LEAF DEFORMATION AND FRUIT PRODUCTION OF GLASSHOUSE GROWN TOMATO (LYCOPERSICON-ESCULENTUM MILL) AS AFFECTED BY CO-2 PLANT-DENSITY AND PRUNING

During summer, glasshouse grown tomato plants (Lycopersicon esculentum Mill.) often demonstrate leaf deformation, reduced leaf area (short leaves) and low Specific Leaf Area (SLA), sometimes accompanied by higher dry matter content of leaves and stems and higher leaf starch content. This so-called "Short Leaves Syndrome" (SLS), which decreases the production capacity, was investigated with emphasis on the effects of CO2 concentration. As a working hypothesis it was postulated that SLS is indirectly caused by an oversupply of assimilates relative to the sink capacity. An experiment was conducted between 10 May and 31 July 1990 in 12 glasshouse compartments. The sink/source ratio was varied by maintaining two levels of CO2, multifactorially combined with two plant densities and three pruning treatments. CO2 enrichment and wider planting enhanced SLS and decreased leaf area and SLA of upper leaves. Leaf pruning and fruit pruning, however, did not give clear effects on vegetative characteristics, although the impact on the sink/source ratio was of the same order of magnitude. As a mechanism for these effects, we suggest that SLS is caused by calcium deficiency in the apex, a condition more severe when much phloem sap (with low calcium content) is available, i.e. when the sink/source ratio is lower. Stronger effects of CO2 and plant density than of pruning on the incidence of SLS, may be due to local effects of sink/source relationships or to involvement of other processes, like transpiration. In crops with little SLS-symptoms, CO2 enrichment increased the weight of fruits grown during the treatment period by 31%, whereas in crops with severe SLS, CO2 enrichment aggravated SLS and had no significant effect on fruit production. CO2 enrichment in summer is beneficial if SLS is prevented, which can be achieved by maintaining a higher plant density or, in an early crop, an extra shoot on the plants in spring and summer.