Biomass accumulation and partitioning of tomato under protected cultivation in the humid tropics

Results of a 2-year structural analysis of indeterminate tomato (Lycopersicon esculentum Mill.) cultivated during different seasons under protected cultivation (ventilated greenhouses with PE-film roofs and PE-net walls) in the humid tropics of Central Thailand are presented. Under the prevailing high-radiation conditions (o 35.9 MJ m(-2) outside and 23.7 MJ m(-2) inside of the greenhouse), total dry-mass production was 2.6 g MJ(-1) and similar to regions at greater latitude with much lower global radiation. Plant density (2.1 plants m(-2) in single rows vs. 4.2 plants m(-2) in double rows) had no meaningful effect on biomass production and partitioning on a per-plant basis as well as on internode length and specific leaf area (SLA) indicating that availability of light had only limited effect on growth in closer stands. Although crop growth rate (CGR) was comparable to other studies (3-14 g dry mass m(-2) day(-1)), biomass partitioning into individual plant organs was not. The most striking difference to greenhouse tomato production at greater latitude was the low percentage (16-19%) of total biomass distributed to fruits. Crop responses to lack of sink strength resulting from poor fruit set were deformed leaves and accelerated growth of auxiliary shoots. When canopy density was increased by cultivating tomato with double stems, total fruit biomass per plant was significantly improved by ca. 13%. Within these plants, ca. 100% more biomass was partitioned into fruits of the primary stem than the secondary stem. Since leaf biomass and area did not vary significantly between individual stems, there was indication that secondary stems improved availability of assimilates which promoted biomass partitioning into fruits on primary stems. Besides marginally decreasing greenhouse air temperature through greater transpiration, high plant and stem density maximize assimilation and are, therefore, one measure to improve tomato fruit biomass under hot tropical conditions. The generally low amount of fruit biomass was also due to lack of pollination and, therefore, development of parthenocarpic fruits under high temperatures particularly during night. A practice to improve the latter includes application of growth regulators to improve enlargement of parthenocarpic fruits.