THE EFFECTS OF ENVIRONMENT AND TIME OF HARVEST AT EARLY STAGES OF SELECTION

IN SUGAR CANE I. SELECTION IN PLANT CANE CROP

M. H. R. Julien, Z. Peerun and R. Domaingue Sugar Industry Research Institute, RCduit, Mauritius

ABSTRACT

T h e effects of three dates of harvest at an early stage of selection in a plant crop of sugar cane were studied in three contrasting environments. Brix, weight and cane number were measured in three families each repre- sented by 72 genotypes. Selection pressure was applied at two levels with respect fo four standards of different maturity characteristics.

Cane number and weight per stool were found to be highly variable, while brix proved to be the most reliable criterion for selection.

I t was found that families as a whole and their genotypes displayed specific adaptation to time of harvest but not to environment for brix. Se- lection rates for this character depended on the standard, the family and environment. These rates varied when different standards were used at different harvest dates, emphasizing the need for a critical choice of stan- dard. Further, differential selection rates for the families were observed at different harvest dates but not in different environments. The relative performance of genotypes at two harvest dates depended on the standard used and the selection pressure.

I t is shown that a combination of sampling dates, different standards, and different selection pressure can allow early selection of genotypes with different maturity characteristics.

INTRODUCTION

In sugar cane breeding, a selection programme starts with several thousands of seedlings which are reduced to a few dozens in the final stages. In the early stages, where unreplicated plots are generally used, some traits interact strongly with certain variables, especially the environment. According to Skinner, "' only 15% of the phenotypic variation of a genotype is genetic and the rest in environ- mentally induced. This situation is further complicated if, as it seems, different genetic mechanisms operate under different environmental conditions for the ex- pression of the same trait (Mariotti 6' '). Therefore it is generally agreed that selection should be lenient in the early stages to minimize the risk of discarding valuable genotypes.

Athough the effect of environment on selection in the early stages has been well studied (George; ' Mariotti; Skinner lo ) , less attention has been paid to the effect of time of harvest and the agronomic characteristics of the standards to be used for selection.

In Mauritius, sugar cane is grown in a mosaic of environments spread over a

small area and is harvested from June to December. As it has been shown that commercial varieties show adaptation to both time of harvest and environment (Julien and Delaveau 4), these two factors should be taken into consideration in a selection programme.

In Mauritius, George ' reached the conclusion that it was better to plant seedlings in the humid rather than in the poorer superhumid zone because of higher heritability values obtained there. This practice is still implemented and the next stage of selection, the "Bunch Selection Plot" (BSP), is laid down in either the humid of subhumid areas with single stool plots as in Hawaii (Urata and Warner "). A standard commercial variety is planted at every tenth location within a row. Selection in BSP is based primarily on Brix (refractometric solids), and the standard is M 377156, a widely adapted and late maturing commercial variety. The canes are not weighed but the number of canes per stool is taken into account during selection. In the BSP, brix is recorded early in the season (June) when the canes are 14 months old as it is believed that genetic differences for this trait are then most apparent. Because of the standard low sucrose con- tent early in the season, the selection of both early and late maturing genotypes is possible.

The main objetive of this study was to determine whether in the early stages (BSP) there is a need to select for adaptation to environment and date of harvest. Other objectives were to determine:

1. Whether brix, number of canes per stool, or weight of cane per stool is the best criterion for selection.

2. The performance of genotypes relative to standards of different maturity characteristics.

3. The selection rates at different selection pressures across environments and dates of harvest.

4. The relevance of quantitative genetic parameters to selection. 5 . The merits of selection in plant cane as opposed to selection in first ratoon

cane.

Results relevant to the last two objectives will be discussed in subsequent papers.

MATERIALS AND METHODS

Experimental Design

A split-plot design was adopted and consisted of two blocks of three main plots with three sub-plots each. The experiment was replicated in three contrasting environments (Table 1).

The main plot treatments were 3 dates of harvest: July (early), September (middle) and November (late). Progenies from one of the following crosses were planted in each sub-plot.

M 13/56 x Polycross M 55-5/60 x Polycross S 17 x M 201159 ,

Seventy-two genotypes, taken at random from each of these crosses, represen- ted a family. Two three-eyed cuttings were used to plant each genotype and

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Table 1. Characteristics of the Three Experimental Zones

Locality Zone Altitude

Annual Soil * Rainfall

(m) . (rnrn)

Belle Rive (BR) Superhumid F1 475 3 200 Mon Dtsert (MDA) Humid L2 425 2 000 Pamplemousses (PES) Subhurnid L2 70 1 400

* Parish and Feillaft.

standard as single stools spaced at 0,75 m X 1,5 m, in a completely randomized manner. The same genotypes for a given family were planted in all treatments and environments.

Each sub-plot consisted of four rows of 30 locations. Four standards, as given below were replicated three times in each row within the following positions: (1-4), (14-17), and (27-30). A line of standard across rows was considered as a column.

The three female parents as well as the late maturing variety M 377156 were used as standards. M 555160 is also a late maturing variety whereas S 17 and M 13/56 are considered early varieties with high and low sucrose content respecti- vely.

'Data Recorded

Cane number, weight and % refractometric solids (Brix) were recorded for each st011 in the plant cane crop. Two composite samples of cane juice were obtained from the middle internode of each of five canes and Brix was read with a hand refractometer.

Analysis of Data

Families

For each sub-plot the mean and the coefficient of variation (CV,) for each character were computed for each family independently from standards. These derived variates were analysed as for a split-plot design for each environment and followed by a pooled analysis across environments.

Standards

For each sub-plot, the mean and coefficient of variation for each standard were computed. The derived data were analysed as for a split-split-plot; the second split treatment being allocated to the 4 standard varieties. Analysis was also performed for each environment and across environments.

Relative Performance of Genotypes

The performance of each genotype relative to each of the standards and at both selection pressures was compared at early and late harvest as well as in different environments.

Selection

This was studied only for the character brix. Data for each standard were analysed separately for each main plot to determi-

ne variance due to rows, columns and residual. The statistical model used will be described in a further communication (Lim Shin Chong and Julien ').

This analysis gives rise to the following four alternatives:

1. Rows and columns not significant; selection is based on the general mean of each standard in main plot.

2. Only rows significant; selection of genotypes operates for each row indepen- dently and is based on the mean of the standard in each row for a given main plot.

3. Only columns significant; selection operates on genotypes lying between any two columns and is based on the average of the respective column means of standard for each main plot.

4. Columns and rows significant; selection operates on genotypes lying between two adjacent standards within a row and is based on the mean of these standards.

Two selection pressures were applied; at high pressure varieties were selected in the upper tail area of the distribution at a probability of >> 75%, while at low pressure they were selected at P >> 25%. The percentage selection for each family was derived with respect to each of the four standards and analysed as a split-split-plot, ,

RESULTS AND DISCUSSION

Performance of Families

Significant differences between families were observed in all environments for mean brix and cane number but not for mean weight. Time of harvest and the interaction of time of harvest with families were significant only for brix, irrespec- tive of environ,ment (Table 2). Significance of the family x harvest interaction is explained by a higher rate of increase of brix from early to late harvest in family S 17 compared to the two others in all environments.

Differences between environments were observed for mean brix and weight (highest values being recorded in the subhumid zone) but not for cane number (Table 3). The interaction family X environment was not significant, indicating that the relative performance of families was similar in all environments.

The coefficient of phenotypic variation (CV,) was significantly different bet- ween families for the characters brix and cane number. Environment affected CV only for cane number and weight, lowest values being recorded in the subkumid zone. No differences in CV, between dates of harvest were observed

Table 2. Mean Square for Mean Brix of Each Family in the Three Environments

Environment Source d. f.

BR MDA PES

Harvest 2 14,79 * 20,90 * 36,O * * Family 2 0,66 * * 0,59 * * 1,16 * * * F X H 4 0,43 * * 0,39 * * 0,21 * *

* Significant at P < 0,Ol. * * Significant at P < 0,05.

* * * Significant at P < 0,001.

(Table 3). CV, was highest for weight (46%), intermediate for cane number (37%) and lowets for brix (5%).

Table 3. Mean Squares for Mean and for Phenotypic Coefficient of Variation (CV,) for,brix, Cane Number and Weight of Families

Mean Source

c VP d.f.

Brix Cane No. Weight Brix Cane No. Weight

Environment (E) 2 44,83 * 3,59 33,74 * * * 0,79 76,44* 124,8 * * * Harvest (H) 2 34,07 * 0,88 8,46 6,91 7,28 13,69 Family (F) 2 17,OO * 44,65 * 0,70 2,23 " * * 60.4 * * * 120,35 H X E 4 0,90 0,54 0,60 1,11 13,89 27,04 F X E 4 0,28 0,75 0,43 0.14 22,21 20,55 F X H 4 6,71 * * 0,35 0,18 0,23 0.81 17,84 F x H x E 8 0,35 0,52 0.56 0,14 5,26 12.28

* Significant at P < 0,001. * * Significant at P < 0,Ol.

* * * Significant at P < 0,05.

Performance of standards

Brix

There were significant differences in mean brix between standards at different harvest dates (Table 4). The significant interaction of standard X harvest is explained as follows:

1. Higher rate of increase'of brix from early to late harvest in standards M 377156 i and M 555160 compared to S 17. / 2. Relatively slow rate of increase of brix from early to late harvest for standard I

i M 13/56 (Fig. 1). 3. High value of'brix for standard S 17 in July compared to other standars.

1 7 I I I

Jul Sept Nov

Figure 1. Evolution of brix for the four standard varieties.

These results confirm the agronomic characteristics of the standards as descri- bed in the materials and methods section.

Significant differences between environments for mean brix were also obser- ved. The highest values of mean brix for standards were recorded in the subhu- mid zone irrespective of time of harvest. The interaction of standard with envi- ronment was not significant.

The coefficient of variation (CVE) of brix was not affected by environment or time of harvest and was not different for the standards. The average value of this parameter was 3 3 % (Table 4).

Cane number and weight

Differences between standards for mean values of these characters were obser- ved in all environments. Although no significant interaction with harvest date was apparent, the standard x environment interaction was significant. This is explained by a higher cane number and weight in standards M 13/56 and M 377156 in the subhumid zone whereas in the super humid zone standard S 17 gave the highest cane number and weight.

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Coefficient of variation (CVE) was significantly different between the standards for both characters. Highest values CVE for cane number were recorded for M 13/56 and M 377156 (36 and 38% respectively) compared to only 21% in M 5551 60. The trend was similar for .weight, but with high.e-r values e.g. 1.50% f s r M 13%56 and 26% for M 55.5160. This parameter was not affected by time of harvest o r environment for both characters (Table 4).

Table 4. Mean Squares for Mean and For Coefficient of Environmental Variation. (CV,) for brix, Cane Number and Weight fo'r the four Standards -

Mean CV, Source d.f.

Brix Cane No. Weight Brix Cane No. Weight

Envirionment (E) 2 235,95 ' 10,81 229.62 ' 3.08 99,37 283,98 Harvest (H) 2 137.56 ' 2.77 106.97 ' 11.51 138,15 140,53 Standard (S) 3 4.35 ' 404.26 ' 282.40 ' 0.32 1 700.99 ' 2 909,50 ' H x E 4 5.01 " 1.61 16.38 2.95 240,34 '" 201.09 S X E 6 0.23 20.65 ' 22.01 ' 0.31 133.75 298,60 S X H 6 0.65' 0.51 6.47 0.94 37.41 49.91 S X H X E 12 0.07 1.03 2.89 0.66 70.06 70.84

* Significant at P < 0.00.1. * * Significant at P < 0.05.

Selection rates

Selection rates at high pressure varied with the environment, the standard used, the family and to a lesser extent the time of harvest (Table 5 and 6) . Percentage selection was similar in the humid and superhumid environments (20- 21) and significantly lower in the subhumid environment (12). In all environ- ments percentage selection varied with standards, being highest with M 13156 as standard and lowest with M 555160 and S 17 as standards. The highest selection % was recorded in family M 555160 and lowest in S 17, irrespective of standards and environments. Differences between selection rates at different ti- mes of harvest were significant only in the subhumid environment where it was highest early in the season.

The following interactions between the above factores were significant: envi- ronment x standard, harvest X standard and harvest X family (Table 6). Thus, selection rates with respect to S 17 and M 13/56 were highest in the superhumid zone and lowest in tyhe subhymid zone while it was stable across environments with the two other standards; higher rates of selection were obtained with stan- dards M 377156 and M 555160 early in all environments while selection rate was stable with respect to S 17 and erratic with standard M 13/56; higher rates of selection were recorded early compared to late in families M 555160 and M 13156 while it was> the converse in family S 17.

Similar results were obtained at low selection pressure.

Table 5. Effect of Time of Harvest and Standard on % Selection * for three families -

Standard Harvest Family Mean G. Mean

M 13/56 M 377156 M 555160 S 17

M 555160 35,3 35,2 30,8 17,7 29,8 July M 13/56 37,3 33,7 232,5 11,2 26,2 21,l

S 17 10,3 9,7 7,7 1 8 7,4 -

M 555160 46,3 15,5 7 3 10,7 20,l September M 13/56 42,5 623 5 3 8,7 16,O 17,4

S 17 45,7 8,7 4 8 6,O 16,2

M 555160 40,O 6,3 11 ,O 11,5 17,l November M 13156 30,O 8,7 5,7 6,O 12,5 15,6

S 17 41,3 10,7 7,3 9,5 17,l

Mean 36,5 15,O 11,5 9 2

* Mean of the three environments and at P >> 0,75.

Table 6 . Mean Square for % selection

Source E H F S H x E S x E F x E H x F H x S F x S --

d.f. 2 2 2 3 4 6 4 4 6 6 Mean square 836,8 280,8 681,O 4 221,5 261,6 178,2 34,3 6,O 590,4 7,8 Significance * NS * * * * * NS * NS x * * * NS

All second order interactions were not significant.

Differential Selection

The relative performance of genotypes between early and late harvest and across environments was studied at two selection pressures.

Time of Harvest

A t high selection pressure (P >> 75%) the majority (92%) of varieties selected early were rejected late and the majority (96%) of those rejected early were also rejected late with respect to all standards except M 13/56. With respect to this standard a high % (48-60) selected early were also selected late in the humid and superhumid zones and about 45% of those rejected early were selected late (Fig. 2).

% Selected late % Rejected early

% Rejected late

Figure 2. Performance in late harvest of genotypes rejected early at high (H) and low (L) selection pressures, with respect to standards.

At low selection pressure (P >> 25%) and in all, environments about 50% of varieties selected early were also selected late with respect to all standards except M 13/56. With the latter as standard 80% selection was obtained in late harvest in the superhumid and humid zones. The performance at late harvest of varieties rejected early depended on the evironment and standard used. In general about 73% of varieties rejected early were selected late with respect to M 13/56, and a much lower percentage (37-45%) with respect to the other standards (Fig. 3).

Since a very low number of varieties was selected late at P >> 75%, their performance early was not followed. Of the varieties selected late at low selec-

% Selected

% Rejected

late

late

Figure 3. Performance in late harvest of genotypes selected early at low selection pressure with respect to four standards.

tion pressure, the majority (63-69%) was selected early with respect to all stan- dards except S 17. Only 37% of those selected late were also selected early with respect to S 17 (Fig. 4).

Environments

The differential response to selection across environments was examined only for those genotypes which were rejected. The two selection pressures were consi- dered for that study.

At high selection pressure and for both early and late harvest the mahority (83%) rejected in any one environment was rejected in the other.

At low selection pressure early in the season, the % of rejected varieties in a given environment that repeated their performance in the other environment was 67-89 with S 17 as standard and only 50% with the other standards.

The performance of varieties rejected at late harvest was complex but in general the mahority (70%) of varieties repeated their performance.

Forboth families and standards in plant cane, the coefficients of variation for cane number and weight were very high irrespective of enfironment, as found by Walker. This could be due to the small size of the lot, competition between f' genotypes and uneven germination of buds (Skinner ). Consequently, weight should probably not be used as a criterion for selection at this early stage, a view that concurs with the findings of George.

Further, the coefficient of variation for cane number varied with the standards

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% Selected early

% Rejected early

Figure 4. Performance in early harvest of genotypes selected late at low selection pressure with respect to four standards.

and acceptable values of about 20% were recorded for some. Thus, cane number could be used as an aid to selection at this stage if an appropriate standard were adopted. This confirms earlier results which had shown correlation between cane number at this stage with cane weight at the subsequent stage where larger plots (1 row of 5 m) are measured (Julien et al, unpublished). The very low CV of brix for both standards and families (5% or less) shows that this character is reliable for selection at this stage.

Although George showed that families displayed specific adaptation for the environment of the criterion of mean brix, the interaction family X environment in the present study was not significant. However the following differences in met- hodology should be noted between the two investigations:

1. First ratoon crop from a seedling population compared to a plant cane crop grown from cuttings in the present study.

2. Random sample of seedlings from families were used to replicate across envi- ronments instead of the same genotypes replicated in this study.

The results of the present study are in accordance with those obtained by Julien and Delaveau who showed that the interaction of variety with environ- ment was not significant for brix and Pol % DM. It thus appears that there is no need to replicate across environments at this stage.

The significance, irrespective of environment, of the interaction family x har- vest indicates that the efficiency of selection for brix would be reduced if based on only one harvest date. It could result in families adapted to other dates or harvest being lost altogether. The interaction of variety X harvest has also been found to be highly significant by Julien and Delaveau. These results emphazise

the need to replicate in time rather than in space for this particular stage of selection where brix is generally the main selection criterion.

Although selection rates for the three families was not influenced by harvest date, the interaction family X harvest was highly significant, indicating that diffe- rential rates of selection will operate in different families at different harvest dates. This situation is further complicated by the fact t h a t ~ t h e standard used appears to be the main determinant of selection rate and its effect is significantly greater than those of harvest date and environment.

The interaction of time of harvest with standard was highly significant for selection rate because of the different maturity behaviour 'of the standards used. This emphazises the need for a critical choce of a standard for selection for brix and points to the desirability of using a group of standars with different maturity characteristics. Furthermore, the interaction family x standard for se- lectin rates was not significant, showing that the different standards are giving the same pattern of selection across families at a given harvest date.

Relative performance of genotypes within families at two harvest dates was found to be dependent on the seleition pressure as well as on the standard. Thus, at high selection pressure, selection was specific for 95% of genotypes for a given harvest date when three of the standards were considered. On the other hand, at low selection pressure, selection was specific for only about half of the genoty- pes. This tendency was more pronounced for the standard M 13/56 which allowed 75% of genotypes to be selected for both early and late harvest at low selection pressure and 35% at high selection pressure. This is explained by the specific maturity characteristic of M 13/56. Thus, the maturity characteristic of the stan- dard used is one of the main factors influencing which genotypes are selected at different harvest dates.

Differences in adaptation to time of harvest between and within families having been observed for the character brix which is closely related to sucrose content (Julien 7, selection for genotypes with different maturity characteristics should be undertaken in a breeding programme. This can be achieved at an early stage in one or more of the following measures are adopted:

1. , Recording of brix early and late. 2. Selection with respect to a standard with late maturity early in the season but

a low selection pressure. 3. Selection with respect to a standard with low sucrose content early using a

higher selection pressure.

ACKNOWLEDGEMENTS

The authors wish to thank Mr. J . D . de Rivaltz de Saint Antoine, Director, MSIRI and Drs. C. Ricaud and J. R . Williams for their critical review of the paper.

Our thanks are extended to Mr. J . L. C. Y. Lim Shing Chong, Mr. J . R . Moutia, Mr. G . Fong San Pin, Mr. D. Bissessur, Mrs. L. Rivet for their help in .

the collection and analysis of data.

REFERENCES

George, E. F. (1962): A Further Study of Saccharurn progenies in Contrasting Environments. Proc. of the XI Congress of the ISSCT. Mauricio, pp. 488-504. George, E . F. (1965): An Experiment to Assess the Effect of Competition Between Sugar Cane Clones at the Microplot Stage of Selection. Proc. of the XI1 Congress of the ISSCT. Puerto Rico, pp. 920-930. Julien, M. H. R. (1974): An Evaluation of Methods Used for Maturity Testing. Proc. of the XV Congress of the ISSCT. South Africa, pp. 991-999. Julien, M. H . R. and Delaveau, P. (1977): The Effects of Time of Harvest on the Partitioning of Dry Matter in Three Sugar Cane Varieties Grown in Contrasting Environments. Proc. of the XVI Congress of the ISSCT. Brazil, pp. 1755-1770. Lim Shin Chong, L. C. Y. and Julien, M. H. R . : The Analysis of Data for Mass Selection (manuscript). Mariotti, J . A. (1974): The Effect of Environments on the Effectiveness of Clonal Selection in Sugar Cane. Proc. of the XV Congress of the ISSCT. South Africa, pp. 89-95. Mariotti, J . A . (1978): Effect of Sampling Date on Efficiency of Selection for Quality Traits in Sugar Cane. ISSCT Sugar Cane Breeders' Newsletter 428-10. Parish, D. H. and Feillaft?, S. M. (1965): Notes on the 1:100 000 Soil Map of Mauritius. Maurit. Sug. Ind. Res. Inst. Occasional Paper, 22:43. Skinner, J. C. (1961): Sugar Cane Selection Experiments. 2. Competition Between Varieties. Tech. Commun. Bur. Sug. Exp. Sta. Qld. (I), p. 26. Skinner, J . C. (1971): Selection in Sugar Cane: A Review. Proc. of the XIV Congress of the ISSCT. Louisiana, pp. 149-162. Urata, R. and Warner, J . N. (1959): Criteria for Sugar Cane Selection in Hawaii. Proc. of the X Congress of the ISSCT. Hawaii, pp. 702-708. Walker, D. I. T . (1962): Family Performance at Early Selection Stages as a Guide to the Breeding Programme. Proc. of the XI Congress of the ISSCT. Mauricio, pp. 469-483.

INFLUENCE DU MILIEU ET D L'EPOQUE DE LA RECOLTE AUX PREMIERES ETAPES DE SELECTION

DE LA CANNE A SUCRE I, SELECTION DANS LA RECOLTE DES CANNES

DE NOUVELLE PLANTATION

M. H. R. Julien, Z. Peerun et R. Domaingue Institut de recherches de l'industrie sucrikre, RCduit, Maurice

Les auteurs ont CtudiC l'influence de trois dates de rCcolte aux premie- res Ctapes de selection de la canne ti sucre de nouvelle plantation, dans trois milieux diffkrents. Le brix, le poids et le nombre de tiges ont CtC CvaluCs dans trois familles, representee chacune par 72 gknotypes. La pression de sClection a CtC appliquCe ii deux niveaux sur quatre tCmoins aux diffCrents caractkres de maturation.

Le nombre et le poids des tiges par touffe se sont montrCs trks varia-

bles, tandis que le brix s'est aver6 le critere le plus fiable pour la sklection. Le test a demontre que le brix des familles et de leurs gknotypes, dans

leur ensemble, n'a pas CtC affect6 par 1'Cpoque de la rCcolte mais par le milieu. Les taux de selection pour ce caractere ont 6tC on,-fonction-~des temoins, des familles et du milieu. Ces taux ont change lorsque divers temoins ont CtC utilisCs aux diffkrentes dates de rdcolte, ce qui met en relief le besoin de faire une selection critique des tCmoins. En outre, des taux differentiels de sklection ont 6tk observes dans les familles selon les dates de rCcolte mais non selon le milieu. Le comportement relatif des genotypes -i deux dates de rkcolte a dCpendu du tkmoin utilisk et de la pression de selection.

L'experience a dCmontr6 qu'une combinaison de dates d'kchantillonna- ge, de temoins differents et de pressions diverses de selection permet de faire une selection precoce des gknotypes aux differents caracteres de maturation.

EFECTOS DEL AMBIENTE Y EPOCA DE LA COSECHA EN LAS ETAPAS INICIALES DE LA

SELECCION EN LA CANA DE AZUCAR I. SELECCION EN EL CULTIVO DE LA CANA

PLANTA

M. H. R. Julien, Z. Peerun y R. Domaingue

Instituto de Investigaciones de la Industria Azucarera, RCduit, Mauricio

RESUMEN

Se estudiaron 10s efectos de tres fechas de cosechas en una etapa inicial de seleccion de cultivos de caiia planta en tres ambientes contrastantes. El brix, peso y cantidad de caiias fueron valorados en tres familias, cada una representada por 72 genotipos. Se aplic6 presion de seleccion a dos niveles con respecto a cuatro normas de distintas caracteristicas de maduracion.

La cantidad de caiias y peso por plant611 aparecieron como altamente variables, en tanto que el brix result6 ser la pauta mis confiable para la seleccion.

Se ha110 que las familias, en su conjunto, y sus genotipos, demostraron una adaptacidn especifica a la Cpoca de la cosecha, per0 no respecto a1 brix en lo relativo a1 ambiente. Las tasas de seleccion para este caricter depen- dieron de la norma, la familia y el medio ambiente. Estas tasas variaron cuando se emplearon normas diferentes en distintas fechas de la cosecha, haciendo Cnfasis en la necesidad de realizar una seleccidn critica de las normas. Ademis se observaron tasas diferenciales de selecci6n para las familias segun las distintas fechas de la cosecha, per0 no respecto a diferen-

tes ambientes. El relativo comportamiento de 10s genotipos en dos fechas de cosecha dependi6 de la norma utilizada y la presi6n de selecci6n.

Se ha demostrado que una combinaci6n de fechas de muestreo, normas diferentes y distintas presiones 'de selecci6n pueden permitir una selecci6n precoz de genotipos con distintas caracteristicas de maduraci6n.