CARBON IMPORT INTO DEVELOPING OVULES OF PISUM-SATIVUM - THE ROLE OF THE WATER RELATIONS OF THE SEED COAT

We tested the hypothesis that the transport of carbon to developing pe a ovules is controlled by the water potential of the seed coat, in bot h the short-term (minutes to hours) and long-term (days). At 14 d afte r anthesis, when the embryo just fills the seed coat, the osmotic pres sure of seed coat apoplast solution was about 1 MPa (equivalent to 400 mOsmol kg-1). Transport of carbon into perfused attached seed coats a t this stage of development was monitored with radioactive carbon-11. After a small (50 mOsmol kg-1) increment in the osmotic pressure of th e bathing solution, transport of carbon increased abruptly, but after about 100 min it returned towards pretreatment values. Therefore, alth ough osmotic pressure in the sink apoplast initially affected carbon i mport, as expected from the Munch hypothesis, we concluded that it was not a factor able to control import. At the same time, seed coat cell turgor, measured with the pressure probe, initially decreased but res umed pretreatment values within about 20 min, implying that turgor-reg ulation in the sink maintained import despite the change of apoplastic concentration. In the longer term, from 4 to 33 d after anthesis, bot h the water potential and the inferred turgor of the cells of the seed coat approached zero so that the driving force for influx increased, although the rate of carbon influx was declining. Therefore, despite t he importance for carbon inflow of turgor in sieve tubes in the sink, the turgor of the entire tissue was not a factor which varied to contr ol import via any direct effect on pressure gradients in the sieve tub es.