• MATERIAL AND METHODS
• General description
• Experiment 1
• Experiment 2
• Measurements
• RESULTS AND DISCUSSION
• Experiment 1
• Leaf fluorescence (Fv/Fm; FLU) of corn under excess water irrigation

Corn (Zea mays L.) is one of the three globally important food crops beside rice and wheat in many countries. It is also used as an animal feed as silage or concentrated feed. Particularly, silage corn is feasible in Korea in terms of economical value and environmental suitability, because of its high productivity and total digestible nutrients (TDN).

Recently, rice consumption per person is rapidly decreasing year after another. Based on that the Korean government suggested a rice-replacing substitute crop should be developed to satisfy and improve the increasing food self-sufficiency of the country. However, corn could not grow well in flood conditions such as rice-fields or high under-ground water levels. Therefore, to establish corn that is capable of growing in paddy fields, new varieties need to be developed with capacities and adaptabilities to excess water conditions.

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 by its high palatability and high feed value as a forage crop for cows (Ko et al., 1986; Kim et al., 1998; Son et al., 2006). Nevertheless, Korea mostly depends on import of corn for animal feeding, because of the limited available area of uplands. This limited acreage of uplands for silage corn production in Korea can be extended in the lowlands. Large area of lowlands can be available after drainage of paddy fields following rice production, which can be used to increase total production of silage corn. However, excess soil moisture is common in lowlands thus eventually hampers corn growth and leads to large yield decline. Lodging resistance in corn might be improved by increasing of both spread and bending strength of the roots (Crook and Ennos, 1993).

Leaf fluorescence is considered a key measure for the response of leaf photosynthesis to various environmental stresses (Baker, 1991) and has been known to be mainly damaged by photo-inhibition (Barber and Andersson, 1991). Under natural conditions where soil water stress develops gradually, high irradiance may potentially impair plant’s photosynthetic capacity and induce photo-inhibition if a plant is exposed to a higher light intensity than that experienced during earlier growing stage (Congming and Zhang, 1998). Therefore, it is important to evaluate photosystem II (PS II) activity under excessive water conditions.

There were no studies that evaluated leaf fluorescence of corn under excess water conditions for fast selection of resistance against excess water stress. Hence, pot experiments were conducted to determine water irrigation duration and irrigation timing in early growth stage of CHs under phytotron.

MATERIAL AND METHODS

General description

Two pot experiments were conducted in phytotron with 111 corn hybrids (Table 1) at the National Institute of Crop Science, RDA, Korea. The 1^st experiment was executed to select a valuable corn progeny at the 3^rd leaf stage, while the 2^nd experiment to determine the critical time of maximum irrigation amount and to select useful CHs under excess water conditions at the 5^th leaf stage. Soil characteristics are shown in Table 2. Air temperature in both experiments (1 and 2) was constantly maintained at 21±3°C and 24±3°C, respectively and relative humidity of 70±3%. All plants were watered every day and after 4 weeks of incubation, corn seedlings were investigated for excess water stress treatments.

Table 1. Corn hybrids selected for Experiment 1 and 2.

Table. 2. Initial soil physicochemical characteristics.

Experiment 1

Air temperature during day and night was kept at 21±3 and relative humidity at 70±3% for the initial growth stage, which was simulated to outdoor temperature before the 3^rd leaf growth stage of corn. All plants were watered every two days. Excess water irrigation treatments were initiated from the 3^rd leaf stage, i.e., 38 days after seeding, and the amount of irrigation was 80 mm at the 1^st day and 50 mm for the following 4 days. For the irrigation treatment, rainfalls were simulated using a split irrigator connected with a plastic hose at a height of 1 m above the ground with a rate of 20 mm per hour. The 1/5000a Wagner pots were used for the experiment and five corn seeds were sown at 2 cm soil depth then thinned to two seedlings that were maintained in each pot.

Experiment 2

Air temperature during day and night was kept at 24±3 and relative humidity at 70±3% for the initial growth stage, which was simulated to outdoor temperature before the 5^th leaf stage of corn, i.e., 44 days after seeding. The irrigation treatment was started at the 5^th leaf stage of corn and the amount of irrigation was 80 mm at the 1st day and 50 mm for following 4 days, then the amount of irrigation was adjusted from the top of pots, which was 3 cm above soil to allow over-irrigation water to flow out. Simulated rainfall was done with a split irrigator at a height of 1 m aboveground with the rate of 20 mm per hour. The 1/3000 a Wagner pots were used and five corn seeds were sown at 2 cm soil depth then thinned to two seedlings that were maintained in each pot.

Measurements

Leaf fluorescence was measured to evaluate excess water stress with two replications at 36 days after seeding. Specifically in dark-adapted states, Chlorophyll fluorescence was measured at room temperature (21±3°C in experiment 1 and 24±3°C in experiment 2) with a portable fluorometer after leaves were dark-adapted for 30 min. The fluorometer was connected to a leaf-clip holder with a trifurcated fiberoptic and to a computer with data acquisition software. The clip was installed 2/3 from the top of leaf blade except midrib on the youngest or fully expanded leaf. The fluorometer (PAM-2000), leaf-clip holder (2030-B), trifurcated fiberoptic (2010-F), and data acquisition software (DA-2000) were from Walz, Germany.

RESULTS AND DISCUSSION

Experiment 1

Leaf fluorescence (Fv/Fm; FLU) of corn under excess water irrigation

Under excess water conditions, the stress level of most upland plants can be quantified by measuring leaf fluorescence.

In the 1^st experiment, the FLU values varied between 0.75-0.85. The FLU was mostly 0.85 before irrigation, however it rapidly decreased on 1^st day after irrigation, i.e., 39th day after seeding, showing a range of 0.63-0.78 (Fig. 1, left). Treatment (hybrid) No. 37, 53, 85, and 112 showed higher FLU of more than 0.78, while No. 16, 46, 80, 86, 94, 95, 102 and 104 showed lower than 0.7. The later group was considered PG (physiological growth) with weak resistance to excess water stress.

Fig. 1. Leaf fluorescence (Fv/Fm) of hybrid corn after 39-d at 21±3°C (left) and 45-d at 24±3°C (right), 1-d after irrigation start in both experiment.

In the 2^nd experiment under excess water conditions, CHs in pots expressed FLU values mostly 0.85 before water irrigation, but rapidly decreased after 1^st day of irrigation treatment, i.e., 45^th day after seeding and values ranged between 0.55 and 0.70 (Fig. 1, right). The crossed PGs were more sensitive to wet stress than conventional corn cultivar or progeny. For example, FLU value of No 76 and 101 was higher (0.73) than those of No. 13 and 67 (< 0.48).

In the 1^st experiment, six days after irrigation, i.e., 44 days after seeding, soil surface was dried during evapo-transpiration. Leaf FLU was measured again to determine the recovery efficiency after wet stress. The FLU ranged from 0.23 to 0.83. CHs No. 5, 13, 29, 39, 55, 59, 80, 100, 107 and 116 showed FLU higher than 0.8, while No. 3, 14, 43, 61, 67, 101 and 103 expressed FLU values lower than 0.4 (Fig. 2, left). However, it was not easy to classify the FLU values because they were mostly located in a very narrow range between 0.74 and 0.78.

Fig. 2. Leaf fluorescence (Fv/Fm) of hybrid corn after 44-d at 21±3°C (left) and 50-d at 24±3°C (right), 5 and 9-d after irrigation start in exp. 1 and exp. 2, respectively

In the 2^nd experiment, FLU values were not affected in irrigation treatment, therefore they were put in the same figure (left and right) and comparably expressed a little similar range between experiment 1 and experiment 2. In excess water condition, the value of FLU ranged between 0.33-0.80 at 6 days after irrigation treatment. In pot experiment, the FLU values mostly at the range of 0.55-0.75, however CHs No. 21, 65, 69, 75, 79, 91, 96, 109, 111, 116, and 117 showed FLU values more than 0.75. These values were densely positioned around 0.70, which meant that decreased leaf fluorescence was increased with increasing growth duration. Exceptions from this group were CHs No. 1, , 9, 38, 78, and 86 that scored FLU values less than 0.50(Fig. 2, right).

In both experiments, the FLU values were rapidly decreased after the application of each experiment specific treatment at 6 days after irrigation treatment. Anyway, for the selection purpose of CHs resistant to excess water stress based on range differences of FLU values, the 1^st experiment was more useful than the 2^nd experiment considering time and economic point of view. 

In the 1^st experiment, eleven days after irrigation treatment, soil surface water almost dried by evapo-transpiration and at this time FLU values were measured again to evaluate recovery efficiency after wet stress. FLU values ranged between 0.35 and 0.83, however, CHs No. 5, 6, 7, 9, 38 and 106 scored FLU values above 0.8, whereas No. 49, 51 and 53 were lower than 0.45 (Fig. 3, left).

Fig. 3. Leaf fluorescence (Fv/Fm) of hybrid corn after 49-d at 21±3°C (left) and 55-d at 24±3°C (right), 7 and 12-d after irrigation start in exp. 1. and exp. 2, respectively.

In the 2^nd experiment, most of PGs FLU values ranged between 0.60 and 0.80, however, it was more informative than values obtained 5 days after irrigation and proved convenient to classify FLU values (Fig. 3, right).

In the 1^st experiment at 16 days after irrigation treatment, water of soil surface almost dried by evapo-transpiration and at this time FLU values were measured again to evaluate the recovery efficiency after excess water stress. FLU values ranged between 0.50 and 0.80. CHs No. 36, 46, 104, 110 and 118 expressed FLU values above 0.75, but No. 14, 22, 25, 45, 50, and 90 were below 0.50 (Fig. 4, left). Most FLU values of PGs ranged between 0.55 and 0.75, which facilitated the process of classification and definition of excess water stressed plants. In this experiment, FLU values rapidly decreased after water irrigation, which was recovered after soil drying then it was decreased again. The reasons for this decrease, recover and decrease of FLU values might be related to corn growth stage, which was 46 days after seeding and soil nutrients might also be enough for re-growth. Soil nutrients were decreased by corn growing. Therefore, corn FLU low values after excess water conditions at initial growth stages might be avoided by soil drying and application of enough nutrients.

Fig. 4. Leaf fluorescence (Fv/Fm) of hybrid corn after 54-d at 21±3°C (left) and 60-d at 24±3°C(right), 11 and 15-d after irrigation start in exp. 1. And exp. 2, respectively.

In the 2^nd experiment, FLU values ranged between 0.20 and 0.82 and were mostly higher than 0.70 at 16 days after irrigation. FLU values were lower than 0.60 in CHs No. 16, 20, 57, 66, 72, 81 and 106, which means the recover speed or degree was decreased after excess water damage (Fig. 4, right). However, at 16 days after irrigation, soil surface water almost dried by evapo-transpiration and at this time FLU values were measured again to determine the recovery efficiency after excess water stress.

In conclusion, for fast selection of corn resistant to excess water stress, the 3^rd leaf stage is more suitable than the 5^th leaf stage under phytotron conditions in pots, which could be similar to field conditions (Cho et al., 2011), because the experimental temperature was simulated to a similar level of field conditions for corn growth stages.