Phloem transport of tall tree and its diurnal variations

To understand diurnal variations in phloem transport in tall trees, ¹³C-CO₂ pulse labelling experiments were conducted on a 20.4 m tall Chamaecyparis obtusa tree. ¹³C-CO₂ labelling were carried out in the morning and afternoon at close dates in autumn of 2020 and 2022, and in the isotope composition of stem respired CO₂ was measured at four heights (17.0, 13.5, 6.23, 1.85 m) along the stem using a carbon stable isotope ratio analyzer (Picarro G2131-i). Small bark samples were taken to observe the anatomy of the phloem and measure the sieve tube radius on the labelled tree.

The rate of phloem transport at different positions along the stem differed from day to day. The phloem transport rate was inferred to be low or zero in sunny condition and high (0.15-0.24 m h^-1) in cloudy weather and at night. This is consistent with the fact that the sink-source hydrostatic pressure difference (driving force) was higher at night than on sunny days. Daytime and nighttime turgor was calculated using data on midday and predawn water potential, osmotic potential, and gravity potential from similarly sized trees from same site. Using the anatomy data, the maximum phloem velocity in the night was calculated as 0.28 (m h^-1).

Both approaches revealed that phloem transport of carbohydrates in tall cypress trees took five or six days to reach the roots after they were produced by leaf photosynthesis, and that the rate of phloem transport increased and decreased overtime during transportation depending on weather conditions and day-night alternation.