Seasonal uptake, storage, and remobilization of nitrogen (N) are of critical importance for plant growth. The use of N reserves for new growth in the spring is especially important for sweet cherry (Prunus avium L.), for which new shoot and fruit growth is concomitant and fruit development occurs during a relatively short bloom-to-ripening period. Sweet cherries grafted on precocious, dwarfing rootstocks such as the interspecific (P. cerasus × P. canescens) hybrids Gisela® 5 and 6 tend to produce large crops but smaller fruit when crop load is not balanced with adequate leaf area. Study objectives were to: 1) characterize natural N remobilization during fall and winter to canopy reproductive and vegetative meristems; 2) determine the effect of fall foliar urea applications on storage N levels in flowering spurs; 3) determine whether differential storage N levels influence spur leaf formation in spring; and 4) determine whether fall foliar urea applications affect the development of cold-hardiness. During fall, total N in leaves decreased by up to 51% [dry weight (DW)] and increased in canopy organs such as flower spurs by up to 27% (DW). The N concentration in flower spurs increased further in spring by up to 150% (DW). Fall foliar applications of urea increased storage N levels in flowering spurs (up to 40%), shoot tips (up to 20%), and bark (up to 29%). Premature defoliation decreased storage N in these tissues by up to 30%. Spur leaf size in the spring was associated with storage N levels; fall foliar urea treatments increased spur leaf area by up to 24%. Foliar urea applications increased flower spur N levels most when applied in late summer to early fall. Such applications also affected the development of cold acclimation in cherry shoots positively during fall; those treated with urea were up to 4.25 °C more cold-hardy than those on untreated trees.
Theoharis Ouzounis and Gregory A. Lang
Theoharis Ouzounis, Eva Rosenqvist and Carl-Otto Ottosen
With the expeditious development of optoelectronics, the light-emitting diode (LED) technology as supplementary light has shown great advancement in protected cultivation. One of the greatest challenges for the LED as alternative light source for greenhouses and closed environments is the diversity of the way experiments are conducted that often makes results difficult to compare. In this review, we aim to give an overview of the impacts of light spectra on plant physiology and on secondary metabolism in relation to greenhouse production. We indicate the possibility of a targeted use of LEDs to shape plants morphologically, increase the amount of protective metabolites to enhance food quality and taste, and potentially trigger defense mechanisms of plants. The outcome shows a direct transfer of knowledge obtained in controlled environments to greenhouses to be difficult, as the natural light will reduce the effects of specific spectra with species or cultivar-specific differences. To use the existing high-efficiency LED units in greenhouses might be both energy saving and beneficial to plants as they contain higher blue light portion than traditional high-pressure sodium (HPS) lamps, but the design of light modules for closed environment might need to be developed in terms of dynamic light level and spectral composition during the day to secure plants with desired quality with respect to growth, postharvest performance, and specific metabolites.