• MATERIALS AND METHODS
• General description
• Management
• Measurements
• RESULTS
• Discussion
• 적 요
• ACKNOWLEDGMENTS

Corn (Zea mays L.) is one of the three globally important crops (i.e., corn, rice and wheat) as a staple food in many countries. It is also used as an animal feed in the form of silage or concentrated feed. Particularly in Korea, silage corn is economically and environmentally feasible because of its high productivity and total digestible nutrients (TDN) (Son et al., 2006).

In recent years, rice consumption per person is rapidly decreasing year after year, which lead the Korean government to suggest a rice-substitute crop should be developed bearing in mind satisfaction and improvement of increasing food self sufficiency. However, corn could not grow well under flood conditions such as rice fields or high levels of under-ground water. Therefore, for the establishment of corn in paddy fields, new varieties that are adaptable to excess water conditions should be developed (Cho et al., 2011; Cho and Son, 2012).

Corn is considered the most valuable crop as animal silage in the world and its productivity is relatively high among major crops. Corn is also known for its high palatability by cow and high feed value as a forage crop (Ko et al., 1986; Kim et al., 1998; Son et al., 2006). Nevertheless, South Korea mostly depends on imported corn for animal feeding, because of limited availability of upland areas in general and acreage of uplands for silage corn production. To increase total production of silage corn, large areas of lowlands can be made available by water draining following rice production. However, excess soil moisture is common in lowlands that eventually hamper corn growth and lead to large decline of yield. Lodging resistance of corn might be improved by increased spread and bending strength of roots (Crook and Ennos, 1993). Selection on combining ability of silage corn was studied by Lee (1987), however, the study mostly focused on silage yield and agronomic characteristics in the paddy fields.

Hence, an experiment was executed to select valuable corn hybrids or sources at the early growth stage, 3^rd leaf stage, under rain conditions and excess water stress to simulate conditions in the Korean paddy fields in early summer.

MATERIALS AND METHODS

General description

This experiment was conducted in pots under a phytotron included 111 corn hybrids (Table 1) at the National Institute of Crop Science, RDA, Korea. Soil characteristics were shown in Table 2. Air temperature of experiment was con-stantly maintained at 21±3°C and relative humidity at 70±3%. Plants were watered every day and after 4 weeks seedlings were collected for water stress treatment (see next section for details).

Table 1. Corn hybrids selected for experiment and their assigned numbers.

Table 2. Initial soil physicochemical characteristics.

Management

Air temperature was kept at 21±3°C during day and night that simulated outdoor temperature and relative humidity at 70±3% for the initial growth stage of corn. Plants were watered every day then excess water irrigation treatment was initiated from the 3^rd leaf stage, i.e., 28 days after seeding. Irrigation amount was 80 mm at the 1^st day and 50 mm for the following 10 days. Irrigation treatment was carried out to simulate rainfall using a split irrigator connected to a plastic hose at 1 m high from the ground with a rate of 20-mm per hour. Wagner pots of 1/5000a were used in this experiment and five corn seeds were sown at 2 cm depth in soil. Later on and after germination, plants were thinned and maintained as two seedlings in each pot.

Measurements

Growth parameters Plant height was measured between the soil surface and tip of top leaf blade before plant sampling, and then a working order was followed after sampling. Culm diameter was determined as a mean of thickest and thinnest culm at 10 cm above soil. Weight balance was measured by keeping the plant even on a standing bar (1 cm in diameter) then measure the balance point as a percentage based on length and weight. Leaf blades were separated from plants and leaf area was measured using LAI-3100(LI-COR inc., Lincoln, NE, USA). Leaf blades, culms and roots were dried in an oven at 75°C for two days then weighed to determine dry weights.

RESULTS

Major characteristics of corn hybrids under excess water conditions at 21±3°C.

Growth related characteristics of 111 CHs, which were for cold area cultivation, but they will be grown in paddy fields after breeding, were evaluated in this excess water treatment for each growth characteristic and standardized to other germplsms. They were evaluated for the following parameters; plant height, dry weight of shoot, root, and leaf blade as well as leaf area, plant weight balance, and culm diameter.

Plant height after 11 days (39 days after seeding) of excess water treatment was tallest (130 cm) in CH no.62 followed by 12, 7, 87, 107, 85, 41, 40, 43, 106, and 109 (Fig. 1), but no.96 was the shortest followed by 80, 72, 66, 8, 18, 6, 24, 16, 56, 57, 98, 97, 103, 104, and 116. However, more than 80% of plants were at the range of 80-100 cm height.

Fig. 1. Plant height (cm) of corn hybrids after 11 days of excess water treatment at 21±3°C.

Dry weight of above-soil (shoot) plants ranged from 1.25 to 4.5 g/plant (Fig. 2). CH no. 59 was the heaviest followed by 42, 34, 44, 11, 85, 24, 104, 81, 100, 108, 6, 32, 102, 64, 36, 98, and 94, while the lightest was no. 25 followed by 7, 26, 31, 90, 101, 87, 70, 50, 23, 47, 97, and 99, and they were less than 2.0 g/plant. CH no. 85 ranked as 5^th based on both plant height and dry weight; however, the other top 10 CHs did not rank on both measurements. These results indicated that an increased plant height was not correlated with plant thickness or hardness when grown under such conditions at 21±3°C.

Fig. 2. Dry weight of corn hybrids after 11 days of excess water treatment at 21±3°C.

Dry weight of leaf blade ranged between 0.8 and 3.2 g/ plant. It was more than 2.5 g in CH no. 85, 11, 32, 34, 6, 108, and 104, but in no. 25, 7, 26, 70, 87, 22, 31, 90, and 101 it was below 1.1g (Fig. 3). Broad range of leaf dry weight means the capacity of increasing crop growth rate (CGR) can easily be classified by water treatment in early growth stage of CHs.

Fig. 3. Dry weight of leaf blade of corn hybrids after 11 days of excess water treatment at 21±3°C.

Leaf area of corn hybrids ranged variably between 370 and 1070cm²/plant and CHs no. 2, 27, 89, 81, 91, 12, 7, 31, and 87 scored the widest area and no. 24, 96, 80, 50, 3, 6, and 16 scored the smallest area (Fig. 4). That was almost 3 folds difference of leaf area, which also means that the classification of CHs can be easy by water treatment in early growth stage.

Fig. 4. Leaf area of corn hybrids after 11 days of excess water treatment at 21±3°C.

Weight balance that appeared as low percentage means that culm weight positioned at the lower part and it was heavier than leaf blade weight. It was higher than 27% in CH no. 96, 55, 11, 9, and 74, but was lower than 23% in CH no. 88, 94, 114, 42, 28, 23, 6, 49, 62, 77, and 112, which were considered as low positioned culm weight and heavy or their leaf blade weight was lighter (Fig. 5). Weight balance was found to be negatively correlated or not correlated with plant growth parameters such as plant height, culm thickness, leaf area and dry weight, and others.

Fig. 5. Weight balance of corn hybrids after 11 days of excess water treatment at 21±3°C.

Dry weight of the basal 10cm of culm from corn hybrids ranged between 0.3 and 1.35 g/plant. It was more than 1 g in CH no. 42, 100, 24, 44, 34, 12, 102, and 36, but it was below 0.4 g in no. 25, 13, 23, 7, 9, 52, 50, 97, and 99, which were considered weak regarding lodging resistance (Fig. 6).

Fig. 6. Dry weight of the lower 10cm of culm from corn hybrids after 11 days of excess water treatment at 21±3°C.

Culm diameter at 10 cm above soil of corn hybrids ranged between 6.2 and 10.8 mm. It was higher than 9.5 mm in CH no. 91, 87, 89, 7, 31, 32, 33, 94, and 105, but it was lower than 7.0 mm in no. 98, 24, 115, 108, 14, 6, 96, and 119 (Fig. 7). Similar to the general concept, culm diameter is positively related to lodging resistance.

Fig. 7. Changes of culm diameter at 10cm above soil of corn hybrids after 11 days of excess water treatment at 21±3°C.

Dry weight of root from corn hybrids mostly ranged between 0.8 and 3.4 g/plant. It was more than 3.0 g in CH no. 113, 12, 87, 81, and 75, but it was lower than 1.0 g in no. 53, 24, and 16 (Fig. 8).

Fig. 8. Dry weight of root from corn hybrids after 11 days of excess water treatment at 21±3°C.

Dry weight of leaf blade (Fig. 3) was heaviest in CH no.85 followed by 11, 32, 34, 6, 108, 104, 44, 59, 81, 98, 42, 84, 109, 4, 19, and 100, while it was lightest in no. 25 followed by 7, 70, 26, 87, 31, 22, 101, 90, 99, 95, 79, 14, and 47.

The correlation coefficient among growth characteristics of corn hybrids were compared and it was highly positive and correlated between; 1) plant height and culm diameter, 2) plant height and root weight, 3) plant height and leaf area, 4) culm diameter and root weight, 5) culm diameter and leaf area, and 6) root weight and leaf area. However, it was negative in the case of plant height and weight balance. Weight balance and culm diameter and weight balance and leaf area were not related (Table 3).

Table 3. Correlation coefficient of growth characteristics of hybrid corn after excess water treatment.

Discussion

Genetic variability among CHs under excess water stress was observed in this study and reported by others as well (Porto, 1997; Zaidi et al., 2003). Such variability might be a result of CHs and their different abilities to utilize available nutrients under anaerobic conditions.

Excess soil moisture causes inhibition of early stage root growth of corn that leads to a higher shoot-root ratio, which was suggestively linked to nitrate reductase activity in roots (Zaidi et al., 2003). Peroxidase activity was also found to take part in the biochemical inhibition of plant growth subjected to unfavorable environmental conditions such as excess water stress. It was reported that superoxide dismutase, ascorbate peroxidase and catalase coordinate activities and play a central protective role in the O₂^- and H₂O₂ scavenging process and that coordinated activity is related, at least in part, to waterlogging-induced oxidative stress tolerance in maize seedlings (Bin et al., 2010). Corn plant roots suffer under extreme oxygen stress or hypoxia that lead to anoxia when exposed to prolonged excess soil moisture situation, which suppress plant growth and development. Water stress also reduce dry matter accumulation, leaf area development, transpiration, and affect anthesis and silking, which result in poor grain formation and poor yield. The presence of excess water above field capacity in the rhizosphere was reported to negatively affect maize at every growth stage, though the extent of effect varied at different growth stages and susceptibility declined with further developmental stages (Mukhtar et al., 1990; Zaidi, et al., 2004). Early and increased nodal root development contributes significantly to seedling growth under excess soil moisture and they were found to have large air spaces in cortical regions (Rathore et al., 1997).

Plant height, culm weight and culm diameter are among morphological traits of aerial parts of the corn plant that are correlated with lodging resistance, though it depends on plant material. Although, plant height is considered the best trait for indirect assessment of lodging resistance, a combination of plant height and culm stiffness as indirect morphological traits could be adopted for lodging resistance (Zuber et al., 1999; Keller et al., 1999). Corn leaf area was mechanistically linked with corn plant water status (Yang et al., 2009).

Recently, there was no convenient means to identify maize genotypes or hybrids that are tolerant to waterlogging or excess water stress (Loaiza and Ramirez, 1993). It is difficult to find a unique index at maize seedling stage due the limited information of metabolic mechanisms and biochemical basis. Hence, this study tried to identify tolerant and susceptible corn hybrids based on phenotypic parameters under excess water stress, and the results of evaluated parameters will be an index for primary screening (Yong-zhong et al., 2010).

적 요

옥수수 교잡종 111종의 습해저항성과 생육특성을 규명하기 위하여 인공기상동에서 3엽기에 인공강우 조건(1일차 80 mm, 이후 50 mm, 온도 21±3°C) 에서 본 시험을 수행하였다.

조사항목은 생육특성으로서 지상부, 뿌리, 엽신의 건물중을 조사하였으며 식물체의 중심고, 간직경을 측정 하였다. 교잡종 중에서 62번이 간장이 가장 길었으며 96번이 가장 짧았다. 무게 중심고는 96번이 가장 컸으며 88번이 가장 짧았다. 건물중은 59번이 가장 무거웠으며 25번이 가장 낮았다. 간직경은 91번이 가장 두꺼웠으며 24번이 가장 가늘었다. 엽신건물중은 42번이 가장 컸고 25번이 가장 낮았다. 뿌리 건물중은 16번이 가장 높았고 24번이 가장 낮았다. 엽신건물중은 85번이 가장 높았고 25번이 가장 낮았다. 각 항목별 상관을 구한결과 간직경, 초장, 뿌리건물중은 엽신건물중과 정상관을 나타냈고 간직경과 초장은 뿌리건물중과 정상관을 보였으나 초장과 무게중심고는 부의 상관을 보였다. 무게중심고와 간직경 또는 간직경은 뿌리건물중과 상관을 보이지 않았다.

ACKNOWLEDGMENTS

Young-Han Yun, Won-Ha Yang, Bon-Cheol Goo, and Sung-Eun Jung are highly acknowledged for their technical support and guidance. This work was partially supported by a grant from Gyeongnam National University of Science and Technology and the samples were processed at the National Institute of Crop Science, RDA, Korea, to whom we are grateful.