The Complex Interplay Between Shoots and Roots

AMHERST, Mass.–A recent post by Jason Lanier on behalf of the University of Massachusetts Turf Program explains the complex interplay between shoots (the green above-ground portion of the plant) and the root system. At the most basic level, each of these plant components has a crucial role to play:Shoots conduct photosynthesis, the conversion of atmospheric carbon to carbohydrates using energy from the sun. Roots obtain moisture and mineral nutrients from the soil. All parts of the plant conduct respiration, or respire, the process by which carbohydrate resources created by photosynthesis are consumed to release energy for biological processes. Root tissue, of course, is not exposed to sunlight and does not photosynthesize. Consequently, the root system is completely reliant on above-ground shoots for a sufficient supply of carbohydrates to support normal root growth and functioning. In turn, shoots depend on roots for a source of mineral nutrition and precious water. Plant scientists often use a metric called the shoot:root ratio (sometimes root:shoot ratio) to quantify the amount of shoot matter present in proportion to the corresponding extent of root mass. There are a few key considerations involved in maintaining a viable shoot:root relationship that can assist significantly in achieving turf management goals: The first is mowing height: particularly efficient photosynthetic functioning is required in managed grasses in comparison with other plants, because significant amounts of leaf tissue are regularly removed by mowing. Furthermore, the majority of turf areas are perennial systems and thus carbohydrate production and storage over seasons must also be accounted for. The amount of shoot tissue present dictates photosynthetic capacity and is therefore directly proportional to the amount of root mass and depth that can be maintained over time. Long story short, lower mowing heights result in less total leaf area which translates to a shallower and/or less extensive root system. This is a main reason why it is generally better to mow at the highest mowing height appropriate for the species present and the turf use. The second is fertility: Among all of the mineral nutrients required for plant growth, turfgrasses are most responsive to fertilizer nitrogen (N). Nitrogen is a required building block of proteins and amino acids, which are the basis for plant life. Tillering rates and turf density increase as N level increases, up to an optimum point. Beyond this point, shoots can increasingly monopolize valuable carbohydrates which are used to fuel excessive shoot growth that occurs at the expense of root growth. Also, root-inhibiting thatch may accumulate more quickly with high N fertility due to increased formation of lateral stems. For these reasons and also environmental concerns related to excessive nitrogen rates, modern recommendations call for reducing nitrogen input to the lowest possible level required to sustain the desired level of turf performance. Managers can exert some control within a turf system with cultural practices, but considerably less control is typically possible over environmental stress factors that impact shoot:root ratios. In summer, significant heat and drought are frequently the most active stresses for turf. High soil temperatures can lead to extensive root dieback, and this lost root tissue is not readily replaced because of generally unfavorable conditions and the fact that the plant must devote a substantial share of resources to stress resistance. Root systems compromised due to heat are less able to keep up with plant water use demands, and the effect is compounded by lower overall moisture availability during the summer months. Irrigation frequency, therefore, must increase if the goal is to prevent moisture stress and eventual drought dormancy. The most direct avenue to contend with summer root dieback is to build a strong root system before summer stress sets in, by taking all available steps to encourage vigorous rooting in the spring. Source: UMass Turf Extension