes did not affect the ratio of GA8/GA1 in the shoot tip. The results indicate that altered stem elongation of the pea plants in response to diurnal temperature alternations may be mediated by changes in endogenous levels of GA1. The GA1 levels may be controlled by an effect of DIF on both biosynthetic and inactivation steps.

The application of gibberellins (GA) reduces the difference in stem elongation observed under a low day (DT) and high night temperature (NT) combination (negative DIF) compared with the opposite regime, a high DT/low NT (positive DIF). The aim of this work was to investigate possible thermoperiodic effects on GA metabolism and tissue sensitivity to GA by comparing the response to exogenously applied GA (in particular, GA 1 and GA 3 ) in pea plants (Pisum sativum cv. Torsdag) grown under contrasting DIF. Control plants not treated with growth inhibitors or additional GA were 38% shorter under negative (DT/NT 13/21°C) than positive DIF (DT/NT 21/13°C) because of shorter internodes. Additional GA 1 or GA 3 decreased the difference between positive and negative DIF. In pea plants dwarfed with paclobutrazol, which inhibits GA biosynthesis at an early step, the response to GA 1 was reduced more strongly by negative compared with positive DIF than the response to GA 3 . The induced stem elongation by GA 19 and GA 20 did not deviate significantly from the response to GA 1 . Plants treated with prohexadionecalcium, an inhibitor of both the production and the inactivation of GA 1 , grew equally tall under the two temperature regimes in response to both GA 1 and GA 3 . We hypothesize that the reduced response to GA 1 compared with GA 3 in paclobutrazol-treated plants grown under negative DIF is caused by a higher rate of 2␤-hydroxylation of GA 1 into GA 8 under negative than positive DIF. This contributes to lower levels of GA 1 and consequently shorter stems and internodes in pea plants grown under negative than positive DIF. Differences in tissue sensitivity to GA alone cannot account for this specific thermoperiodic effect on stem elongation. Key Words. DIF-Gibberellin-Inactivation-Pea-R e s p o n s e -S t e m e l o n g a t i o n -T e m p e r a t u r eThermoperiodismDaily alterations in day (DT) and night temperature (NT) affect stem and internode elongation in long day plants as well as in short day plants Heins 1995, Myster and. In general, plants grow tall with long internodes when the DT is higher than the NT, whereas a short phenotype with short internodes is developed under a DT lower than the NT. These thermoperiodic responses have been utilized for control of plant height in commercial plant production in climatecontrolled conditions (Bakken and Flønes 1995, Erwin et al. 1989, Moe 1994. The two opposite temperature combinations have been referred to as temperature regimes with a positive (positive DIF, high DT/low NT) and negative difference between DT and NT (negative DIF, low DT/high NT), respectively.Many reports (Ihlebekk et al. 1995, Moe 1990, Pinthus and Meiri 1979, Tangerås 1979, Zieslin and Tsujita 1988 have suggested that the effects of daily temperature alternations on stem elongation are related to the metabolism and sensitivity to gibberellin (GA), a plant hormone required for stem elongation in plants (Graebe 1987). These studies have examined mainly the responses to external applications of GAs in...

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