Abstract:
Low-temperature stress is an important limiting factor affecting citrus growth and fruit
yields. Therefore, increasing citrus cold stress tolerance may enhance the growth, yield, and quality of
citrus production in marginal areas. The objective of this study was to determine the efficacy of silicon
(Si) fertilizer application on cold-tolerance enhancement in citrus. Two citrus cultivars (Delta and
Nules) were subjected to Si fertilization (control, 1000 mg L−1) and cold-stress temperature treatments
(control and 0 ◦C for 72 h) using a 2 × 2 × 2 factorial treatment structure with six replications. Leaf
gas exchange and chlorophyll fluorescence parameters, such as net photosynthetic rate (A), stomatal
conductance (gs), transpiration rate (Tr), internal CO2 concentration (Ci), intrinsic water-use efficiency
(iWUE), minimal fluorescence (Fo), maximum fluorescence (Fm), maximum quantum efficiency of
PSII primary photochemistry of dark-adapted leaves (Fv/Fm), maximum quantum efficiency of
PSII primary photochemistry of dark-/light-adapted leaves (F’v/F’m), electron transport rate (ETR),
non-photochemical quenching (NPQ), and the relative measure of electron transport to oxygen
molecules (ETR/A), were measured. The application of Si drenching to trees that were subsequently
exposed to cold stress reduced gs, Tr, and Ci but improved iWUE and Fo in both cultivars compared
to the Si-untreated trees. In addition, specific adaptation mechanisms were found in the two citrus
species; NPQ and ETR were improved in Si-treated Valencia trees, while A, Fm, and ETR/A were
improved in Clementine trees under chilling stress conditions. The current research findings indicate
the potential of Si application to enhance cold stress tolerance in citrus, which can provide a strategy
for growing citrus in arid and semi-arid regions that may experience cold stress. Overall, after the
application of silicon drenching, the cold-sensitive citrus Valencia cultivar became as cold-tolerant as
the cold-tolerant Clementine cultivar.