Crop-weed interactions in saline environments

European Journal of Agronomy 99 (2018) 51–61

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European Journal of Agronomy journal homepage: www.elsevier.com/locate/eja

Crop-weed interactions in saline environments a

b

a,⁎

V. Cirillo , R. Masin , A. Maggio , G. Zanin a b

T

b

University of Napoli Federico II, Department of Agricultural Sciences, Via Università 100, 80055 Portici, Napoli, Italy University of Padova, Department of Agronomy, Animals, Food, Natural Resources and Environment, Agripolis – Viale dell’Università 16, 35020 Legnaro, Padova, Italy

A R T I C LE I N FO

A B S T R A C T

Keywords: Salinity Weeds Abiotic stress Crop management Salt stress

Soil salinization is one of the most critical environmental factors affecting crop yield. It is estimated that 20% of cultivated land and 33% of irrigated agricultural land are affected by salinity. In the last decades, considerable effort to manage saline agro-ecosystems has focused on 1) controlling soil salinity to minimize/reduce the accumulation of salts in the root zone and 2) improving plants ability to cope with osmotic and ionic stress. Less attention has been given to other components of the agro-ecosystem including weed populations, which also react and adapt to soil salinization and indirectly affect plant growth and yield. Weeds represent an increasing challenge for crop systems since they have high genetic resilience and adaptation ability to adverse environmental conditions such as soil salinization. In this review, we assess current knowledge on salinity tolerance of weeds in agricultural contexts and discuss critical components of crop-weed interactions that may increase weeds competitiveness under salinity. Compared to crop species, weeds generally exhibit greater salt tolerance due to high intraspecific variability, associated with diverse physiological adaptation mechanisms (e.g. phenotipic plasticity, seed heteromorphism, allelopathy). Weed competitiveness in saline soils may be enhanced by their earlier emergence, faster growth rates and synergies occurring between soil salts and allelochemicals released by weeds. In the future, a better understanding of crop-weed relationships and molecular, physiological and agronomic stress responses under salinity is essential to design efficient strategies to achieve weed control under altered climatic and environmental conditions.

1. Introduction Soil salinization is one of the most critical environmental factors affecting crop yield. Salinization of agricultural land is a consequence of climate change, competition for natural resources due to an increasing world population, and inadequate irrigation management (Rengasamy, 2006; Godfray et al., 2010). It is estimated that 20% of cultivated land and 33% of irrigated agricultural land are affected by salinity, a phenomenon that is expanding at an annual rate of 10% (Shrivastava and Kumar, 2015). This problem has been observed not only in the driest areas of the world but also in temperate regions, where there is increasing concern for secondary salinization due to seawater intrusion and consequent use of brackish waters for irrigation. Improving crop management under limited resources and environmental constraints is a primary target of agriculture specialists who need to respond to an increasing demand for food (FAO, 2009). Problems associated with soil salinization include, at the soil level, a reduction in water infiltration and soil hydraulic conductivity, surface crusting, soil structure degradation and overall loss of arable lands (Warrence et al., 2002). At the plant level salinity causes osmotic stress due to high salt concentration

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Corresponding author. E-mail address: [email protected] (A. Maggio).

https://doi.org/10.1016/j.eja.2018.06.009 Received 29 January 2018; Received in revised form 10 May 2018; Accepted 26 June 2018 1161-0301/ © 2018 Elsevier B.V. All rights reserved.

in the soil water surrounding the root zone, followed by ionic stress due to the accumulation of Na+ and Cl− in plant tissues, leading to decreased growth and yield (Munns and Tester, 2008). In recent decades, management of saline agro-ecosystems has focused on 1) controlling soil salinity to minimize/reduce the accumulation of salts in the root zone (Shrivastava and Kumar, 2015) and 2) improving plants ability to cope with osmotic and ionic stress (Jamil et al., 2011). Less attention has been given to other components of the agro-ecosystem such as the soil microbiome (de Souza Silva and Fay, 2012; Canfora et al., 2017), soil mesofauna (Thakur et al., 2014; Pereira et al., 2015), and weed populations (Li et al., 2011; Hakim et al., 2011; Ma et al., 2014), which also respond to soil salinization and indirectly affect plant growth and yield. Among these, weeds play a central role. Weed control contributes for around 10% of cultivation costs in agriculture, but this cost can vary greatly depending on weed species, cultivation systems and locations (Bastiaans et al., 2000; Zimdahl, 2004). Weeds represent an increasing challenge for crop systems because, compared to cultivated crops, they have conserved higher genetic resilience and adaptation ability to adverse environmental conditions (Clements et al., 2004; Chen et al., 2015; Mitchell et al., 2016; Lu et al., 2016). The genetic diversity that

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conditions (Maggio et al., 2001; Jalali et al., 2017). This model has been used to rank crop species with respect to their salinity tolerance (Tanji and Kielen, 2002). Using the same approach, we did a metadata analysis to categorize the most common weeds and compare them with cultivated plants. In Table 1, 43 weed species are listed with an indication of specific salinity tolerance threshold (Th), slope (Sl) and category of tolerance based on Maas and Hoffman (1977). A first comparison of the data in Table 1 with the relevant literature for crop salt tolerance indicates that weeds, with respect to germination and growth parameters (fresh, dry weight biomass or shoot length), are generally more tolerant than most crops (44% of the species in Table 1 is classified as T or ET and 76% as MT, T or ET). At germination, a critical stage in crop-weed interactions, characteristic stress tolerance parameters vary substantially among weeds. Mean values of thresholds and slopes were 5.29 dS m−1 and 6.76% per dS m−1, respectively, ranging from extremely tolerant species such as Kochia scoparia with Th = 18.18 dS m−1 and Sl = 1.07% per dS m-1 to very sensitive species such as Orobanche cernua with Th = 0.94 dS m−1 and Sl = 16.9% per dS m−1. To better visualize the relative crop-weed salinity tolerance, we used the two characteristic parameters (threshold, Th and slope, Sl) to calculate the Electrical Conductivity (EC) level at which each species had a 50% reduction (EC50) in dry weight biomass compared to nonsalinized controls (Figs. 1 and 2) (Steppuhn et al., 2005). The average EC50 for the 18 most common weed species worldwide shown in Fig. 2 was 16.4 dS m-1, while the average EC50 for 35 common crop species (Tanji and Kielen, 2002) was 10.6 dS m-1, further indicating that weeds are more tolerant to root-zone salinity than crops. Specific response functions for some common crops and associated weed species revealed important differences in terms of salinity tolerance threshold (Th) and responses to increasing soil salinity after Th (Sl) (Fig. 3). Corn and soybean are more sensitive than their associated weed species. With respect to corn (Fig. 3A), Cynodon dactylon, Echinochloa crus-galli and Setaria italica have higher Th, while Digitaria sanguinalis and Setaria italica present lower Sl. As for soybean (Fig. 3B), Sorghum halepense has higher Th whereas Echinochloa crus-galli, Xanthium strumarium and Portulaca oleracea manifest both higher Th and lower Sl (Essa, 2002). In contrast, wheat (Fig. 3C) shows similar tolerance to Lolium perenne, higher tolerance than Bromus tectorum (only for Th) and higher tolerance compared to Avena fatua only in terms of Th, yet lower tolerance in terms of Sl. Rice (Fig. 3D) tolerates salinity better than Echinochloa colona, Oryza sativa (weedy species) and Cyperus iria in terms of Th, whereas it shows a remarkable decline in yield (Sl) compared to these weeds, at salinity higher than their specific tolerance thresholds (Aslam et al., 1993). Rice is much more sensitive than Echinochloa crus-galli in terms of both Th and Sl, indicating an important interspecific variability (compare E. colona vs. E. crus-galli). The salinity tolerance difference between cultivated rice and its relative wild species is consistent with a significant loss of competitiveness vs. weeds that has occurred during the selection of cultivated varieties. Finally, sugar beet shows higher salinity tolerance compared to Xanthium strumarium, Avena fatua and Sorghum halepense in terms of slope (Sl), whereas it has a slightly reduced threshold compared to those weeds (Fig. 2E). While a first level assessment demonstrates that weeds may cope better than crops with increasing salinization, it is also important to unravel the bio-functional basis for weed competitiveness and invasion. In the following sections we review the multiple determinants that may enhance weeds competitiveness and spread as these may serve as leverage points to elaborate adequate control strategies in saline environments.

differentiates high yielding performant cultivated crops from wild weed species has already been shown to increase the competitiveness of the latter in several agricultural contexts (Patterson, 1995; Concenço et al., 2012). Moreover, the consequences of climate change, including the decline in quality and abundance of natural resources, may further alter crop-weed relationships and consequently affect crop yield (Peters et al., 2014). In this review we assess current knowledge on salinity tolerance of weeds and discuss critical components of crop-weed interactions that may increase weeds competitiveness and spread under salinity. The ultimate goal was to highlight functional traits and responses which could allow us to better understand how increasing salinity may affect crop-weed competition and how weed management should best evolve to cope with these constraints in the future. 2. Salinity tolerance of weeds and crops Plant responses to salinity can be assessed based on different criteria depending on the specific research and agronomic objectives. For instance, plant survival vs. adaptation to high root-zone salinity implies different physiological mechanisms and consequent effects on crop yield (Rodriguez et al., 2010; Ali and Yun, 2017). Also, the relevance of stress adaptation responses will depend on agricultural contexts and/or transitory (seasonal) vs. long-term salinization (Maggio et al., 2004; De Pascale et al., 2012). A growth model to describe the yield response of plants to increasing salinity of the root zone has been proposed by Maas and Hoffman in 1977 and, despite some limitations (Maggio et al., 2002; Steppuhn et al., 2005), it remains useful for a first level assessment of the relative tolerance of different species to salinity. The model is based on two parameters that can be defined by plotting the relative yield as a continuous function of root zone salinity (Fig. 1). At low soil salinity concentrations, yields are generally not affected or, in some cases, even enhanced by salinity. In contrast, increases of soil salinity beyond a certain threshold” will cause a slow decrease in yield. This response is typically described by two intersecting linear regions. The first is a tolerance plateau with zero slope (yield response at low salt concentration), whereas the second linear region is concentration-dependent with a characteristic ‘slope’ which defines the yield reduction per unit increase in salinity. The intersecting point of the two lines identifies a characteristic ‘threshold’ that is the maximum soil salinity that does not reduce yield below that obtained under non-saline

Fig. 1. Boundaries of salinity tolerance categories, from sensitive to extremely tolerant, obtained based on the Maas and Hoffman relationship (1977) (Tanji and Kielen, 2002). Classification of soybean and purslane, based on data from Fig. 2, are reported as an example. The inset shows the Maas and Hoffman relationship: Yr = 100 –b (ECe- a) where a = the salinity threshold expressed in dS/m; b = the slope expressed in percent per dS/m; and ECe = the mean electrical conductivity of a saturated paste taken from the root zone.

3. Competitive advantage of weeds in saline soils Plant tolerance to Na+ and Cl− ions varies widely, from very sensitive glycophytes such as chickpea to tolerant halophytic species like stem-succulent Tecticornia species (English and Colmer, 2013; Flowers and Colmer, 2015). Although there is still a debate on the tolerance 52

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Table 1 Salt stress tolerance of worldwide most common weeds based on Maas and Hoffman threshold-slope model (1977). Ratings (from sensitive to extremely tolerant) were defined by the boundaries reported in Fig. 1. Ratings have been calculated consideringthe relative threshold and slope at which the tolerance parameter (germination, shoot DW, etc.) showed a 50% reduction (Rhoades et al., 1992; Steppuhn et al., 2005). DW = Dry Weight; FW = Fresh Weight. Botanical name

Common name

Tolerance based on

Thresholda

Slopeb

Ratingc

References

Abutilon theophrasti Medik. Ambrosia artemisiifolia L. Artemisia scoparia Waldst. & Kitam. Atriplex prostrata Boucher Avena fatua L. Avena fatua L. Bidens alba (L.) DC. Bromus tectorum L. Carduus nutans L. Cenchrus pauciflorusd Benth. Cenchrus paucifloruse Benth. Cephalaria syriaca (L.) Shrad. Chenopodium acuminatum Willd. Chenopodium album L. Chenopodium glaucum L. Chenopodium glaucum L. Chloris virgata Sw. Convolvulus arvensis L. Conyza canadensis (L.) Cronquist Cynodon dactylon (L.) Pers. Cyperus iria L. Digitaria sanguinalis (L.) Scop. Echinochloa colona (L.) Link Echinochloa crus-galli (L.) P.Beauv. Echinochloa crus-galli (L.) P.Beauv. Echinochloa oryzicola Vasinger Eleusine indica (L.) Gaertn. Eragrostis pilosa (L.) P.Beauv. Galium aparine L. Hibiscus tridactylites Lindl. Imperata cylindrica (L.) Raeusch. Kochia scoparia (L.) Shrad. Lepidium vesicarium L. Lolium perenne L. Lolium perenne L. Lolium perenne L. Orobanche cernua Loefl. Orobanche cernua Loefl. Oryza sativa L. Phalaris minor Retz. Plantago coronopus L. Polygonum aviculare L. Portulaca oleracea L. Portulaca oleraceaf L. Rapistrum rugosum (L.) All. Setaria italica (L.) P.Beauv. Silybum marianum (L.) Gaertn. Sonchus oleraceus (L.) L. Sorghum halepense (L.) Pers. Striga hermonthica (Delile) Benth. Xanthium strumarium L.

Velvetleaf Annual ragweed Virgate wormwood Spear-leaved Orache Common wild oat Common wild oat Shepherd's needles Cheatgrass Musk thistle Field Sandbur Field Sandbur Syrian scabious Willdenow Lambsquarters Oak-leaved goosefoot Oak-leaved goosefoot Feathertop Field bindweed Horseweed Bermuda grass Nutsedge Crabgrass Junglerice Barnyard grass Barnyard grass Early watergrass Indian goosegrass Indian lovegrass Catchweed Bladder hibiscus Cogongrass Burningbush Bladdery pepperwort Perennial ryegrass Perennial ryegrass Perennial ryegrass Nodding broomrape Nodding broomrape Weedy rice Little seed canary grass Plantain Common knotgrass Purslane Purslane Turnip Weed Foxtail millet Cardus marianus Annual sowthustle Johnson grass Purple witchweed Rough cocklebur

Germination Shoot DW Germination Seedling FW Germination Shoot DW Germination Shoot DW Germination Germination Germination Germination Germination Germination Germination Germination Germination Germination Germination Shoot DW Shoot Length Total DW Shoot DW Germination Shoot DW Shoot DW Germination Germination Germination Germination Shoot DW Germination Germination Shoot DW Shoot DW Germination Germination Total DW Shoot Length Germination Germination Germination Shoot DW Shoot DW Germination Seedling DW Germination Germination Total DW Germination Total DW

4.09 6.55 3.03 9.09 0.72 4.14 0.91 3.05 5.00 8.59 6.30 19.13 2.27 2.65 9.10 14.29 4.55 1.09 3.64 6.90 0.53 1.80 2.38 9.44 6.00 0.57 0.70 4.55 2.80 7.82 1.00 18.18 8.18 5.6 6.00 10.78 0.94 2.27 1.80 1.65 1.44 9.09 1.42 9.11 1.34 3.85 2.50 2.31 2.09 1.75 3.58

6.11 1.38 13.54 3.08 3.76 3.10 3.55 8.26 3.74 3.60 3.92 8 6.29 1.17 2.20 25.67 4.40 4.43 8.80 6.40 4.17 2.61 4.80 6.43 5.33 3.52 5.72 2.51 11.96 5.14 2.91 1.07 6.88 7.6 4.40 7.56 16.90 22.00 3.31 6.05 9.66 4.80 4.41 4.79 6.74 3.58 6.59 7.56 6.24 8.05 4.40

MT ET MS ET MT T MT MS T ET T ET MT ET ET T T MT MS MT MT T MT T T MT MS ET MS T T ET T MT T T S S T MT MS T MT T MS T MT MS MT MS MT

Sadeghloo et al. (2013) Eom et al. (2013) Li et al. (2011) Bueno et al. (2015) Dinari et al. (2013) Kashmir et al. (2016) Ramirez et al. (2012) Rasmuson et al. (2002) Kaya et al. (2009) Zhang et al. (2017) Zhang et al. (2017) Kaya et al. (2009) Li et al. (2011) Yao et al. (2010a) Duan et al. (2004) Li et al. (2011) Li et al. (2011) Tanveer et al. (2013) Nandula et al. (2006) Tanji and Kielen (2002) Hakim et al. (2011) Šerá et al. (2011) Chauhan et al. (2013) Aslam et al. (1987) Chauhan et al. (2013) Kim et al. (1999) Chauhan and Johnson (2008) Li et al. (2011) Wang et al. (2016) Chauhan (2016) Hameed et al. (2009) Khan et al. (2001) Amini et al. (2016) Tanji and Kielen (2002) Nizam (2011) Nizam (2011) Al-Khateeb et al. (2003) Al-Khateeb et al. (2005) Hakim et al. (2011) Dinari et al. (2013) Bueno et al. (2015) Khan and Hungar (1998) Kafi and Rahimi (2011) Grieve and Suarez (1997) Chauhan et al. (2006a) Veeranagamallaiah et al. (2008) Sedghi et al. (2010) Chauhan et al. (2006b) Sinha et al. (1986) Hassan et al. (2010) Devi et al. (2016)

a b c d e f

Threshold unit= dS m−1. Slope unit= % per dS m−1. T = tolerant; MT = moderately tolerant; ET = extremely tolerant; MS = moderately sensible; S = Sensible. Mango type seeds. Plum type seeds. Sulphate salinity stress.

3.1. Intraspecific variability

limit that differentiates glycophyte from halophytes (Flowers and Colmer, 2008), most crop species are glycophytes whereas common weeds, although not properly classified as halophytes, have conserved some halophytic traits that may give them some competitive advantages vs. crop species. Experimental evidence suggests that these traits, alone or in combination, may enhance the process of weed spread and competition.

Weeds show great genetic diversity and remarkable plasticity with respect to different sources of salinization. Populations of Ambrosia artemisiifolia exposed to deicing salts accumulated along roadways during winter were found to be much more tolerant to salinity than those found in agricultural fields (Di Tommaso, 2004). The average germination of roadside populations at 400 mM NaCl was 31%, whereas the germination of field populations at the same salt concentration was only 3%. Moreover, roadside plants of Ambrosia showed 53

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Fig. 2. EC levels of weed and crop species that cause 50% dry weight reduction. Data based on Table 1 and Tanji and Kielen (2002).

3.2. Seed heteromorphism and sex-specific responses

early spring emergence, which provided them a competitive advantage over other weeds that emerged along the roadways later in the season. Similar results were reported by Rahman and Ungar (1990) for Echinochloa crus-galli. Environmental conditions experienced by parental plants may therefore influence the level of stress tolerance in their progeny. Bromus tectorum plants grown in saline habitats had higher colonization ability in saline soils than populations of the same species from other (non-saline) sites (Scott et al., 2010). Molecular analysis has revealed that salt tolerant variants of Bromus tectorum were the result of an environmental selection that pre-adapted specific genotypes to live in extreme habitats (transgenerational ‘memory’). Environmentally induced heritable change of quantitative traits may indeed increase genomic flexibility and environmental adaptation even in successive generations (Herman and Sultan, 2011; Molinier et al., 2006). Two populations of Chenopodium album, from mesic (Denmark) and xeric (Iran) environments had different germination ability in response to salinity. More than 42% of xeric seeds were able to germinate at 30 dS m−1, while mesic seeds were unable to germinate at the same salinity level (Eslami, 2011). Germination responses could therefore be the result of natural selection, maternal effects or both, and should raise some alert with respect to the introduction of alien species and/or stress tolerant ecotypes of known species that may endanger agricultural systems. Intraspecific variability has also been attributed to common and ornamental accessions of purslane (Portulaca oleracea). In this case stem-root anatomical modifications were associated with different levels of tolerance (Alam et al., 2015).

Seed heteromorphism is a phenomenon in which a single plant produces different morphophysiological types of seeds. Under changing environmental conditions, seed heteromorphism has an important function for the maintenance of weed populations in the agroecosystem (Lenser et al., 2016). Smaller seeds of Convolvulus arvensis were more sensitive to environmental stresses, including salinity, in comparison to big and medium-size seeds (Tanveer et al., 2013). Big seeds started germinating, and reached 50% germination, in a shorter time with higher germination percentage and germination index. Seed heteromorphism may therefore help weed plants to adapt to different and/or adverse environmental conditions that may interfere with germination, such as high salinity. Similar results were obtained by Zhang et al. (2017) in Cenchrus pauciflorus, a grass species with important potential invasiveness. Functional seed heteromorphism has also been documented for Chenopodium album, one of the most common weeds worldwide. In this species, the percentage of black and brown seeds on the same plant was affected by salt stress conditions experienced by the mother plant (Yao et al., 2010b). Brown seeds were bigger, mostly non-dormant and with higher salt tolerance (up to 300 mM NaCl) compared to black seeds. Under saline conditions, Chenopodium album produces fewer seeds with a greater proportion of brown seeds with reduced dormancy. In contrast, under non saline conditions, Chenopodium album produces more seeds with a greater proportion of dormant black seeds. From an evolutionary perspective, it has been proposed that seedlings from brown and black seeds are likely programmed for “high-risk” and “low risk” strategies, respectively (Tanveer and Shah, 2017). Dioecy also improves the ability to cope with limited resources 54

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Fig. 3. Representation of the threshold-slope linear regressions based on Maas and Hoffman (1977) for corn (Zea mays L.), soybean (Glycine max (L.) Merr.), wheat (Triticum aestivum L.), rice (Oryza sativa L.) and sugarbeet (Beta vulgaris L.) and their most detrimental weeds (Maas and Hoffman, 1977; Francois et al., 1986; Essa, 2002; Dadkhah, 2011).

Goodin, 1972; Khan and Ungar, 1984). The interactive effect of temperature and salinity on germination may have particular relevance with respect to climate change. Rising temperatures may exacerbate the effects of salinity on seed germination and enhance competition among different species (Kenkel et al., 1999; Giménez Luque et al., 2013). In saline soils, germination early in the growing season may affect the dynamics of annual weed populations. When salinity builds up in the soil profile during the spring-summer, as a consequence of irrigation practices and intense evapotranspiration, young seedlings generally do not survive. In contrast, seeds that do not germinate in the spring, due to high soil salinity, become dormant and build a more persistent seed bank (Tobe et al., 2000; Ungar, 2001). These seeds will generally germinate at the beginning of the following spring when soil salinity has been reduced by the fall/winter rainfall. The interaction between

under stressful environmental conditions (Bawa, 1980). Female and male plants of dioecious species have different stress adaptation potentials, with female plants generally performing worse than male plants (Juvany and Munné-Bosch, 2015). There are, however, a few exceptions. Male plants of Mercurialis annua, a native species of the Mediterranean basin, are less tolerant to salinity and have shorter lifespan than female plants. For this specific case, higher activity of ascorbate peroxidase and higher ascorbate concentration in young leaves were both associated with higher salinity tolerance of female plants (Orlofsky et al., 2016).

3.3. Weed response to thermoperiod and rainfall Temperature affects seeds germination under salinity (Francois and 55

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for germination of parasitic weeds. Salt stress induced variations of the dynamics of germination and development may call for revised weed control strategies. Increasing soil NaCl levels may affect both germination rate and emergence time in wheat and its associated weeds Carduus nutans and Cephalaria syriaca (Kaya et al., 2009). Significant effects of salinity were observed starting from 20 dS m−1, a relatively high EC level. Specifically, salinity delayed the emergence of the two weed species and inhibited the seedlings growth rate compared to wheat. This response has important implications in terms of weed control, which should consequently be delayed until most weeds emerge. The Duration of Tolerated Competition (DTC), Weed Free Period (WFP) and Critical Period for Weed Control (CPWC) are critical parameters for understanding weed-crop relationships and the accuracy of weed control timing (Otto et al., 2009). These parameters may be altered by soil salinity, as demonstrated by Hakim et al. (2013) with respect to the relative competitiveness of three salt tolerant weeds species (Echinochloa colona, Cyperus iria and Jussiaea linifolia) vs. a salt tolerant rice variety in a greenhouse experiment. The CPWC increased with salinity ranging from 14 days to 55 days at 0 dS m−1 (5% yield loss), from 12 days to 64 days at 4 dS m−1 and from 7 days to 80 days at 8 dS m−1 (Hakim et al., 2013). These results suggest that, in rice, weed management may require more intensive operations in saline compared to non-saline soils. Based on these findings, it can be anticipated that the weed control window under saline conditions should likely be extended for at least one month. Therefore, identifying the tolerance level of weed species at germination and early growth stages is pivotal for successful weed control in rice fields subjected to salinization. Field experiments to study possible CPWC variations at increasing salinity have never been conducted to our knowledge. Different sources of salinization may also have different effects on weed-crop competition. Secondary salinization can be a consequence of irrigation with water withdrawn from coastal aquifers contaminated by seawater intrusions, land subsidence, rising groundwater table and salt water intrusions from sea into rivers. In some cases, salinity can be more concentrated in the subsoil, in other cases it can be more homogeneously distributed along the soil profile (e.g. as a consequence of irrigation with saline water). When salinity is due to seawater intrusions and it affects the subsoil (Antonellini et al., 2008), seed germination of weeds and/or crops is not likely to be compromised. Negative effects are evident only when the root systems expand further down in the soil profile. Therefore, the effects of salinization on plants must be assessed with respect to species, phenological phases and specific agricultural contexts.

thermoperiod and salinity may therefore strengthen species-specific survival. Khan and Ungar (1998) have reported that the distribution of Polygonum aviculare in moderately saline soils depended on its ability to geminate and grow during spring months. 3.4. Crop-weed competition and weed control strategies In agricultural contexts, competitiveness in saline soils is the result of multiple components including different physiological responses of weeds and crops with respect to germination, emergence and growth. Early germination is amongst the most critical traits since it may give a competitive advantage either to weeds or crops. Some rice weeds like Cyperus iria and Echinochloa colona can germinate up to 32 dS m−1 whereas Leptochloa chinensis, an alien weed expanding in Italian rice fields, does not germinate at 24 dS m−1 (Hakim et al., 2011). Germination ability and timing of germination at high salinity can both affect weed competition. At low salinity (4 dS m−1), the mean germination time for E. crus-galli, E. colona and O. sativa (weedy rice) can be delayed by 1–2 days with respect to 0 dS m−1, whereas at 24 dS m−1 this delay can go up to 7–9 days. This difference can significantly change the outcome of crop-weed competition. After germination, faster growth rates can also increase weed competitiveness. However, slow growth rates have been associated with long-term salinity tolerance and colonization of saline habitats (Maggio et al., 2002; Orsini et al., 2010). Slow growth rates have been reported for the salt tolerant pasture grasses Hordeum jubatum, Dactylis glomerata and Phalaris arundinacea (Israelsen et al., 2011). Later in the growth season, agricultural practices may also affect crop-weed interactions. For example, saline irrigation may reduce weed density and biomass and could be considered as a control strategy when the specific crop-weed relationships are known (Bilalis et al., 2014). Weed tolerance to saline irrigation has been ranked from highest to lowest for the following species: Cyperus rotundus > Portulaca oleracea > Echinochloa crus-galli > Chenopodium album > Cynodon dactylon > Amaranthus retroflexus. Specific crop-weed competitions in response to salinity have been documented for rice and wheat (Ungar, 1974; Chauhan et al., 2013). E. crus galli and E. colona produced more shoot biomass in saline soil than the high salt tolerant rice cultivar FL 478. The EC required to inhibit 50% shoot biomass in rice was 10 dS m−1, whereas they were 15 and 13 dS m−1 for E. crus galli and E. colona, respectively. The two weed species produced root biomass even at 24 dS m−1, an electrical conductivity that was lethal for rice. These results further demonstrate that, in contrast to cultivated varieties, wild species may have conserved physiological, morphological and molecular stress response mechanisms that allow plant survival and adaptation to extreme environments (Maggio et al., 2006; Orsini et al., 2010). Competition effects could also appear at different developmental stages. The tolerance threshold of E. crus galli at germination stage is high compared to the glycophytic Triticum aestivum, and is closer to the moderately salt-tolerant halophytic Hordeum jubatum (Ungar, 1974). E. crus galli is more sensitive to salinity stress at the seedling stage than at the germination phase. In contrast, halophytic-like species exhibit higher tolerance to salinity at advanced growth stages. For instance, sugar beet seedlings generally cannot survive at high salinity levels, whereas mature plants are quite tolerant to salinity (Rahman and Ungar, 1990). There are a few cases in which salinization can actually benefit crop vs. weed competition. Salinity may reduce and delay Striga hermonthica emergence in sorghum fields (Hassan et al., 2010). Soils saturated with NaCl water at a concentration of 50 or 75 mM reduced Striga infestations by 65 and 100%, respectively. With 50 mM NaCl, Striga emergence was delayed by more than two weeks. Also the emergence of Orobanche cernua can be completely prevented by 75 mM NaCl in irrigated tomato (Al-Khateeb et al., 2005). Conceivably, salinity may have a direct inhibition of weed seeds germination and/or an indirect action via inhibition/modification of crop root exudates, which are required

3.5. Salinity and allelopathy Problems arising from salinity are more severe when salinity and other stresses such as the presence of allelochemicals from neighbour plants co-occur. The production of allelochemicals and its effects on plants may depend on environmental factors including soil water content, pH, presence of high salt concentrations in the soil, and soil organic matter content (Al-Turki and Dick, 2003). Salinity stress alters plant metabolism and the synthesis of primary and secondary metabolites, including allelochemicals (Siemens et al., 2002). A whole range of synergies may occur between soil salts and these molecules. Salinity may enhance the phytotoxicity of allelochemicals released during the decomposition of plant residues (Khalid et al., 2002) and it has been shown to augment inhibitory effects of Tribulus terrestris on growth and yield of Citrullus vulgaris (El-Darier and Youssef, 2017). Similarly, the allelopathic potential of canola (Brassica napus cv Hyola401) against soybean has been shown to increase under salinity (Maryam et al., 2008). Wheat seedlings were also more sensitive to salt stress when this was provided in combination with Convolvulus arvensis extracts (Tabrizi et al., 2012). In this case, growth inhibition was most evident at the seedling stage, indicating a phenology-dependent sensitivity to such 56

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information to develop useful models for the management/timing of mechanical and chemical treatments (Masin et al., 2014). The relative prevalence of tolerant species could be used as an indicator for monitoring salt water infiltrations, which is an increasing concern for coastal areas worldwide (Van-Camp et al., 2004; Antonellini et al., 2008; Arslan and Demir, 2013).

synergistic effects at least in wheat. Likewise for rice, the presence of weed extracts may enhance the effects of NaCl alone (Alam and Shaikh, 2007). Leaf extracts from Chenopodium murale, alone or in combination with NaCl, did not affect rice germination but they significantly inhibited shoot and root length, especially when salinity and weed extracts acted together. Whether NaCl enhanced the activity of toxic allelopathic molecules from Chenopodium murale or vice versa, it needs to be established. However, Sánchez-Moreiras (2004) demonstrated that inhibition of the hydroxamic acid BOA, an allelochemical released by plants, was more effective as a plant growth inhibitor when applied in combination with salt and water stress.

4. Herbicide efficacy in saline environments It is important to know whether herbicides efficacy changes with increasing soil salinity. Scattered and contrasting information on the response of weeds to herbicides in saline soils is available. Papiernik et al. (2003) have quantified the efficacy of chlorimuron and imazethapyr for controlling Echinochloa crus-galli, Cyperus esculentus, Xanthium strumarium, Portulaca oleracea and Ipomoea hedearcea at two salinity levels (2 and 7 dS m−1). Salinity decreased height and shoot biomass only in Cyperus esculentus, whereas no effect was in general observed on weed density after 20 days from herbicide application. Based on these results, chemical weed control may not require adjustment in moderately saline soils. Similarly, trifluralin treatments, supplied at increasing soil salinity did not alter shoot and root development in soybean, and also had no effect on root nodulation (Maftoun et al., 1982). In contrast, Sacala et al. (2008) have documented in hydroponic culture a significant interaction between glufosinate applications and NaCl stress in maize, which led to a significant reduction of leaf ammonium content compared to the sole herbicide treatment. Considering that the phytotoxicity of glufosinate is due to ammonium accumulation, there may be an antagonism between salinity and glufosinate. In some cases, herbicides may exert some protective effects on salt stressed crops. 2,4-D applications can attenuate saline stress in rice plants via modifications of their antioxidant capacity and cation transporters efficiency in the roots (Islam et al., 2017). The relative interaction/interference of this response with the 2,4-D herbicide efficacy remains to be established. Salinity may also interfere with herbicide-soil relationships. González-Márquez and Hansen (2014) have reported that the presence of sodium ions in the soil solution facilitated the adsorption and reduced desorption of atrazine. This may have important consequences with respect to the herbicide persistence in the soil, the relative control efficacy and overall environmental impact. Soil salinization also interferes with microbial activity (Tripathi et al., 2006; Yuan et al., 2007), yet little is known on how this may affect the degradation of herbicides. Jing et al. (2018) have recently found that the degradation of two soybean herbicides, lactofen (and its metabolite desethyl lactofen) and acifluorfen, was slower in salty soils because of inhibitory effects of salt on soil microorganisms. Considering that increasing salinity of soils and irrigation water are closely linked to climate change, it is essential to understand how all environmental variables affected by global change would interact with salinization phenomena and crop-weed management (Kumar et al., 2017; Chandrasena, 2009; Peters et al., 2014; Amare, 2016; Singh et al., 2016; Varanasi and Vara Prasad, 2016; Ramesh et al., 2017). At increasing soil temperature, the persistence of soil active herbicides decreases. Based on soil temperature data for the period 1980–2001, Bailey (2003) has demonstrated that the soil persistence of isoproturon, applied in pre-emergence, decreased below the efficacy threshold 30 days earlier in 1995–2000 compared to 19801985. These findings were correlated to higher average temperatures observed in the time window 1995–2000. Since weed and crop emergence in saline soils are both delayed, it is conceivable that lower herbicide persistence associated with global warming would likely lead to reduced herbicides efficacy. For these reasons, it will be necessary to re-program herbicide treatments to better match the time of crop emergence or to anticipate using them as early post-emergence treatments. In contrast, summer drought associated to increasing temperature can improve herbicide persistence with an increased risk of carryover effects on following crops. Absorption of post-emergence

3.6. Salinity and invasiveness The progression of soil salinization may greatly affect plant ecology and biogeography by impacting vegetation patterns and diversification. It may therefore play a role in weed invasion and/or spread (Bui, 2013). Salinity could affect plant invasion if native and non-native species present different levels of tolerance. The relationship between salinity and invasiveness of non-native species has been largely overlooked and the few available reports on this topic often present contrasting results. The relative salinity tolerance of native and non-native species may enhance or restrain invasiveness in different environments. High salinity may counteract invasion by non-native plants if these are less tolerant than native species. Conversely, low salinity may facilitate invasion of non-native species if these are more efficient in terms of water and nutrient uptake in a given environment (Hoopes and Hall, 2002). In sand dunes, seeds of the invasive legume Acacia longifolia were more salt tolerant than the native legume Ulex europaeus, which could explain the remarkable invasiveness of A. longifolia observed in Portuguese coastal areas (Morais et al., 2012). In contrast, Kolb and Alpert (2003) found that salinity was negatively associated with the invasion by Bromus diandrus, a non-native species of California grassland. In this specific case, the competition of B. carinatus, a native grass, against B. diandrus was not enhanced by increasing soil salinity, indicating that both species had similar levels of tolerance. There is a notable diversity among non-native species responses to salt stress. In marshes of southern California, most non-native species are confined into areas of low salinity (Noe and Zeller, 2001). However, Richards et al. (2008) have associated the increased salt tolerance of Reynoutria japonica populations, which have been invading salt marshes in California, to their outstanding physiological and genetic plasticity. Results from Onen et al. (2017) also demonstrated that higher salinity tolerance may accelerate common ragweed invasion. Further determinants that may confer salt stress adaptation ability to weeds or crops include their stomatal response to external stimuli. Increasing atmospheric CO2 levels cause stomatal closure and reductions in stomatal density, which in turn may differentially increase salt stress tolerance among different plant species (Reuveni et al., 1997; Maggio et al., 2002; Pinero et al., 2016). Similar responses can be observed with respect to water shortage. Ozaslan et al. (2016) assessed the growth performance of two invasive Physalis species, P. philadelphica and P. angulata, along a soil salinity gradient. Up to 3 dS m−1 the two weeds behaved similarly. At higher salinity (6 and 12 dS m-1), P. philadelphica was more tolerant compared to P. angulata. It was suggested that the dominance of each species depended on environmental conditions and the interaction between multiple stresses with P. philadelphica having a competitive advantage under optimal water availability and P. angulata favored under water shortage. Thus, invasiveness of different species with increasing soil salinity should also be related to specific pedoclimatic conditions and climate change pressure. The adaptive mechanisms of salt tolerance in plants at cellular and organismal levels are numerous. It is clear that the genetic diversity of weeds may give them a competitive advantage in terms of response and adaptation to salinity (Munns and Gilliham, 2015). The study of weed infestation along a salinity gradient could provide important 57

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herbicides depends on both atmospheric temperature and relative humidity. In saline soils, plants have waxier cuticles, leaf pubescence, greater mesophyll thickness, increased stomatal closure and reduced root biomass, all of which may reduce herbicide adsorption/translocation and consequently herbicide efficacy. Ziska and Teasdle (2000) and Ziska et al. (2004) have shown that with increasing atmospheric CO2, glyphosate efficacy on Cirsium arvense was reduced due to a dilution effect (higher CO2 increases plant biomass and leaf area). CO2 induced biomass production may be, however, counteracted by biomass reduction caused by increased salinity, and consequently may nullify the dilution effect. Under these conditions the herbicide efficacy may appear unaltered. Nevertheless, foliar adsorption may decrease due to anatomical/physiological changes (increased leaf thickness, production of waxes, stomatal closure) induced by soil salinity. This can be mitigated by using specific adjuvants. The interaction between other abiotic stresses and salinity certainly deserves further attention in the face of climate change and weed control. 3 5. Conclusions In this review we attempted to provide an overview of crop-weed relationships under increasing soil salinization, a phenomenon associated with climate change and demographic growth. The key findings of this review were: 4 1 Weeds generally exhibit higher salt tolerance compared to crop species. In saline environments, critical tolerance and competitiveness determinants include remarkable phenotypic plasticity, allelopathic potential, production of heteromorphic seeds, seed longevity, earlier emergence, and faster growth rates respect to crops. For the Mediterranean basin, weed species that may likely increase their competitiveness based on these traits are listed in Table 2. It is therefore critically important to start monitoring changes in the composition of the flora in areas particularly exposed to salinization (e.g. coastal regions of the Mediterranean basin) so as to understand/anticipate whether and how plant populations may evolve. 2 Strategies for weed control are also affected by soil salinization. When weeds are more salt tolerant than crops, CPWC becomes longer (up to two weeks) making crops much more vulnerable. This will require a more intensive and/or tailored weed control strategy. Moreover, DTC depends on weed density (higher density determines

5

6

shorter DTC and longer CWPC, and vice versa) and emergence dynamics (faster emergence determines shorter DTC and longer CWPC, and vice versa). Therefore, salinity causes opposite effects on weed competition since a reduced weed density will limit competition while faster emergence will increase competition. Considering the large soil seed bank, which generally guarantees a relatively sizable weed density even in saline soils, the most dangerous consequence of soil salinization seems to be higher weed competition due to faster emergence. As a consequence, timing of weed control should be adapted to slower crop emergence and delayed closing of the canopy, both of which occur under salinity. Salinity may also alter weed emergence patterns, especially in those species emerging later in the winter. It is expected that, in saline soils, weed emergence will occur mostly at the beginning of the spring, when the winter rainfall may have leached down salts along the soil profile. If crops have a moderate/high salt tolerance, a delay of the sowing time could help in controlling most emerged weeds at the time of seedbed preparation. Under increasing salinization, herbicide persistence will tend to increase in reaction to adverse effects of salinity on soil microbial communities. The agronomic and environmental consequences of this effect must be taken into account in future weed management strategies. Moreover, temperature increase due to climate change will very likely have a greater effect on herbicides efficacy in spring/ summer compared to fall treatments. Seed bank persistence in salt affected cultivated fields will tend to increase because un-germinated seeds will be subjected to salt-induced dormancy. Crop rotations and strategies to minimize seed rain will become even more critical for effective weed management. Improving crop salt tolerance is also crucial for rational weed management in the face of climate change. Sustainable weed management may require specific breeding strategies to potentiate those traits that would enhance crops vs. weeds competition. Improving crop salt stress per se (Chantre Nongpiur et al., 2016) is necessary but may be not sufficient. Pyramiding salt and herbicide tolerance traits may be worthy of further investigation. Recently, co-overexpression of salt stress and phosphinothricin tolerance has been obtained in mung bean (Kumar et al., 2017). Future breeding programs specifically targeted to weed control may also consider potentiating in crop species those traits that make weeds more competitive, such as their allelopatic potential. Finally, more research is needed on molecular and physiological

Table 2 Physiological and ecological characteristics of some weed species for which it can be hypothesized, based on salt tolerance-invasiveness traits, their diffusion in the Mediterranean basin upon increasing salinity. Salt Tolerance

Seed Heter.a

Allel. Potential

Propag. Form

Photos. Pathway

Germin. Time

Seed Longevity

References

Ambrosia artemisiifolia L.

ET

yes

yes

Th

C3

spring

M/H

Atriplex prostrata Boucher

ET

yes

–

Th

C3

spring

M/H

Cyperus iria L.

MT

–

yes

Th

C4

spring

L

Chenopodium album L.

ET

yes

yes

Th

C3

spring

H

Convolvulus arvensis L.

MT

yes

yes

G

C3

summer

M

Echinochloa crus galli (L.) P.Beauv. Portulaca oleracea L.

T

–

yes

Th

C4

spring

M/H

Thompson et al. (1997), Fumanal et al. (2007), Vidotto et al. (2013). Thompson et al. (1997), Carter and Ungar (2003) Drost and Doll (1980), Thompson et al. (1997), Ayeni et al. (2015); Chopra et al. (2017). Thompson et al. (1997), Rezaie and Yarnia (2009); Yao et al. (2010b). Thompson et al. (1997), Tabrizi et al. (2012); Tanveer et al. (2013). Thompson et al. (1997); Guo et al. (2017).

MT

–

yes

Th

C4

summer

H

Polygonum aviculare L.

T

–

yes

Th

C3

spring

H

Thompson et al. (1997); Haar and Fennimore (2003); Shehata (2014). Alsaadawi et al. (1983), Thompson et al. (1997).

a Seed Heter. = Seed Heteromorphism; Allel. Potential = Allelopathic Potential; Propag. Form = Propagation Form; Photos. Pathway = Photosynthetic Pathway; Germin. Time = Germination Time; T = tolerant; MT = moderately tolerant; ET = extremely tolerant; Th = therophyte; G = geophyte; L = low; M = medium; H = high.

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aspects of crop-weed interactions, which could bring new insights for advancing sustainable weed management strategies in response to environmental stresses.

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