Endogenous cytokinin levels and growth responses to extended photoperiods for creeping bentgrass under heat stress

Summer bentgrass decline is associated with inhibition of plant growth and cytokinin production. This study was designed to investigate whether shoot and root growth and cytokinin production for creeping bentgrass (Agrostis stolonifera L.) exposed to heat stress would be improved by extending photoperiod. 'Penncross' plants were initially grown in growth chambers at 20/15°C (day/night) and at a 14-h photoperiod for 60 d, and then exposed to 33/28°C (heat stress) with three different photoperiods: 14 (control), 18, and 22 h (extended photoperiod) daily for 32 d. Root number, fresh weight, viability, turf quality, and shoot extension rate declined during heat stress at the 14-h photoperiod. The decline was delayed and suppressed by extending photoperiod from 14 to 18 or 22 h. Root growth, turf quality, and shoot extension rate increased with extending photoperiod. Cytokinin content in roots and leaves also decreased during heat stress at the 14-h photoperiod. Plants at the 22- and 18-h photoperiods maintained higher cytokinin content in roots at 16 and 32 d of heat stress. The differences in total cytokinin content in leaves among photoperiod treatments were mainly due to the variations in trans-zeatin [6-(4-hydroxy-3-methylbut-2-enylamino) purine], zeatin riboside {6-[(E)-4-hydroxy-3-methylbut-2-enylamino]-9-β-D-ribofuranosylpurine} and isopentenyl adenosine [iPA, 6-(3,3-dimethylallylamino)-9-β-D-ribofuranosylpurine]. Cytokinin content in leaves at the 14- and 18-h photoperiods decreased at 16 d of stress, but at the 22-h photoperiod it did not decrease until 32 d. These results demonstrated that extended photoperiod increased root and shoot growth and endogenous cytokinin levels under heat stress, suggesting that extended photoperiod could alleviate heat injury in creeping bentgrass.