Changing sink demand affects the area but not the specific activity of assimilate sources in cantaloupe (Cucumis melo L.)

To better understand source-sink interactions, this work focused on the inf luence of fruit number on leaf area and photosynthetic activity in cantalou pe. To this end, flowers were removed over 2 years on two Charentais cultiv ars to obtain single-fruit plants and plants with an unrestricted fruit loa d (which set two to five fruits and constituted control plants). At the who le plant scale, net photosynthesis was reduced by about 30 % under high fru it load. At the leaf scale, a submodel of stomatal conductance was fitted t o the data and was included in a rectangular hyperbola model of leaf photos ynthesis. Maximum leaf net photosynthesis averaged 14.83 mu mol CO2 m(-2) s (-1) at 1000 mu mol quanta m(-2) s(-1). Light use efficiency was not affect ed by fruit load and equalled 0.040 mol CO2 mol(-1) quanta. Leaf area of pl ants with unrestricted fruit load decreased after 24 days from pollination, while the leaf area of single-fruit plants was still increasing. The decre ase was due to production of fewer new leaves per day, whereas the number o f senescent leaves and the size of individual leaves were not affected by t he treatment. Under high fruit load, cultivar Galoubet developed a larger p rojected leaf area than cultivar Talma. Thus it is concluded that: (1) larg e cantaloupe fruits may divert a large amount of assimilates away from, and grow at the expense of, the canopy; and (2) photosynthesis of the canopy w as lowered because leaf area was reduced whereas photosynthetic rate of lea ves was not altered. (C) 1998 Annals of Botany Company.