Mutant dn Influences Dry Matter Distribution, Assimilate Partitioning and Flowering in Lathyrus odoratus L.

The flowering mutant dn in sweet pea was used as a tool to study¹⁴C-assimilate and dry matter partitioning with respect to nutrientdiversion theories on the control of flower initiation. Wildtype plants (Dn^h) are photoperiodic and exhibit late floweringand profuse basal branching in short days while mutant plants(dn) are day neutral, early flowering and devoid of basal laterals.In short days, dn plants exported a significantly greater proportionof assimilate acropetally than (Dn^h) plants and the upper portionof dn plants had a greater dry weight. These differences werereduced dramatically when basal laterals were excised regularlyfrom the (Dn^h) plants although the difference in flowering remained.However, the effect of dn on resource allocation within theapical region may be more important in regard to flowering thanthe effect on acropetal versus basipetal movement. In shortdays, the dn plants partitioned significantly more resourcesinto their internodes and petioles, and less into their leaflets,than Dn^h plants as shown by dry weight and ¹⁴C-assimilate measurements.These differences were apparent from as early as node 7 up tothe node of flower initiation in dn plants (node 30) and theywere not eliminated by removal of basal laterals from Dn^h plants.Differences between dn and Dn^h plants in partitioning and floweringwere largely eliminated under long days. The fact that in thisspecies a single gene influences both resource allocation andflower initiation lends further support to nutrient diversionhypotheses on the control of flowering. Key words: Assimilate partitioning, branching, flowering, mutant, sweet pea     

Internode Length in Pisum: the Response of le na Scions Grafted to Na Stocks

The recessive gene na results in peas with extremely short internodes(phenotype nana). In intact plants, na prevents expression ofthe Le/le gene pair which determine the tall/dwarf difference.na blocks a step early in the gibberellin biosynthetic pathwaywhile le prevents conversion of gibberellin A₂₀ to GA₁. Whengrafted to leafy Na stocks, le na and Le na scions become phenotypicallydwarf and tall, respectively. Hence, Na stocks provide a graft-transmissiblesubstance which promotes elongation of na scions and allowsexpression of the Le/le difference. Pisum, internode length, grafting, genotype     

Flowering in Pisum: the Sites and Possible Mechanisms of the Vernalization Response

Grafting experiments with several genotypes provide evidencethat vernalization acts through at least two mechanisms. Vernalization of the stock promoted flowering by 26 nodes ingenotype If e Sn Hr and 5.5 nodes in genotype If e Sn hr buthad no detectable effect in genotype If e sn hr. Cold treatmentappears to cause a higher ratio of promoter to inhibitor, atleast in part, through low temperature repression of Sn activity.This mechanism is particularly evident in the cotyledons sincethey form a major area of Sn activity during vernalization.Continuous light was shown previously to prevent Sn forminginhibitor. It seems therefore that both photoperiod and vernalizationhave an effect through the Sn gene. Vernalization of the shoot promoted flowering by 19 nodes ingenotype If e Sn Hr, 3 nodes genotype If e Sn hr, and 1 nodein genotype If e sn hr1 grafted to an If e Sn hr stock. Theshoot effect may result from one or possibly two mechanisms.Firstly, vernalization may lower the threshold ratio of promoterto inhibitor required at the apex for floral initiation. Thesame change in threshold could result in changes in the floweringnode of quite different magnitude depending on the rate of changein the hormonal levels in the different genotypes. Secondly,vernalization may disturb the ageing process relative to theplastochronic age leading to an earlier (nodewise) decline ininhibitor level.     

Flowering in Pisum: A Sixth Locus, Dne

A sixth major flowering gene, dne, is identified in the gardenpea (Pisum salivum L.). Linkage tests show dne is located onchromosome 3 near locus st. The ability to respond to photoperioddepends on the joint presence of the dominant genes Sn and Dnewhich together confer a long day habit. Genotypes Sn dne, snDne and sn dne are all essentially day-neutral although by examiningseveral flowering criteria under strictly controlled conditionssome small responses could be demonstrated. Like sn, dne reducedthe response to vernalization when substituted into genotypeSn Dne. It is suggested genes Sn and Dne both control steps in a biosyntheticpathway which leads to the production of a graft-transmissibleinhibitor of flowering and apical senescence. Stocks of genotypeSn dne and sn Dne promoted flowering in Sn Dne scions whileSn Dne stocks delayed flowering in Sn dne (and sn Dne) scions.However, reciprocal grafting between genotypes Sn dne and snDne gave no evidence of physiological complementarity correspondingto the genetic complementarity ofSn and Dne. Initial resultssuggest dne may be less effective than sn at blocking inhibitorproduction but this requires confirmation.     

The Influence of Genes ar and n on Senescence in Pisum sativum L.

The effect of genes ar (violet flowers, small hilum) and n (thick,fleshy pod wall) on whole plant in senescence peas was examinedby comparing Ar- with arar and N- with nn plants in segregatingprogenies. Homozygosity for ar or n significantly delayed the time whenthe plants were ready for harvest of their entire seed crop.These genes did not delay either the onset of reproduction orthe onset of apical arrest in the first instance. However, whereasAr- N- plants almost invariably senesced and died as the firstseed crop matured, the majority of arar and/or nn plants entereda period of secondary growth and a further fruiting cycle. Comparedwith Ar- plants, arar plants had over twice as many pods andseeds but individual seeds were 58 per cent lighter and totalseed yield (wt) was 19 per cent less. Pod length was unaffected.Compared with N-plants, nn plants had shorter pods (16 per cent),fewer seeds per pod (21 per cent), smaller seeds (20 per cent)and a lower total seed yield (wt 14 per cent less). It appearsthat ar and n impose a lower metabolic drain per reproductivenode as a consequence of their effects on hilum anatomy andpod morphology, respectively. These mutants disrupt the normalpattern of monocarpic senescence by breaking the coordinationbetween apical arrest and subsequent events. The developingseed crop delimited by the first arrest fails to cause plantdeath, possibly because sink size is less than in normal counterparts. Pisum sativum L, garden pea, senescence, hilum, pod, seed size, genetics     

A Comparison of the Flowering and Branching Control Systems in Lathyrus odoratus L. and Pisum sativum L.

In the sweet pea (Lathyrus odoratus L.) the difference in floweringbehaviourbetween photoperiodic (long-day) and day-neutral cultivars appearsto be due to a difference in their ability to produce a graft-transmissiblefloral inhibitor. The flowering control systems in the sweetpea and the garden pea (Pisum sativum L.) appear to be verysimilar on the basis of inter-generic graft results. It is suggestedthat the major flowering genes Dn* in L. odoratus and Sn andDne in P. sativum control steps in a biochemical pathway commonto these two species (which are related at the tribal level)and that the product of this pathway inhibits flowering andpromotes outgrowth of basal laterals in both species.     

Inheritance of cyanide-resistant respiration in two cultivars of pea (Pisum sativum L.)

Abstract Two pea cultivars (Pisum sativum L., cvs. Alaska and Progress No. 9) shown previously to differ with regard to the appearance of the cyanide-resistant (alternative) pathway of respiration in axis tissue, were found to show this same difference in mature leaf tissue and in epicotyl mitochondria. The possible relationship between dwarf growth form and lack of alternative respiration in cv. Progress No. 9 was tested in two ways. When dwarfism was alleviated in Progress No. 9 by application of exogenous gibberellin A₁, no appearance of the alternative pathway was observed. In a survey of eight other dwarf pea cultivars, five were found to have an alternative pathway comparable to that shown by the tall cv. Alaska, while three lacked the pathway (cf. Progress No. 9). In reciprocal crosses between Alaska and Progress No. 9, the alternative pathway capacity of F₁ progeny resembled that of the maternal parent. This pattern was maintained in all the F₂ generation, indicating maternal inheritance of the trait. These data suggest that alternative respiration in pea is, to some extent, under the control of an organellar genome.     

Internode Length in Pisum: Variation in Response to a Daylength Extension with Incandescent Light

Lines representing a range of internode length and floweringgenotypes in Pisum sativum L. were grown in 8 h of daylightfollowed by either 16 h of darkness or incandescent light. Thestem elongation response index (RI = length in 24 h ÷length in 8 h) was least in the very short internode nana types,which are grossly deficient in gibberellins (GAs), and the verylong internode slender types, which behave as if saturated withGAs. The common tall (genotype Le) and dwarf (le) types (lepartially blocks conversion of GA₂₀ to the active form, GA₁)were all markedly responsive but the peak RI (based on the mostresponsive internode) was less in tall lines (1.79 to 2.78)than in dwarf lines (2.32 to 5.01) and the peak RI tended tooccur about three to four internodes earlier in tall than indwarf lines. The cry⁸ mutation reduced the RI. (Duplicate lengthloci La and Cry are probably concerned with GA reception.) Amongle dwarf lines, genotype La cry⁸, was generally less responsivethan La Cry, La cry^c and la Cry. Data from crosses showed thaton either an le La or le la background cry⁸ segregates had alower RI than cry⁸ segregates. On an le la background, cry⁸plants were shorter than cry^c plants, cry⁸ was partially dominantto cry⁸ and segregation was clear only in long days. On an lela background, cry^c plants were shorter than cry^c plants, cry⁸was partially dominant to cry⁸ and segregation was clear inlong or short days. The very high peak RI (5.0) of the microcryptodwarfline, L57, appeared to result, in part, from a marked foreshorteningof internodes 4 to 10 in the 8 h regime. In the 24 h regimeL57 (lm) had a fairly similar growth pattern to normal (Lm)cryptodwarf types. The peak RI tended to occur at a lower internode in early thanlate flowering lines, especially among dwarf types, and genotypeswith a day neutral flowering habit (genotype sn or dne) wereless responsive than their photoperiodic counterparts (Sn Dne). White fluorescent light, given as a daylength extension, wasmuch less effective than incandescent light at stimulating stemelongation suggesting control through the phytochrome equilibrium(P_tr/P_total). Pisum sativum, garden pea, daylength extension, flowering, genotype, gibberellin, hormone receptor, incandescent light, internode length, phytochrome, stem elongation     

Flowering in Pisum: Kinetics of the Vernalization Response in Genotype If e Sn Hr

Previous work has shown that vernalization acts at two sites,one in the cotyledons and one in the shoot, in young plantsof genotype Ife Sn Hr. During the present study the size ofthe vernalization responses in both the cotyledons and shootincreased as the temperature was lowered from 17 to 3 °C.This occurred regardless of whether the treatment was givenfor the same chronological period of time or for the same physiologicalperiod of time. Vernalization treatment was effective from thetime the seeds were developing in the pods on the maternal plantuntil at least 20 leaves were expanded and became graduallymore effective as the length of the treatment was increasedfrom 2 to 5 weeks. High pre– or post–vernalizationtemperatures can reduce the cotyledon effect and to a lesserextent the shoot effect of vernalization. Devernalization occurredto a larger extent in low light intensities and darkness thanin high light intensities. No stabilization of the vernalizationeffects in the cotyledons or shoot appeared to occur at normalgrowing temperatures (15–25 °C). These results arediscussed in terms of the previously hypothesized mechanismsfor the cotyledon and shoot effects of vernalization. Pisum sativum, flowering, vernalization     

Flowering in Pisum: the Effect of Genotype, Plant Age, Photoperiod, and Number of Inductive Cycles

The genotypes If e Sn hr, Lf e Sn hr, and If e Sn Hr requirefewer inductive cycles as they age. It is suggested that thisresults from a decrease in the activity of the Sn gene in theleaves as they age, resulting in a higher ratio of promoterto inhibitor. Gene Lf does not affect the rate of this agingbut it does increase the number of inductive cycles requiredfor flower induction over the first 5 weeks of growth. The geneHr has no effect until week 4 but thereafter causes a reductionin the effect of age on the Sn gene. The genotype If e Sn Hrcan be induced by a single inductive cycle (32 h of light) fora relatively long period. The length of dark period required for the expression of theSn gene is shown to be less than 4 h providing a relativelylong photoperiod precedes the dark period. It appears that noper manent induction of tissue by photoperiods favourable toflowering occurs in peas. The critical photoperiod for plantsof genotype if e Sn Hr is shown to be between 12 and 14 h atl7·5 °C and the usefulness of the term ‘criticalphotoperiod’ is discussed with respect to quantitativelong-day plants.