Effect of Radiation Quality and Relative Humidity on Intumescence Injury and Growth of Tomato Seedlings

Intumescence is a physiological disorder that affects some tomato ( Solanum lycopersicum ) cultivars grown in environments lacking ultraviolet radiation. Both far-red (FR) radiation and blue light have been shown to help mitigate this disorder. Thus, the objectives of this study were to characterize and compare intumescence injury and growth of various tomato cultivars propagated under different radiation qualities (Expt. 1) and to evaluate plant responses to the interactive effect of radiation quality and relative humidity (RH) (Expt. 2). Seedlings of six cultivars in Expt. 1 were grown under broad band white light (W), W and blue with (WBFR) or without (WB) FR radiation, and blue and red light with FR radiation (BRFR). Seedlings of four cultivars in Expt. 2 were grown under W or WBFR and a low (≈50%) or high (≈95%) RH. In both experiments, seedlings were grown under a daily light integral of ≈13 mol·m ‒2 ·d ‒1 (200 ± 4 μmol·m ‒2 ·s ‒1 for 18 h·d −1 ). FR radiation was provided using 20 ± 2 μmol·m −2 ·s −1 delivered throughout the entire photoperiod or at the end-of-day (EOD) in Expts. 1 or 2, respectively. Intumescence was generally suppressed when seedlings in Expt. 1 were grown under BRFR and WBFR, which also corresponded with the general response to stomatal conductance ( g s ). In contrast, seedlings grown under W had the highest incidence of intumescence, ranging from 23% to 69% across cultivars. The high blue photon flux (PF) ratio in WB was not effective at suppressing intumescence injury without FR radiation, although incidence and severity were lower compared with W. In Expt. 2, intumescence incidence was generally lower in seedlings grown under WBFR, and RH had small effects on intumescence. In both experiments, younger leaves were relatively less affected by intumescence, suggesting that a developmental factor is associated with the disorder. As expected, providing FR radiation resulted in a general increase in stem height across cultivars and in both experiments. The high RH provided in Expt. 2 also resulted in an increase in stem height. However, seedlings under low RH produced larger leaves, lower specific leaf area, and more shoot dry mass than those under high RH. Overall, our findings show that applying FR radiation helps suppress intumescence, but strategies are needed to minimize issues with excessive stem elongation.