1. Regulation of Ion Uptake and Transport:
- Rootstocks can control the uptake, transport, and compartmentalization of ions, such as sodium (Na+) and chloride (Cl-).
- Salt-tolerant rootstocks restrict Na+ uptake or compartmentalize it in root tissues, preventing excessive accumulation in the scion.
- They can also enhance the uptake of beneficial ions like potassium (K+) and calcium (Ca+), maintaining ionic balance and cellular functions in the scion.
2. Water Relations and Osmotic Adjustment:
- Rootstocks can influence the scion's ability to maintain water uptake and minimize water loss under salinity stress.
- Some rootstocks enhance the development of deeper and more extensive root systems, allowing the scion to access water from deeper soil layers.
- Rootstocks can also assist in osmotic adjustment by accumulating compatible solutes, such as proline and glycine betaine, in the scion, which helps maintain cell turgor and cellular functions.
3. Hormonal Signaling:
- Rootstocks can modify the hormonal balance in the grafted plant, impacting the scion's response to salt stress.
- Certain rootstocks alter the production or sensitivity to hormones like abscisic acid (ABA), which plays a role in stomatal closure and water relations under stress conditions.
4. Nutrient Acquisition and Utilization:
- Rootstocks can affect the scion's ability to acquire and utilize essential nutrients.
- Some rootstocks enhance nutrient uptake efficiency, particularly micronutrients, which may be limited under saline conditions.
- Efficient nutrient acquisition supports overall plant growth and resistance to salt stress.
5. Antioxidant Defense Systems:
- Rootstocks can stimulate the scion's antioxidant defense systems, enabling the plant to mitigate the damaging effects of reactive oxygen species (ROS) produced under salt stress.
- Enhanced antioxidant activity helps protect cellular components and maintains cellular integrity.
6. Genetic Traits and Stress-Responsive Genes:
- Rootstocks may possess specific genes or genetic variations that confer salt tolerance and can be transferred to the scion through grafting.
- These genetic traits can activate stress-responsive genes, leading to the production of protective proteins and enzymes that mitigate the adverse effects of salinity.
7. Rhizosphere Interactions and Microbiome:
- Rootstocks can modify the rhizosphere environment, influencing the composition and activity of beneficial microbial communities.
- Some rootstocks promote the colonization of salt-tolerant bacteria and fungi, which can assist in nutrient acquisition, stress tolerance, and disease resistance.
8. Rootstock-Scion Compatibility:
- The compatibility between the rootstock and scion is critical for successful grafting and stress tolerance.
- Compatible rootstock-scion combinations exhibit improved overall plant performance, including salt tolerance, due to efficient nutrient and water transport and reduced graft incompatibility issues.
It's important to note that the specific mechanisms of rootstock-mediated scion salinity tolerance can vary among different rootstock-scion combinations and environmental conditions. Understanding the physiological and molecular basis of rootstock effects helps in selecting appropriate rootstocks for grafting and improving crop performance in saline environments.
