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Agronomy Journal 92:772-779 (2000)
© 2000 American Society of Agronomy

PEA

Genetic Characterization of Flowering of Diverse Cultivars of Pea

José A. Alcaldea, Timothy R. Wheelerb and Rodney J. Summerfieldb

a Dep. de Ciencias de Recursos Naturales, Facultad de Agronomía e Ingeniería Forestal, Pontificia Univ. Católica de Chile, Casilla 306, Santiago 22, Chile
b Dep. of Agric., The Univ. of Reading, Earley Gate, P.O. Box 236, Reading RG6 6AT, UK

jalcalde{at}puc.cl


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Genetically controlled responses to photoperiod and temperature combine to determine wide variations in flowering time of pea (Pisum sativum L.) and aid the breeding of crops adapted to target environments. Our objective was to model the flowering responses of eight diverse cultivars of pea to photoperiod and temperature in order to estimate their probable flowering genotype for the flowering time genes Lf, Sn, E, and Hr. Plants were grown in pots in 11 contrasting semicontrolled and natural environments. Durations from sowing to flowering and nodes of first initiated flowers were recorded. Photothermal flowering responses of each cultivar were quantified using a two-plane photothermal model that linearly relates the rate of progress from sowing to flowering with the mean preflowering values of temperature and/or photoperiod. Flowering genotypes were estimated based on the similarities of these response planes to those of reference lines of known flowering genotypes. The estimated flowering genotypes of the eight cultivars were NZ 6753, lf sn; Bolero, Lf Sn hr; Conway, Lfd Sn hr; Fjord, Lfd Sn Hr; Botánica-INIA, Lf Sn hr; Amarilla-INIA, Lfd Sn hr; Lebu Loma 13, lf e Sn hr; and Catrico SS, Lfd Sn Hr. The flowering responses described by the linear photothermal model provided quantitative information for estimating the flowering genotype complementary to that of the usual semiquantitative approach based on the node of flower initiation. We concluded that the linear photothermal model provides a sound basis for describing the flowering responses of pea cultivars in order to estimate their probable flowering genotype.

Abbreviations: 1/f, rate of progress from sowing to first flower • f, duration in days from sowing to first flower • NFI, node of first initiated flower • RMSD, root mean square of deviation of 1/f • TFI, duration in days from sowing to initiation of first flower


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
PHENOLOGY is one of the most important single factors influencing adaptation and yield of annual crops (Richards, 1991). In pea, flowering time is mainly determined by genotype-specific responses to photoperiod and temperature (Murfet, 1985; Roberts and Summerfield, 1987; Yan and Wallace, 1998). Late cultivars are generally more sensitive to photoperiod than early ones (Pate, 1975; Summerfield and Roberts, 1988). Responsiveness to photoperiod is conferred by genes Sn (Barber, 1959; Murfet, 1971b), Dne (King and Murfet, 1985), and Ppd (Arumingtyas and Murfet, 1994), which are responsible for the production in short days of an as yet unidentified graft-transmissible flowering inhibitor in the leaves and cotyledons (Murfet, 1971c; Murfet and Reid, 1973; Weller et al., 1997). Gene Hr (Murfet, 1973) prolongs the expression of the Sn Dne Ppd system in shoots (Reid and Murfet, 1977; Reid, 1979), while gene E (Murfet, 1971a) reduces its activity in cotyledons (Murfet, 1971c). Gene Lf, with four alleles, governs the sensitivity of the shoot apex to the flowering signal (Murfet, 1971c), and determines minimum nodes of flower initiation of 5, 8, 10, and 15 for lfa, lf, Lf, and Lfd, respectively (Murfet, 1977; Yates and Murfet, 1978).

Variations at these six loci determine the four main maturity classes of pea defined by Murfet (1971a, 1977, 1985). Most cultivars have the genotype Lf E Sn Dne Ppd hr, defined as wild type for convenience (Murfet and Reid, 1993), which results in a late-flowering, quantitative long-day habit (class L). Substitution of hr with Hr gives a late flowering type with a strong response to photoperiod (class LHR), while substitution of Lf with lf (or lfa) results in an early photoperiodic type in which short days do not delay flower initiation, but do delay the appearance of open flowers and also extend the duration of the reproductive phase (class EI). Substitution of sn, dne, or ppd leads to an early-flowering, day-neutral habit (DN class). Mutations at genes Dne and Ppd are less widely distributed among modern cultivars (I.C. Murfet, 1996, personal communication); hence, DN-class cultivars are generally sn.

The classes differ not only in the duration from sowing to flowering (f) but also in related attributes such as the node of first initiated flower (NFI) and the time (in days) of flower initiation (TFI). Within-class differences are determined by major genes, minor modifier genes, and quantitative polygenic systems (Reid and Murfet, 1976; Murfet, 1985). Several genes that govern internode length (such as Le, Na, Lka, and Lw) also act as minor flowering modifiers by influencing the rate of production of nodes and subtended structures (Marx, 1975; Murfet, 1977; Murfet and Reid, 1993).

Knowledge of the flowering genotype is desirable not only in pea breeding lines, but also to predict phenology in order to match the adaptation of a cultivar to its target environment (Aitken, 1974; Richards, 1989). Estimation of the flowering genotype from phenotype observations in pea is normally based on differences in NFI as well as in f in contrasting photoperiods, which are normally imposed with moderately warm (15–20°C) temperatures (Murfet, 1971a, 1971b, 1973). While this approach is well suited for genetic and physiological studies, it does not provide a direct quantitative means of predicting crop phenology in the field, and is not always sufficient to estimate the flowering genotype (Murfet, 1981).

Simple models for the prediction of harvest time of fresh pea based on thermal time accumulation above a specified base temperature (normally between 0 and 5°C) predict with good confidence only across a limited range of environments (Mikkelsen, 1981; Friis et al., 1987; Cervato and Piva, 1989). More robust predictions require knowledge of photothermal responses and genetic differences therein (Summerfield and Roberts, 1988).

The photothermal flowering responses of several cultivars of pea of contrasting maturity type have been shown to conform well to the linear photothermal model (Hadley et al., 1983; Roberts and Summerfield, 1987; Summerfield et al., 1991). This model relates the rate of progress from sowing to flowering (1/f) as a linear function of mean temperature and mean photoperiod between sowing and flowering. It has been used to describe the phenology of a wide range of pea cultivars of known flowering genotype (Alcalde et al., 1999).

The linear photothermal model described by Hadley et al. (1983) and Roberts and Summerfield (1987) is mathematically expressed as

(1)

(2)
where T and P are the respective mean values of preflowering temperature and photoperiod; Pc is the critical photoperiod that is defined by the intersection of the planes described by the two equations; Tb and To are the base and optimum temperatures, respectively; and a, b, a', b' and c' are genotype-specific coefficients.

Our objectives were to model the photothermal flowering responses of diverse cultivars of pea in terms of 1/f, and to estimate the most probable flowering genotype (for genes Lf, Sn, E, and Hr) of these cultivars by comparison with the responses of lines of known flowering genotype (Alcalde et al., 1999). The estimated flowering genotypes also were compared with those derived by analyzing variations in TFI.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Plant Material and Photothermal Environments
Eight cultivars of pea were selected to represent diversity in maturity class, origin, and genetic background. These were `Bolero' (Asgrow Seed Co., Kalamazoo, MI), `NZ 6753' (Canterbury Seed Co., Canterbury, New Zealand), `Conway' (W. Brotherton Seed Co., Moses Lake, WA), `Fjord' (Svalof Weibull AB, Hammenhog, Sweden), `Botánica-INIA' (Instituto de Investigaciones Agropecuarias [INIA], Temuco, Chile), `Amarilla-INIA' (INIA), and two selections from the old Chilean landraces `Lebu Loma 13' and `Catrico SS'. Amarilla-INIA, Lebu Loma 13, and Catrico SS are tall types and, with the exception of the semileafless (af) Fjord, all cultivars have leaves of conventional morphology.

Two plants of each cultivar were grown in 4-L-capacity pots, with a 10:10:3 (v/v) mixture of vermiculite, perlite, and peat, and irrigated to excess three times each day with a 100 mg N kg-1 complete nutrient solution (Summerfield et al., 1977). Daily observations were made to determine the date of first flower (corolla color visible) for each plant; NFI was also recorded. Three replicates (pots) were grown in at least 11 contrasting photothermal environments (Table 1) at Santiago, Chile (33°28' S, 70°36' W; altitude 600 m) from 1996 to 1997. NZ 6753 and Bolero were grown in three and four additional environments, respectively, which consisted of delayed sowing dates in outside environments.


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  		Table 1 Photothermal environments and experimental durations (d) in which the cultivars of pea were evaluated

 
Photoperiod was extended using 100-W incandescent spotlight bulbs, suspended 1.6 m above the pot surface and spaced at 1.7 by 2.1 m to ensure a minimum irradiance of 4 W m-2 at pot level (Summerfield and Roberts, 1987). Lighting in the growth chamber consisted of cool white fluorescent tubes at approximately 65 W m-2 at pot level for 8 h d-1, in addition to incandescent lighting as in the glasshouse environments. Natural photoperiods in the glasshouse and natural environments were taken to be inclusive of civil twilight (true center of the sun 6° below horizon [Summerfield and Roberts, 1987]). Photoperiod was calculated based on astronomical data with latitude and date as inputs (Monteith and Unsworth, 1990). Temperature was recorded hourly from aspirated and shielded thermocouples suspended 0.8 m above the pot surface.

Analysis of Results
The linear photothermal model described by Eq. [1] and [2] was fitted to data of each cultivar individually using the RoDMod software (Watkinson et al., 1994), which uses the MINIM procedure described by Shaw and Wedderburn (1971) for use with FORTRAN 77. Prior to analysis, values of f were transformed to 1/f. Average values of the two plants in each pot were used in the analyses.

Comparison of regression analyses were performed between cultivars to establish similarities in their thermal and photothermal plane responses. Data from the thermal and photothermal planes were analyzed separately, and data of all cultivars in each comparison group were pooled prior to comparison of regression analysis. Responses were considered similar if the residual sum of squares for 1/f of the grouped cultivars was not significantly different (P > 0.05) from the residual sum of squares of the set of individual responses. Base temperatures when (i.e., when ) were compared by calculating the mean Tb of all cultivars; linear responses were then constrained through this value. Significance of changes in residual sum of squares between the constrained and nonconstrained responses was then determined.

The most probable flowering genotypes of these cultivars were established by comparing the actual values of 1/f in any given photothermal regime with the rates predicted from the planes determined for reference lines of known flowering genotype by Alcalde et al. (1999) (Table 2) , using the following equation:

(3)
where RMSD is the root mean square of deviation of 1/f, Oi is the observed rate of progress from sowing to flowering of the cultivar in a given environment, Pi is the predicted rate of progress from sowing to flowering in that environment from the planes derived for a specific reference line, and n is the number of observations. The coefficients of the linear photothermal model of cultivars and reference lines also were compared.


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  		Table 2 Coefficients of the linear photothermal model of reference lines of known flowering genotype determined by Alcalde et al. (1999). Values of a, b, a', b' and c' are x 105.{dagger}

 
Compatibility between the flowering genotypes estimated through their linear photothermal responses and those estimated by analysis of NFI (Murfet, 1973) was also evaluated.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
The cultivars showed wide variation in time from sowing to flowering in all environments evaluated, ranging from 39 to 59 d for NZ 6753 and Conway in the 18 h d-1, 15°C regime (Table 3) . This range increased under short days (12 h d-1, 15°C), from 42 d for NZ 6753 to 95 d for Fjord. The delay of flowering time by short days was greater at warmer temperatures in all cultivars except NZ 6753, and was most pronounced in Fjord and Catrico SS. NZ 6753 was clearly different from the others because it was only slightly affected by photoperiod.


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  		Table 3 Duration (d) from sowing to first flower (f ± SD) in eight cultivars of pea as affected by mean temperature and mean photoperiod from sowing to flowering. Data are means and standard deviations of three pots with two plants in each pot

 
Fitting the linear photothermal model to each cultivar individually accounted for >=85% (P < 0.00001) of the variation in 1/f (Table 4) . The actual rates of progress from sowing to flowering for each cultivar, together with the fitted response planes (Fig. 1) , illustrate how well Eq. [1] and [2] combined to describe the photothermal flowering responses of these cultivars. While NZ 6753 showed the single-plane response typical of DN-class genotypes, all other lines showed a clear two-plane response, defined by Eq. [1] and [2]; i.e., the thermal and photothermal planes, respectively.


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  		Table 4 Coefficients of the linear photothermal model fitted to each cultivar individually. Values of a, b, a', b', c' and SE terms are all x 105.{dagger}

 


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  		Fig. 1 Rate of progress from sowing to flowering in response to mean preflowering values of temperature and photoperiod in cultivars of pea from diverse origins. Points represent the means of three replicates grown in a single photothermal environment. Error bars (where visible) indicate difference between observed and estimated values when larger than the marker

 
Comparison of Regression Analyses
At a relatively warm temperature (19°C), predicted values of 1/f in the photothermal plane under long days (18 h d-1) were very similar for all photoperiod-sensitive cultivars (Fig. 2a, 2c, and 2e) . However, under short days (12 h d-1), cultivars differed significantly (P < 0.05): Bolero and Amarilla-INIA had the largest values of 1/f (Fig. 2a); Botánica-INIA, Lebu Loma 13, and Conway as a group had intermediate rates (Fig. 2c); and Fjord and Catrico SS (Fig. 2e) had the smallest values of 1/f. For example, at 12 h d-1 and 19°C, values of 1/f were around 0.017 d-1 for the first pair, 0.014 d-1 for the second group, and 0.010 d-1 for the third. Temperature sensitivity in the photothermal plane (b') was least for Botánica-INIA (Fig. 2d) but did not vary considerably among cultivars, nor did it follow any particular trend with photoperiod sensitivity.



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  		Fig. 2 Relations derived from Fig. 1 showing fitted photothermal flowering responses to (a, c, and e) mean preflowering photoperiod at 19°C, and to (b, d, and f) mean preflowering temperature under a 12 h d-1 photoperiod, for seven photoperiod-sensitive cultivars of pea

 
Comparison of regression analyses of photothermal planes indicated that Botánica-INIA, Lebu Loma 13, and Conway had similar photothermal plane responses (P > 0.05) (Fig. 2c and 2d) in spite of the smaller sensitivity to temperature of Botánica-INIA. The photothermal planes of Fjord and Catrico SS also were very similar (P > 0.05) (Fig. 2e and 2f). However, photothermal plane responses of Amarilla-INIA and Bolero, which had the largest rates of progress from sowing to flowering in the photothermal domain, differed significantly (P < 0.001) (Fig. 2a and 2b).

Comparison of regression analyses of thermal planes indicated that the similarities between Fjord and Catrico SS also applied to thermal plane responses (P > 0.05), and these also matched those of Conway (Fig. 3b and 3c) . Similarly, Bolero and Botánica-INIA also shared a common thermal plane response (P > 0.05) (Fig. 3a and 3b). No other similarities in thermal plane responses were found, nor was there a common base temperature for all cultivars (P < 0.001). The thermal plane of NZ 6753 predicted much greater values of 1/f at any given combination of mean temperature and photoperiod as compared with the thermal planes of the photoperiod-sensitive cultivars.



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  		Fig. 3 Thermal plane responses of rate of progress from sowing to flowering to mean preflowering temperature in seven photoperiod-sensitive cultivars of pea. Responses of reference lines of known flowering genotype derived by Alcalde et al. (1999) have been added for comparison. Points are the means of three replicates

 
Estimation of Flowering Genotypes
In Fig. 4 , variation in 1/f with mean preflowering temperature for NZ 6753 is compared with the responses of the DN class lines L69 (lfa E sn hr) and L59 (lf E sn hr). The responses of NZ 6753 were intermediate between the two reference lines; however, the deviation measured as RMSD from line L59 was least (Table 5) , and therefore NZ 6753 was estimated to have a lf_sn hr flowering genotype.



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  		Fig. 4 Rate of progress from sowing to flowering in response to mean preflowering temperature in NZ 6753 (circles) as compared with the responses of reference lines L69 (lfa E Sn hr) and L59 (lf E Sn hr). Points are the means of three replicates

 

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  		Table 5 Root mean square deviation (RMSD) of the rate of progress from sowing to flowering (as calculated by Eq. [3]) of eight cultivars of pea as compared with reference lines of known flowering genotype. Italics indicate the lowest RMSD values. Values are all x 105{dagger}

 
Comparisons based on deviations between the actual values of 1/f of the photoperiod-sensitive cultivars and the responses of reference lines for data from the thermal plane (Table 5 and Fig. 3), together with similarities in the magnitude of the coefficients of the linear photothermal model (Tables 2 and 4), indicated that Bolero and Botánica INIA most probably have allele Lf; that Amarilla INIA, Conway, Fjord and Catrico SS have allele Lfd; and that Lebu Loma 13 has allele lf.

Interpretations based on similar comparisons show that deviations from the responses of reference lines within the photothermal plane (Table 5) indicate that the responses of Bolero and Amarilla-INIA were close to those of the L class lines L53 (lf e Sn hr), L24 (Lf e Sn hr), and L31 (Lfd E Sn hr); hence, we propose them to be Sn hr. Responses of Fjord and Catrico SS were very similar to those of L67 (Lfd E Sn Hr); hence, they are likely to be Sn Hr. The responses of Botánica-INIA, Lebu Loma 13, and Conway in the photothermal plane (Fig. 2c and 2d) were intermediate between the responses of the Sn hr and Sn Hr reference lines. However, Botánica-INIA and Lebu Loma 13 had 1/f values in the photothermal plane closer to those of L53 or L31 (Sn hr) than to L63 or L67 (Sn Hr) (Table 5), so we propose that they are Sn hr rather than Sn Hr. Similarly, considering a likely Lfd genotype for Conway, its photothermal flowering responses were closer to those of L31 than L67 (Table 5); hence, we propose a configuration of Sn hr. These genotypes are compatible with the similarities between cultivars as determined by comparison of regression analysis discussed earlier.

The genotypes now proposed for the cultivars investigated with respect to the flowering genes Lf, Sn, and Hr are summarized in Table 6 . Corresponding maturity classes for each cultivar also are indicated together with the reference line that each cultivar most closely resembled. With the exception of Lebu Loma 13, the genotype at the E locus could not be determined due to epistasis of Lf or Lfd over E in photoperiod-sensitive genotypes, and that of sn over E in the DN genotypes (Murfet, 1971a and 1971b).


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  		Table 6 Flowering genotypes of eight cultivars of pea estimated by differences in rates of progress from sowing to flowering as quantified by the linear photothermal model, or by differences in the node of flower initiation (NFI). As a result of combining the two approaches, the most probable flowering genotypes also are presented. Corresponding maturity classes and closest reference lines also are indicated

 
Estimation of Flowering Genotype Based on the Node of Flower Initiation
The observed NFI values for the cultivars investigated together with those of the reference lines of known flowering genotype are presented in Table 7 . The NFI recorded in the 18 h d-1, 15°C environment (regarded as a strongly inductive regime [Murfet, 1973, 1977; Yates and Murfet, 1978]) was used to estimate the genotype at the Lf locus. NZ 6753, with an NFI of 10, very similar to that of L59, was estimated to be lf. Bolero, Botánica-INIA, and Lebu Loma 13 had NFI values intermediate between L53 (lf e Sn hr) and L24 (Lf e Sn hr) (Table 7); hence, they may be either lf or Lf. Conway, Fjord, Amarilla-INIA, and Catrico SS, with mean NFI above 16, were proposed as Lfd.


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  		Table 7 Node of flower initiation (NFI) under various photothermal regimes for eight cultivars of pea and for reference lines of known flowering genotype. Data are means and standard deviations of three replicate pots with two plants in each pot

 
Estimation of the genotype at loci Sn and Hr was based on the increase in NFI under a 12 h d-1 as compared with a 18 h d-1 photoperiod when combined with relatively warm mean temperatures (Table 7). NZ 6753 had no increase in NFI in short days; hence, it may well be sn and so classified as DN. Bolero, Conway, Botánica-INIA, Amarilla-INIA, and Lebu Loma 13 had a clear photoperiodic delay of between 5 and 10 nodes, whereas Catrico SS had a response greater than 40 nodes. The former group is probably Sn hr, whereas Catrico SS is Sn Hr. A loss of apical dominance in Fjord, such that node counts were not directly comparable between lateral branches and the main stem, has been interpreted to indicate a putative Lfd Sn Hr flowering genotype based on the observations of Murfet and Reid (1985).

Strong agreement was found between the flowering genotypes proposed by differences in 1/f as quantified by the linear photothermal model, and by differences in node counts (Table 6). Differences between the two approaches were evident only when differentiating among Lf alleles. This is not surprising, given that Murfet and Reid (1993) proposed the existence of further alleles at this locus with subtly different strengths, which cannot be adequately identified by conventional genetic analysis.

Deducting flowering genotypes from field observations alone is usually hindered by the natural correlation between temperature and photoperiod as the growing season progresses. Additional evaluations in controlled environments, and/or modified field environments (i.e., daylength extensions), are considered key in breaking such a correlation, allowing for a more accurate determination of response planes. The possibility of directly relating the photothermal flowering responses of cultivars of pea to their putative flowering genotype without significant interference from genetic background has important implications for pea breeding and crop adaptation. First, this relation allows for a relatively simple and direct estimation of the flowering genotype of cultivars or selected breeding lines using their thermal and photothermal flowering responses. Second, a knowledge of the putative flowering genotype then permits crop flowering dates under diverse fluctuating photothermal environments to be predicted.


   ACKNOWLEDGMENTS
 
We thank the Facultad de Agronomía, Pontificia Universidad Católica de Chile for granting leave to the senior author and for providing the facilities and funding to conduct this research. We also thank Mr. H. Faiguenbaum (P. Universidad Católica de Chile) and Dr. M. Mera (INIA-Carillanca, Chile) for providing seed and information on the cultivars investigated, and Dr. Aldo Norero for valuable advice during the course of this investigation.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Research partially funded by Facultad de Agronomía e Ingeniería Forestal, Pontificia Univ. Católica de Chile.

Received for publication August 23, 1999.
   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 





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