Carbon and water balances for young fruits of platyopuntias

Questions relating to transpired versus retained water for fruits, the xyle m versus the phloem as water supplier to the fruits, and the importance of fruit photosynthesis for fruit dry mass gain were examined in the field for 6 species of platyopuntias (Nopalea cochenillifera, Opuntia ficus-indica, O, megacantha, O, robusta, O, streptacantha and O, undulata), cacti with fl attened stem segments (cladodes). For plants with fruits midway between flo ral bud appearance and fruit maturation, transpiration was greater at night for the cladodes, as expected for Crassulacean acid metabolism (CAM) plant s, but greater during the daytime for the fruits of all 6 species. Neverthe less, net CO2 uptake by fruits of these platyopuntias occurred predominantl y at night, as expected for CAM plants. The water potential of the young fr uits (average of - 0.41 MPa) was higher than that of the cladodes (average of -0.60 MPa), indicating that water entered the fruits via the phloem rath er than via the xylem. Solution entry into the fruits via the phloem suppli ed the water lost by transpiration and allowed for increases in fruit fresh mass (daily transpiration averaged 3.2-fold higher than daily water conten t increases), while the accumulating solutes were apparently polymerized to account for the higher water potentials of the fruits compared with the cl adodes, The phloem thus acts as the sole supplier of water and the main sup plier of dry mass (90%) to such young fruits of platyopuntias.