Understanding Tlirfgrass Roots — Part 2 While water uptake is critical, nutrient absorption, hormonal balance and proper treatment also play major roles in the health of turfgrass roots enter roots passively along with water. By Richard J. Hull The membrane transporters that couple the expenditure of energy with nutrient ion Editors' note: In Turfgrass Trends for October, Dr. Hull presented Part 1 of his Back to Basics uptake into root cells have a high affinity for information on turfgrass roots, which dis- nutrient ions and this permits the concen- cussed root development and function, growth tration of nutrients patterns, root structure, system operation and within the roots to water uptake. This article finishes his discus- attain levels much This mining the soil for sion. greater than that essential nutrients constitutes present in soil water. Nutrient absorption This mining the one of the primary roles The routes described above for water soil for essential played by plants in the uptake by roots are the ones that nutrient nutrients constitutes ions must also take. Since nutrients are dis- one of the primary overall drama of life. solved in soil water, they are carried along roles played by with water into the apoplasm of plant roots. plants in the overall The apoplastic barriers created by the endo- drama of life. All animal life depends on the dermis and exodermis also effectively block minerals concentrated within plants for the radial transport of nutrient ions into the their source of such nutrients. stele and throughout the plant. Nutrient ion absorption by roots is selec- Ions differ from water in that they carry tive. Those ions essential for plant growth a positive (cation) or negative (anion) (potassium, nitrate, magnesium, sulfate, charge and cannot cross the plasma mem- zinc, etc.) each enter root cells via their own brane into the symplast via aquaporins. membrane transporter. However, they can cross the plasma Other less useful or even toxic ions membrane and become concentrated with- (sodium, aluminum, selenium, etc.) are not in the symplasm via specific transporters accommodated by specific ion transporters energized by electrochemical gradients cre- and thus, do not enter roots readily. ated across the plasma membrane through Because some nutrient elements can be the consumption of ATP. toxic at concentrations only slightly higher The mechanisms of nutrient uptake by than what would be beneficial (boron, man- roots was described in an earlier article ganese, copper, etc.), their cellular levels (Hull, 1995) and will not be repeated here. must be tightly regulated. Specific ion It is only important to know that nutrient transporters are responsible for maintaining uptake requires the use of metabolic ener- optimum ion concentrations. gy by root cells and that nutrients do not Nutrient ions absorbed into the proto- ing to the soil through the apoplast. Thus, water and nutrients LEAVES absorbed by roots are usually retained and loss to the soil nor- mally occurs when cells die or sustain injury. The energy required by roots is delivered from the leaves in the form of simple sugars (sucrose mostly) via intercon- COLEOFTILE nected living tube-like cells called sieve tubes. These, along SEED with companion cells and other ADVENTITIOUS miscellaneous cells, make up the ROOTS phloem that normally is located in the angles between xylem poles (Fig. 2). PRIMARY ROOTS Sugars released from the sieve tubes move via the sym- plasm to all living cells of the root. Releases of organic sub- stances from root cells to the soil is normally a controlled process. Mucilage is released by root Fig. 1. Primary and adventitious roots developing from a turfgrass seedling. cap cells for root lubrication and cells in the absorptive region of roots will excrete organic solutes plasts of epidermal and cortical cells can be during times of nutrient (especially phos- transported via the symplast into cells of the phorus) deficiencies (Marschner, 1995). stele by means of cytoplasmic streaming, The influence of morphological charac- diffusion and the flow of water through teristics of turfgrass roots on nutrient roots powered by transpiration. uptake has received little attention. How- Once in the stele, nutrients ions are again ever, a recent study reported by our group transported across cell membranes into the (Sullivan et al., 2000) has demonstrated dif- apoplast. Here they are carried to the ferences in the weight, length, surface area shoots in large xylem vessels via transpira- and diameter of roots (Fig. 4) among six cul- tional flow eventually to be reabsorbed by tivars of Kentucky bluegrass. living cells of stems and leaves. Below-ground organs were segregated Transpiration, of course, only occurs dur- into fine roots, adventitious roots and rhi- ing the daylight hours so during the night, zomes (Fig. 5). Adventitious roots in this nutrient ions transported into xylem vessels case, were those emerging from upper of the roots are not carried to the shoots but nodes of the crowns and plant stems and rather become concentrated within the ves- comprised only a small portion of the total sels. root system all of which was technically This high solute concentration attracts adventitious, primary roots having been water into xylem vessels creating substantial removed earlier. root pressure that sometimes causes gutta- These root characteristics were correlat- tion droplets to form on leaf tips visible dur- ed with entire plant nitrate uptake. The six ing early morning. The suberized cell walls cultivars differed in their rates of nitrate of the endodermis prevents ions and water uptake and these differences were positive- concentrated in the xylem from back-flow- ly influenced by weight, total length and their response to stress conditions. Roots tNTERCaiUUM SPACE CASPABiAN BANE« PSRtCYCLE XYLÊM VÊSSSt and shoots mutually depend on the contri- butions of the other. Roots cannot grow or function without the energy provided by the photosynthetic production of the shoots. Also, shoot growth is limited by the supply of nutrients and water provided by the roots. It is important that the growth of roots PIASMODESMATA ^ and shoots be coordinated so that the growth of one is not always limited by the APOPtASTSC TRANSPORT performance of the other. If plant growth were controlled by such a balance of limit- EMDOD£ftMJ8 ing factors, one system, roots or shoots, would always be operating at full capacity Fig. 3 Diagram of root (longitudinal view) while the other was limited by inadequate shouring apoplastic and symplastic trans- resources delivered to it. port routes. This would make it difficult for the roots to grow and accumulate reserves if all their surface area of fibrous and adventitious energy were being expended to supply roots. Root diameter was negatively corre- water and nutrients to meet the demands of lated with nitrate uptake indicating that shoots. The same would hold for shoots if total nutrient absorption was mostly a func- their productivity was devoted exclusively tion of fine roots. to meet the needs of roots. Of course in Kentucky bluegrasses, most To coordinate the activities of these two of the root system is composed of fine roots. mutually dependent Nitrate absorption was negatively correlat- plant systems so ed with rhizome weight, length and surface each can grow and The action of ABA allows a area but the relationship was not simple. provide for its pre- There appeared to be an optimum amount sent and future plant to conserve its of rhizome production that resulted in the needs, plants have resources during stress, greatest nutrient uptake. evolved a hormonal This is due to the increased number of control system. making them available for new plants produced at rhizome nodes. There are many growth to resume when con- These plants generate fibrous root systems hormones involved that invade fresh soil and absorb nutrients. in growth coordina- ditions become favorable. However, excess rhizome growth consti- tion but for simplic- tutes a heavy demand for photosynthetic ity, I will concen- energy produced by the shoots and this trate on the two that appear to be most energy is drawn away from root growth involved: cytokinins and abscisic acid resulting in fewer roots and less nitrate (ABA). uptake. Cytokinins are derivatives of the purine Thus, the finest turfgrass roots appear to adenine and they promote cell division and be most responsible for nutrient recovery thereby growth. Cytokinins are synthesized from the soil and rhizomes make a positive in the roots and are carried in the xylem to contribution only to the extent that they the shoots where they counteract the add to the number of functioning roots. inhibiting effects of the hormone aukin. Auxin is made in the shoot apex and moves Hormonal balance down the stem inhibiting the growth of axi- Beside their obvious roles in water and lary buds causing the phenomenon kn6wn nutrient acquisition, roots also participate in as apical dominance. the coordinated growth of plants and in Cytokinins block the inhibitory action of auxin allowing lateral buds to grow and pro- duce branches. Because cytokinins are derived from the roots, it is the basal shoot buds that are most stimulated to grow. This is why grass stems rarely branch and tillers and rhizomes or stolons emerge from the basal buds of the crown (Hull, 2000). If root growth is inhibited by stressful soil conditions or roots are damaged by dis- ease or insects, the supply of cytokinins to the shoot is interrupted and shoot growth is Fig. 2. Grass root structure. inhibited. This is why a stem cutting rarely A. Longitudinal diagram through a young exhibits any growth until it has initiated root shounng developmental regions. adventitious roots. JB. Cross section of a grass root through This control of shoot growth works in the absorption (root hair) region shounng the plant's best interest.