Crop Science 42:406-414 (2002)
© 2002 Crop Science Society of America
CROP PHYSIOLOGY & METABOLISM
Reproductive Development of White Clover (Trifolium repens L.) is Not Impaired by a Moderate Water Deficit That Reduces Vegetative Growth
I. Inflorescence, Floret, and Ovule Production
Christine Bissuel-Belaygue*,a,
Alexander A. Cowanb,
Athole H. Marshallb and
Jacques Weryc
a ENSAR-INRA, UMR Sols-Agronomie-Spatialisation, 65 rue de Saint-Brieuc, 35042 Rennes, France
b Institute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth, Ceredigion, SY23 3EB, UK
c Agro.M-CIRAD-INRA, UMR SYSTEM, 2 place Viala, 34060 Montpellier, France
* Corresponding author (Christine.Bissuel{at}agrorennes.educagri.fr)
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ABSTRACT
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Field experiments have shown that water deficit can either increase or decrease white clover (Trifolium repens L.) seed yield depending on its intensity. In order to explain this behavior, we evaluated the effect of water deficit intensity on the components of plant reproductive potential: number of reproductive stolons produced per plant (NSr), inflorescences per reproductive stolon (NI), viable florets per inflorescence (NFv) and ovules per floret (NO). Four experiments were conducted in greenhouse or growth chamber on white clover plants grown in large or small soil columns, or in small pots with vermiculite. Water supply was managed to maintain relatively constant values for predawn soil water potential and midday leaf relative water content (RWC) during the deficit period (20 to 68 d). Water deficit treatments were classified as moderate (M) or severe (S) on the basis of the reduction of RWC compared with the well-watered plants (C) and of previously established relationships between RWC, soil water potential and vegetative growth. The development of inflorescences, florets and ovules were largely unaffected in M plants, while vegetative growth was depressed by reductions in leaf number per stolon (up to 30%), leaf area (30 to 40%), and by inhibition of stolon branching. Moderate water deficits induced an increase in the percentage of reproductive stolons per plant and reproductive phytomers per stolon. Although this positive effect on the reproductive to vegetative balance was generally observed with S water deficit, the reproductive potential of S plants was strongly depressed relative to C and M plants because of reduced stolon branching, phytomer production, and increased inflorescence and floret abortion. These results show that inflorescence, floret and ovule development of white clover were not impaired by a moderate water deficit that reduced vegetative growth, and suggest good prospects to manage optimal soil-plant status to maximize potential of seed production.
Abbreviations: NFi, number of florets initiated per inflorescence • NFv, number of viable florets per inflorescence • NI, inflorescence number per reproductive stolon • NO, ovule number per floret • NS, stolon number per plant • NSr, reproductive stolon number per plant • P, potential seed number per plant • RWC, relative water content
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INTRODUCTION
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THE DEVELOPMENT OF WHITE CLOVER (Trifolium repens L.) seed production as a profitable crop is impaired by the difficulty of obtaining consistent economic seed yields, especially in Europe (Hides et al., 1984). This yield instability can be attributed partially to the effects of water availability during critical periods of plant development (Clifford, 1986a). Unlimited water supply results in excessive leaf growth (Danyach-Deschamps and Wery, 1988) and a dense canopy that causes flower and ovule abortion (Pasumarty and Thomas, 1990), delays crop maturity (Clifford, 1986a), and favors development of fungal diseases. On the other hand, severe water deficits reduce the number of stolons produced per plant and the number of phytomers (defined as a structural subunit that comprise a leaf, a node, an internode, and the associated axillary bud, Moore and Moser, 1995) produced per stolon (Belaygue et al., 1996). This response could limit the production of inflorescences which is the main component of seed yield (Clifford, 1986b; Danyach-Deschamps and Wery, 1988; Oliva et al., 1994). No literature is available regarding severe deficits in white clover grown for seed, but flower and ovule development are also limited by severe water deficits which induce abortion of reproductive organs as floral buds in soybean [Glycine max (L.) Merr.], (Momen et al., 1979; Westgate and Peterson, 1993) or immature ovules in maize (Zea mays L.), (Westgate, 1994).
Maximum white clover seed yields are generally obtained when the amount of water supplied is slightly lower than the amount required to achieve maximum evapotranspiration of the crop (Zaleski, 1966; Clifford, 1986b; Danyach-Deschamps and Wery, 1988). Oliva et al. (1994) showed that the highest seed yield was achieved when water supply was delayed until 68% of the available soil-water was used by the white clover crop. For alfalfa (Medicago sativa L.) seed crops, Steiner et al. (1992) showed that floral development, pod set and seed yield were maximal when the crop received only 70% of the amount of water required for maximal evapotranspiration during reproductive development. Similar benefits of sub-optimal irrigation have been reported for grain legumes such as pea (Pisum sativum L.) (Turc et al., 1990) and chickpea (Cicer arietinum L.) (Wakrim-Mezrioui and Wery, 1995). Vegetative growth of white clover is highly sensitive to soil water deficit (Oliva et al., 1994), with reduction of leaf area, leaf number and stolon number in proportion to the intensity and duration of water deficit (Belaygue et al., 1996). Danyach-Deschamps and Wery (1988) showed that suppressing one fifth of the amount of irrigation to a well-watered white clover seed crop reduced the leaf biomass by 11% while increasing the number of inflorescences per unit area by 14% and seed yield by 35%. These results suggest that although leaf growth is reduced in proportion to the soil water deficit (Belaygue et al., 1996), there is an optimal level of water deficit required to allow the development of a large number of inflorescences and ovules.
The effect of a moderate water shortage on final seed yield varies from year to year in alfalfa (Steiner et al., 1992) and white clover (Oliva et al., 1994). It is difficult, from the existing literature, to identify the soil-plant water status required to optimize the balance between reproductive and vegetative growth of white clover. The aim is to slow down leaf growth without a significant reduction of the canopy net carbon exchange rate, in order to maximize the reproductive potential of the crop (number of fertile ovules) and to allow the best conditions for ovule fertilization and development.
As the first step towards identifying the optimal soil-plant water status required for white clover seed production, we determined the reproductive potential of the plant over a range of water deficits. Reproductive potential (P) of white clover (i.e., potential seed number per plant) is defined as the number of fertile ovules per plant and calculated as:
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where NSr is the number of reproductive stolons produced per plant, NI the number of inflorescences per reproductive stolon that reached pollination stage, NFv the number of viable florets per inflorescence, and NO the number of ovules per floret. We evaluated the contributions of each term to the drought-induced reduction of reproductive potential under various levels of water deficit.
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MATERIALS AND METHODS
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Experimental Conditions and Water Deficit Treatments
Four experiments were conducted in greenhouse or growth chamber between 1992 and 1995 in Montpellier, France and in Aberystwyth, Wales as previously described in Belaygue et al. (1996). Five genotypes (G1 to G5) differing in leaf type (large, intermediate, or small) and number of ovules per ovary (4 or 6) were selected from cultivar Olwen (Table 1). The required number of plants for each experiment were cloned by vegetative propagation from each selected genotype. Three different systems were used to impose constant levels of water deficit (moderate or severe) during short (20 d) or long periods (68 d) under a large range of environmental conditions: large soil columns in greenhouse (Exp. I and II), and small pots with either vermiculite (Exp. III) or compost (Exp. IV) in growth chambers.
Experiment I: Large Column with Soil— Short and Moderate Water Deficit
Twelve clonal plants of genotype G1 (large leaf type) were grown in a greenhouse, in columns (0.25 m diameter, 0.70 m height) filled with a soil mixture (14% clay, 25% silt, 55% sand, 6% organic matter). Average daily maximum and minimum air temperatures were 21.5°C ± 3 and 14.5°C ± 2, respectively. All plants were defoliated before flowering on 20 Apr. (90 d after planting). Irrigation was scheduled daily to maintain a constant soil water potential, measured in each column with tensiometers at 0.25- and 0.45-m depth. Soil water potential was maintained above -1.5 x 10-3 MPa in six columns (control treatment, C) by supplying 0 to 2 L per plant, depending on the evaporative demand. The other six columns (deficit treatment, D1) were irrigated in the same way until 20 Apr. Irrigation was stopped until the soil water potential reached -80 x 10-3 MPa (5 May), and was reapplied during the following 20 d in order to maintain soil water potential at a constant value of -80 x 10-3 MPa by daily water supply of 0 to 1 L per plant. On 24 May, soil water potential was returned to the control level (-1.5 x 10-3 MPa) for 25 d.
Experiment II: Large Column with Soil— Long and Severe Water Deficit
Sixteen clonal plants of genotype G1 were grown under the same conditions as in Experiment I. Average daily maximum and minimum air temperatures were 21.0°C ± 2.5 and 9.5°C ± 2, respectively. Soil water potential was maintained above -1.5 x 10-3 MPa in eight columns (control treatment, C) by daily water supply of 0 to 3.75 L per plant depending on the evaporative demand. For the other eight columns, irrigation was stopped 90 d after planting until plants showed signs of wilting (134 d after planting), and then was scheduled by supplying 0.5 L per plant each time leaves began to wilt. This resulted in 66 d severe water deficit (D2) managed by periodic water supply every 3 to 5 d.
Experiment III: Small Pot with Vermiculite— Long and Moderate or Severe Water Deficits
Twelve plants of genotype G2 (small leaf type) and 12 plants of genotype G3 (intermediate leaf type) were grown in pots (10 cm diameter, 20 cm height) filled with vermiculite, and placed in a growth room when they were 30 d old. Photosynthetic photon flux density was 400 µmol m-2 s-1 during a 16 h photoperiod, day and night temperatures were 20 and 15°C, respectively, and air vapor pressure deficit was 6 x 10-4 MPa. Pots were placed on the top of columns of foam blocks and sealed at their base with nylon gauze which restricted root growth following the design of Snow and Tingey (1985). Plants were supplied with Jensen's nutrient solution (Jensen, 1942) minus nitrogen through a float valve that controlled the level of nutrient solution inside the foam. Three water supply treatments were maintained for 71 d (four plants per genotype and per treatment) by adjusting the level of nutrient solution below the nylon gauze at 0.05 m in control treatments (C), at 0.15 m in treatments D3 and at 0.25 m in treatments D4.
Experiment IV: Small Column with Compost— Long and Moderate Water Deficits
Eighteen plants of genotype G4 (small leaf type) and 18 plants of genotype G5 (intermediate leaf type) were grown in the same conditions as in experiment III. They were potted into columns of 15 cm diameter and 45 cm height filled with John Innes No.1 potting compost when they were 60 days old. Plants were irrigated on the basis of soil water content, which was checked by weighing columns daily. Three treatments, corresponding to 100% (C), 65% (D5), and 44% (D6) of available soil water were maintained during 68 d.
Characterization of Water Deficit Experienced by the Plants
Leaf water potential was measured with a pressure chamber every one or two weeks before dawn and at noon on one leaf of six or eight plants per treatment (one per plant) in Exp. I and II. Leaf relative water content (RWC) was measured in all experiments every one or two weeks at midday on one leaf of four, six, or eight plants per treatment. For both measurements, leaves were chosen among the youngest fully expanded leaves to avoid the effects of growth or senescence on leaf water status (Belaygue et al., 1996).
Potential Seed Number per Plant
The number of stolons (NS), the number of reproductive stolons with at least one inflorescence (NSr), and the number of inflorescences (e.g. flower heads) were recorded for each plant at the end of the experiment. The average number of inflorescences per reproductive stolon (NI) was calculated as the ratio between the total number of inflorescences per plant and NSr. The number of viable florets per inflorescence (NFv) was determined from a sample of 10 (Exp. II, III and IV) to 24 (Exp. I) inflorescences per plant collected between full bloom stage and maturity. At this period of inflorescence development, viable florets could be easily distinguished from aborted ones. The number of aborted florets per inflorescence was recorded in Exp. I and II and the percentage of floret abortion per inflorescence was calculated. The number of ovules per floret (NO) was recorded by dissecting 120 to 240 ovaries per treatment collected from 10 to 20 inflorescences per treatment depending on the experiment. Reproductive potential (P) (the potential seed number per plant) was defined as the number of fertile ovules per plant and calculated as the product of four terms NSr, NI, NFv, and NO.
The final length of peduncles and axillary leaf petioles were recorded on the mature reproductive phytomers in order to quantify the effect of water deficit on the growth of vegetative and reproductive organs of the same phytomer, as described for leaf lamina area in Belaygue et al. (1996).
Dry Matter Accumulation
Total dry matter was recorded for each plant in Exp. I and II at the end of the experiment. Dry matter accumulation was recorded on 24 stolons per treatment (eight per plant) in Exp. I or 40 stolons per treatment (ten per plant) in Exp. II. All removed biomass was dried in an oven at 60°C for 48 h before weighing.
Statistical Analysis
Experiments involved completely randomized designs with four (Exp. III), six (Exp. I and IV), or eight replicates of one plant (Exp. II). Analysis of variance was performed for each genotype within each experiment. Differences between treatment means were evaluated by genotype, using least significance difference at P 
0.05 (LSD0.05).
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RESULTS
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Water Deficit Experienced by the Plants
Experiment I and II: Soil water potential, predawn leaf water potential, and midday leaf water potential were significantly reduced in water deficit treatments compared with well-watered treatment (Fig. 1 in Belaygue et al., 1996). In these experiments, decrease in leaf relative water content (RWC) was closely related to decrease in plant water potential or in soil water status as measured with soil water potential or predawn leaf water potential (Fig. 1d in Belaygue et al., 1996). This anisohydric behavior of white clover allows the use of leaf RWC to characterize the water deficit experienced by the plant. Leaf RWC was chosen instead of leaf water potential, because it allowed rapid sampling of a large number of leaves (30 min for all the treatments). This was an efficient way to reduce time-induced variability linked to variation of solar radiation and air vapor pressure deficit.
Leaf RWC was maintained in a range of 85 to 94% in the control plants (Fig. 1)
. Differences in leaf RWC between control and water deficit plants were rapidly established, except for Exp. II, where soil water potential decreased slowly because of the low evaporative demand and large soil volume. These differences remained nearly constant from 20 (Exp. I) up to 68 d (Exp. III and IV). According to the approach established in the previous paper (Belaygue et al., 1996), the intensity of water deficit was quantified with the average leaf RWC during the stabilized period of water deficit (Fig. 1).
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Fig. 1. Change with time of leaf relative water content (RWC) for well-watered plants (C) or plants subjected to water deficit (D) in each genotype (G1 to G5) in each experiment (Exp. I to IV). In water deficit treatments, irrigation was stopped or adjusted on Day 0. In Exp. I, columns were fully rehydrated on Day 34. Each point is the mean of four (Exp. III), six (Exp. I and IV) or eight leaf measurements (Exp. II). Bars represent 95% confidence intervals. Dotted vertical lines indicate the beginning and the end of the stabilized water deficit period characterized by stabilized soil water potential (Exp. I), stabilized available soil water (Exp. II and IV) or stabilized water level in column (Exp. III). Values in brackets refer to RWC averaged during water deficit period. Each water deficit treatment (D) was characterized a posteriori as moderate water deficit (filled triangle) or severe water deficit (filled diamond) on the basis of reduction in RWC compared with the control (C, open circle) with the threshold value of 12% reduction in mean RWC that defined the level of water deficit required for maximum inhibition in stolon branching in absence of symptoms of wilting.
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The more severe water deficit treatments that resulted in maximum reduction in plant leaf area (Belaygue et al., 1996), caused symptoms of leaf wilting at midday, particularly on sunny days. These treatments, inducing a drop of mean RWC of more than 12%, in comparison with the control, were classified as severe water deficit (S) (Fig. 1). Those inducing a drop of mean RWC of less than 12% were classified as moderate water deficit (M). No wilting was observed on these plants. Moderate water deficits induced significant reductions compared with control plants in leaf appearance rate (up to 30%) and leaf expansion rate (30 to 40%), and a considerable reduction in stolon branching (Belaygue et al., 1996). From this classification we can see that M water deficits were obtained for a short period in Exp. I and for a long period in Exp. III and IV, while S water deficits were only obtained in Exp. II and III for a long period (Fig. 1). Treatment D6 in Exp. IV was expected to be a S water deficit and was finally classified as M water deficit. This classification was used to compare the effects of moderate (M) and severe (S) water deficit on reproductive development.
Inflorescence Growth and Development
The final length of leaf petiole and inflorescence peduncle were reduced by water deficits (Fig. 2a)
as was internode length (not shown) and leaf lamina area (Belaygue et al., 1996). Across genotypes and treatments of Exp. III, final peduncle length and final leaf petiole length of the same phytomer remained closely related (Fig. 2a), indicating a similar effect of water deficit on leaf and inflorescence expansion. We showed earlier (Belaygue et al., 1996) that the reduction in final leaf area was related to the proportion of expansive development of the lamina subjected to water deficit and to the intensity of water deficit. Normalized reduction in final petiole length (difference in final petiole length between well-watered and water deficit plants divided by petiole length of well-watered plants) was related to normalized reduction in RWC (Fig. 2b) for phytomers which experienced water deficit at least during the period of expansive development of their organs (e.g., from 5.5 to 12.5 plastochrons, Bissuel-Belaygue, 1996). Plastochron is the time required for the initiation of a new phytomer by the apical meristem (Rickman and Klepper, 1995) and it is used as a unit of development time of the various organs of each phytomer. These results show that the reduction of peduncle elongation of an inflorescence was closely related to the intensity of water deficit as well as the petiole growth and the lamina expansion of its axillary leaf.
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Fig. 2. (a) Final length of inflorescence peduncle as a function of final length of axillary leaf petiole, for plants of genotype G2 (filled symbols) and G3 (open symbols) subjected to well-watered treatment (circle), moderate water deficit (triangle) and severe water deficit (diamond) in Exp. III. (b) Normalized reduction in final petiole length as a function of normalized reduction in relative water content (RWC). For each variable, normalized reduction is the difference between mean values in the control and the water deficit treatment divided by the mean value in the control. Each point represents one treatment (well-watered, circle; moderate water deficit, triangle; severe water deficit, diamond) for each genotype (G1 to G5) in each experiment (I to IV).
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Number of Reproductive Stolons per Plant
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The number of reproductive stolons (NSr) initiated per plant during water deficit periods was reduced by the long water deficits (Exp. II, III, and IV, Table 2). Severe water deficits (S) were associated with the largest reductions (22% to 93%), while moderate water deficits (M) had less impact on NSr. For genotype G2 in Exp. III, M water deficit treatment, applied for 68 d, even significantly increased NSr (33%) compared with the control. The total number of stolons initiated per plant (Table 2) was more affected by water deficit than was NSr. For example, in Exp. III, genotype G3 showed a reduction of NS of 64% and 78% in M and S treatments respectively, whereas the corresponding reductions of NSr were 14% and 49%. As a consequence, the percentage of reproductive stolons was increased by water deficits (except in Exp II), despite large differences in the number of stolons produced per plant between the experiments (Table 2). Within a range of moderate water deficits (Exp. IV), small reductions of leaf RWC (Fig. 1) were associated with an increased percentage of reproductive stolons (Table 2). Further reductions of RWC (from M to S treatments) decreased (Exp. II-G1 and Exp. III-G3) or did not change (Exp. III-G2) the percentage of reproductive stolons (Table 2).
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Table 2. Number of reproductive stolons per plant (NSr), number of stolons per plant (NS), percentage of reproductive stolons, number of inflorescences per reproductive stolon (NI), number of phytomers per stolon (NP), percentage of reproductive phytomers produced during the treatment period.
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Number of Inflorescences per Reproductive Stolon
The number of inflorescences per reproductive stolon (NI) was largely unaffected by moderate water deficits (Table 2). On the other hand, the number of phytomers produced per stolon (NP) and the number of stolons produced per plant (NS) were reduced by a moderate water deficit. As a consequence, the percentage of reproductive phytomers produced per plant during a treatment period (NIxNSr / NPxNS) was increased by moderate water deficits (Table 2). Depending on the experiment, the total number of inflorescences produced per plant was increased (Exp. III-G2), not impaired (Exp. I and Exp. III-G3) or decreased (Exp. IV) by M treatments compared with the well-watered treatments (Table 3).
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Table 3. Number of inflorescences per plant, number of viable florets per inflorescence (NFv), number of ovules per floret (NO), and number of ovules per plant (potential seed number, P).
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Severe water deficit reduced the number of inflorescences per reproductive stolon (NI) by 50% (Exp. III-G2) and 65% (Exp. II) compared with the control, while no difference was recorded in Exp. III-G3 (Table 2). As the number of phytomers produced per stolon (NP) and the number of stolons produced per plant (NS) were largely reduced by severe water deficits (Table 2), the number of inflorescences per plant was always reduced by S treatments compared with the control (Table 3).
Number of Viable Florets per Inflorescence
The number of viable florets per inflorescence (NFv) was not affected or slightly increased by moderate water deficits and was appreciably reduced by severe water deficit (10 to 50%, Table 3). The number of florets initiated per inflorescence (NFi) was the same for well-watered and water deficit plants (Table 4). As a consequence, differences in NFv between treatments resulted from some floret abortion at bud stage. All inflorescences that experienced severe water deficits, exhibited some floret abortion (see column p1 for Exp. II in Table 4). Some of these inflorescences failed to develop at an early stage (after buds emerged from the stolon tip but before inflorescences reached the pollination stage and peduncle elongation was complete) and all their florets aborted (not shown). Some floret abortion was recorded on well-watered plants (C) and moderate water deficit plants (M). The percentage of inflorescences that exhibited floret abortion (column p1 in Table 4) and the percentage of aborted florets in these inflorescences (column p2 in Table 4) were greater in C plants than in M plants. These results explain why mean NFv was slightly increased by moderate water deficits compared with the control in Exp. I (Table 3).
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Table 4. Average number of florets initiated per inflorescence (NFi), percentage of inflorescences exhibiting some floret abortions (p1), and percentage of floret abortions on these inflorescences (p2).
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Number of Ovules per Floret
In control plants NO differed between genotypes; four in G3 and G4, and six in G1, G2, and G5 (Table 3). It was not affected by water deficit, except in Exp. III where it was lower in the moderate water deficit treatment.
Potential Seed Number per Plant
Severe water deficit markedly decreased reproductive potential (P) per plant (e.g., ovule number per plant) compared with the control (Table 3). The effect of moderate water deficit on P was less pronounced and was sometimes nil (Exp. I) or positive (Exp. III-G2). The differences in P between treatments resulted, above all, from the differences in the number of inflorescences per plant. For each group of genotypes, with four or six ovules per floret, P was closely related to the inflorescence number per plant, regardless of the moisture treatment (Fig. 3)
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Fig. 3. Potential seed number (i.e., number of ovules per plant) as a function of number of inflorescences per plant for each group of genotypes, having 4 (filled symbols; G3 and G4) or 6 ovules per floret (open symbols; G1, G2, and G5). Each point represents one treatment (well-watered, circle; moderate water deficit, triangle; severe water deficit, diamond) for each genotype in each experiment.
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Dry Matter Accumulation per Stolon and per Plant
Dry matter accumulation per plant was reduced by water deficit in comparison to the control (Table 5). Long and severe water deficit (Exp. II) was associated with the largest reduction (88%) while short and moderate water deficit (Exp. I) had less impact (22%).
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Table 5. Dry matter accumulation per plant and per stolon (e.g. nodes, petioles, leaves and inflorescences) during the treatment period in experiments I and II.
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Dry matter accumulation per stolon during the water deficit period was similar in C and M plants (Table 5), indicating that differences in plant dry matter resulted mainly from the differences in number of stolons (NS) initiated per plant during the water deficit period (Table 2). Severe water deficit reduced simultaneously the number of stolons (Table 2) and dry matter accumulation per stolon (Table 5). A limited number of measurements of net leaf photosynthesis in similar experiments (data not shown) suggest that moderate water deficit did not alter photosynthetic rate (3 to 18% increase in M plants, not significant) and severe water deficit strongly reduced it (significant decrease of 37 to 55% in S plants).
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DISCUSSION
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Reproductive Development Is Not Impaired by a Moderate Water Deficit that Reduces Vegetative Growth
Vegetative growth and reproductive development showed differential responses to water deficit. Any reduction of RWC was associated with a decrease of phytomer and stolon growth (Belaygue et al., 1996). A moderate water deficit (less than 12% reduction of RWC and no symptom of leaf wilting), was sufficient to greatly reduce stolon branching and to induce a 30% reduction in the final size of the vegetative organs of the phytomers (internode, leaf lamina, and leaf petiole) and of the peduncle length of the inflorescences (Fig. 2a), indicating that reproductive organs experienced similar water deficit as vegetative organs. Nevertheless, moderate water deficit did not affect the development of the reproductive organs of these inflorescences (florets, ovules) and increased the proportion of reproductive stolons per plant compared with well-watered plants (Table 2). These responses, combined with the reduction in leaf expansion and the inhibition of stolon branching (Belaygue et al., 1996), increased reproductive-to-vegetative ratio both at the plant level and at the phytomer level (Table 2). The production of phytomers per stolon, and consequently the number of inflorescences per stolon, were not affected by moderate water deficit. These results, obtained at stolon level, can explain the behavior of white clover seed crops if we consider them as a population of stolons (Clifford,1986b; Danyach-Deschamps and Wery, 1988; Oliva et al., 1994). These authors showed in field experiments that a certain level of water deficit was necessary to obtain the highest density of inflorescences and maximum seed yields. A soil water potential of -80 x 10-3 MPa in the rooting zone (corresponding to the moderate water deficits in Exp. I) may provide the optimal level of soil-plant dehydration required to induce the highest proportion of inflorescences per unit area without affecting development of florets and ovules, thereby giving the maximum number of ovules per unit area for a white clover seed crop.
The effect of severe water deficit on the components of final seed yield has been studied for different species and the results show that both flower bud and young pod development are highly sensitive stages of the reproductive development in grain legumes such as soybean [Glycine max (L.) Merr.], (Westgate and Peterson, 1993) or pea (Pisum sativum L.), (Ney et al., 1994). These authors concluded that final loss in seed yield was due primarily to fewer pods per plant. In white clover, the reduction in potential seed number (P) in severe water deficit plants resulted primarily from a limited number of inflorescences produced per plant (Table 3). This is in agreement with the results obtained on white clover seed crops grown in the field by Clifford (1986a), Danyach-Deschamps and Wery (1988) and Oliva et al. (1994), who reported that final seed yield was closely related to the number of inflorescences per unit area. We conclude from our studies that severe water deficit impaired the production of reproductive stolons per plant (NSr) by reducing stolon branching, and the production of inflorescences per stolon by reducing the rate of node appearance and ultimately the final number of nodes per stolon (Belaygue et al., 1996). Thus, the number of inflorescences per unit area, which is the primary component of seed yield, is significantly reduced by severe water deficit. Inflorescence and floret abortion (Table 4) also contributes to the reduction of potential seed number (P) as observed in our experiments where P was decreased by 56 to 99% in plants submitted to severe water deficit compared with well-watered plants (Table 3). Three of the four components of the reproductive potential (NSr, NI and NFv in Eq. [1]) were greatly reduced by severe water deficit. Under field conditions, this would result in a limited number of ovules per unit area thereby limiting seed yield.
Implications for Assimilate Partitioning between Reproductive and Vegetative Sinks
Moderate water deficit reduced the number of stolons (Table 2) and thereby the plant biomass but did not affect the dry matter accumulation per stolon during the water deficit period (Table 5). As also suggested by a limited number of measurements of net leaf photosynthesis (data not shown), we can make the hypothesis that photoassimilate availability was not affected by moderate water deficit. This is in agreement with numerous studies that suggest photosynthetic capacity is not significantly impaired under mild water deficits (Vu et al., 1987; Battistelli et al., 1992; Cornic et al., 1992 cited by Daie, 1996). Kaiser (1987) reported that plants were able to maintain maximal photosynthetic rates even when leaf tissue dehydrated to 50% because CO2 concentrations were maintained at saturating levels. Our results suggest that moderate water deficit may not limit stolon carbon supply, but the priority of assimilate partitioning during reproductive development may be modified as the growth and development (and so carbon demand) of vegetative organs is significantly reduced. Reduced competition between sink organs could be facilitated by hormonal regulation of sink strength of the developing reproductive organs as in other indeterminate species like tomato (Lycoperiscon esculentum L.), (Ho, 1996). This may result in a diversion of assimilates from the vegetative organs developing in each stolon bud to the inflorescences. This also could explain why reproductive phytomers developed under moderate water deficit produced an inflorescence with a reduced peduncle length (Fig. 2) and with a higher proportion of viable florets (Table 3) compared with well-watered conditions.
Severe water deficits considerably decreased plant dry matter production (Table 5) by inhibiting stolon production (NS in Table 2) and by reducing leaf area (Belaygue et al., 1996), photosynthetic rate (data not shown) and dry matter accumulation per stolon (Table 5). This lack of carbon supply at the stolon level could significantly impact overall growth and major components of potential seed yield and could explain the considerable number of inflorescence and floret abortions in severe water deficits.
Implication for Irrigation Management
As we improve our understanding of the effects of soil water status on the processes of vegetative growth and reproductive development and of the effects of these processes on potential seed yield per plant, better control of crop development can be achieved by proper irrigation management. Our results have led us to assess that limiting the amount of water supplied, in order to maintain soil water potential between -60 x 10-3 MPa (Danyach-Deschamps and Wery, 1988) and -80 x 10-3 MPa (Exp. I) in the active rooting zone, from the initiation of flowering and during the whole period of reproductive development, would favor inflorescence production and development and would result in the highest seed yield potential (number of fertile ovules per plant) in white clover seed crops. As shown in a companion paper (Bissuel-Belaygue et al., 2002), this moderate water deficit had no adverse effect on fertilization efficiency, seed set, and seed filling. These results might aid in irrigation management of white clover seed crops and possibly other legume seed crops known to have an excessive leaf growth under well-watered conditions.
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ACKNOWLEDGMENTS
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Helpful suggestions from the editor and the anonymous reviewers are gratefully acknowledged. The authors thanks Philippe Naudin for technical assistance in preparation and maintenance of greenhouse experiments.
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NOTES
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This work was supported by INRA, IGER via Biotechnology and Biological Science Research Council (BBSRC), RAGT company (Rodez, France) and European Union (EC Air 3 CT92-0279).
Received for publication February 18, 2001.
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C. Bissuel-Belaygue, A. A. Cowan, A. H. Marshall, and J. Wery
Reproductive Development of White Clover (Trifolium repens L.) is Not Impaired by a Moderate Water Deficit That Reduces Vegetative Growth: II. Fertilization Efficiency and Seed Set
Crop Sci.,
March 1, 2002;
42(2):
414 - 422.
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ABSTRACT
INTRODUCTION
