Nitrogen Reserve Mobilization during Regrowth of Medicago sativa L. (Relationships between Availability and Regrowth Yield)

An experiment was designed to study the role of N and C reserves on regrowth of the shoots following defoliation of forage species. Starch and N accumulation in root and crown tissue of nonnodulated Medicago sativa L. were modified during regrowth by applying different levels of N and different cutting heights. Plants were obtained with similar crown and root dry weights, but having either low starch and high tissue N or high starch and low tissue N. The plants were then submitted to a second defoliation and supplied with optimal N nutrition, and N flow from reserve was quantified using pulse-chase 15N labeling. Maximum yields following the second regrowth were obtained from those plants having a high tissue N, despite their low level of nonstructural carbohydrate. When N in the roots and crown exceeded 5 mg N plant-1 at the beginning of regrowth, about 68% was translocated to regrowing shoots. Highly significant correlations were also found between the amounts of N available in roots and crown at the beginning of regrowth and (a) the amount of N that was mobilized to new tissues, (b) the amount of N taken up during the regrowth period, and (c) the final shoot yield after 24 d of regrowth. No similar correlations were found for plants that varied in their initial starch content of roots and crown. It is suggested that N reserves were used mainly during the first 10 d after defoliation, and that the resulting aerial growth during this period should be sufficient to restore N2 fixation and/or N uptake to levels equal to those prior to defoliation. These data emphasize (a) the importance of root N reserves in initiating and sustaining new shoot growth, and (b) the need for a re-evaluation of the contribution of C reserves to shoot regrowth.     

Do compensatory shoot growth and mycorrhizal symbionts act as competing above- and below-ground sinks after simulated grazing?

Since mycorrhizal symbionts are dependent on host carbon availability, shading and grazing of the host plant often decrease mycorrhizal colonization in host roots. We conducted field experiments on an intensively mycorrhizal host, Autumn gentian, Gentianella amarella to investigate the effects of neighbor removal and simulated grazing on host growth and reproduction as well as mycorrhizal colonization. In the neighbor removal experiment, we manually removed the above-ground parts of companion plants of gentians. Reduced competition tended to improve the performance of both the host and the symbionts. Total fungal as well as arbuscular, hyphal, and coiled hyphal colonizations were higher in gentians with neighbors removed. Simulated grazing (clipping off 50% of shoot height) reduced host shoot and seed biomass, whereas the responses of the root symbionts were most often positive or neutral. In the first experiment (with late-flowering plants), clipping increased arbuscular, hyphal, and total colonizations but decreased dark septate endophyte colonization. In the second experiment (with both early- and late-flowering plants), clipping did not affect total mycorrhizal colonization. Higher arbuscular, hyphal, and total colonizations were found in late-flowering gentians compared to early-flowering ones. Early-flowering plants, on the other hand, tolerated simulated grazing better compared to late-flowering plants, which may indicate a higher cost of the symbionts for the late-flowering plants. Above-ground herbivory tends to increase carbon limitation, and under these conditions, regrowing shoots and the fungal symbionts may appear as alternative, competing sinks for the host’s limited carbon reserves.     

Branching responses in Silphium integrifolium (Asteraceae) following mechanical or gall damage to apical meristems and neighbor removal

Branching in plants increases plant access to light and provides pathways for regrowth following damage or loss of the apical meristem. We conducted two experiments in an eastern Kansas tallgrass prairie to determine how apical meristem loss (by clipping), apical meristem damage (by insect galling), and increased light availability affected growth, reproduction, and branching in Silphium integrifolium (Asteraceae). The first experiment compared clipping with galling. Clipping increased axillary shoot numbers, while galling increased axillary shoot lengths, reflecting different allocation responses among damage types and inhibition of branching by galls. However, total capitulum production was less in all gall/clip treatments than in intact shoots. The second experiment compared clipping with mowing the surrounding vegetation to increase light availability. Mowing increased total leaf, total capitulum, and axillary shoot length and axillary capitulum production in clipped and unclipped plants and in large vs. small shoots. The presence of the neighboring canopy, not of an intact apical meristem, was therefore the stronger limitation on leaf and capitulum production. These experiments suggest that damage and light competition affected both branching frequency and the partitioning of resources among shoots, branches, and leaves. Because Silphium's growth form is widespread, similar responses may occur in other grassland forbs.     

Partitioning of Nitrogen Derived from N2 Fixation and Reserves in Nodulated Medicago sativa L. During Regrowth

Nodul{macron}ted alfalfa plants were grown hydroponically. Inorder to quantify N₂ fixation and remobilization of N reservesduring regrowth the plants were pulse-chase-labelled with ¹⁵N.Starch and ethanol-soluble sugar contents were analysed to examinechanges associated with those of N compounds. Shoot removalcaused a severe decline in N₂ fixation and starch reserves within6 d after cutting. The tap root was the major storage site formetabolizable carbohydrate compounds used for regrowth; initiallyits starch content decreased and after 14 d started to recoverreaching 50% of the initial value on day 24. Recovery of N₂fixation followed the same pattern as shoot regrowth. Afteran initial decline during the first 10 d following shoot removal,the N₂ fixation, leaf area and shoot dry weight increased sorapidly that their levels on day 24 exceeded initial values.Distribution of ¹⁵N within the plant clearly showed that a significantamount of endogenous nitrogen in the roots was used by regrowingshoots. The greatest use of N reserves (about 80% of N incrementin the regrowing shoot) occurred during the first 10 d and thencompensated for the low N₂ fixation. The distribution of N derivedeither from fixation or from reserves of source organs (taproots and lateral roots) clearly showed that shoots are thestronger sink for nitrogen during regrowth. In non-defoliatedplants, the tap roots and stems were weak sinks for N from reserves.By contrast, relative distribution within the plant of N assimilatedin nodules was unaffected by defoliation treatment. Key words: Medicago sativa L., N₂ fixation, N remobilization, N₂ partitioning, regrowth     

Compensatory growth of two clonal dwarf shrubs, Arctostaphylos uva-ursi and Vaccinium uliginosum in a heavy metal polluted environment

The effect of artificial shoot clipping on the vegetative growth and sexual reproduction of the evergreen bearberry, Arctostaphylos uva-ursi, and the deciduous bog bilberry, Vaccinium uliginosum, was studied in the vicinity of a copper-nickel smelter in SW Finland. According to the research hypothesis, heavy metal induced shoot death breaks the apical dominance in the clones growing in a polluted environment. This causes activation of dormant axillary and adventitious buds and an increase in branching on the older parts of the stem. Regrowth after shoot death was studied by clipping off all the current-year shoots from experimental branches in autumn (1994) and spring (1995). Within-clone and between-clone control branches were used to test the data.Both species displayed a considerable ability to activate dormant meristems after the damage. Regrowth of the current shoots during the next growing season (1995) was about 80% compared to the within-clone control in both species after autumn clipping. Shoot clipping in early summer was more detrimental for both species, and the regrowth of A. uva-ursi was less than that of V. uliginosum. Differences in the storage reserves and source-sink mechanisms of carbon allocation between evergreen and deciduous species probably explain their distinct response. When the removed biomass was added to the living biomass of the branches, there was overcompensation in the total dry weight of A. uva-ursi after autumn clipping, and the weight was almost 90% of the control after spring clipping. The total dry weight of V. uliginosum also equalled that of the control when the removed biomass was added. No berries developed on either species in the year following the autumn treatment, because clipping removed all the flower buds. Spring clipping had no effect on the sexual reproduction of A. uva-ursi, but decreased the berry production of V. uliginosum. The degree of compensatory growth of both species was only slightly affected by the distance from the smelter. It is suggested that dormant bud activation, rapid regrowth and plastic branching contribute to the resistance mechanism to heavy metals.     

Mobilization of nitrogen reserves during regrowth of defoliated Trifolium repens L. and identification of potential vegetative storage proteins

Although it is well established that carbon reserves contributeto shoot regrowth of leguminous forage species, little informationis available on nitrogen reserves except in Medicaqo sativaL. and Trifolium subterraneum L. In this study, reserves werelabelled with ¹⁵N to demonstrate the mobilization of endogenousnitrogen from roots and stolons to regrowing leaves and newstolons during 24 d of regrowth in white clover (Thfolium repensL.). About 55% and 70%, respectively, of the nitrogen contentsof these organs were mobilized to support the regrowth of leaves.During the first 6 d, nitrogen in regrowing leaves came mainlyfrom N reserves of organs remaining after defoliation. Afterthese first 6 d of regrowth, most of the shoot nitrogen wasderived from exogenous nitrogen taken up while the contributionof nitrogen reserves decreased. After defoliation, the buffer-solubleprotein content of roots and stolons decreased by 32% duringthe first 6 d of regrowth. To identify putative vegetative storageproteins, soluble proteins were separated using SDS-PAGE ortwo-dimensional electrophoresis. One protein of 17.3 kDa instolons and two proteins of 15 kDa in roots seemed to behaveas vegetative storage proteins. These three polypeptides, initiallyfound at high concentrations, decreased in relative abundanceto a large extent during early regrowth and then were accumulatedagain in roots and stolons once normal growth was re-established. Key words: White clover, regrowth, ¹⁵N-labelled, vegetative storage proteins, electrophoresis     

Potential soil carbon sequestration in a semiarid Mediterranean agroecosystem under climate change: Quantifying management and climate effects

Repeated defoliation and flooding trigger opposite plant morphologies, prostrated and erect ones, respectively; while both induce the consumption of carbohydrate reserves to sustain plant recovery. This study is aimed at evaluating the effects of the combination of defoliation frequency and flooding on plant regrowth and levels of crown reserves of Lotus tenuis Waldst. & Kit., a forage legume of increasing importance in grazing areas prone to soil flooding. Adult plants of L. tenuis were subjected to 40 days of flooding at a water depth of 4 cm in combination with increasing defoliation frequencies by clipping shoot mass above water level. The following plant responses were assessed: tissue porosity, plant height, biomass of the different organs, and utilization of water-soluble carbohydrates (WSCs) and starch in the crown. Flooding consistently increased plant height independently of the defoliation frequency. This response was associated with a preferential location of shoot biomass above water level and a reduction in root biomass accumulation. As a result, a second defoliation in the middle of the flooding period was more intense among plants that are taller due to flooding. These plants lost ca. 90% of their leaf biomass vs. ca. 50% among non-flooded plants. The continuous de-submergence shoot response of frequently defoliated plants was attained in accordance to a decrease of their crown reserves. Consequently, these plants registered only 27.8% of WSCs and 9.1% of starch concentrations with respect to controls. Under such stressful conditions, plants showed a marked reduction in their regrowth as evidenced by the lowest biomass in all plant compartments: shoot, crowns and roots. Increasing defoliation frequency negatively affects the tolerance of the forage legume L. tenuis to flooding stress. Our results reveal a trade-off between the common increase in plant height to emerge from water and the amount of shoot removed to tolerate defoliation. When both factors are combined and defoliation persists, plant regrowth would be constrained by the reduction of crown reserves.     

Carbohydrate Reserve and Dark Carbon Fixation in the Brown Macroalga, Laminaria hyperborea

Tissue discs originating from young, growing blade areas and from adult, mature frond regions of the brown macroalga Laminaria hyperborea (Fosl.) Gunn. (Phaeophyceae, Laminariales) were investigated with particular regard to photosynthesis, dark respiration, dark carbon fixation, and carbohydrate reserves. It was found that the mannitol/laminaran reserve of the young, developing blade meets the requirements of dark respiratory metabolism for only 7-10d at 10±2 °C under continuous darkness. A concomitant decrease in the potential for (β-carboxylation of phosphoenolpyruvate by phosphoenolpyruvate carboxykinase (EC 4.1.1.32) occurred along with the depletion of the stored carbohydrate. Restoring the intracellular pool of reserve carbohydrates by photosynthesis and by feeding of exogenously supplied mannitol resulted in a short term recovery of the rates of dark fixation. These findings support the view that (i) in the dark the substrate of (β-carboxylation is mainly derived from mannitol (along with glycolytic degradation of laminaran) and (ii) the young blade is not able to maintain its own carbon balance under the environmental conditions during midwinter and early spring, but relies on a carbon flow from the old blade.     

Seasonal changes in photosynthesis in the desiccation-tolerant fern Polypodium virginianum

Summary Seasonal changes in photosynthesis were examined in the desiccation-tolerant fern Polypodium virginianum growing in a forest understory along cliff edges of the Niagara Escarpment in southern Ontario, Canada. For plants growing in situ, the photosynthetic response to irradiance was examined on a seasonal basis, to determine the degree to which the utilization of light changed over the growing season. Experiments were executed on control plants, on previously desiccated then rehydrated plants, and on continuously hydrated plants to determine if prior desiccation influenced the response to light. Soil and xylem water potential and temperature were monitored and used as covariates in analyses. The results showed that carbon gain in the spring greatly exceeded that of any other season. Despite this, there was little change in the photosynthetic response to light on a seasonal basis even though plants were exposed to highly variable and highly limited light most of the time. Prior desiccation had a slight influence on photosynthetic rate but not on other photosynthetic parameters such as the light compensation point and L_half. Temperature was a significant seasonal covariate and additional experiments conducted in the laboratory showed that the response of photosynthesis to temperature was broad. Xylem water potential was correlated with seasonal changes in relative humidity. The results suggest that P. virginanum persists in hostile cliff-edge habitats by being able to exploit high-light periods in the spring and by thereafter maintaining a low but relatively constant rate of carbon despite prior exposure to fluctuating supplies of light and water.     

Effect of defoliation upon root growth,phosphate absorption and respiration in nutrient-limited tundra graminoids

Summary Moderate experimental defoliation stimulated root respiration and phosphate absorption in two tundra graminoids, Eriophorum vaginatum and Carex aquatilis, growing under nutrient-limited field conditions in northern Alaska. The increase in phosphate absorption rate following defoliation of Eriophorum was associated with a decrease in root phosphate and available carbohydrate contents per unit root length but a constant root nitrogen content. Only after four repeated defoliations did phosphate absorption rate decrease below control levels. We suggest that the stimulation of root respiration and phosphate absorption immediately following defoliation resulted from lowered root phosphorus status as nutrient reserves were reallocated to support shoot regrowth. Root growth was affected more severely by defoliation than was root activity. Two or more defoliations reduced root elongation, initiation and weight per unit length, but root mortality increased only after four defoliations. Carex aquatilis, a species with large belowground biomass, was less sensitive to defoliation than Eriophorum. Phosphate absorption rate increased only after four defoliations in this species, and root elongation, initiation and mortality were affected only by the most severe clipping regimes. Responses of plants to repeated defoliation over two growing seasons were consistent with results of short-term studies.     

Management of below-ground biomass of <Emphasis Type="Italic">Typha angustifolia</Emphasis> by harvesting shoots above the water surface on different summer days

A dynamic model of regrowth in Typha angustifolia after cutting shoots above the water surface was formulated by characterizing the phenology and mobilization of resources from below-ground to above-ground organs after the cutting. The model parameters were determined by two cutting experiments to investigate the different strategies with flowering and non-flowering shoots after cutting in 2001 and by four cutting experiments to elucidate the regrowth characteristics after cutting on different days from June to September in 2002. A difference was evident both for flowering and non-flowering shoots and for each cutting day. From June to August, non-flowering shoots regrew immediately after cutting, but flowering shoots did not. The shoot regrowth height, number of leaves and shoot biomass were higher with the earlier cutting. The model was validated using the below-ground biomass observed in December 2002 and below-ground dynamics observed in 2003. In the low-flowering shoot zone of the stands, in which the percentage of flowering shoots was small (around 10%), the decrease in below-ground biomass became larger from June (20%) to August (60%). Cutting the high-flowering shoot zone (flowering shoots: 78%) in July 2001, just 1 week after peduncle formation, decreased the below-ground biomass by about 50%. In the low-flowering shoot zone, cutting just before senescence is better for decreasing below-ground biomass with a smaller rate of flowering shoots. The difference of below-ground biomass reduction in non-flowering shoots is mainly due to the decrease in downward translocation (DWT) of above-ground material to below-ground organs during senescence, because of the decrease in regrowth biomass. As for flowering shoots, the decrease in the photosynthate transportation from above-ground to below-ground organs and that of DWT are closely related because they cannot grow again within the season.     

Effects of simulated winter browsing on mountain birch foliar chemistry and on the performance of insect herbivores

Winter browsing by mammalian herbivores is known to induce a variety of morphological and physiological changes in plants. Browsing has been suggested to decrease the carbohydrate reserves in woody plants, which might lead to reduced tannin production in leaves during the following summer, and consequently, to increased herbivore damage on leaves. We conducted a clipping experiment with mature mountain birch trees and measured the effects of clipping on birch growth, leaf chemistry and toughness, as well as on the performance of insect herbivores. Leaves grew larger and heavier per unit area in the clipped ramets and had a higher content of proteins than leaves in the control trees. Clipping treatment did not affect the total content of sugars in the leaves (mg g^?1), suggesting that a moderate level of clipping did not significantly reduce the carbohydrate pools of fully‐grown mountain birch trees. Furthermore, the contents of proanthocyanidins (condensed tannins) and gallotannins were slightly higher in the leaves of clipped ramets, contrary to the hypothesis of reduced tannin production. The effects of clipping treatment on leaf and shoot growth and on foliar chemistry were mainly restricted to the clipped ramets, without spreading to untreated ramets within the same tree individual. The effects of clipping on leaf characters varied during the growing season; for instance, leaf toughness in clipped ramets was higher than toughness in control trees and ramets only when leaves were mature. Accordingly, clipping had inconsistent effects on insect herbivores feeding at different times of the growing season. The generally small impact of clipping on herbivore performance suggests that the low intensity of natural browsing at the study area, simulated by our clipping treatment, does not have strong consequences for the population dynamics of insect herbivores on mountain birch via enhanced population growth caused by browsing‐induced changes in food quality.     

Seasonal dynamics of photosynthesis and total carbon gain in bearing and nonbearing pistachio (<Emphasis Type="Italic">Pistacia vera</Emphasis> L.) shoots

Seasonal changes in leaf gas exchange, assimilation response to light and leaf area were monitored in bearing and nonbearing pistachio shoots. Shoot bearing status did not directly affect leaf photosynthetic rate. However, photosynthetic light-response curves strongly varied during the season demonstrating the dominant effect of the tree’s seasonal phenology on assimilation. Early in the season low photosynthetic rates were associated with high rates of dark respiration indicating limited photosynthesis in the young leaves. As leaves matured, dark respiration decreased and assimilation reached maximum values. Photosynthetic efficiency was strongly reduced late in the season due to leaf age and senescence. Fruit load precipitated an early leaf senescence and drop that resulted in a 53% decrease in leaf area in bearing vs. nonbearing shoots, strongly decreasing the seasonal photosynthetic performance of bearing shoots. Bearing shoots produced a 26% lower seasonal carbon gain compared to nonbearing shoots.     

Consumption of wheat seed reserves during germination and early growth as affected by soil water potential

Seed size and weight are important criteria for determining seedling vigour and stand establishment. Evolution of seed dry weight of wheat (Triticum aestivum L.) during germination and early growth was examined because poor stands are often associated with the depletion and exhaustion of seed reserves. Two laboratory experiments were conducted on filter paper and in soil at three water potentials using wheat seeds. Seed, root, and shoot dry weights were recorded at approximately one-day intervals. Coleoptile and first leaf lengths were also measured at all sampling periods. Wheat seedlings grown on filter paper in the dark grew to a length of 90 to 100 mm with 50% of the initial seed weight remaining after eight days when the experiment was terminated. In soil, wheat seedlings grew 15 mm with 25% of the initial seed weight remaining. Seed reserves were depleted more quickly when the soil was wet because seedlings grew more quickly. There were significant and similar negative relationships between seed weight and coleoptile length of wheat seedlings grown on filter paper and in soil. There was no effect of soil water potential on the relationship between seed weight and shoot length. Therefore, it was concluded that poor wheat stands are not likely to occur due to depletion of seed reserves under field conditions without mechanical obstacles.     

The relative importance of carbohydrate and nitrogen for the resprouting ability of Quercus crispula seedlings

BACKGROUND AND AIMS: Plants need some kind of stored resources to resprout after shoot destruction. The aim of this study was to determine the relative importance of carbohydrate and nitrogen (N) storage levels for their ability to resprout. METHODS: A shoot clipping experiment was conducted on Quercus crispula seedlings, which were grown in a factorial experimental design, with two light levels (40% and 3% of full light) and three nutrient concentrations (low, medium and high). KEY RESULTS: At the time of shoot clipping (the end of spring leaf expansion), seedlings exposed to 40% light had an average total non-structural carbohydrate (TNC) concentration of 17.0% in their roots compared with 4.9% in the roots of seedlings exposed to 3% light, and the average amount of TNC (TNC pools) in the roots was 203.8 mg and 20.0 mg at 40% light and 3% light, respectively. In contrast, root N concentration averaged 2.3% in the 3% light treatment compared with 1.2% in the 40% light treatment, and it increased with successive rises in nutrient concentrations at both light levels. Regardless of the nutrient status, at the 40% light level >80% of the seedlings resprouted after shoot clipping. Few seedlings, however, resprouted at the 3% light level, particularly in the medium- and high-nutrient treatments. Furthermore, both root TNC concentrations and TNC pools decreased after resprouting, but the amount of root N remained constant. CONCLUSIONS: These results suggest that carbohydrate storage has a stronger influence on resprouting in Quercus crispula than N storage. However, the size of the resprouting shoot was positively correlated with the amount of both N and TNC in roots. The level of N storage is, therefore, also important for the growth of resprouting shoots.     

Nitrogen Turnover and Assimilation during Regrowth in Trifolium subterraneum L. and Bromus mollis L

Subterranean clover (Trifolium subterraneum L. cv Woogenellup) and soft chess grass (Bromus mollis L. cv Blando) were grown in monocultures with ¹⁵NH₄Cl added to the soil to study nitrogen movement during regrowth following shoot removal. Four clipping treatments were imposed. Essentially all available ¹⁵N was assimilated from the soil prior to the first shoot harvest. Measurements of total reduced nitrogen and ¹⁵N contained within that nitrogen fraction in roots, crowns, and shoots at each harvest showed large, significant (P ≤ 0.001) declines in excess ¹⁵N of crowns and roots in both species between the first and fourth harvests. There was no significant decline in total reduced nitrogen in the same organs over that period. Similar responses were evident in plants defoliated three times. The simplest interpretation of these data is that reduced nitrogen compounds turn over in plant roots and crowns during shoot regrowth. Calculations for grass and clover plants clipped four times during the growing season indicated that 100 to 143% of the nitrogen present in crowns and roots turned over between the first and fourth shoot harvest in both species, assuming nitrogen in those organs was replaced with nitrogen containing the lowest available concentration of ¹⁵N. If other potential sources of nitrogen were used for the calculations, it was necessary to postulate that larger amounts of total nitrogen flowed through the crown and root to produce the measured dilution of ¹⁵N compounds. These data provide the first quantitative estimates of the amount of internal nitrogen used by plants, in addition to soil nitrogen or N₂, to regenerate shoots after defoliation.     

C and N allocation in soil under ryegrass and alfalfa estimated by 13C and 15N labelling

Background and Aims

Below-ground translocated carbon (C) released as rhizodeposits is an important driver for microbial mobilization of nitrogen (N) for plants. We investigated how a limited substrate supply due to reduced photoassimilation alters the allocation of recently assimilated C in plant and soil pools under legume and non-legume species.

Methods

A non-legume (Lolium perenne) and a legume (Medicago sativa) were labelled with ¹⁵N before the plants were clipped or shaded, and labelled twice with ¹³CO₂ thereafter. Ten days after clipping and shading, the ¹⁵N and ¹³C in shoots, roots, soil, dissolved organic nitrogen (DON) and carbon (DOC) and in microbial biomass, as well as the ¹³C in soil CO₂ were analyzed.

Results

After clipping, about 50 % more ¹³C was allocated to regrowing shoots, resulting in a lower translocation to roots compared to the unclipped control. Clipping also reduced the total soil CO₂ efflux under both species and the ¹³C recovery of soil CO₂ under L. perenne. The ¹⁵N recovery increased in the shoots of M. sativa after clipping, because storage compounds were remobilized from the roots and/or the N uptake from the soil increased. After shading, the assimilated ¹³C was preferentially retained in the shoots of both species. This caused a decreased ¹³C recovery in the roots of M. sativa. Similarly, the total soil CO₂ efflux under M. sativa decreased more than 50 % after shading. The ¹⁵N recovery in plant and soil pools showed that shading has no effect on the N uptake and N remobilization for L. perenne, but, the ¹⁵N recovery increased in the shoot of M. sativa.

Conclusions

The experiment showed that the dominating effect on C and N allocation after clipping is the need of C and N for shoot regrowth, whereas the dominating effect after shading is the reduced substrate supply for growth and respiration. Only slight differences could be observed between L. perenne and M. sativa in the C and N distribution after clipping or shading.     

Root cooling strongly affects diel leaf growth dynamics,water and carbohydrate relations in Ricinus communis

In laboratory and greenhouse experiments with potted plants, shoots and roots are exposed to temperature regimes throughout a 24 h (diel) cycle that can differ strongly from the regime under which these plants have evolved. In the field, roots are often exposed to lower temperatures than shoots. When the root‐zone temperature in Ricinus communis was decreased below a threshold value, leaf growth occurred preferentially at night and was strongly inhibited during the day. Overall, leaf expansion, shoot biomass growth, root elongation and ramification decreased rapidly, carbon fluxes from shoot to root were diminished and carbohydrate contents of both root and shoot increased. Further, transpiration rate was not affected, yet hydrostatic tensions in shoot xylem increased. When root temperature was increased again, xylem tension reduced, leaf growth recovered rapidly, carbon fluxes from shoot to root increased, and carbohydrate pools were depleted. We hypothesize that the decreased uptake of water in cool roots diminishes the growth potential of the entire plant – especially diurnally, when the growing leaf loses water via transpiration. As a consequence, leaf growth and metabolite concentrations can vary enormously, depending on root‐zone temperature and its heterogeneity inside pots.     

Effect of shoot removal on remobilization of carbon and nitrogen during regrowth of nitrogen‐fixing alfalfa

The contribution of carbon and nitrogen reserves to regrowth following shoot removal has been studied in the past. However, important gaps remain in understanding the effect of shoot cutting on nodule performance and its relevance during regrowth. In this study, isotopic labelling was conducted at root and canopy levels with both ¹⁵N₂ and ¹³C‐depleted CO₂ on exclusively nitrogen‐fixing alfalfa plants. As expected, our results indicate that the roots were the main sink organs before shoots were removed. Seven days after regrowth the carbon and nitrogen stored in the roots was invested in shoot biomass formation and partitioned to the nodules. The large depletion in nodule carbohydrate availability suggests that root‐derived carbon compounds were delivered towards nodules in order to sustain respiratory activity. In addition to the limited carbohydrate availability, the upregulation of nodule peroxidases showed that oxidative stress was also involved during poor nodule performance. Fourteen days after cutting, and as a consequence of the stimulated photosynthetic and N₂‐fixing machinery, availability of C_new and N_new strongly diminished in the plants due to their replacement by C and N assimilated during the post‐labelling period. In summary, our study indicated that during the first week of regrowth, root‐derived C and N remobilization did not overcome C‐ and N‐limitation in nodules and leaves. However, 14 days after cutting, leaf and nodule performance were re‐established.