Nitrogen Foliar Application Growth Response In Rice Cultivars

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Growth characteristics of three Indonesian rice cultivars (Inpara 5, IR 64, and Si Kuning) were evaluated under 7 and 14 days submergence condition. Nitrogen foliar treatment of 2300 ppm was applied to rice plants and split into two application times: before (N1) and after (N2) submergence. Growth analysis then was calculated separately for both under submerged condition and recovery period. Results showed that tiller number was identified as an important character determining plant survival under submergence and growth after de-submergence. Greater tiller number after de-submergence would lead to an improved leaf area ratio (LAR) and finally will affect relative growth rate (RGR) and the accumulation of total dry weight (TDW). Therefore, N2 treatment was assumed as an effective effort to enhance tiller number and maintain plant growth after de-submergence. Nevertheless, under longer submergence duration, the application of N1 appeared to give better performance.

Introduction

Rice is a well-adapted species of plant to flooded condition. However, excessive flooding may result in plant stress leading to decline of productivity. Flooding is a common problem occurred in lowland area across South and Southeast Asia, including swamp areas in Indonesia. Swamp rice cultivation in Indonesia has suffered from an adverse impact from high flooding which occurs in various stages of plant cycle. The risk of flooding to rice plant is not only during early growth but also on the later stage or even during reproductive stage causing the farmers suffer from massive harvest loss. Poorly drained fresh-water swamp is the most prone area to flash-flooding that generally will last less than a few weeks and is caused by heavy rain but the depth is not very deep. Flash floods that result in stagnant flooding or complete submergence of paddy fields can cause yield losses from 10% to 100% depending on water depth, duration of the flooding event, turbidity, temperature and the developmental stage of the plant.

The increase in the frequency and intensity of flash floods due to changing global weather patterns (makes the development of more flood tolerant rice varieties critical. Nowadays, breeders are making efforts to develop and release new rice cultivars that have submergence tolerance character. However, this effort will take longer time due to the complexity of floodwater characteristics among different environments. Furthermore, existing submergence cultivars in Indonesia, such as INPARA cultivars, are not preferable for both farmers and consumers due to the unfavorable taste. Farmers still cultivate popular commercial cultivars which are less adaptive to flooding injury. Consequently, agronomists should consider cultivation technique to cope with this submergence problem. 26 Proper nutrient adjustment then is considered as one of the subjects that can be modified to adapt to this situation. Nitrogen (N) is one of the essential macronutrients for plant growth and yield. Many studies have reported the importance of N in plant body to show better performance either in plant elongation, dry matter production or tillering. Ehara et al (1993) mentioned that application of 2300 ppm N would give the best effect on rice growth. The application of N fertilizer to improve plant tolerance under submerged condition has also been investigated.

As reported by Rao et al. (1985), basal fertilization with N promoted early vigor allowing plants to tolerate submergence at later stages of growth. Sharma (1995) also stated that top dressing of N after recession of flood water improved recovery of plants and partially compensated for the mortality due to flooding. Furthermore, the combination of N application through plant foliage along with soil application is believed more efficient compared to application only through soil. However, very limited references of the effect of foliar N application to growth of rice under submergence were found. Thus, the study was carried out to examine the effect of foliar application of high N concentration on growth parameters of rice under submerged condition.

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Discussion

In this experiment, the evaluation of growth parameters could be mostly performed only for 7 days submergence duration since most of rice plants under 14 days submergence could not survive until the end of experiment. Therefore, the evaluation of N2 treatment was also very limited. Some studies reported that survival during submergence was closely related to plant elongation growth that led to plant avoidance mechanism under submerged condition. Plant length increment was not so apparent in 7 days submergence among N treatments, except for N1 in IR64 which later resulted in drastic decrease of plant length during recovery period. Generally, plant length increase was slower or relatively constant, and even declined due to detached leaf or enhanced leaf senescence occurred after de-submergence. Nevertheless, enhanced stem elongation was actually an unfavorable character for submergence rice.

Kawano et al. (2009) reported that tolerance was greater in cultivars where acceleration of elongation caused by submergence was minimal. Elongation growth competed with maintenance processes for energy and hence reduced survival during submergence. The report was also validated by result of this study as shown by IR 64 which had the best survival under 14 days submergence since it had minimal plant length under submergence, while Inpara 5 which was basically a sub-1 rice cultivar and anticipated to have better survival performed an unexpected result. The application of N1 treatment improved survival of Inpara 5 by enhancing stem elongation under submergence. As described above, N1 treatment had significantly affected TDW under submergence conditions. However, the results were varied at the end of experiment. The relationship between TDW and RGR then was investigated as given in Fig. 9 and showed significant positive correlations for both under submergence and during recovery conditions. As RGR was a product of leaf area ratio (LAR) and net assimilation rate (NAR), observing the trend of both traits would be beneficial in differentiating the roles of morphological and physiological response of rice plant under submergence condition.

By comparing the tendency in NAR results, it was understandable that RGR change was mostly attributed by the alteration in NAR. Under both 7 and 14 days submergence, NAR was increased by the application of N1 as seen in Inpara 5 and IR64 and this trend was similar to RGR as aforementioned. Hence, the opposite result was shown by Si Kuning by having smaller NAR compared to N0 indicating that RGR change of Si Kuning was not determined by the change in NAR and more subjected by the change in LAR, though the increase was quite small. Increase in LAR due to N1 treatment in Si Kuning defined plant investment in leaves and larger LAR was indicative of plant emphasizing on leaf production.

Inpara 5 actually also had increased its LAR under 14 days submergence. However, greater increase was initiated by NAR attributing to a superior impact to RGR change. From the results, it was also noted that there was a negative relationship between NAR and LAR caused by N1 treatment under submerged condition regarded as a compensatory response between these two variables. Some studies also reported the occurrence of compensatory effect in rice. Yan and Wang (2009) reported the compensatory effect in rice seedling induced by water deficit. Compensatory effect on yield was also found in rice planted at different seedling densities. Contributions from both NAR and LAR to RGR then were further investigated and found that both variables affected RGR though the impact was not similar. From the results, it was also considered that during recovery period, NAR gave a greater impact to RGR rather than LAR.

Gautam et al. (2014) reported that a quick re-growth following submergence was a desirable quality as it could ensure production of sufficient biomass for plant productivity indicating the importance of plant growth during recovery period. As NAR during recovery showed the great impact to RGR as mentioned, NAR related traits then were identified. Some studies reported that NAR of rice was affected by leaf morphogenesis such as thinning of the leaf blade when nutrients were supplied sufficiently. Therefore, the relationship between NAR and SLA was also evaluated. However, the result showed that the correlation coefficient between NAR and SLA was significant (r = 0. 530, P = 0. 076) only under submergence and insignificant during recovery period (data not shown).

The present study also attempted to investigate the relationship between RTR and RGR as they both showed a similar tendency. The relation, however, was insignificant (data not shown). Similar insignificant result was shown between RGR and tiller number (r = 0. 424, P = 0. 080). Hence, when tiller number was associated with LAR, the correlation coefficient showed significant result (r = 0. 815, P < 0. 01). Furthermore, the role of tiller number was more apparent to TDW for both under submergence and during recovery period with r = 0. 577, P < 0. 05 and r = 0. 828, P < 0. 01 respectively.

These findings indicated that tiller number was an important character determining plant adaptation under submergence and survival after de-submerged condition. Corresponding to this result, Fageria et al. (1997) clarified that greater tiller number would increase the leaf area index (LAI) (or LAR in this case) and consequently the radiation interception capacity resulting in high biomass production. Similar results were also reported by some studies indicating the contribution of tiller number to dry matter production. Moreover, tiller number was also considered as a substantial trait affecting yield. The former study in submergence-prone swamp area in South Sumatra also discovered that fewer tillers led to less panicle number per hill resulting in a lower yield due to the absence of N fertilizer.

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