Written by Nadia A. Valverdi and Lee Kalcsits, WSU Tree Fruit Research & Extension Center, Wenatchee, WA. July 2020 leer en español
Like many tree fruits, apple trees in commercial orchards are a combination of two genetically different plants, which is why they are called composite plants. The belowground section is the rootstock, and the above-ground portion is called the scion. These two portions of the trees are selected based on different characteristics. The scion is the commercial cultivar which will be chosen due to consumers’ preferences and along with high yield, fruit quality, and disease resistance. The rootstock, on the other hand, is more often chosen based on vigor, precocity, soil disease resistance, nutrient uptake capacity, and rootstock-scion compatibility.
Mineral nutrients are essential for plant survival. Nutrients are acquired by the root system from the soil and carried to the scion through the transpiration stream in xylem vessels. Mineral nutrients differ in their mobility in both the soil and the plant, i.e. phosphorus is mobile in the plant but has low mobility in the soil. Firstly, nutrients need to be available in the soil solution for uptake to occur. Mainly, nutrients are acquired, along with water, by mass flow or diffusion. Nutrients acquisition from soil can also occur via direct contact with roots or symbiotic organisms like mycorrhizae (Table 1).
Once inside the root system, mineral nutrients are loaded into the transpiration stream in the xylem system of the plant, and allocated to plant transpiring organs, i.e. leaves and young fruits. Many mineral nutrients can be remobilized to different organs through the phloem system (Fig. 1). However, nutrients have different mobilities within the plant. For example, calcium is mobile in the soil but is considered mostly immobile in the plant because it can only be moved through the xylem system while not through the phloem. This means that once it reaches the transpiring organ, it cannot be remobilized through the phloem to other plant organs. This lack of mobility is thought to be one of the underlying causes of calcium-related disorders like bitter pit in apple or cork spot in pear.
Figure 1. Representation of plant vascular systems, xylem and phloem, and their flow direction.
Nutrient related disorders such as bitter pit and soggy breakdown in apples are the result of a mineral nutrients imbalance in fruit. The relationship between these disorders and mineral nutrients have been extensively described. For example, mineral nutrients such as potassium, magnesium, and nitrogen and their ratio compared to calcium concentrations in the fruit have long been used as indicators of susceptibility to physiological disorders, and that can frequently occur in some cultivars, like Honeycrisp apple.
Abiotic stress conditions are the primary cause of crop loss worldwide, adversely affecting plant growth and productivity. Soil conditions can also induce stress in the plants. These conditions can be a result of low or high soil moisture content, nutrient availability, soil pH, physical impediments such as caliche layers, soil organic matter content, among others. This environmental induced stress can lead to a wide range of responses from the plants, including morphological, physiological, biochemical, and molecular changes. Therefore, interactions between roots and the soil environment can be significant drives of long-term orchard performance. For this, developing rootstocks that perform well across a range of soil types and environmental conditions is essential.
Both the scion and rootstock can respond differently to abiotic stress conditions stimulated by different environmental factors. This is why a study was conducted at the Tree Fruit Research and Extension Center in Wenatchee to evaluate the responses of ‘Gala’ and ‘Honeycrisp’ apple trees grafted on four different rootstocks, G41, G890, M9-T337, and Bud-9. The experiment was a greenhouse-based, and the objective was to determine how root-based abiotic stress and rootstock selection affects nutrient uptake in apple trees. For all the eight combinations of scion/rootstock, three treatments were applied: a drought stress treatment where trees where irrigated at 50% of field capacity; a heat stress treatment where the root zone temperature of the trees was increased by approximately 5 °C, and a control treatment where trees were not stressed in either way. We found that scion influenced nitrogen distribution. ‘Gala’ trees accumulated more nitrogen in the roots whereas ‘Honeycrisp’ trees accumulated more nitrogen in the leaves. Rootstocks also affected the partitioning of nutrients between below- and above-ground tissues. G890 allocated more nutrients to the roots, Bud-9, more to the stems and M9-T337 allocated more to leaves (Table 2). Both elevated soil temperature and low water content reduced nutrient content and partitioning to the leaves, and more interestingly, elevated soil temperature affected nutrient ratios when scions were grafted onto G41 (Fig. 2).
These results are valuable for understanding how scions and rootstocks interact with the soil environment and also provides information on how nutrient management decisions may change with different soil abiotic stress conditions and rootstock-cultivar combinations.
Table 2. Nutrient partitioning (%)
Figure 2. Leaf nutrient ratios for ‘Gala’ and ‘Honeycrisp’ apple trees grafted on G41, G890, M9-T377, and Bud-9 rootstocks under abiotic stress treatments. Reprinted from Valverdi, N. A., Cheng, L., & Kalcsits, L. (2019). Apple Scion and Rootstock Contribute to Nutrient Uptake and Partitioning under Different Belowground Environments. Agronomy, 9(8), 415.