Echinochloa crus-galli (L.) P. Beauv.

Fogliatto, Silvia, Patrucco, Lorenzo, Milan, Marco & Vidotto, Francesco, 2021, Sensitivity to salinity at the emergence and seedling stages of barnyardgrass (EchinochloQ crus-gQlli), weedy rice (OryzQ sQtivQ), and rice with different tolerances to ALS-inhibiting herbicides, Weed Science (Cambridge, England) 69 (1), pp. 39-51 : 42-45

Echinochloa crus-galli

Seedling Development

Plant Emergence. The regression analysis carried out on plant emergence for different salt concentrations established that emergence decreased in all E. crus-galli populations for increasing salt levels, albeit at different magnitudes ( Figure 1 ). The percentages of emerged seedlings were already different at 0 mM, with population s2 showing the highest emergence rate, that is, about 86%, and population r2 the lowest, with an average emergence value of about 27%. The emergence rates of populations s1, s3, and r 1 in the control (0 mM) were 50%, 55.5%, and 55.6%, respectively. The different initial rate of emergence between populations was also visible when parameter d (upper asymptote) was compared across the curves ( Table 1 ). In fact, parameter d for population s2, which had the highest emergence rate, was significantly different compared with all the other populations; moreover, populations r1 and r2 also differed in their initial emergences. The CompParm function also highlighted that parameter b, which is the curve slope, was different between all the resistant populations and populations s2 and s3. Parameter e, the point of inflection of the curve, was instead similar for all curves ( Table 1 ). The most similar curves were those of populations s1 and s3 and those of populations s1 and r1, as they showed nonsignificantly different curve parameters and SI values, which is also a comparison between emergence in a specific point of the curve ( Table 1 ).

All populations showed an increase in emergence, compared with the control, for 50 mM, probably due to a hormetic effect of the salt; this increase was moderate in some populations, such as in s2, which only showed a 3% increase (88.8% emergence vs. 86.1% at 0 mM), while it was more pronounced in others, such as in population r2, which displayed a 70% increase in emergence (47.2% emergence vs. 27.7% at 0 mM). However, at 100 mM, only population r1 showed an increase in emergence, that is, of about 20% compared with the control (66.7% emergence vs. 55.5% at 0 mM), while all others showed a reduction in emergence. The decrease in emergence was even more notable for higher salt rates; in fact, about 30% emergence reduction or higher was observed in populations s1 and s3 compared with the control, while population r2 showed the greatest reduction (80%).

The NaCl concentration required to reduce emergence by 50% ranged from 123.58 mM NaCl (population r2) to 287.76 mM (population s1). It was observed, from a pairwise comparison of the SIs, that population r2 was affected significantly more than the sensitive populations (s1, s2, and s3) ( Table 1 ).

Echinochlo a crus-galli is one of the most common and problematic weeds for rice production throughout the world, and previous studies demonstrated that the species is quite tolerant to salinity ( Chauhan et al. 2013; Cirillo et al. 2018; Hakim et al. 2011; Serra et al. 2018). Although most plants can cope with salinity during germination, they are generally more sensitive at the emergence and early growth stages ( Läuchli and Grattan 2007). A previous study conducted on Italian ALS inhibitor–resistant and ALS inhibitor–sensitive E. crus-galli populations found that germination occurred up to 400 mM NaCl, although with lower values, that is, about 13% ( Serra et al. 2018). Moreover, in the same study, values of up to 90% were recorded for the germination of sensitive populations for up to 250 mM NaCl, while an average germination of 70% was observed for resistant populations ( Serra et al. 2018). Previous studies also found a high tolerance of the Echinochloa genus during germination for two populations of Echinochloa glabrescens Munro ex Hook f. collected in the Philippines, which showed a germination of more than 60% for 200 mM NaCl, with an EC50 value of between 250 and more than 300 mM (Opena ˜et al. 2014). These values were similar to those found by Serra et al. (2018) and had the same magnitude as the values found in this study for the emergence of sensitive populations.

In this study, values of between 36% and 44% were observed for the emergence of sensitive populations for the highest salt level, while percentages lower than 20% were recorded for the resistant populations. Thus, emergence seems to be affected more by salinity than germination, as observed in the germination test conducted previously on the same populations; in addition, the emergence of herbicide-resistant populations was reduced to a great extent by high salt concentrations compared with the sensitive ones, as already observed for germination in a previous study ( Serra et al. 2018).

Plant Height, Shoot, and Root Weight. Echinochloa crus-galli seedling development was hampered by the salinity conditions, as seedling emergence occurred for NaCl concentrations above 150 mM, but this was followed by the total desiccation of the plants. However, population r2 was already affected at 150 mM NaCl, as the seedlings were able to emerge at this concentration but were unable to grow.

Variability in plant heights among the populations was already observed in the control, in which the height ranged between about 20 cm (population s1) and almost 43 cm (population r1) ( Table 2 ). Increasing salt concentrations did not significantly change the plant height in population s1 or r2, while a general decrease in height was observed in the other populations. Increase in plant height was observed at 50 mM, compared with 0 mM, in all the sensitive populations, with increased values that ranged between about 5% (populations s2 and s3) and about 49% (population s1). The resistant population r2 showed a 38% plant height decrease at 50 mM, and thus an absence of hormesis can be hypothesized; plant heights in population r1 were only slightly reduced (decreased by about 10%). Only population s1 still showed an increase in plant height at 100 mM compared with the control (about 38%), while all the other populations behaved similarly to one another, that is, their height was reduced. Apart from r2, all the other populations showed a more marked height reduction for the highest salt concentration (150 mM), which was more moderate in populations s1 and s2.

Shoot weight was affected more than plant height by increasing salt concentrations, as weight reductions occurred ( Table 2 ). Population s1 showed a higher tolerance to salt, and it was the only one to show an increase in shoot weight at 50 mM (63% increase compared with the control); population r1 did not show any reduction in weight for this salt concentration. All populations showed similar weight reduction at 100 mM, that is, above 70% compared with the control, apart from population s1, which only showed a reduction of about 14%. Population r1 showed an intermediate behavior, with a weight reduction of about 46%. Apart from population r2, which failed to grow, no significant differences in weight reduction were observed among the populations for 150 mM, with reduction values between 62% in population s1 and 88% in population s3, compared with the control.

Root weight also reflected the higher salt tolerance observed for population s1 and the lower salt tolerance observed for population r2. A probable hormetic effect was observed for 50mM in population s1, whose root weight increased by about 54% compared with 0 mM, while the other populations showed a moderate (s2, r1, and r2) or a more marked (s2) root weight decrease. All populations showed a root weight reduction at 100 mM, which was moderate in populations s1 (about 36%) and r1 (about 46%), but more evident in all the others, reaching even higher reduction values than 80% (populations s3 and r2). As already observed for the shoot weight, the root weight decrease was similar among all populations for 150 mM NaCl.

In general, seedling growth (plant height, shoot and root weight) was affected more by salinity at 150 mM in the ALS inhibitor– resistant population r2, which was not able to mature, compared with the sensitive populations, as already observed for emergence; in fact, even though no significant differences were found between resistant and sensitive populations at 50 mM and 100 mM, growth was completely hampered in r2 at the high salt concentration. Among the sensitive populations, s1 was the most tolerant to salt, showing the smallest reduction for all parameters. However, these differences were only detectable up to 100 mM NaCl.

Reductions in plant height and in the biomass of E. crus-galli for increasing salinity levels were observed in previous studies ( Chauhan et al. 2013; Hakim et al. 2011; Serra et al. 2018). A study carried out on E. crus-galli seeds, collected in environments with different degrees of salinity, showed that the seeds were able to germinate up to 1.5% NaCl, while once they emerged, they only survived about 20 d at a concentration of about 1% NaCl ( Rahman and Ungar 1990). A previous study of ours confirmed these results and highlighted that E. crus-galli demonstrates a lower tolerance at the early seedling stage than at the emergence and germination stages ( Serra et al. 2018). Plant weight was affected more than height, as a generally more marked reduction was observed in the case of shoot and root weight for increasing salt levels; this was already observed in the same species in a previous study in which plant height was found to be more tolerant to salt than biomass ( Wilson and Read 2006). A moderate reduction in plant height in response to salt allows E. crus-galli to be competitive in saline environments, even when the plant biomass is reduced, and it has been demonstrated that most of the competitive ability of this species is due to its fast growth and ability to shade other plants as a result of its height ( Pearcy et al. 1981; Wilson and Read 2006). Moreover, the allocation of resources toward plant height instead of biomass may be an adaptation of this species to abiotic stresses that permits the species to succeed in difficult environments ( Wilson and Read 2006).

In this study, the root weight reduction in all populations showed a similar trend to that of the shoot weight reduction. Such a similarity was also observed in another study on the same species; however, in that study, E. crus-galli was still able to produce roots at 24 dS m −1 EC, while in present experiment, no plants survived after 150 mM NaCl, which could approximately correspond to 15 dS m −1 EC ( Chauhan et al. 2013; Government of South Australia 2015).

Chlorophyll and Carotenoid Content

The chlorophyll a content increased in all populations as the salt level increased, except in population r2, which showed a moderate reduction, with a maximum of 3%, compared with the control ( Table 3 ). The greatest increase in chlorophyll a (about 40% compared with the control) was observed in population s1 for all salt concentrations, followed by population r1, while populations s2 and r1 showed intermediate values with a more moderate increase. The chlorophyll a content increased slightly in population s3, reaching a maximum of 5% for 150 mM NaCl. The chlorophyll b content displayed an even greater increase for higher salt concentrations compared with the control. The only exception was population s3, which showed reduction in the chlorophyll b content of about 20% for 50 and 100 mM and about 40% for 150 mM NaCl compared with the control. All the other populations showed a marked increase in chlorophyll b content for 50 mM compared with the control, with values above 90% in populations s1 and s2. Populations s1 and r1 showed a further increase for 100 mM, that is, above 100%, compared with the control, while populations s2 and r2 showed lower values. Apart from populations s3 and r2, the chlorophyll b content was still higher for 150 mM in all the other populations than that observed at 0 mM NaCl, albeit with lower values than observed with the other salt concentrations.

Carotenoid content showed a more variable behavior among the populations; decrease in carotenoid content was in fact observed for increasing salt rates for populations s2, r1, and r2, while an increase was detected for populations s1 and s3 compared with the control. A more pronounced decrease was displayed by the ALS inhibitor–resistant populations for all salt concentrations, while population s1 showed the highest increase in carotenoid content, that is, an increase of more than 100% for 150 mM.

Salinity can affect photosynthesis by inducing stomatal closure and a reduction in carbon assimilation, thereby resulting in growth limitation in the short term ( Acosta-Motos et al. 2017; Parida and Das 2005). In the long term, salinity can cause the accumulation of salt in the leaves and a reduction in the chlorophyll and carotenoid contents ( Acosta-Motos et al. 2017). In our study, the photosynthetic pigments increased in the majority of the populations under salt stress, so it is probable that the salinity stress did not last long enough to cause a reduction in the pigment content, as this is generally a long-term response to salt ( Acosta-Motos et al. 2017; Stepien and Johnson 2009). However, not only were the chlorophyll and carotenoid contents not reduced under saline conditions, they also showed an increase, thus indicating the tolerance of E. crus-galli to salt; a rise in chlorophyll under saline conditions is considered a marker of salt tolerance in plants, as observed in other salt-tolerant species ( Acosta-Motos et al. 2015; Ashraf and Harris 2013; Stepien and Johnson 2009).

In our study, population r2 displayed a reduction in both chlorophyll a and carotenoid contents, thus confirming it was the most sensitive to salt. Another indicator of salt tolerance is the high chlorophyll a / b ratio, which is known to be an indicator of health and nonstressed plants ( Duarte et al. 2013). In our study, most populations showed a decreasing value for the chlorophyll a / b ratio as the salt concentration increased up to 100 mM ( Table 3 ); only population s3 showed an increase in this ratio at increasing salt concentrations, while a more variable behavior was shown by population r2. This could indicate that even though such plants have a certain degree of salt tolerance, salinity induced stress and slowed down photosynthesis, as also demonstrated by the reduction in seedling weight.

Carotenoid content was reduced more compared with chlorophyll content in presence of salt; a reduction in carotenoids has been found to be caused by both oxidation due to ROS and destruction of carotenoids due to salt ( Abdel Latef et al. 2020). However, populations s1 and s3 showed an increase in carotenoid content for increasing levels of salt, thus demonstrating they were less sensitive to salinity; the high carotenoid content in these populations could have protected the chlorophyll, thereby preventing its photooxidation ( Toscano et al. 2019).

Figures

Figure 1. Dose–response emergence curve with the average data points of the different EchinochloQ crus-gQlli populations against the salt concentration.Curve parameter estimates (Equation 1): s1 (b = 1.94, d = 55.85, e = 287.76), s2 (b = 1.52, d = 89.91, e = 222.71), s3 (b = 1.13, d = 89.91, e = 282.58), r1(b = 3.96, d = 67.67, e = 196.55), r2 (b = 6.85,d = 94.64,e = 123.58). The salt concentration required to reduce emergence by 50% (EC50) is shown below the graph. Only the significant pairwise comparisons between EC50 (SI index) are shown (Equation 2).

Tables

Table 1. Curve parameter (Equation 1) and sensitivity index (SI) (Equation 2) comparison between EchinochloQ spp. populations (estimate and P-values).

	Parameter b comparison		Parameter d comparison		Parameter e comparison		SI
Population comparison	Estimate	P-value	Estimate	P-value	Estimate	P-value	Estimate	P-value
s1/s2 s1/s3 s1/r1 s1/r2 s2/s3 s2/r1 s2/r2 s3/r1 s3/r2 r1/r2	1.28 1.72 0.49 0.54 1.34 0.38 0.42 0.28 0.31 1.10	0.772 0.649 0.184 0.350 0.726 0.003* 0.050* 0.001* 0.014* 0.898	0.62 0.96 0.82 1.49 1.55 1.33 2.40 0.85 1.55 1.81	0.000* 0.843 0.165 0.130 0.034* 0.049* 0.004* 0.310 0.121 0.027*	1.29 1.01 1.46 2.31 0.79 1.13 1.80 1.43 2.28 1.59	0.581 0.977 0.387 0.177 0.641 0.623 0.162 0.577 0.337 0.151	1.29 1.02 0.68 0.43 0.79 0.88 0.56 0.69 0.44 1.59	0.581 0.977 0.207 0.002* 0.641 0.577 0.013* 0.424 0.031* 0.151

*Significant comparisons at P ≤ 0.05.

Table 2. Plant height and shoot and root weight of the different EchinochloQ crus-gQlli populations and the percentage difference of these growth parameters for each salt concentration compared with the control.

	NaCl		Plant height difference compared with		Shoot weight difference compared		Root weight difference compared
Population	concentration	Plant heighta	the controlb	Shoot weighta	with the controlb	Root weighta	with the controlb
	mM	cm	%	g	%	g	%
s1	0	20.80	—	2.73	—	2.62 ab	—
	50	31.04	49.23 c	4.46	63.29 b	4.04 b	54.20 b
	100	28.85	38.70 b	2.36	−13.65 b	1.69 a	−35.69 b
	150	19.25	−7.45 B	1.02	−62.50	0.68 a	−74.04
s2	0	37.89 ab	—	15.79 c	—	15.35 b	—
	50	40.31 b	6.38 b	10.14 bc	−35.79 a	10.57 ab	−31.15 ab
	100	30.28 ab	−20.09 a	4.16 ab	−73.64 a	4.29 a	−72.05 a
	150	27.85 a	−26.50 B	2.73 a	−82.71	5.29 a	−65.53
s3	0	41.81 b	—	11.84 b	—	12.30 b	—
	50	44.14 b	5.58 b	10.35 b	−12.58 a	9.46 ab	−40.73 a
	100	32.19 ab	−23.01 a	2.75 a	−76.80 a	3.12 a	−80.48 a
	150	21.64 a	−48.25 A	1.34 a	−88.68	1.89 a	−88.14
r1	0	44.92 b	—	12.77 b	—	13.08 b	—
	50	40.45 b	−9.95 ab	12.74 b	−0.26 ab	16.81 b	−8.67 ab
	100	35.45 ab	−21.07 a	6.83 ab	−46.51 ab	9.84 ab	−46.53 ab
	150	25.36 a	−43.55 A	2.02 a	−84.18	3.04 a	−83.47
r2	0	32.86	—	3.64 c	—	3.20 b	—
	50	20.65	−37.16 a	2.12 b	−41.84 a	2.73 ab	−14.60 ab
	100	22.00	−33.05 a	0.73 a	−80.02 a	0.56 a	−82.38 a
	150	—	—	—	—	—

aLowercase letters refer to a comparison of the plant height and shoot and root weight of E. crus-galli for NaCl concentrations within the same population.

bThe plant height and shoot and root weight variations of E. crus-galli among populations are indicated in lowercase letters in italics, lowercase letters in bold, and uppercase letters for 50, 100, and 150 mM concentrations, respectively. Different letters indicate significantly different values, according to Tukey’ s honest significant difference test. Where no letters are present, the comparison was not significant.