1. James L. Brewbaker

1. Hawaii Foundation Seeds, Department of Tropical Plant and Soil Science, College of Tropical Agriculture and Human Sources, College of Hawaii, Honolulu, HI 96822

Temperate sweet corn inbreds and hybrids routinely fail within the tropics. Not one of them show resistance in Hawaii to maize mosaic virus (MMV) in order to the racial mixtures of southern rust (Puccinia polysora Underw.), common rust (P. sorghi Schw.), or northern leaf blight [Exserohilum turcicum (Pass.) LS]. Damage is severe from tropical earworms (Helicoverpa zea Boddie) and fusarium rots (Fusarium verticillioides Sheld.), and seeds should be fungicide-coated to germinate satisfactorily. Additionally, couple of temperate hybrids yield well underneath the short days and temperatures from the tropics. Additional problems may include stalk rots, downy mildews, along with other infections and blights. The six tropical supersweet inbreds of corn (Zea mays L.) released here by Hawaii Foundation Seeds (HFS) happen to be bred to deal with these constraints and function parents of commercially marketed hybrids.

Origin

Inbreds Hi75, Hi76, Hi79, Hi80, Hi81, and Hi82 represent the very first tropical supersweet inbreds to be sold openly. Breeding was initiated in 1962 and presently averages 33 generations of selection per inbred. The inbreds were initially reported in ’09 by HFS (world wide web.ctahr.hawaii.edu/hfs ). Formerly released from Hawaii were 16 tropically adapted sweet corn populations according to su1, sh2, bt1. and bt2 genes (Brewbaker, 1998 ). These populations were bred for everyone as open-pollinated varieties for growers and because the grounds for inbred development. The six inbreds released derive largely from all of these populations and involved 25% temperate parentage.

Are all homozygous for that linked complex (bt1 c2) of genes for top-sucrose endosperm (brittle-1 ) and insufficient anthocyanin in plant (without color-2 ). Each was selected for prime quality and mixing ability for yield in commercially marketed hybrids under tropical conditions of short days, low light, and disease and insect pressure. They resemble field corns as opposed to the distinctive temperate American sweet corns that possess the gt (grassy-tiller ) gene that confers tillers and lengthy husk leaves (Brewbaker and Josue, 2007 ). Five from the inbreds were transformed into white-colored kernels (y locus) and also to the multiple mutant (bt a2)(sh2 a1).

Sweet corn is grown year-round in Hawaii and frequently grown weekly. This can lead to intense pressure from illnesses, insects, and abiotic stresses. Hawaii breeding nurseries were grown basically monthly throughout the 40 years of the inbred development. No fungicides or insecticides were ever put on seeds or plants. Disease and insect pressure thus managed to get frequently impossible to reap ears from temperate sweet corn hybrids utilized as checks. All breeding populations were advanced through recurrent pedigree selection in fields from the Waimanalo Research Station on Oahu in Hawaii (lat. 21° N, elev. 20 m, 24.5 °C average annual temperature). Ecological conditions ranged in the short-day (11 h), cloudy, high-rain fall winter towards the lengthy-day, low-rain fall, and temperature (mean 28 °C) summer time.

Each season was characterised by specific disease epibiotics. These annual variations can lead to a 50% decrease in maize grain yields during the cold months (Brewbaker, 2003 ). The congenial ecological conditions at Waimanalo encouraged weekly observation and field taste-testing. The writer took part in breeding all cycles of selection barefoot (Srinivasan, 1999 ). Inbreds as well as their hybrids were also evaluated regularly underneath the turcicum blight pressure in the Mealani Research Station on Hawaii Island (elev. 850 m, 17 °C average annual temperature).

Two illnesses were pre-eminent and needed genetic advance. They were the all pervading leafhopper-transmitted MMV (Brewbaker, 1979 Ming et al. 1997 ) and kernel and seedling rots caused by Fusarium verticillioides Sheld. (formerly F. moniliforme. perfect stage Gibberella fujikuroi ). Southern rust (Puccinia polysora Underw.) was racially different and very severe in winters since 1993. Under these conditions, not really a single temperate sweet or dent corn was resistant. Common rust (Puccinia sorghi Schw.) was seasonally severe as well as involved races pathogenic to any or all known monogenes for resistance (Kim et al. 1988 Pataky et al. 2001 ). Populations of earworms (Helicoverpa zea Boddie) were full of every breeding population. No commercial temperate hybrids demonstrated potential to deal with the earworms or these four illnesses. Other common illnesses in breeding nurseries were brown place (Physoderma maidis Miyabe) and microbial leaf blight [Acidovorax avenae (Manns) Willems, also referred to as Pseudomonas avenae Manns]. Northern leaf blight [Exserohilum turcicum (Pass.) LS] was severe in the highland Mealani station where all inbreds were evaluated. Less frequent in breeding nurseries were rotting Erwinia caratovora f. sp. zeae. aflatoxin-inducing Aspergillus spp. common smut [Ustilago maydis (Electricity) Cda.], and maydis blight [Bipolaris maydis (Nisik.) Shoem.]. They were illnesses that the released inbreds demonstrated tolerance however that will not have been major factors in genetic advance under selection.

Inbred development was begun in 1962 with different tropical number of sweet corn (sugary-1 gene) referred to as ‘Hawaiian Sugar’ and bred by Dr. A.J. Mangelsdorf of Hawaii Sugar Planters’ Association in the mid 1940s. Early evaluations of the variety as well as temperate sweet corn inbreds and hybrids in Hawaii (Brewbaker, 1965 ) revealed impossible problems of adaptability to biotic and abiotic stresses. Consequently, the first breeding centered on growth and development of open-pollinated varieties produced from ‘Hawaiian Sugar’ and tropical field corns rich in tolerance of specific illnesses and stresses. 16 of those varieties were subsequently released (Brewbaker, 1998 ), including six according to sugary-1. seven around the high-sucrose gene bt1 (brittle-1 ), one on bt2 (brittle-2 ), and 2 on sh2 (shrunken-2 ). Market preference for that high-sucrose or supersweet corns brought to early conversions of sugary-1 varieties by backcrossing and selfing to genes bt1 and sh2. A sh2 composite named ‘Hawaiian Supersweet #6’ was marketed for any couple of years (Brewbaker and Banafunzi, 1975 ) and the other sh2 composite, ‘Hawaiian Supersweet #2’, was transformed into ‘Thai Supersweet’ and dominated the Thai market in early 1970s. Later the bt1 composite Hi bt Comp 3 was launched as ‘Hawaiian Supersweet #9’ (Brewbaker, 1977. 1998 ). Breeding of sh2 was stopped in Hawaii because of seed quality problems linked largely to poor field germination and inclination towards fusarium seedling rots. Zan and Brewbaker (1999) studied near-isogenic hybrids using these along with other genes and demonstrated brittle-1 to become better than shrunken-2 in germination, fusarium tolerance, and storage existence under diverse conditions in Hawaii. The issues with shrunken-2 made an appearance to connect with its highly collapsed mature kernels with large vacuolar space and rates of leachate in soils (Zan and Brewbaker, 1999 ). The genes sh2 and bt2 govern ADPG-pyrophosphorylases that block starch synthesis and make very sweet phenotypes. Tries to breed using these genes along with the gene sugary-enhancer (se ) that delays kernel dry-lower largely unsuccessful because of severe kernel rots. However, breeders around australia and Thailand have been successful in marketing sh2 hybrids under farm problems that lack fusarium pressure, unlike Hawaii’s year-round growers. Sweet corn production in Thailand is dominated about equally by hybrids ATS-5 (sh2) and ATS-8 (bt1) from Dr. Taweesak Pulam (Brewbaker et al. 2006 ).

‘Hawaiian Supersweet #9’ (Hi bt Comp 3) grew to become integral to growth and development of most Hawaii inbreds (Brewbaker, 1977 ). It combined top quality with exceptional tolerance of earworms as well as fusarium-related seedling and ear rots. Advanced through 14 cycles of recurrent selection since its release, this variety remains marketed and grown round the tropics. Two cycles of selection within this composite involved field bite tests of 400 selfs of superior plants with 100 ears selected for advanced breeding. This led to major advance for tenderness (Ito and Brewbaker, 1981 ) as well as for uniformity colored and search. A conversion to silvery white-colored kernels was completed and marketed as ‘Hawaiian Supersweet Silver’ (Hi bt Comp 9) which has offered within the conversions of inbreds to white-colored kernels (Brewbaker, 2006 ). Another critical open-pollinated genetic source of tropical breeders may be the composite NE-EDRbt (PI 619436) bred in Hawaii with 50% temperate parentage, presently in cycle 22 of selection (Brewbaker and Scully, 2002 ). This composite can also be obtainable in sugary-1 as well as in shrunken-2 versions.

Quality evaluations from the six inbreds were according to testcrosses with bite tests within the field at 18 to twenty d after pollination. Test-mix parents varied but always involved the inbred now designated Hi82, our most senior inbred and something with below-average tenderness. Undesirable flavors and inedible pericarp were quite uncommon during inbreeding thinking about the variety of tropical maize races without anyone’s knowledge from the Hawaii populations. We attributed this that most maize races were utilised as human food. Caribbean races performed a significant role in origin of ‘Hawaiian Sugar’ plus they were considered to be harvested routinely like a eco-friendly vegetable and located to possess thin pericarps (Brewbaker et al. 1996 ). The tiresome measurements of pericarp thickness were taken periodically throughout the breeding, but genetic advance for tenderness found depend heavily around the more enjoyable bite tests of ears within the field (Ito and Brewbaker, 1981 ).

Breeding nurseries involved 5-m plots grown in a density of 140,000/ha and then thinned to under 70,000/ha. Roguing for agronomic traits centered on brace-root formation, lodging resistance, and ear position—traits by which tropical maize frequently rates poorly. Plants were selected for prolificacy, high leaf-area indices, dark plant coloration, early canopy closure, and remain-eco-friendly. In none of those populations was the gt (grassy-tiller ) gene retained using its tillers and lengthy husk leaves that characterize temperate sweets and grassy mutants like tillered and teosinte-branched (Brewbaker and Josue, 2007 ). Erect-leaf plants were favored and ear selection took it’s origin from tapered 14-row ears with superior husk cover with no kernel rot or tip emergence. Kernels with dark yellow endosperms were favored.

Table 1 provides web sites the foundation from the six inbreds. Incorporated would be the prior names from the source inbreds and also the composite populations or hybrids that they originated. The figures of cycles of breeding are recorded within the table and therefore are detailed within the following section.

Origin of Hawaii inbreds.

The six released inbreds demonstrated a higher amount of genetic diversity as evidenced by cluster analysis (Fig. 1 ) by heterosis of the hybrids. Genetic similarity coefficients within this figure were calculated by Nourse (2000) from simple sequence repeat data on five brittle-1 inbreds from Hawaii, two from Thailand, two from Australia, and something temperate sh2 inbred Ia453. Inbreds are denoted within the figure by their original designations, i.e. 190 (Hi75), 217 (Hi76), bA11 (Hi79), Hi36 (Hi80), Hi37 (Hi81), and Hi38 (Hi82). Inbreds 190 (Hi75) and 217 (Hi76) demonstrated the nearest relationship. Also carefully related were inbreds Hi36 (Hi80) and Hi37 (Hi81) as were two inbreds from Thai breeder T. Pulam, bA11 (Hi79) and it is related bF47. Inbreds Kbt113 and Kbt132 from Ian Martin in Queensland, Australia, were produced from Hi bt Comp 3 as well as clustered with Hi82 (Hi38). The temperate Iowa inbred Ia453 was most different. Inbreds bF47, Kbt113, and Kbt132 weren’t advanced in Hawaii breeding.

Genetic similarity coefficients for ten supersweet corn inbreds.

Inbred Hi76 includes a pedigree of 16 generations of breeding. It had been selected in 1994 together with Hi75 as outstanding S7 inbreds from Hi bt SYN 2. It’s carefully associated with Hi75 (Fig. 1 ) and far from the description of Hi75 applies straight to Hi76. Hi75 was recognized as plot 94-217 and subsequently went through nine cycles of selection and evaluated being an inbred in additional than 30 trials so that as a hybrid in lots of more. It resembles Hi75 in rust resistance and remain-eco-friendly and it is similarly tender and vulnerable to fusarium seedling mortality. Additionally, it shares Hi75’s attractiveness to aphids and it has dark yellow kernels wealthy in beta-carotenoids. Hi76 has eco-friendly inner silks and it is the most well-liked inbred because the female parent.

Inbred Hi79 includes a pedigree of 21 generations of breeding. It originated in inbred bA11 bred by Dr. T. Pulam in Thailand from hybrids of Hi bt Comp 3 (‘Hawaiian Supersweet #9’) and Thai Composite 3-4 that took it’s origin from tropical flint composite Suwan 1. Inbred bA11 was one parent of Dr. Pulam’s broadly grown, 16-row, processing hybrid ATS-2, just one mix that laid the building blocks for Thailand’s burgeoning supersweet corn industry (Brewbaker et al. 2006 ). Hawaii breeding started in 1998 having a concentrate on conversion to eco-friendly silks and tassels along with improved potential to deal with MMV and fusarium. The Thai inbred was entered with ‘Hawaiian Supersweet Silver’ (Brewbaker, 2006 ) adopted by four backcrosses and 17 generations of selfing and sibbing. Eco-friendly inner silks were achieved obviously because of exchanging the initial (bt A2 ) linked genotype of bA11 for that (bt a2 ) in the Hawaiian parent. Inbred Hi79 is characterised by superior mixing ability for kernel tenderness, but kernels come with an associated high sensitivity to fusarium seedling blight. In fusarium-wealthy soils, the inbred germinates poorly and seedlings are slow to build up healthy root systems. The vegetation is notable for stay-eco-friendly, prolificacy, the persistence of wealthy eco-friendly color after physiological maturity, and superior ability to tolerate turcicum blight. The tassels are large about two times the branch quantity of other inbreds (Table 2 ), which is suspected to hold the gene Brta (branched tassel ) recognized by Brewbaker and Yu (2009). The 16-row ears are somewhat irregular as well as on large cobs (25 mm), and also the kernels are deep (13 mm). Its hybrids are known as ‘HiThai’ in Hawaii and therefore are of outstanding tenderness. They’re attractive within the field with heavy 16-row ears in a low position on tall plants. Marketed hybrid ‘HiThai2’ may be the modified single-mix (Hi75 Hi76) Hi79.

Inbred Hi80 includes a pedigree of 39 generations of breeding and it was formerly referred to as Hi36c1. Its precursor, Hi36, was selected in 1979 along with Hi37 (now Hi81) as outstanding inbreds by itself so that as parents in test crosses to Hi82, then referred to as AA8bt. Hi36 and Hi37 was initially S6 selfed progenies from Hi bt Comp 3 where the inbred AA8bt comprised 25% of their pedigree. The precursor of Hi80 was entered in 1980 to some supply of eco-friendly inner silks, backcrossed five occasions, selfed and sibbed 12 cycles, and designated Hi36a in 1997. During 15 subsequent cycles of sib and self-pollination, it had been made homogeneous for top tenderness, dark yellow kernels, high fusarium tolerance, and prolificacy. The outstanding subline was designated Hi36c1. Hi36 enters the broadly grown three-way hybrid ‘Hawaiian Supersweet #10’ with pedigree [(Hi80 Hi81) Hi82]. Sister lines Hi36c2 with pale yellow kernels (verified to become gene y11 ) and Hi36cy1 with white-colored kernels (gene y1 ) are utilized in breeding bicolors. Growers understand the bicolor expression of hybrids with y11 partially since it aids identification of market maturity, even though it is less striking than bicolor hybrid ‘Sweet Jenny’ [(Hi75 Hi76) Hi36cy1].

Inbred Hi81 includes a pedigree of 38 generations of breeding and it was formerly referred to as Hi37c2. Its precursor, Hi37, was selected in 1979 along with Hi36 (now Hi80) and it is origin from Hi bt Comp 3 and early breeding were just like individuals described for Hi80. The initial Hi37 was entered in 1980 to some supply of eco-friendly inner silks, backcrossed five occasions, selfed and sibbed 12 cycles, and designated Hi37a in 1997. During 15 subsequent cycles of sib and self-pollination, it had been selected for erect plant form, fusarium tolerance, and improved yield and quality in test crosses. Many related sublines of the along with other inbreds happen to be retained and compared for test mix performance. The outstanding subline was designated Hi37c2 and it is parent from the broadly grown three-way hybrid ‘Hawaiian Supersweet #10’. Inbred Hi81 has high general mixing ability (GCA) values for ear length and yield as well as for ability to tolerate fusarium and blight but lower scores for tenderness and quality. Conversions happen to be designed to white-colored kernels.

Inbred Hi82 includes a lengthy pedigree of 72 generations of breeding and it was formerly referred to as Hi38c1. Hi82 traces to some sugary-1 inbred known as AA8 derived in 1967 from ‘Hawaiian Sugar’ as S6 line A19-6-1-1-1-1. It had been fully up against the MMV which had crippled sweet corn production in Hawaii. AA8 grew to become parents of first couple of commercial tropical sweet corn hybrids ‘Hi38’ and ‘Hi68’, single crosses previously marketed by Northrup-King and according to temperate parents P39 and Il442 (Brewbaker, 1968 ). These early hybrids were designated ‘hapa-Hawaiian’ (i.e. half-Hawaiian) as well as their failure to supply commercially acceptable disease resistance brought for their substitute with hybrids according to 100% tropical parentage. AA8 ended up being transformed into bt1 and renamed AA8bt. It had been notable for potential to deal with fusarium that kills temperate sh2 inbreds in Hawaii’s wet winter soils. Later AA8bt was transformed into Ht1 and Rp1-d genes for turcicum blight and customary rust resistance, however these soon unsuccessful because of racial evolution. After 14 generations of choice for tenderness, yield, and horticultural traits, AA8bt was renamed Hi38 in 1979. It had been selected because the male parent for that first commercial discharge of ‘Hawaiian Supersweet #10’ and grew to become the main test mix parent for those new inbreds, entering the pedigree of synthetics for example Hi bt Syn 2 (Brewbaker, 1998 ). Eight generations involving countless sublines were put into improve general potential to deal with common rust (Rp1-d getting unsuccessful) and also to improve testcross performance, ear tip cover, and earworm resistance also it was renamed Hi38a later. Another cycle of eight generations of improvement brought to Hi38b and involved the incorporation of C cytoplasm (later discarded), erect leaves, and stay-eco-friendly. Your final cycle of 10 generations was centered on eco-friendly inner silk and mixing ability for tenderness. Some 23 superior sublines was evaluated as lines by itself as well as in test mix yield trials throughout the years 1993 to 1997, leading to selection and discharge of Hi38c. Related sublines which have been retained in HFS include Hi38-47 rich in tenderness and Hi38-71 that’s homozygous for any recessive aphid resistance gene (So et al. 2009 ). Hi38 seemed to be transformed into white-colored kernels through crosses in 1990 to some Nigerian white-colored flint inbred TZi17 adopted by six backcrosses over 28 generations. White-colored-seeded subline Hi38cy10 is marked by upright leaves and greatly elevated potential to deal with southern rust in contrast to its Hi82 parent (Table 3 ), and commercial white-colored hybrid ‘Sweet Cynthia’ may be the single mix Hi36cy1 Hi38cy10.

Performance

Horticultural traits from the six inbreds are summarized in Table 2 with comparative data for Golden Mix Bantam grown within the same Hawaii nurseries. Data are associated with midseason, 12-h day and 25 °C conditions. All inbreds flower 7 days after typical temperate sweet corns. When grown under 4 h added light in Hawaii fields, these were delayed only one week to flowering. Heights within the table will be to the top tassel and also to the internode at the bottom of uppermost ear. All inbreds have low ear position on erect plants with higher brace rooting. None could be characterised as “stiff-stalk,” a trait that’s suspected to become correlated with thick pericarps (Brewbaker et al. 1996 Wang and Brewbaker, 2001 ). Husk figures exceed individuals of temperate sweet corns and vary from 10 to 13 because of selection to reduce damage from insects (Brewbaker and Kim, 1979 ). Inbred Hi79 has 16 rows of kernels, getting been selected for processing. Another five inbreds average 14 rows that spiral slightly in the ear tip and supply a tapered tip, also minimizing earworm injuries. Inbred Hi82 has got the longest ears using more than 40 kernels per row and has a tendency to create hybrids with tip emergence. Inbreds Hi79 and Hi80 have bigger kernels which is reflected within their hybrids. Average kernel depth at sweet corn stage is 9 mm in most inbreds except the deep-kerneled Hi79 (13 mm) which has 16 rows on the large indurate cob. All six inbreds were selected for dark yellow kernels. Conversions exist to white-colored kernels (gene y1 ) of inbreds except Hi79. Hi80 can also be symbolized with a pale yellow brother or sister (gene y11 ). Inbreds Hi76 and Hi82 demonstrated abnormally high amounts of zeaxanthin (pro-vit a) in Australian studies of macular degeneration. Dry kernels are lightweight (11 to fifteen g per 100 kernels) along with a typical grower recommendation is 4 kg of seed per acre presuming 80% emergence. Their F1 hybrids (brittle-1 ) average from 15-20 g per 100 kernels and fieldcorn outcrosses average from 25 to 35 g per 100 kernels. Viability loss was very rapid kept in storage with 50% mortality levels 12 months at 25 °C and a pair of years at 18 °C. Lengthy-term storage has been around freezers. Fusarium loads were finest from seed harvested in Hawaii’s wet winters, which was directly reflected in seed viability, emergence vigor, and delayed flowering.

Performance from the six inbreds with regards to significant tropical illnesses and insects is summarized in Table 3. Ability to tolerate fusarium (F. verticillioides ) during germination and emergence was very best in Hi80, Hi81, and Hi82 and perhaps correlated using their relatively low tenderness values. The highly tender and related Hi75 and Hi76 (Fig. 1 ) were probably the most prone to fusarium and were also abnormally appealing to aphids (So et al. 2010 ). However, no inbreds displayed significant kernel or ear rots caused by fusarium or Aspergillus spp. All inbreds had sufficient tolerance of polysora rust (Puccinia polysora ). Tolerance of turcicum blight (Exserohilum turcicum ) was considered insufficient only in Hi80 and Hi82. All inbreds demonstrated high ability to tolerate physoderma stalk rots and also to microbial leaf blight, hot-weather illnesses that may be serious on temperate sweet corns. All inbreds were bred homozygous for Mv locus conferring potential to deal with MMV, and all sorts of are thought to be resistant against strains of maize dwarf mosaic virus and sugarcane mosaic virus within the tropics (I. Martin, unpublished data).

Availability

Foundation seeds of those inbreds can be found from Hawaii Foundation Seeds, Dept. of Tropical Plant and Soil Science, CTAHR, U. Hawaii, 3190 Maile Way, Honolulu, HI 96822 (world wide web.ctahr.hawaii.edu/hfs ).

Notes

We acknowledge many thanks the help of many graduated pupils, publish docs, and field staff in the Waimanalo (Manager Roger Corrales) and Mealani (Manager Milton Yamasaki) Research Stations in the last 40 years. Notable contributions happen to be produced by former students Drs. Aleksander Josue, Soon Kwon Kim, Myoung Hoon Lee, Manuel Logroo, Hyeun Graphical user interface Moon, Chifume Nagai, Yoonsup So, Sarah Nourse Styan, Bingtian Wang, Guo Hwa Zan, yet others (Srinivasan, 1999 ).

• Received for publication May 27, 2010.
• Recognized for publication This summer 13, 2010.

Literature Reported