Understanding Tlirfgrass — 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 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 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 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 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 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 . 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 and tillers and rhizomes or 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. If roots are injured tissue types. or inhibited, shoot growth ceases making its photosynthetic products available to regen- inhibits shoot growth, greatly reducing the erate roots and reestablish the supply of use of photosynthetic products by the water and nutrients for the shoot. shoot. If shoots continued to grow when roots This makes more photosynthate avail- were hurting, there would be little energy able to support root growth which occurs available to restore the roots, the shoots not in the drought stressed shallow roots would eventually suffer from lack of water but rather the deep roots that have ade- or nutrients and the plant likely would die. quate water and can respond to additional Abscisic acid (ABA) is an inhibiting hor- supplies of energy. mone. It normally retards growth, induces In this way, the first roots to experience dormancy in buds, causes stomates to close drought stress use ABA as a signal molecule and generally induces a plant to become informing the plant to reduce its water use inactive. ABA synthesis can occur in any and allocate more of its energy to growing plant organ and is often promoted by stress deep roots. This permits plants to respond conditions. to drying conditions in a timely manner and The action of ABA allows a plant to con- alter their growth pattern so as to avoid gen- serve its resources during stress, making eral drought injury. them available for growth to resume when Understanding this plant response conditions become favorable. The role of mechanism has permitted turf managers to ABA in root function is well illustrated by schedule irrigation for optimum water use the phenomenon of early drought response efficiency. (Davies and Zhang, 1991). When a plant begins to experience a lack Getting optimum root growth of water this is first perceived by the shal- from turfgrass low roots. These roots become stressed for Understanding turfgrass roots, their struc- water and begin to synthesize ABA which is ture and function, can constitute informa- transported to the shoots. This occurs tion useful to the turf manager for growing before the shoots have suffered any loss of more vigorous and stress tolerant grass. It water potential since deeper roots are pro- might be helpful to link management prac- viding all the water required by the plant. tices with the basic requirements for good However, the delivery of ABA to the root growth. shoot causes stomates to close in the leaves 1. Roots need space. To respond to envi- dramatically reducing transpiration and ronmental signals and prepare for stressful slowing the rate of water loss. ABA also conditions, a turfgrass root system needs a sufficiently large soil volume so it can adjust the greatest height consistent with the use its growth pattern and better utilize avail- requirements of turf is a good guide. able resources. Implicit with this require- Increased mowing height during the hottest ment is a growing medium that is con- summer months will partly compensate for ducive to root growth. In short, it must be increased photorespiration. well aerated, of suitable structure, have the Selecting grasses with leaf angles that capacity to hold an abundance of available will retain more leaf surface even if subject- water and be lacking in toxic substances. ed to close mowing will also help maintain A reasonable cation exchange capacity favorable energy relations. Limit durations to insure nutrient retention would also be of deep shade. Cool shade is attractive and good. Thus, a good blend of sand and silt refreshing during hot summer days but turf along with 4-6% organic matter, a pH of 5.5 needs light. to 6.5 and favorable particle structure to Long durations of full sunlight are not permit gas exchange should be provided. essential and may even be harmful if excess A well designed sand/peat green should heating and drought occur. Thin to supply these conditions but a growth medi- provide moderate or broken shade. This um based on native soil may need to be will help the trees and improve air circula- amended to become optimum. tion while insuring sufficient light energy The root zone must be sufficiently large for the turf. to accommodate stress induced changes in 3. Roots compete for energy. During the root system architecture. The depth should heat of summer, the turf manager should be be such (1 to 1.5 ft.) that drought stimulat- aware that roots are not the only sink for the ed downward root extension can occur. If photosynthetic product of leaves. Actually, these soil conditions change over time, roots of cool-season turfgrasses are normal- management practices should insure they ly not very strong sinks during the hot are brought back to optimum levels. months often receiving less than 5% of total Aeration will help maintain good soil photosynthate (Hull, 1996). structure, insure good aeration and incorpo- If shoot growth is stimulated during rate organic matter to greater depths. It summer, the roots experience even greater might be good to remember that root competition for available energy and their turnover is the best source of soil organic growth decreases and mortality likely will matter. increase. Nitrogen fertilization should be Thus, conditions that favor deep root avoided during the hottest summer months. growth will also help maintain a favorable Its metabolism will consume energy and it organic content and a suitable water hold- will invariably stimulate shoot growth that ing capacity. Scheduling irrigation so as to will compete with roots for limited avail- permit turf to suffer mild drought stress able energy. before supplying water will encourage deep If turf needs a green-up, try an iron appli- rooting. cation with a litde nitrogen. That will stim- 2. Roots need energy. Roots are not pho- ulate chlorophyll synthesis and might even tosynthetic. Therefore, they depend improve the plant's energy status. absolutely on photosynthetic energy cap- 4. Heat is the enemy of roots. Heat is espe- tured by the shoots. It is good to consider cially hard on roots of cool-season turfgrass- that the amount of photosynthetic energy es (Huang, 2000). If soil temperatures available to a plant depends on its leaf sur- exceed 70 -75° F, roots decline rapidly. This face, the duration of light and, in cool-sea- is probably due to less available energy son grasses, the extent by which production translocated from the shoots but whatever is decreased by heat stimulated photorespi- the cause, root heating should be avoided. ration. Again, a higher mowing height will pro- Sound turf management should encour- vide more turf mass to insulate the soil bet- age photosynthetic production and mini- ter. Irrigate during the evening or at night. mize whatever detracts from it. Mowing at That will cool the soil and allow time for energy transport to roots, slow root respira- efforts involving the installation of fans can tion and permit some recovery. Partial improve air circulation and this will shade during the heat of afternoon will also increase transpiration and cool the grass. reduce the heat load on turf. There are many ways turf management Maintaining adequate water so transpi- practices can be modified to favor good root ration can cool the grass is critical during the growth and reduce summer decline. If a hot season. Equipping putting greens with manager understands the structure and forced internal ventilation has the potential function of the turf root system, many of keeping turf roots several degrees cooler. desirable cultural modifications that would Equipment designed to draw excess water encourage good root conditions will through the turf by applying a subsurface become evident. This article is submitted vacuum can be useful for providing root with that hope. system ventilation. With some modifica- tion, the system can be run in reverse and — The author is Professor of Plant Science at force cooled air up through the turf. the University of Rhode Island, Kingston. He can be reached at: 401/874-2791. This technology offers considerable promise for easing the management of greens in hot locations. Less ambitious

REFERENCES

Davies, W.J. and J. Zhang. 1991. Root signals and McCully, M.E. 1999. Roots in soil: unearthing the the regulation of growth and development of complexities of roots and their rhizospheres. Annual plants in drying soil. Annual Rev. Plant Physiol. Rev. Plant Physiol. Plant Mol. Biol. 50:695-718. Plant Mol. Biol. 42:55-76. Schreiber, L.; K. Hartmann; M. Skrabs and J. Zeier. Huang, Bingru. 2000. Summer decline of cool- 1999. Apoplastic barriers in roots: chemical compo- season turfgrasses: heat stress and cultural man- sition of endodermal and hypodermal cell walls. agement. TurfGrass Trends 9(6): 1 -5. Jour. Experimental 50:1267-1280.

Hull, R.J. and H. Liu. 1995. Nutrient uptake: Some Sheffer, K.M.; J.H. Dunn and D.D. Minner. 1987. turfgrasses do it better than others. TurfGrass Trends Summer drought response and rooting depth of 4(6):7-13 three cool-season turfgrasses. HortScience 22:296-297. Hull, R.J. 1996. Managing turf for maximum root growth. TurfGrass Trends 5(2): 1-9. Sifers, S.J.; J.B. Beard and J.M. DiPaola. 1985. Spring root decline (SRD): discovery, description and Hull, R.J. 1999a. Water: how turfgrasses obtain and causes. Proceedings Fifth Internat. Turfgrass use moisture. TurfGrass Trends 8(8): 1-5. Research Conf., Avigon, France, July 1-5,1985. Hull, R.J. 1999b. Summer decline: can cool-season pages 777-788, F. Lemaire (ed.) Inter. Turfgrass Soc. turfgrasses take the heat? TurfGrass Trends 8(10): 1-7. & Institut National de la Recherche Agronomique, Paris. Hull, R.J. 2000. Understanding the turfgrass crown. TurfGrass Trends 9(5): 1-9. Stuckey, I.H. 1941. Seasonal growth of grass roots. Amer. Jour. Botany. 28:486-491. Marschner, H. 1995. Mineral Nutrition of Higher Plants, second edition. Academic Press, San Diego, Sullivan, W.M.; Z. Jiang and R.J. Hull. 2000. Root CA. 889 pages. morphology and its relationship with nitrate uptake in Kentucky bluegrass. Crop Science 40:765-772. Maurel, C. 1997. Aquaporins and water perme- ability of plant membranes. Ann. Rev Plant Physiol. Turgeon, A.J. 1999. Turfgrass Management, Fifth Plant Mol. Biol. 48:399-429. Edition. Prentice Hall, Upper Saddle River, NJ BUILD EXPECTATIONS

By Curt Harler turns out to have unexpected talent as a mechanic. Or the quiet one who didn't say he recent uproar six words all summer turns out to be a whiz over the "unex- handling the job-scheduler on the comput- pectedly" high er. By next season he's in the front office test scores since Cali- working with PCs. fornia banned bilin- It's just a reminder that we should give gual education is not everyone a chance to shine. There likely is about speaking Span- hidden talent on your team. But it requires ish or Vietnamese in taking time to find out just what a person's the classroom. The lesson here - and it aspirations are. Many workers, especially applies to turfgrass managers and the rest of newer ones, are simply too shy to speak up. us -- is about expectations. Seek all of your employees out informally, For those of you not familiar, in June perhaps on a lunch break, and find out what In Future Issues 1998 California voters overwhelmingly their goals are. Some will have none. But passed Proposition 227 which banned bilin- others will surprise you. • The value of clip- gual education in the state's schools. Every- The second factor at work is acceptance. pings one from President Clinton to local school No one - student, summer help, supervisor • Primo update teachers predicted disaster for non-English - likes to be branded as different. It is both speaking students. distracting and humiliating. Whether in • Surfactant applica- The children surprised everyone. The school or on the golf course, it interferes tions opposite happened. Their math scores went with getting the job done. • Phosphorus runoff up 14 points. Reading (in English) scores Expect the most from people. True, update jumped nine points. What happened? you'll occasionally be disappointed; but for Two things were at work here. First, it the most part you'll be pleasantly surprised. • Snow mold seems that those in charge had very low And your workers will be delighted. • Grasses we love- expectations of a group of normal kids. Part 2 We've all seen it happen - the guy who was always raking leaves in the background

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