New maize inbreds in Argentina. I: Heat unit requirements. --Corcuera, VR, Bernatené, EA, Naranjo, CA During the last growing season 2000/2001 a field trial was sown at the Inst. Fitotécnico de Santa Catalina to evaluate different maize inbreds developed during the last decade for several agronomic traits. The list of materials is shown in Table 1. The inbreds studied can be divided by their endosperm type into the following categories: 1) waxy maize, 2) opaque-2 maize, 3) +/wx maize and 4) opaque-2/wx maize.

Different evolutive cycle traits were measured and analyzed, but in this article we shall only refer to Heat Unit (C) requirements necessary for silking (stage R1, Hanway scale). Heat Units (C) were calculated according to the formula proposed by the US Weather Bureau (USWB, 1958):

HUR (C) = SDHU

DHU = ( min temp C + max temp C) / 2 - 10 C1
HUR = Heat Unit Requirements during the whole cycle
DHU = Daily Heat Units
1: 10 C was taken as the growing base temperature of these materials.

We prefer to base the discussion on HUR instead of number of days to silking because, as is well known, it makes it possible to compare amongst genotypes evaluated in different years and locations.

Most of the waxy maize selected as "foundationals" at the beginning of this breeding programme in 1990 were precocious (418 C 518 C for silking). On the other hand, while the "foundationals" opaque-2 type showed a long evolutive cycle (750 C 870 C for silking), some normal endosperm maize denoted medium or short evolutive cycle (484 C 568 C). All data collected since 1990 up-to-date through the inbreeding and selection process point out that whether inbreeding tends to lengthen evolutive cycle traits, it was possible to obtain new short evolutive cycle (472 C 535 C) opaque-2 maize inbreds (93.75 % homozygosity) (see Table 2 and Graphs 1 and 4). Then selection by precocity was successful in these materials. In a similar way, the different S3 S7 waxy inbreds (87.5 % to 99.2% homozygosity) complete their thermal requirements to R1 with 451 C-609 C (see Table 2 and Graphs 2 and 3).

Inbred 3074 merits separate analysis. It was developed from the original backcross (SCV1 x A255) x SCV1. During the growing season 2000/01, the S2 and S3 generations of this inbred were evaluated, as were an irradiated sample of the S3 and S4 generations. Gamma irradiation consisted of a dose of 150 Gy of Co60 using a Gamma-cell. The S2 showed the lower HUR for silking (451.5 C) whilst S3 showed the highest (581.1 C). The differences between values are statistically significant at 1% probability based on t Student. The irradiated generations showed different values: S3 (498.7 C) and S4 (513.5 C). Then, the results are clear that the irradiated generations of the inbred have a shorter cycle than their non-irradiated counterpart. According to the data presented in this article one could state the following:

1. When the inbreeding process progressively lengthens the evolutive cycle, precocious materials can be obtained by selection.

2. Gamma irradiation was a useful tool to generate new precocity in some materials.

3. New precocious inbreds were obtained by quality breeding and are now being tested to produce commercial hybrids.

Table 1. List of materials evaluated by their HUR in a field trial during 2000/01.
 
  PEDIGREE ENDOSPERM GENERATION PEDIGREE ENDOSPERM GENERATION
  2000-3098 OPAQUE2 INBRED 2000-3092 OPAQUE2 INBRED
  2000-3088 OPAQUE2 S7 2000-3130 OPAQUE2 S1
  2000-3141 NOR/O2 S1 2000-3142 NOR/O2 S1
  2000-3132 NOR/O2 S1 2000-3133 NOR/O2 S1
  2000-3134 O2/wx S1 2000-3135 O2/wx S4
  2000-3096b O2/wx S3 2000-3096c O2/wx S4
  2000-3096a O2/wx S3 2000-3136b O2/wx S3
  2000-3137 NOR/O2 S1 2000-3138 NOR/O2 S1
  2000-3139 OPAQUE2 S1 2000-3022c NOR/wx S7
  2000-3022b NOR/wx S7 2000-3022a NOR/wx S7
  2000-3072 NOR/wx S3 2000-3003 NOR/wx S3
  2000-3024 NOR/wx S4 2000-3014 NOR/wx S4
  2000-3074a NOR/wx S2 2000-3074b NOR/wx S3
  2000-3074c* NOR/wx S4 2000-3074d* NOR/wx S3
  2000-3016a WAXY S7 2000-3016b WAXY S4
  2000-3002a WAXY S4 2000-3002c WAXY S6
  2000-3078a WAXY S4 2000-3078b WAXY S4
  2000-3136a O2/wx S2 2000-3078d NOR/wx S4

*: irradiated materials, 150 Gy Co60.
 
 

Table 2. Comparisons of means for Heat Units for silking (C).
 
               
  a. Opaque-2 maizes   b. Opaque-2/waxy maizes
  Genotype Average Groups *   Genotype Average Groups *
  3098 585.9 A   3136b 608.6 A
  3130 571.3 AB   3136a 599.1 A
  3092 562.3 AB   3133 591.8 A
  3088 559.2 AB   3135 587.0 A
  3141 542.2 ABC   3137 585.0 A
  3132 539.0 ABC   3134 574.9 A
  3139 535.1 ABC   3096b 566.9 A
  3142 505.5 BC   3096c 490.4 B
  3138 471.6 C   3096a 484.8 B
               
  c. +/waxy maizes   d. Waxy maizes
  Genotype Average Groups *   Genotype Average Groups *
  3078d 615.1 A   3078b 609.5 A
  3022b 595.0 AB   3002c 535.6 B
  3074b 580.5 ABC   3020 535.6 B
  3022a 578.1 BC   3016b 526.8 B
  3022c 575.4 BC   3002a 525.9 B
  3003 552.9 CD   3078a 512.1 B
  3072 551.6 CD   3016a 451.2 C
  3024 538.3 DE        
  3074c 513.1 F        
  3074d 498.7 F        
  3014 487.1 F        
  3074a 451.6 G        

* Averages followed by the same letter belong to the same group and do not differ significantly at 1% of probability.

Graph 1.

Graph 2.

Graph 3.

Graph 4.
 
 


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