Circadian regulation of grapevine root and shoot growth and their modulation by photoperiod and temperature

Some plant species demonstrate a pronounced 24 h rhythm in fine root growth but the endogenous and exogenous factors that regulate these diel cycles are unclear. Photoperiod and temperature are known to interact with diel patterns in shoot growth but it is uncertain how these environmental factors are interrelated with below-ground growth. In this particular study, the fine root system of two grapevine species was monitored over a period of ten days with a high resolution scanner, under constant soil moisture and three different photoperiod regimes. Pronounced diel rhythms in shoot and root growth rates were apparent under a fixed 14 h photoperiod. Maximal root growth rate occurred 1-2 h prior to- and until 2 h after the onset of darkness. Subsequently, during the latter part of the dark period, root growth rate decreased and reached minimal values at the onset of the light period. Relative to 22 °C, exposure to a 30 °C air and soil temperature halved root growth but stimulated shoot growth. Notably, the shoot extension rate peak shifted from late afternoon to midnight at this higher temperature zone. When plants were exposed to a delayed photoperiod or progressively shortening photoperiod, the diel changes in root growth rate followed the same pattern as in the fixed photoperiod, regardless of whether the plant was in light or dark. This suggests that light was not the predominant trigger for stimulating root elongation. Conversely, shoot growth rates were not fixed to a clock, with minimum growth consistently at the completion of the dark period regardless of the time of day. In summary, fine root growth of grapevines was found to have a pronounced diel pattern and an endogenous circadian clock appears to orchestrate this rhythm. Soil temperature modified the amplitude of this pattern, but we argue here that, as evidenced from exhausted starch reserves within root tips by early morning, carbon supply from photosynthesis is also required to maintain maximum root growth.