Hort 436 Energy Lecture

Review

Photosynthesis in a C3 turf

Two parts to photosynthesis reaction. A light reaction to capture energy and a dark reaction to fix CO₂.

Due to the affinity of the RUBISCO enzyme for O₂, photorespiration also takes place. Does not take place in C4 turf grasses.

Photosynthesis in a C4 turf

Chloroplasts in both bundle-sheath cells and mesophyll cells.

CO₂ concentration increases in bundle sheath cells with an increase in photosynthetic efficiency.

There is increased photosynthetic efficiency under conditions of high light and elevated temperature making C-4 plants more efficient in their use of water.

In low light or cool temperatures, C3 plants are more competitive.

Because more CO₂ passes through open stomata per unit time into C-4 leaves, less water is lost for each gram of CO₂ assimilated.

N use efficiency

An additional distinction between C-4 photosynthesis over the C-3 pathway is the efficiency by which plants use N.

Nitrogen use efficiency is expressed as the quantity of dry matter produced per unit of N present in the tissue.

A nutrient efficiency value can be derived from the N content of leaf tissue by the equation:

N use efficiency (mg dry matter/mg N) =

Photosynthetic N use efficiency is defined as the CO₂ fixed per unit of N in a unit leaf area or leaf mass.

Both expressions of nutrient use efficiency normally show C-4 grasses to be more efficient in their use of leaf N than C-3 grasses.

The basis for greater N efficiency in C-4 grasses is linked to a smaller allocation of leaf N to the CO₂ carboxylating enzyme, RuBP carboxylase.

In most C-3 plants, RuBP carboxylase is the most abundant protein comprising more than 50% of total leaf N.

By comparison, the leaves of C-4 plants allocate only 10 to 15% of their protein to RuBP carboxylase in bundle sheath cells.

The primary CO₂ trapping enzyme, PEP carboxylase, in mesophyll cells is almost as large as RuBP carboxylase but it constitutes only about 10% of leaf protein.

The quantity of N in "CO₂ assimilating enzymes" in C-4 plants is about one-half of what it is in C-3 plants.

The obvious advantages of C-4 photosynthesis for greater heat tolerance, reduced water use, and increased N use efficiency of turfgrasses has thus far prompted little research directed to breeding cool-season grasses that possess C-4 characteristics.

The potential for such genetic manipulation is limited because most turfgrass genera are either C-3 or C-4 with no intermediate species having been identified.

CARBOHYDRATE DYNAMICS

Nonstructural carbohydrates constitute the energy currency within turfgrass plants. ( think of it like your checkbook)

Growth, differentiation, and maintenance are purchased with carbohydrates obtained from current photosynthate or that banked in storage organs.

The total nonstructural carbohydrate (TNC) content of tissues is often used as an indicator of the physiological status of turfgrasses.

TNC is composed of glucose, sucrose, fructose and the polysaccharides fructan or starch.

Glucose

Fructose

Sucrose

Fructan

Conditions that favor rapid growth and metabolic activity result in a lower TNC level than conditions that tend to depress growth while permitting near-maximum photosynthetic rates.

The latter condition causes an accumulation of carbohydrates normally in the form of glucose or fructose polymers.

Grasses native and introduced to North America can be classified on the basis of the nonstructural polysaccharides that accumulate in their stem bases.

Grasses of a tropical or subtropical origin (C-4, warm-season grasses) accumulate glucose polymers (starch) or sucrose.

Temporate zone grasses (C-3, cool-season grasses) accumulate mostly fructose polymers (fructans) along with small amounts of starch and sucrose.

The degree of fructan polymerization varies between cool-season grass genera.

Fructans can be removed from perennial ryegrass and tall fescue tissues in 65% ethanol. This indicates the fructan's are generally short chained.

Kentucky bluegrass and redtop (Agrostis alba L.) contained more highly polymerized fructans that could be extracted only with dilute ethanol solutions or pure water.

These fractions contained long-chain fructan molecules consisting of about 260 fructose units while the fructans in ryegrass and tall fescue were short-chains consisting of 26 fructose molecules or less.

Diurnal TNC variation has been measured in Kentucky bluegrass leaves exhibiting a mid-day peak followed by a decline later in the day.

The TNC content of leaves paralleled changes in light energy but exhibited a 2-h lag. Other studies, in which grass leaves were analyzed during a constant light period, demonstrated an almost linear increase in TNC for up to 16 h.

During periods of darkness, TNC is mobilized and translocated from the leaf blades to sheath, crown, and rhizome tissues.

By the end of the dark period, leaf carbohydrate levels normally have returned to their concentration of the previous morning.

Consequently, grass leaves do not serve as permanent or long-term storage sites for TNC.

TNC found in leaf tissues are the result of temporary accumulation occurring when photosynthesis exceeds export.

The major TNC found in turfgrass shoots consists of the monsaccharides glucose and fructose, the disaccharide sucrose, various oligosaccharides short-chain fructans, starch, and long-chain fructans.

seasonal TNC accumulation from Narra et al., 2004

In most grasses, early photosynthetic product is partitioned between sucrose and starch during periods of light.

When sucrose accumulates in the cytosol of leaf mesophyll cells because synthesis exceeds transport from the cells, fructose 2,6-bisphosphate is synthesized from fructose-6-phosphate. This blocks further sucrose synthesis, causing triose phosphates to accumulate in the chloroplasts and become channelled toward the synthesis of starch

In warm-season turfgrasses, this dichotomy in carbon partitioning between starch and sucrose appears to be fairly simple with starch accumulating when metabolic sinks become weakened due to inhibited growth or when photosynthate production exceeds demand

However, because C-4 grasses are highly efficient in exporting photosynthate from leaves, starch rarely accumulates in leaves but concentrates mostly in storage organs.

The fructose 2,6-bisphosphate control site would inhibit further sucrose synthesis, shunting triose phosphates to starch within the chloroplasts.

The resulting accumulation of large starch grains within the chloroplasts causes internal shading or membrane distortion that will reduce the CER and lower the efficiency of photosynthetic energy conversion.

To accommodate this condition, most cool-season grasses have developed a mechanism for TNC storage within leaf cells.

Turfgrasses have been shown to increase their TNC content when subjected to certain stresses.

An increase in nonstructural carbohydrates in stem and leaf bases of Kentucky bluegrass was observed when air or soil temperatures were reduced to 10°C.

Growth was probably inhibited more than CO₂ fixation and fructans accumulated.

An increase in the water-soluble carbohydrate (fructan) fraction in creeping bentgrass leaf tissue was observed from plants grown at high temperatures (30-40 °C). It probably is safe to assume that fructans contributed to the TNC increase.

Respiration rates increased with temperature but photosynthesis, measured as 0₂ evolution, increased more. Consequently, the carbon needed for a TNC increase was available and fructan accumulation may have provided some protection against high-temperature injury.

Mobilization of fructans in stem bases can be induced by defoliation.

A sharp decrease in the fructan concentration in stem bases of orchardgrass (Dactylis glomerata L.) was observed within one day after mowing which removed most leaf tissue. (Scalping injury)

The impact of a routine mowing on the photosynthate partitioning within turfgrass plants appears to be minimal and of short duration,

Recovery from mowing probably makes little or no demands on the reserve fructans in leaf bases and stems.

Observations of short-term fluctuations in fructan levels following mowing should answer this question.

The application of N fertilizers to turf has long been known to lower the TNC concentration of leaf and crown tissues.

There is a decline in polymeric carbohydrates (fructans) in the leaf blades of Merion Kentucky bluegrass and Toronto creeping bentgrass following the application of N fertilizers. Monosaccharides failed to exhibit a consistent response to N additions.

Several authors found that elevated N fertility depressed fructan levels in stem and leaf bases more than any other carbohydrate fraction

This consistent response to N fertility likely results from the increased shoot growth stimulated by N and the additional demand on photosynthetic energy required to reduce and assimilate N. Thus, those carbohydrate fractions that represent photosynthate in excess of need will be most affected by N additions.

Monosaccharides, which constitute a biochemical currency in energy demanding reactions, may actually increase when metabolic rates are stimulated.

Careful analysis of the TNC profile may provide insight into the metabolic status of turfgrasses and the extent to which growth may be limited by energy supply.

Energy cycling and growth in turfgrasses

In crop plants energy cycling is a complex and important process. Seed production and grain filling are complex and vitally important processes causing large shifts in the patterns of carbohydrate and nutrient translocation and use.

Turf in contrast has a brief immature, seedling stage followed by a long-term continuous vegetative state with seasonal fluctuations imposed upon a fairly constant process. A more simple process but one worth considering.

Photosynthesis and Energy dynamics

N efficiency

C-3 grasses: almost 50% of leaf N is in the RuBP enzyme.
C-4 grasses: only 10-15% of leaf N in RuBP carboxylase.
Also, 10 % as PEP carboxylase.
Still, only 1/2 the leaf N invested is CO₂ assismilation enzymes for C-4 vs C-3.

A major reason for the better N efficiency in C-4 grasses.

The N efficiency of C-4 grasses seems to result in less coupling between leaf expansion rate and leaf N content; while C-3 grasses tend to tightly link leaf expansion to leaf N content. If N is limiting it make sense to slow down leaf growth rate so as not to use up something in short supply.

Thus, C-3 and C-4 grasses should be managed differently for N nutrition. Avoid high N levels for C-3 grasses since so much N is lost during mowing. Use lower N, particularly in summer.

Consequences of high N are worse for cool-season grasses than warm-season. Consequences of low N (deficient) are worse for warm-season grasses than cool.

Also, as explained earlier, plants spend energy to reduce NO_3^- to NH₄⁺. Plants need 1 NADH, 1 ATP, and 1 e^- from photosynthetic electron flow to assimilate NO₃ into glutamine.

This is one reason why late fall fertilization is so valuable for cool-season grasses. When leaf growth has stopped, energy from photosynthesis is readily available to assimilate NO₃^- into amino acids with little N loss from the plant.

For KB

By 21 days after germination - roots = 33% of plant dry weight and was fairly
constant thereafter.

CH₂O partitioning to roots of red fescue declined over a 56 d. period from 27% to 10% of fixed carbon.

Stem fraction 25% plant dry weight through 70D. Percent of fixed CO₂ increased from 20-26% during the first WAG and reached 50% by week 3 in KB and week 5 in RF.

Leaf blades contained 60% of plant dry weight.

Tillering - higher heights of cut increases tillering.

Do tillers communicate? PR plants show ability to move photosynthate among tillers. May allow the community to respond to disease and insects.

Root growth & N fertility in cool season turf.

N decreases root growth. Why? NO₃ reduction to NH₄ requires a great deal of energy also N stimulates more shoot growth.

This effect is reduced in the late fall when low air temps decrease shoot growth but photosynthesis continues at relatively high rates.

Carbo's are produced and translocated to the roots at 2-10 x more rapidly than during growing season.

However high N rates during winter months (when photosynthate is lowered) inhibits rooting.

For warm-season grasses - less inhibitory effect of N rates - consistent with higher photosynthate rates.

Average root extension rate for St. Augustine and bermudagrass was 5x that CB.
Fall root growth declined rapidly as soil-temps cool.

But continues for about 30 d after shoot growth stops.
No root growth in winter.

Spring root decline in warm-season grasses

In early spring when leaf growth begins, warm-season plants may have all roots turn brown and senescence takes up to 3 wk to establish new roots.

This root decline does not occur every year but only in seasons when soil warming was rapid.

When springs are cool, no root decline occurs.
Sifers and Beard explain this root decline as a diversion of carbohydrates from roots to shoots.

Also possible that enzymes responsible for starch breakdown do not function quickly enough to supply carbos under these conditions.