Solar radiation threshold and growth of mini tomato plants in mild autumn/winter condition

The value 8.46 MJ m−2 d−1 is generally indicated as the radiation threshold for vegetable crops in tropical and subtropical climates. However, several studies indicate that mini tomato plants can be cultivated in greenhouse under lower solar radiation conditions. Therefore, the establishment of the actual radiation threshold for this group of tomatoes is necessary. The objective of this study was to define the relative influence of solar radiation and temperature on plant growth, and determine the lowest value of incoming global solar radiation inside the greenhouse by which there is a significant plant dry matter accumulation in the mild autumn/winter of southern Brazil. Mini tomato plants of hybrids Cherry Wanda® and Grape Dolcetto® were grown in NFT hydroponic system. Nineteen plant samples were collected periodically to obtain plant dry matter, leaf area, number of leaves, plant height and absolute and relative growth rates. The variables were related to the data of incoming global solar radiation and temperature inside the greenhouse. The results were submitted to analyses of variance, regression and clustering test. Cherry and Grape plants presented similar responses to the variation of solar radiation and temperature. However, Cherry plants presented greater leaf area and growth under these conditions. Temperature had an indirect influence on plant growth, while incoming solar radiation presented a direct influence on all evaluated variables. The solar radiation threshold for both cultivars of mini tomato was 3.63 MJ m−2 d−1.