Seasonal evolution of the quality of fresh glasshouse tomatoes under Mediterranean conditions, as affected by air vapour pressure deficit and plant fruit load

Changes in yield and quality of fresh tomatoes in response to air vapour pr essure deficit (VPD) and plant fruit load were studied under Mediterranean summer conditions. Plants thinned to three or six fruits per truss were gro wn in two compartments, one at a VPD below 1.5 kPa, the other without VPD c ontrol. The seasonal trend in fruit yield and quality was assessed from Apr il to September by weekly measurement of number. fresh weight and dry matte r content of harvested fruits, together with the occurrence of blossom-end- rot (BER) and cracking. On two occasions, in July and September, sugar and acid content was measured at three ripening stages. The seasonal decrease i n fresh yield was attenuated at low VPD? because of higher individual fruit fresh weight, especially at low fruit load. Low VPD decreased occurrence o f BER but like low fruit load, it increased fruit cracking. Fruit dry matte r content was lower at low VPD. but was unaffected by fruit load. Sugar con tent and the ratio of sugars:acids was increased at high VPD and low fruit load, with interactive effects depending on season and ripening stage. The influence of VPD on acid content differed with fruit load and also changed during ripening and between seasons. Results showed that water was the main limiting factor for growth of fruits picked in July; at this time, reducin g fruit load to promote mean fruit size had negative effects on BER and cra cking. Reducing VPD reduced BER but had a negative effect on cracking and d iluted both the dry matter and sugar content. For fruits harvested later in summer, these negative effects were attenuated because fruit growth was al so carbon limited. (C) 2000 Annals of Botany Company.