Effect and after-effect of water stress on the distribution of newly-fixed C-photoassimilate in micropropagated apple plants

The effect and after-effect of water stress on the distribution of photoassimilate, fixed at different times during and after water stress, were investigated in one-year-old micropropagated ‘Gala’ apple plants (Malus domestica Borkh.) by feeding mature leaves with ¹⁴CO₂. Plants grown in Hoagland's nutrition solution were subjected either to water stress at moderate intensity, induced by polyethylene glycol (PEG6000), for 15 days (prolonged water stress, PWS) or for 3 days, and then transferred to the solution without PEG for a recovery of 12 days (rewatering after water stress, RAWS), compared with plants under normal water conditions (CK). Three parameters were used to quantify the ¹⁴C-photoassimilate distribution: specific radioactivity (SRA), which gives a measure of the actual amount of photoassimilates obtained by organs; the proportion distribution value, which reflects the relative distribution pattern among different organs within a given plant; the distribution coefficient (K), which gives an indication of the competitive sink strength, respectively. The carbon fixation rate and export capacity of source leaves were significantly reduced 24 h after initiating water stress, while the strength of sink organs (K) was increased in roots but decreased in other organs (shoot apex, phloem, xylem) with different response velocities. For the first 5 days of water stress, the enhanced sink strength of fine roots in PWS plants compensated for the reduced carbon availability due to the decreased total carbon fixation. As a result, fine roots obtained the same or even greater amounts of ¹⁴C-photoassimilates (SRA) than those of CK plants, but the compensatory effect became insufficient as water stress continued. The rapid decrease in the distribution parameters of shoot apexes under water stress indicates that shoot apexes are highly sensitive to water stress. The shift of ¹⁴C-photoassimilates from above-ground to the roots, caused by the sensitivity of shoot apexes and the enhanced sink strength of roots under water stress, should be advantageous to maintaining root growth and tolerance to water stress. Upon rewatering, 5- and 3-day lags occurred for the carbon fixation rate and export capacity, respectively, in order to complete recovery to pre-stress levels after removing water stress. In contrast, the percentage distribution values and SRA of ¹⁴C-photoassimilates obtained by fine roots in RAWS plants did not return to the pre-stress levels, but instead remained at higher levels than those of CK plants during the rewatering stage. This greater investment of photoassimilate into the roots during the rewatering period might provide abundant carbon substrates and energy for the restoration of the metabolic activity of the roots. Apparently, there was also an after-effect of water stress on the other organ sink strengths, as revealed by the delayed recovery of SRA, the proportion distribution value and K in RAWS plants, depending on organ sensitivities to water stress. On the other hand, the percentage of ¹⁴C-photoassimilate distributed to the phloem declined linearly with the increased retention in the labeled leaf, confirming that the phloem was just a pathway for transporting photoassimilate, and the higher K revealed during water stress did not signify a stronger sink strength of water-stressed phloem.