OREGON STATE UNIVERSITY
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Once the roots and initial leaf are established, additional leaves are produced from a compressed stem, like a telescope pressed to its shortest position. This unelongated stem is near the soil surface (crown) and is called a growing point. The location of the main growing point below grazing height is what enables grass to regrow many times throughout the season. This growing point will continue to produce leaves until conditions induce it to transition to the reproductive stage.
A grass leaf consists of a blade supported by a sheath. These two components are joined by connective tissue called the collar. The leaf emerges sheath first with the blade appearing later and continuing to grow.
Grass plants have two types of meristems (cells capable of dividing) in the growing point. The top one, the apical meristem, accounts for growth in the stem. The lower one, the intercalary meristem, produces leaf primordia. The leaf primordia eventually divides into two systems that develop leaf blades and leaf sheaths.
Growth of leaf blades begins with cells dividing but eventually is mainly from cell elongation. The pacing of leaf growth results in leaf tips being older than leaf sheaths. As a blade unrolls or unfolds (depending on the species) the blade flattens but the sheath remains rolled or folded, surrounded by older leaf sheaths. If a leaf blade is defoliated, the base and sheath may still be elongating and expanding. The collar includes an intercalary meristematic zone which accounts for blade growth. Growth continues in blade tissue until the sheath has completed its development. Thereafter, if the mature blade is severed there will be no regeneration. If the leaf is severed beneath the collar (through sheath tissue), there can be no regeneration of the blade due to the absence of the intercalary meristem essential for blade renewal.
New leaves emerge alternately on opposite sides of the plant. Successive leaves grow from a higher point on the crown so the sheath occurs at a higher position. The number of leaves on a stem tends to remain constant with new ones produced as older leaves die (senesce). Young leaves feed off the carbohydrate reserves in storage or in other leaves. Fully expanded leaves are the best energy producers during photosynthesis.
During the vegetative phase (prior to stem internode development), the intercalary meristem of most blades remains near the soil surface because of limited sheath elongation. Thus, only blade tissue is vulnerable to removal. As long as the blade meristem in the collar is preserved, regeneration of blade tissue will occur.
It is helpful to remember that most of the parts of a grass plant are compressed inside. The nodes and internodes (joints) of grass are there, leaf tissues are there. Immature blades are enclosed by the sheaths of the subtending leaves, forming a pseudo stem or whorl. Conditions and management determine which parts will be expressed.
The number of leaves formed during the vegetative stage varies with species and typically ranges from 5 to 7.
The grass leaf is comprised of two separate organs, a blade and a supporting sheath. Each leaf primordia arises from a layer of intercalary meristematic tissue in the shoot apex. Each layer of meristem subsequently divides into two zones, one at the base of the blade and one at the base of the sheath. Removal of the blade meristem (the collar) prevents blade renewal regardless of growth stage. If intensive defoliation occurs during the vegetative stage, when perhaps only the basal leaf sheath has elongated, leaf blade reduction will be minimal. However, if intensive defoliation occurs during the transition stage, when internode elongation has commenced, more leaf collars will be removed resulting in reduced leaf blade area and reduced photosynthesis. Further, this defoliation may also remove the shoot apex thereby completely interrupting shoot growth.
The old adage "take a half, leave a half" represents sound advice. But why? Because it ensures sufficient leaf area in the remaining sward to preserve blade meristems essential for blade renewal. But, knowing where the growing points are in your grass plants can help you provide optimal management. Defoliating above the growing point will allow for continued leaf formation which will maintain high rates of photosynthesis necessary for optimal production and allow for optimal yield of the pasture.
Sheath elongation results in elevated collars (blade meristems). This signals the need for special management precautions, because severe defoliation will destroy the collar which contains this critical regrowth meristem.
Below are a few examples of practical applications of the above material. There are others in the Management section of this project.
Situation
A mixture of tall grass prairie species was seeded in the spring of 1998 and burned in November of that year. The intent in 1999 was to allow the prairie species to produce a seed crop in order to thicken the stand. Foxtail (Setaria spp.) became an eyesore, so for cosmetic purposes, the prairie was mowed at early heading of the foxtails. Big bluestem and switchgrass had reached the transition stage (culm internode elongation had commenced), however, their rudimentary seed heads were still below the mowing height and apical meristems remained as part of the stubble. Little bluestem and sideoats grama were less advanced, probably in the early transition stage.
The outcome
The foxtail was not hampered by mowing because of the tendency for new tillers to arise from nodes near the base of the stem. Weed control measures thus failed. The flowering stems of big bluestem and switchgrass continued development, however, there were very few leaf blades on these culms. Mowing caught these grasses with considerable sheath elongaton so it was possible to sever the grass leaf below the meristematic collar zone. Many stems were entirely without leaf blades.
Discussion
Leaf blades severed below the collar make no further growth whether or not the blade has reached maximum size. With relatively immature blades, severing the blade above the collar zone results in varying degrees of continued blade elongation.
When mowing destroys leaf blades, but the shoot primordium (rudimentary seed head with associated undeveloped culm internodes) remains as part of the stubble, the recovery growth consists largely of naked culms. The grass is said to have been "denuded." Had the mower been set two inches lower, many of the culms would have been "decapitated" with no further opportunity for culm develoment because the above ground meristematic zone would have been destroyed.
In the late vegetative stage when sheath elongation results in exposure of several leaf collars, management precautions are in order because with intense grazing pressure it is possible to remove the exposed collars and associated blade meristems without removal of the shoot apex. When this occurs subsequent recovery growth will consist chiefly of bladeless flowering stems. These precautions are of greater inportance during the transition stage when this is problem is more likely to occur.