ROOTS ARE IMPORTANT TOO: ROOT PHYSIOLOGY AND FUNCTION IN THE ORCHARD

Kalcsits, L

lee.kalcsits@wsu.edu

Department of Horticulture, Washington State University, Wenatchee, USA Tree Fruit Research and Extension Center, Wenatchee, USA

KEYWORDS Rootstocks, nutrient uptake, drought tolerance, root tips, root architecture, soil temperature, soil pH, soil water content. ABSTRACT Root health affects orchard performance, yield potential and quality. The most recognizable effect that roots have on growth of fruit trees is from using dwarfing rootstocks that promote precosity, reduced shoot vigor and limit overall tree size. This habit allows for more efficient management of the canopy and increased planting densities. The presence of dwarfing rootstocks have allowed the development of modern, trellised orchard systems to support high fruit production. However, some dwarfing rootstocks have slow or limited root growth potential. The impact of this limited root growth on water and nutrient uptake and resistance to soil-borne pathogens is relatively unknown. During planting and establishment of new orchards, this can be a challenge, particularly in poor soil environments where water and nutrients are being supplied almost exclusively through drip or microsprinkler irrigation and root vigor is restricted. Poor root establishment and slow root growth can limit canopy establishment in early years and increase the susceptibility of roots to periodic environmental stress because of the small, shallow root systems. When an orchard is first planted, early growth depends on root establishment to provide the water and nutrients required for the canopy to fill the space in the orchard. Early growth and development is critical to the productive success of an orchard. There are two primary types of roots, foraging or fibrous roots and structural roots that are responsible for forming the root architecture that fibrous roots branch off of and forage in the soil. Pioneer roots are less affected by short-term fluctuations in environment. Fibrous root systems regularly die and regenerate in trees during the course of a season or from one season to the next.

Roots are responsible for water and nutrient uptake from the soil. Water and nutrients can follow two pathways of movement through the root into the xylem to be transported to the above-ground parts of the tree, either between the cells (apoplastic pathway) or through the cells (sympastic pathway) (Figure 1). Apoplastic movement requires less within cell transport and is therefore a more rapid path to uptake of water and nutrients to the xylem. In roots, fine white root tips are responsible for most of the water and nutrient uptake from the soil (Figure 2). As root tips mature, a waxy layer forms in the hypodermis and endodermis of roots that limits water and nutrient uptake. Therefore, to have good water and nutrient uptake, there needs to be constant generation of white root tips. Soil interacts with roots and soil chemical and physical characteristics can affect root growth, managing orchard soil to be well drained, uniform and with a balanced pH are the first steps to promote healthy root growth.

Root growth starts early in the spring prior to bud flush as soil temperatures warm. This provides early water uptake to supply the developing flowers and leaves. Early season nutrient requirements are largely met by stored nutrients in the roots and wood. As the growing season progresses and the energy sink of the plant switches from growing roots and stems to developing fruit, root growth decreases. High soil temperatures (above 25°C or 80°F) can be a significant source of stress for roots. For dwarfing rootstocks with shallow, small root systems, the impact of temperature is greater. Increased stress may cause root mortality and/or decreased root tip growth leading to a reduced capacity to take up water and nutrients from the soil. Sources of stress may include disease, drought, heat, pH, salinity, and flooding among others.

Figure 1. Uptake pathway of water in roots

Figure 2: Diagram of a root tip. Source: biology.tutorvista.com

Unlike aboveground growth, roots do not become dormant and will continue growing as long as conditions remain favorable and there is an energy reserve that can be used. Therefore, root growth can continue long after the upper part of the tree has gone dormant and can scavenge nutrients later in the season to store for growth and development the following season. Under conditions that lead to root death during the summer, the autumn can act as a period to catch up and re-establish an active root system for the following season. Maintaining an environment that promotes root growth throughout the season and limits below ground stress ensures that the trees can access water and nutrients at times during the season when the demand for those resources is high. As planting densities continue to increase and technology allows for more precise control of orchard systems, it is critical to manage the timing of nutrient and water uptake.

ROOTS ARE IMPORTANT TOO! Root Function and Physiology in the Orchard

Lee Kalcsits, Assistant Professor

Department of Horticulture

Washington State University

Tree Fruit Research and Extension Center

Wenatchee, WA

What do roots do?

• Anchorage• With dwarfing trees, it is important to use trellis, not just because the wood is weak but the root

system is also small.

• Water• Main avenue for water uptake. Even a plant that is growing in wet soil can show symptoms of

water stress

• Nutrition• Main source of nutrient uptake for plants• Some nutrient

• Energy and Nutrient Storage• Roots are a critical place for energy and nutrient storage during winter

• Ecology• Roots support a community of microorganisms that are both good and bad and are important for

Root Systems

Irrigation System

Soil Texture

Rootstock

Water Table

Nutrition

Root Systems

Fibrous Roots

Pioneer Root

Root Hairs

Root Anatomy

bio.miami.edu sciencehelpdesk.com

Water and Nutrient Uptake

Schematic representation of nutrient acquisition and internal cycling by trees.

Millard P , and Grelet G Tree Physiol 2010;30:1083-1095

Root-Soil Interface

Nutrient Availability in Soil

Ecology

Conclusions

• Root systems are constantly changing throughout the season and between seasons

• Healthy roots are critical for water and nutrient uptake

• Roots influence seasonal storage of nutrients and energy for following season

• Soil environment affects the growth and function of roots