Functional roles of root plasticity and its contribution to water uptake and dry matter production of CSSLs with the genetic background of KDML105 under soil moisture fluctuation

ABSTRACT Soil moisture fluctuation (SMF) stress due to erratic rainfall in rainfed lowland (RFL) rice ecosystems negatively affect production. Under such condition, root plasticity is one of the key traits that play important roles for plant adaptation. This study aimed to evaluate root plasticity expression and its functional roles in water uptake, dry matter production and yield under SMF using three chromosome segment substitution lines (CSSLs) with major genetic background of KDML105 and a common substituted segment in chromosome 8. The CSSLs showed greater shoot dry matter production than KDML105 under SMF, which was attributed to the maintenance of stomatal conductance resulting in higher grain yield. The root system development based on total root length of the CSSLs were significantly higher than that of KDML105 due to the promoted production of nodal and lateral roots. These results implied that the common substituted segments in chromosome 8 of the 3 CSSLs may be responsible for the expression of their root plasticity under SMF and contributed to the increase in water uptake and consequently dry matter production and yield. These CSSLs could be used as a good source of genetic material for drought resistance breeding programs targeting rainfed lowland condition with fluctuating soil moisture environments and for further genetic studies to elucidate mechanisms underlying root plasticity.

In rice, the root plasticity such as promoted lateral root development were shown to be exhibited in response to varying water-deficit stress intensities (Kameoka, Suralta, Mitsuya, & Yamauchi, 2015;Kano, Inukai, Kitano, & Yamauchi, 2011;Kano-Nakata et al., 2013;Kano-Nakata, Inukai, Wade, Siopongco, & Yamauchi, 2011;Menge et al., 2016;Tran et al., 2014), continuous cycles of alternating waterlogged and drought stress (Niones et al., 2012;Owusu-Nketia et al., 2018), rewatering after drought (Bañoc, Yamauchi, Kamoshita, Wade, & Pardales, 2000a;Sandhu et al., 2016;Siopongco, Yamauchi, Salekdeh, Bennett, & Wade, 2005; and transient drought preceded by waterlogged and vice versa (Suralta et al., 2010;Suralta & Yamauchi, 2008), and to contribute to the maintenance of dry matter production and yield under such conditions. Khao Dawk Mali 105 (KDML105) is an elite cultivar of aromatic rice mainly grown in rainfed lowland areas of North and Northeast Thailand which are prone to abiotic and biotic stresses such as salinity, drought, submergence, diseases and pest (Jantaboon et al., 2011;Jongdee, Pantuwan, Fukai, & Fischer, 2006;Kanjoo et al., 2012;Siangliw, Jongdee, Pantuwan, & Toojinda, 2007;Toojinda, Siangliw, Tragoonrung, & Vanavichit, 2003;Toojinda et al., 2005). Several studies have been carried out on the improvement of KDML105 under such stresses to increase grain yield. Through markerassisted selection, the development of pyramiding lines tolerant to submergence and resistant to brown plant hopper (Korinsak et al., 2016), backcrossed introgression lines tolerant to drought (Siangliw et al., 2007) as well as chromosome segment substitution lines (CSSLs) tolerant to drought and salinity (Kanjoo et al., 2011(Kanjoo et al., , 2012 have been reported. KDML105 was shown to be susceptible to severe water deficit conditions (Cabuslay, Ito, & Alejar, 2002;Kanjoo et al., 2012;Kumar et al., 2006). A series of our studies showed that KDML105 is well adapted to the rainfed lowland conditions (Azhiri-Sigari, Fukai & Cooper, 1995;Kano-Nakata et al., 2013;Wade, Fukai, Samson, Ali, & Mazid, 1999) where not solely simple/progressive drought but SMF stress is also a major constraint to production. It has quick responses in root development particularly in the promoted production of lateral roots under various water stress conditions. For instance, under SMF conditions, it can increase its efficiency in converting dry matter to root length to promote production of lateral and nodal roots, which supports leaf expansion (Bañoc et al., 2000a(Bañoc et al., , 2000b. Kameoka et al. (2015) also reported that KDML 105 has the ability to promote lateral root production at the shallow soil layer despite of higher water availability in deep soil, which indicates its high adaptability to the conditions of limited soil depth or impermeable hardpan where soil moisture is available mainly in shallow layer. In addition, we also pointed out the functional significance of plasticity in root aerenchyma formation during the wet period under SMF condition (Suralta et al., 2008b;Niones et al., 2012), which was shown to promote lateral development during the subsequent dry period under the same condition. As a consequence, such ability to promote lateral root development due to plasticity, which may be associated with plasticity in aerenchyma formation could be important trait for enhancing water and nutrient uptake during water deficit periods under SMF condition. This can lead to the improvement of KDML105 by increasing the biomass production and hence the yield.
Chromosome segment substitution lines (CSSLs) is a novel mapping population since they are genetic resources containing major genetic background similar to that of the recurrent parent but with introgressed chromosome segments from the donor. CSSLs have been used to effectively identify root plasticity traits, with reduced confounding effects by the variation in genetic backgrounds due to other traits, in response to water deficit of different intensities Tran et al., 2014) and SMF conditions (Niones et al., 2012;Suralta et al., 2008b;Suralta & Yamauchi, 2008). Kanjoo et al. (2012) developed CSSLs carrying drought tolerant (DT) QTLs on chromosomes 1, 4 and 8 (Lanceras, Pantuwan, Jongdee, & Toojinda, 2004) in the genetic background of KDML105. These CSSLs showed better adaptation and higher yield than the recurrent parent, KDML105 under drought stress conditions although the mechanism of such adaptation has not yet been studied. In this aspect, we assumed that root plasticity may be involved as one of the major mechanisms.
In this study, we hypothesized that selected KDML105 CSSLs with common substituted segments on chromosome 8 would produce more dry matter and yield more under SMF stress conditions, which could be attributed to the expression of root system plasticity. Thus, this study aimed to evaluate root plasticity expression of the selected CSSLs of KDML105 under SMF condition and its functional roles in water uptake, dry matter production and grain yield.

Field experiment
The selected CSSLs; CSSL64, CSSL72, CSSL79 together with their recurrent parent, KDML105 were evaluated under two soil moisture treatments: continuously waterlogged (CWL as control) and rainfed lowland (RFL) conditions with soil moisture fluctuation (SMF) during the wet season in 2016. The experiment was conducted at the experimental field of Kasetsart University, Kamphangsaen Campus, Thailand (14.0236°N, 99.9749°E) using a randomized complete block design (RCBD) with four replications. The experimental plot size was 20 m × 5 m with spacing of 0.2 m for each treatment plot. The soil used was heavy clay. Chemical fertilizers, NPK 46-0-0 was applied at 17 days after transplanting (DAT), and NPK 16-18-2 applied at 42-52 DAT at a rate of 90 kg N ha −1 .
Seeds of each genotype were put in paper bags and placed in an oven at 50°C for 48 h to break dormancy prior to sowing. The seeds were then sown in black plastic trays with soil under well-watered conditions. Twenty-eight day old healthy seedlings of each genotype were transplanted in the field at one seedling per hill on the 29th August, 2016. The transplanted seedlings were allowed to recover from transplanting shock for 28 days with frequent watering after which the seedlings were exposed to two water treatments; CWL and RFL conditions, and grown until maturity.
Soil moisture treatments In CWL, the water level was maintained at 5 cm depth above the soil surface from transplanting until maturity.
In RFL, the field was irrigated similar to that of the CWL until 28 days after transplanting (DAT) and then the water was drained and shifted its water supply purely from the rainfall.
Soil water potential was recorded using a soil tensiometer (Daiki soil and moisture, Daiki Rika Kogyo Co., Japan). The soil O 2 concentration was measured with a soil oxygen meter (E.M.J., Decagon, Utah, USA). Six soil tensiometers and one oxygen sensor were installed at 20 cm soil depth in RFL plot only.
The heading date was recorded as the number of days when 50% of the panicles of all plants in a plot already emerged (55 DAT).

Shoot and root measurements
The shoot and root samples were collected at heading (55 DAT) and 15 DAH. Shoots were cut from the base and oven-dried at 80°C for 48 h before recording the dry weight. The root system was extracted as described by Niones et al. (2012) and  using a monolith stainless cylinder (20 cm diameter× 40 cm height) (Kang, Morita, & Yamazaki, 1994) up to 20 cm soil depth. The collected root samples were washed free of soil with gentle running water and stored in 70% ethanol for further measurements. The number of nodal roots at the base was manually counted. For the total root length (TRL) measurements, root samples were spread evenly on a transparent tray without overlapping. Digital images were then taken using an image scanner (EPSON Perfection V700 Photo) at 400 dpi resolution. The TRL was analyzed using WinRhizo software (Regent Instruments Inc., Saint-Foy, Canada) (Kameoka et al., 2015;Menge et al., 2016). A pixel threshold value of 175 was set for the root length analysis (Nguyen et al., 2018;Suralta et al., 2018). The root lengths were analyzed according to their diameter classes to estimate the total length of lateral roots having a diameter of <0.3 mm (Yamauchi, Pardales, & Kono, 1996).

Yield and yield component measurements
Forty eight plants per genotype per water treatment were sampled at maturity (35 DAH) for the yield and yield components measurements. Panicles were separated from the shoots, counted and oven-dried at 80°C for 48 h before weighing. The spikelets were manually separated from the panicles. The spikelets were classified into filled (with developed grains) and unfilled (without developed grains) spikelets and counted separately. The 1000 grains were collected from each sample to record the 1000 grain weight. Yield was determined as the weight of filled grains per plant (adjusted to 14% grain moisture content).

Statistical analysis
Differences between mean values were compared using the least significant difference (LSD) test at p < 0.05 level to compare genotypes within each water treatment using Microsoft Excel Statistics 2013 for Windows. The relationships between root traits and shoot traits were determined using regression analysis.

Pot experiment
To accurately evaluate the water uptake of the three CSSLs (CSSL64, CSSL72, and CSSL79) and their recurrent parent, KDML105, plastic pots (20 cm in height and 25 cm in diameter) were used. Each plastic pot was filled with 6.0 kg of air-dried heavy clay soil and placed under greenhouse of Rice Gene Discovery Unit, Kasetsart University, Kamphangsaen Campus, Thailand using RCBD with four replications.
Seeds of each genotype were put in paper bags and placed in an oven at 50°C for 48 h to break the dormancy prior to sowing. Ten seeds per genotype were directly sown in the plastic pots on the 22nd August, 2016. The seedlings were thinned to one seedling per pot at 20 days after sowing (DAS). Each pot represented one replication.

Soil moisture treatments
The plants were exposed to two water treatments; CWL (as control) and SMF (as stress) conditions. In CWL, the water level was maintained at 5 cm depth above the soil surface from sowing until the end of the experiment. In SMF, the plants were first waterlogged similar to that of the CWL until 30 DAS. Thereafter, the water was drained until the leaves started to roll (52 DAS), and then rewatered back to 5 cm water level above the soil surface (Niones et al., 2012;Owusu-Nketia et al., 2018) for ten days. This was done repeatedly until the termination of the experiment at 85 DAS (20 DAH). The pots were weighed daily using a digital balance to record the wet mass of the soil until leaf rolling. The water uptake for each plant per pot was calculated as the proportion of water weight estimated as the difference between the wet weights of the soil excluding the pot on a given day to the dry weight of soil (Suralta, Lucob, Perez, Niones, & Nguyen, 2015).

Shoot and root measurements
The shoot and root samples were collected at 85 DAS. Shoots were cut from the base and oven-dried at 80°C for 48 h before weighing. The root samples were washed free of soil with gentle running water. The cleaned root samples were stored in 70% ethanol for further measurements described in the field experiment.
Root measurements were carried out in a similar manner described in Field Experiment.

Field experiment
Soil moisture fluctuation dynamics The soil water potential and oxygen concentration dynamics under RFL conditions are shown in Figure 2. Soil water potential and oxygen concentration were recorded from 27 DAT until 90 DAT. The soil water potential fluctuated from 0.0 to −7.6 kPa with oxygen concentration of 1.6 ppm from 30 to 35 DAT, after which the soil oxygen concentration dropped to 0 ppm The soil water potential then dropped to −7.4 kPa with soil oxygen concentration of 1.5 ppm at 45 DAT. Afterwards, the soil oxygen concentration decreased to 1.2 ppm with water potential of 0.0 kPa at 50 DAT due to rainfall. During the heading stage at 55 DAT, the soil water potential dropped to −29.0 kPa with soil oxygen concentration of 4.3 ppm. The soil oxygen concentration and soil water potential then dropped to 0.0 ppm and 0.0 kPa, respectively at 74 DAT. However, due to absence of rain, the soil oxygen concentration increased to 17.3 ppm with soil water potential of −72.0 kPa from 75 DAT to the termination of the experiment at 90 DAT.

Shoot growth and development
Generally, all the CSSLs had a significant increase in shoot dry weight (SDW) under RFL, relative to their CWL counterparts while KDML 105 showed reduction. Specifically, CSSL64 increased SDW by 17 and 28% at heading and 15 DAH, respectively. Likewise, CSSL72 had significant increase by 19% at heading stage and 10% at 15 DAH. Furthermore, CSSL79 had significant increase in SDW by 18 and 21% at heading and 15 DAH respectively. On the other hand, KDML 105 had reduced SDW by 19 and 16% under RFL relative to its CWL counterpart at heading and 15 DAH, respectively (Tables 1 and 2). The three CSSLs (CSSL64, CSSL72 and CSSL79) produced similar SDW with the recurrent parent, KDML 105 at heading and 15 DAH under CWL (Tables 1 and 2). Under RFL, on the other hand, these CSSLs produced significantly greater SDW by 43% (CSSL64), 36% (CSSL72) and 44% (CSSL79) than KDML105 at heading stage. Furthermore, these CSSL also produced significantly greater SDW by 62% (CSSL62), 34% (CSSL72) and 50% (CSSL79) than KDML105 at 15 DAH (Tables 1 and 2). Moreover, the CSSLs had an increase in number of tillers under RFL relative to CWL conditions while KDML105 had reduced tillering (Tables 1 and 2). Furthermore, there were no significant differences in tillering ability between the CSSLs and KDML 105 under CWL. On the contrary, the CSSLs significantly produced higher number of tillers than KDML 105 under RFL both at heading and 15 DAH. Under both water treatments, the tiller production of the genotypes was slightly reduced at 15 DAH as compared with heading stage.

Stomatal conductance
In CWL, the stomatal conductance of CSSL64, CSSL72 and CSSL79 was not significantly different among genotypes regardless of growth stages (Tables 1 and 2). In contrast, these CSSLs had significantly higher stomatal conductance than KDML105 at heading during drought stress and even at 15 DAH during rewatering (occurrence of rainfall). In general, under RFL, the CSSLs maintained their stomatal conductance after rewatering whereas that of KDML105 was reduced at 15 DAH as compared with CWL.
Root system development Root system development based on TRL was not significantly different among CSSLs and KDML105 under CWL (Tables 1 and 2). On the contrary, the CSSLs had significantly greater root system than KDML105 under RFL. Compared to KDML105, CSSL64, CSSL72 and CSSL 79 had significantly greater TRL by 50, 54 and 58%, respectively, at heading and by 75, 65 and 74%, respectively at 15 DAH.
Also, for nodal root production (Tables 1 and 2), there were no significant differences between the CSSLs and KDML105 under CWL. Under RFL condition, the number of nodal roots (NRN) of the genotypes  slightly increased from heading to 15 DAH although the CSSLs showed significantly higher nodal root production than KDML105. CSSL64 had significantly higher nodal root production than KDML105 by 86% at heading and 81% at 15 DAH. CSSL72 showed significantly higher NRN than KDML105 by 78 and 68%, respectively, at heading and 15 DAH. Also, CSSL79 had significantly higher NRN than KDML105 by 91 and 78% at heading and 15 DAH, respectively.

Correlation among root and shoot traits
The correlations between TRL and stomatal conductance (Figure 3), TRL and SDW (Fig. 44), and stomatal conductance and SDW ( Figure 5) of the three selected CSSLs were positive and significant at heading and 15 DAH under RFL conditions.

Yield and yield components
The yield and yield components of the CSSLs and KDML105 are presented in Table 3. There were no significant differences among the genotypes under CWL condition. Under RFL condition, CSSL64, CSSL72 and CSSL79 had significantly higher yields as well as the yield components such as total number of spikelets, filled number of spikelets, percentage filled spikelets and the number of panicles than KDML105. However, there were no significant differences in 1000 grain weight among the genotypes except CSSL64, which was significantly lower than CSSL72.

Pot experiment
Shoot dry matter production, root system development and water uptake For shoot and root growth of the genotypes, a similar trend was observed as in Field Experiment. Under CWL condition, there were no significant differences among genotypes for SDW, TRL and NRN (Table 4). The CSSLs, CSSL64, CSSL72 and CSSL79 had significantly higher SDW, TRL and NRN than KDML105 under SMF   In contrast, KDML105 had significant reduction in SDW and TRL by 12 and 8%, respectively, under SMF as compared with CWL. There was also reduction in NRN under SMF by 6% compared with CWL although not significant (Table 4).
The total water uptake of the CSSLs, CSSL64, CSSL72 and CSSL79 under SMF condition was significantly higher than their recurrent parent, KDML105 (Table 4).

Correlation among root and shoot traits and water uptake
The TRL of the three selected CSSLs and KDML105 was positively and significantly correlated with total water uptake (Figure 7(a)). There was also significant and positive relationship between total water uptake and SDW (Figure 7(b)).

Discussion
In this study, CSSLs carrying substituted segments on chromosomes 8 in KDML105 genetic background were evaluated for root plasticity expression under SMF condition and its functional roles in water uptake, dry matter production and grain yield. We found out that these CSSLs performed better than their recurrent parent in terms of root and shoot growth under only RFL condition in the field and SMF condition in pot experiment but not under favorable CWL conditions. Moreover, these CSSLs had the ability to increase root and shoot growth under RFL, relative to CWL conditions. This indicates that the introgressed segments into the genetic background of KDML105 may have QTLs which could be responsible for the expression of root plasticity responses to SMF and consequently increased water uptake during the transient drought periods and the overall dry matter production.
Several studies have shown the reduction in shoot dry matter production and grain yield of rice plants grown under SMF condition (Belder, Spiertz, Bouman, Lu, & Tuong, 2005;Niones et al., 2012;Owusu-Nketia et al., 2018;Suralta et al., 2008bSuralta et al., , 2010 while such fluctuating soil moisture environment including aerobic culture could cause increase in dry matter production (Katsura, Okami, Mizunuma, & Kato, 2010) as well as grain yield (Kato, Okami, & Katsura, 2009) as compared with flooded conditions. In this study, CSSL64, CSSL72 and CSSL79 had significantly higher grain yield than the recurrent parent, KDML105 under RFL condition but not under CWL condition (Table 3). This increase in grain yield could be attributed to increase in yield components such as number of panicles, total number of spikelets and filled spikelets. The significantly higher number of filled spikelets led to a decrease in percentage spikelet sterility and hence the higher grain yield. . These results of increased yield and yield components of the three CSSLs under RFL condition was consistent with the previous studies by Kanjoo et al. (2012), in which these three CSSLs showed increased grain yield also under drought stress. On the other hand, the yield reduction of KDML105 under RFL condition was found to be caused by increased number of unfilled spikelets and high percentage spikelet sterility. During the heading stage (55 DAT), the soil became dry and the water potential reached at −29.0 kPa (Figure 2). KDML 105 had significantly shorter TRL and thus most probably shallower root system under RFL conditions than CWL conditions while the three CSSLs maintained their root system development under RFL conditions at heading stage (Table 1). These facts indicate that KDML 105 exhibited less plasticity in root system development in response to RFL conditions, which could have adversely caused spikelet sterility.
In addition, the maintenance or increase in dry matter production under SMF condition could result in higher grain yield (Katsura et al., 2010;Niones et al., 2012;Owusu-Nketia et al., 2018;Suralta et al., 2010). The significant increase in shoot dry matter production of CSSL64, CSSL72 and CSSL79 under SMF conditions as compared with CWL in both experiments in this study contributed to their higher grain yields than KDML105 (Tables 1 and 2). This can be attributed to the increased production of tillers (Kanjoo et al., 2012) as well as water uptake and stomatal conductance (Kano-Nakata et al., 2013;Nguyen et al., 2018;Suralta et al., 2015;Tran et al., 2014). The significant and positive relationships between stomatal conductance and shoot dry matter ( Figure 5), under RFL condition in the field and that of total water uptake and shoot dry matter production (Figure 7(b)) under SMF in pot experiment indicate that water uptake and stomatal conductance enhanced dry matter production and consequently the grain yield of CSSL64, CSSL72 and CSSL79, which was significantly higher than that of KDML105. This maintenance or increase in stomatal conductance which subsequently increased water supply to the leaves could be a consequence of promoted root growth under SMF conditions. Significant and positive relationships between TRL and stomatal conductance (Figure 3), TRL and shoot dry matter production and grain yield (Figures 4  and 6(a)) as well as TRL and total water uptake (Figure 7 (a)) of CSSL64, CSSL72 and CSSL79 under SMF conditions suggest that water uptake, stomatal conductance and dry matter production were enhanced by increased root length due to the expressed plasticity.
Previous studies have shown experimental evidences of genotypic variations in root plasticity expressed in response to heterogeneous soil environments for the adaptation of rice plants (Bañoc et al., 2000a;Siopongco et al., 2008;Kano-Nakata et al., 2013;2017;Kameoka et al., 2015;Menge et al., 2016;Niones et al., 2012;Nguyen et al., 2018;Owusu-Nketia et al., 2018;Suralta et al., 2010Suralta et al., , 2016Suralta et al., , 2018Tran et al., 2014). In this study, the CSSLs showed no significant differences in root system development with the recurrent parent under CWL in both experiments. In contrast, root system development in terms of total root length of CSSL64, CSSL72 and CSSL79 were significantly greater than KDML105 under SMF condition. The greater root system of the CSSLs was due to the promoted nodal root production from the tillers and elongation, and promoted lateral root elongation (Tables 1 and 2). These observations were similar to those reported by Niones et al. (2012), Kano-Nakata et al. (2013) and Owusu-Nketia et al. (2018).
These results infer that the root plasticity resulting in promoted nodal root development and increased total root length of which lateral roots constitute greater portion (Wang, Siopongco, Wade, & Yamauchi, 2009;Yamauchi, Kono, & Tatsumi, 1987), could play a major role in the adaptation of rice plants to soil moisture fluctuation conditions (Bañoc et al., 2000a(Bañoc et al., , 2000bNiones et al., 2012;Owusu-Nketia et al., 2018). Thus, the ability of the plant to maintain greater root system (Bañoc et al., 2000b)  increase in root surface area, thereby enhancing water uptake (Kano-Nakata et al., 2013;Kato et al., 2011;Kato, Kamoshita, Yamagishi, Imoto, & Abe, 2007;Siopongco et al., 2005;Siopongco, Yamauchi, Salekdeh, Bennett, & Wade, 2006;Suralta et al., 2010). This could lead to maintenance of dry matter production and yield. In addition, there were significant and positive correlation between TRL and grain yield (Figure 6(a)) as well as TRL and percentage filled spikelet (Figure 6(b)) of particularly CSSL64, CSSL72 and CSSL79 under RFL condition. These results suggest that the promoted root development due to plasticity contributed to increase in grain yield through grain filling. Moreover, the similar performance of the three CSSLs under SMF condition could be due to the common substituted segments on chromosome 8 (Figure 1), which are derived from DH103 allele and introgressed into KDML105 genetic background. These common substituted segments may regulate root plasticity in response to SMF stress condition. Such regulation could have contributed to the greater root system (Tables 1 and 2) of the CSSLs as a result of promoted and elongated nodal and lateral roots, which led to increase in water uptake and consequently the dry matter production and yield. Furthermore, the common substituted segments coincided with QTLs for biological yield, panicle number and plant height (Lanceras et al., 2004), harvest index (Bernier et al., 2007), root to shoot ratio (Champoux et al., 1995), and deep root biomass (Courtois et al., 2013) under constant drought stress conditions using other rice mapping populations. Likewise, the substituted segments of the CSSLs also overlapped with QTLs associated with specific water use (Kato, Hirotsu, Nemoto, & Yamagishi, 2008), root volume (Qu et al., 2008), and root number (Ray et al., 1996;Zheng et al., 2000). However, QTLs detected under SMF condition for root number Suralta et al., 2015) and total root length  did not coincide with the substituted segments of the CSSLs on chromosome 8. This suggests that these QTLs regulating root development may be dependent on specific rice mapping population. The common substituted regions of the CSSLs could play important roles in adaptation to soil moisture fluctuation stress which occurs in rainfed lowland areas. However, further genetic analysis on detection of QTLs using these CSSLs is still needed to confirm the location and validate the effect of QTLs for root plasticity.

Conclusion
In our study, CSSLs carrying DT-QTL on chromosomes 4 and 8 were evaluated for the expression of root plasticity and its functional roles in water uptake, dry matter production and yield. The CSSLs, CSSL64, CSSL72 and CSS79 showed greater shoot dry matter production than KDML105, which were credited to their increase in tiller production as well as higher stomatal conductance and water uptake. In addition, the root system development of the CSSLs expressed as total root length were higher than KDML105 due to the promoted nodal and lateral roots production and elongation under rainfed condition. Interestingly, under rainfed condition, the shoot dry weight of the CSSLs were higher than that of the flooded condition and this could be attributed to the introgressed DT-QTL segment thereby leading to increase in yield. Therefore, such genetic variation in the expression of root plasticity is essential in improving the adaptability of rice plants grown under fluctuating soil moisture environments such as rainfed lowlands and could be useful genetic material for breeding programs for rainfed lowland rice.