Contribution of phytohormones in alleviating the impact of sub-optimal temperature stress on grafted tomato

Increased tomato tolerance to sub-optimal temperature (T) through grafting onto wild tomato species that are cold-tolerant could extend the growing period in the field and unheated greenhouses and reduce energy costs in heated greenhouses. Phytohormones seem to be involved in the tolerance of tomato to sub-optimal T stress. Hence, the selection of rootstock/scion combinations with enhanced tolerance to sub-optimal T requires a better understanding of the root-to-shoot interactions with respect to hormonal transport and signalling. To attain this goal, six trials were conducted employing reciprocal grafting of standard tomato cultivars and mutants that are deficient in the biosynthesis/catabolism of abscisic acid (ABA), cytokinin (CK) or salicylic acid (SA) or low-sensitive to ethylene (ET) or auxin (IAA). The exposure of tomato to sub-optimal day/night T (17/14 °C) decreased drastically the rates of shoot elongation and leaf area expansion in all trials in comparison with optimal day/night T levels (22/18 °C). With respect to the genetic combination of rootstock and scion genotypes, it was found that ABA produced in both plant parts exerts a protective role to tomato shoot growth under sub-optimal T stress, while the contribution of ABA was only partially reflected by leaf ABA levels. The use of an ET-insensitive genotype either as rootstock or as scion improved also sub-optimal T tolerance. Sufficient IAA sensitivity in tomato shoot seems to be positively related to shoot elongation rates under sub-optimal T conditions. The use of a mutant with enhanced CK catabolism either as rootstock or as scion restricted leaf expansion at optimal T but had no impact on leaf expansion under sub-optimal T conditions. Impaired SA biosynthesis in the shoot was associated with a higher susceptibility to sub-optimal T in terms of shoot elongation whereas leaf expansion was similarly affected by SA biosynthesis at both T regimes. In conclusion, ABA seems to contribute to enhanced tomato tolerance to sub-optimal T and to indirect control of endogenous ET level while IAA and SA in the shoot promote shoot elongation in tomato plants grown under sub-optimal T conditions.