Effect of carbon dioxide enrichment and salinity on photosynthesis and yield in melon

Melon (Cucumis melo L. Cv Parnon) grown in rockwool culture in the greenhouse was CO2 enriched, for 5 h every morning, at 400, 800 and 1200 mu mol mol(-1) and trickle-irrigated with nutrient solutions amended with 0, 25 and 50 mM NaCl. High CO2 level increased fruit yield, the increase being greater in unsalinated plants than in salinated. With total shoot fresh weight, the increase was greater in salinated plants. CO2 enrichment also increased leaf growth and the chlorophyll content of the measured leaves. Addition of NaCl in the nutrient solution caused significant reduction in total yield, the reduction being greater at higher concentrations of CO2, At 25 mM NaCl, the decrease in yield resulted mainly from the smaller fruit size, but at 50 mM yield reduction was due both to smaller fruit size and to fewer fruits per plant. Addition of NaCl caused significant reduction in total shoot fresh weight in all cases, the reduction being greater at the lower level of CO2. Salinity also, significantly reduced leaf surface irrespective of CO2 level. Chlorophyll content was reduced by NaCl mainly at the level of 50 mM NaCl. A stronger correlation was found between salinity and shoot fresh weight, plant height and leaf surface area, than salinity and yield and other characteristics. Measurements of gas exchange showed that, for the above mentioned CO2 and NaCl concentrations, net assimilation was affected by CO2 to a greater degree than by salinity. Stomatal conductance was most affected by salinity at a concentration of 50 mM NaCl. (C) 1999 Elsevier Science B.V. All rights reserved.