Season-Dependent Fruit Loading: Effect on Dry Mass,Water, and Nitrogen Allocation in Tomato Plants

Tomato plants were grown hydroponically in greenhouse under common commercial cultivation practice during two seasons: winter, with lower temperatures (LT) and summer, with higher temperatures of (HT), and results were compared. The effect of season on fruit load was drastic and the hypothesis was advanced that nitrogen (N) homeostasis at the whole plant level might be significantly affected, as measured by the extractable N of the stem. During LT, dry-mass accumulation occurred more or less at the same rate in all plant parts. The high fruit load at HT altered the picture, with dry-mass accumulation present at different rates in the various plant organs. Low temperatures positively affected root weight, which was significantly higher; influenced root morphology; and negatively affected fruit load. There were significant differences between the two seasons at the time of flowering, fruit setting, and fruit maturation in terms of the number of leaves and inflorescences on the main stem and in the rate of their appearance. In summer, water content of leaves and roots was lower, but not that of stem. Also, water content of roots increased considerably increase in the last five weeks, which coincided with the temperature and fruit-load decrease at the end of the season. Nitrogen concentration of leaves was higher than that of roots throughout HT, while extractable N concentration increased significantly at the middle part of the stem, where it bore the main fruit load. This effect was more profound when more fruits were ripening. Low temperature conditions were characterized by a significant decrease of root water content, while extractable N allocation was not significantly affected and root total N was higher.