Growth responses of Lantana ( Lantana camara L.) varieties to varying water availability and light conditions

: Lantana camara L. (Verbenaceae) is an invasive alien species in many countries that causes economic losses and harmful ecological impacts on biodiversity. Its varieties with that ornamental L. camara varieties are not drought and shade tolerant as the naturalised varieties, and therefore their potential invasiveness may not be as high as the wild varieties that pose a great threat to the biodiversity of Sri Lanka. water chlorophyll conductance,


INTRODUCTION
Lantana camara L. (Verbenaceae) was spread across its geographical borders, the islands of West Indies, and Central and South America through deliberate introductions by the Europeans nearly two centuries ago (Howard, 1970;Cronk & Fuller, 1995) and at present is reported to be naturalised and invasive in many recipient countries (Stirton, 1977;Day et al., 2003). The ecological impacts imposed by L. camara on ecosystems by arresting succession (Lamb, 1991), changing structure, composition of communities and disrupting ecosystem functions (Holm et al., 1979;Gentle & Duggin, 1997;Gooden et al., 2009;Aravind et al., 2010;Singh, 2012) reduce productivity of the land and impose high economic costs for control (Howard, 1970;Swarbrick et al., 1998), alien species (Lowe et al., 2000). Despite these negative impacts, L. camara (hereafter referred to as Lantana) mainly through hybridisation to develop new forms or varieties to meet the demand in ornamental plant trade (Smith & Smith, 1982;Graaff, 1986;Li et al., 2004). soil, humidity, air temperature and light conditions (Swarbrick et al. moisture, humidity and temperature is high (Holm et al., 1991;Swarbrick et al., 1998). Its successful survival as a woody invasive perennial has been attributed to allelopathy, vegetative propagation, high reproductive output and versatility, and long range seed dispersal capabilities (Sharma et al., 2005). Broad range of tolerance to edaphic and climatic conditions also have been key traits that contributed Lantana to be naturalised and invasive in its introduced environments (Swarbrick et al., 1998;Day et al., 2003;Sharma et al., 2007;Taylor March 2018 Journal of the National Science Foundation of Sri Lanka 46 (1) et al., 2012). For example it grows as a bush up to 2 -4 m height in open unshaded sunny environments (Day et al., 2003), and as a liana up to 15 m when light intensity is low (Lowe et al., 2000) exhibiting very plastic responses to different intensities of sunlight (Carrion-Tacuri & Rubio-Casal, 2011). It thrives in a variety of soil types in which soil moisture is readily available, but also tolerates long period of drought (Munir, 1996). Thus these traits have been widely applied in niche and species modelling to predict potential range expansion of Lantana in diverse biogeographic regions as a response to climate change (Taylor et al., 2012). However, Lantana has not been successfully controlled in almost all introduced environments probably due to the scarcity of knowledge Zalucki et al., 2007).
In Sri Lanka L. camara var. splendens and L. camara var. camara have been listed as invasive Lantana, which became naturalised following introduction as an ornamental plant (Wijesundera, 2010). Nevertheless, new arrivals of Lantana have become popular garden plants which, at present, are propagated and distributed through plant nurseries. As the taxonomy, phenotypes and biology of Lantana group in Sri Lanka is poorly explored, we focused our investigations from an ecological-eco-physiological point of view to reveal how these closely related Lantana varieties deal with drought (reduced water) and shade (reduced light) conditions, as capacity and light conditions in optimum. Hence, this study was aimed at understanding the growth responses of naturalised invasive Lantana and ornamental Lantana varieties under varying water and light availability levels. It was hypothesised that differences reproductive characteristics, leaf growth rate, water and chlorophyll content, defoliation, stomatal conductance, photosynthetic tissue mass and biomass partitioning of plant parts of Lantana varieties.

Plant material
The two ornamental Lantana varieties selected based on public demand (as revealed by nurserymen and plant sellers) included a variety with a straight growing habit (hereafter referred to as wild) Lantana varieties were L. camara var. splendens producing orange-red colour L. camara var. camara producing (Wijesundera, 2010) ( Figure 1).  Leaf growth rate (LGR) was determined (n = 10) as a ratio of leaf extension to the initial length (Ewing et al., 1995), i.e. LGR = (L2 -L1)/L1, where L1 is the length of the leaf soon after bud break and L2 is the length of the leaf after full expansion. Stomatal conductance was measured (n = 6) at the 10 th wk during midday using a porometer (AP4-UM-3, Cambridge, UK). Stomatal resistance was considered as the reciprocal value of stomatal conductance (Sack & Scoffoni, 2012;Goyal & Sharma, 2015). At the end of 12 wks plants were uprooted carefully and average length and width of cleaned root mass was recorded. The approximate root volume was

Light availability
Eighty Lantana plants were arranged according to randomised complete block design in which four Lantana varieties (n = 5 per variety) were kept as four different treatments of shade conditions, 35 % (S2), 50 % (S3) and 75 % (S4) constructed using standard shade nets. Control (S1) plants were unshaded and received maximum sunlight. All plants were allowed to obtain rain water for their survival. The light intensity of treatments were measured thrice a week between were obtained for length of the main stem and number th week, length of petiole, width and length of the oldest leaf of the branch located at the second node from the apex and the length area was calculated using the ellipse formula (Carrion-Tacuri & Rubio-Casal, 2011). Photosynthetic pigment content (chlorophyll a, chlorophyll b and carotenoids) of fully expanded leaves were determined by soaking 0.2 g of leaf in 5 mL of 80 % aqueous acetone for 4 h followed by measuring absorbance using a spectrophotometer (G10S UV-Vis, Genesys, USA) at 3 wavelengths (663.2, 646.8 and 470.0 nm), respectively. Concentration of pigments were calculated using the absorbance value, -1 , c is the concentration in mgg -1 and d is the path length of the corvette in centimetres (Sims & Gamon, 2002). Photosynthetic tissue mass was measured by obtaining transverse sections of two fully expanded leaves per plant. Length of the palisade parenchyma and spongy light microscope (AxiocamERc 5s, ZEISS, Germany). To identify the stomatal density of abaxial epidermal peels, another two fully expanded leaves were treated with 88 % lactic acid in 100 C for 30 min. Stomatal density was recorded by counting the number of stomata microscope.

Data analysis
All data on growth parameters were subjected to analysis of variance (ANOVA) using SPSS version p < 0.05. Regression analysis was conducted to examine the relationships between the leaf water content with soil water content and leaf growth rate in water stress experiment and to examine the relationship between shade level and mean chlorophyll a and b concentrations.

Water availability
The average soil moisture of Lantana pots changed from 39.08 % to 8.16 % across the watering treatments W1-W4 and Lantana varieties responded differently to varying soil water availabilities (Table 1). Treatments W1 and W2 provided favourable conditions for both wild and ornamental Lantana varieties to grow and survive, but further dryness imposed by W3 and W4 treatments negatively impacted on their growth and survival at different scales. Treatment W4 led to the death of all OW ornamental Lantana plants and 80 % of OP by the end of experiment. In contrast, all plants of wild varieties survived.
All Lantana varieties commonly showed a progressive reduction in main stem length, LGR and LWC under reduced water availability, however the responses varied within and between wild and ornamental varieties. Higher LWC was maintained by wild Lantana varieties than the ornamental varieties across the soil moisture gradient and especially at low soil moisture levels ( Figure 2a). Moreover it was evident that both wild  The root volume of wild Lantana varieties increased with reducing water availability and variety WY reported than that of WR. It was also evident that the proportion allocation of shoot biomass of wild Lantana varieties heavily decreased with low water availability exhibiting a sharp gradual increase in their root: shoot ratio. The results indicated that such consistent growth responses were not reported from ornamental Lantana varieties along with a reduction of soil moisture gradient although could cope with the dryness of soil much better than ornamental Lantana varieties by overcoming 'water ground growth. They also showed increased defoliation probably to maintain a high water content of leaves under limited soil water conditions, thus would have had a better osmotic regulation than that of the ornamental varieties. Wild Lantana varieties were also able to adjust stomatal conductance to reduce water loss by making available reported that Lantana varieties tolerate long drought periods by defoliation and recover during favourable seasons (Baars & Neser, 1999). Our results also revealed that defoliation would have been the more plastic (as well as common) plant response. Increased stomatal resistance by stomatal closure also has been one of the  It has been reported that the photosynthetic stress responses of Lantana were related to surface soil temperatures, thus L. camara is unable to survive in areas reaching > 60 C mid-day temperatures and rainfall < 600 mm year -1 (Fenshan, 1996). Therefore it has been considered not as a drought tolerant, but a drought avoider (Castillo et al., 2007). The existence of functional types such as drought tolerants and drought avoiders within the same genus has been reported by Castillo et al. (2007). In this regard, plant morphology of root system, photosynthesis stress levels, hydraulic conduct and stomatal conductance, leaf water potential, a great plasticity than the other traits (Dickson & Tomlinson, 1996;Nandini, 1999). We also suggest the existence of drought avoidance and intolerance within the Lantana group as exhibited by different varieties of Lantana camara. As per the results wild WY Lantana than that of WR. Our observations and personal communications with experts (S. Wijesundara, Personal ) have supported the idea that WY is often found in wetter parts and WR in drier parts in Sri Lanka. This could be a possible reason for WY to show higher LWC and LGR compared to WR, and similarly WR to show a higher defoliation and stomatal resistance to low water availability than WY.

Light availability
The un-shaded treatment received full sunlight within experiment period. Plants under S2 shade level received of sunlight. All Lantana plants except for 40 % of the plants of OW variety exposed to S4 survived throughout the experiment.
Both wild Lantana varieties exhibited progressive increase of the main stem length with decreasing light intensity, while both ornamental varieties recorded to have longer stems with increasing light intensity ( Table 2). The strangling Lantana variety OP was much responsive to varying sunlight than the straight growing variety OW except when they were under excessive between the wild varieties. Internodes and petiole lengths shade. Moreover they produced larger leaves (a high leaf area) under shade, whereas leaves of ornamental varieties did not respond in a similar manner. Stomatal resistance of all Lantana plants reported to be high in unshaded conditions (WY reported the highest) and gradually declined with increased shade. The density of stomata of wild Lantana varieties increased with reducing light intensities, while that of ornamental Lantana varieties did not change. The same pattern of response was shown by chlorophyll a and chlorophyll b contents of Lantana leaves along the decreasing sunlight gradient. Wild Lantana varieties exhibited higher contents of chlorophyll a and b at moderate light intensities (particularly S2 and S3), whereas the chlorophyll content of ornamental Lantana varieties at moderate light intensities were almost similar to that of the un-shaded condition (Figures 3a and 3b).   The failures reported in controlling Lantana invasion in more than 60 countries worldwide have been partly attributed to the uncertainties of its taxonomy, biology and ecology (Howard, 1970;Taylor, 1989; Sharma et al colour have been the primary features that distinguished ornamental plant varieties without much investigations into eco-physiological differences and related growth responses. Thus the potential of these plants becoming invasive is less discussed. As many plant traits exhibit plasticity with respect to varying levels of exploitation of environmental resources, the favourable conditions for growth and spread of the Lantana taxa, or in other words the degree of tolerance, needs to be understood. In this regard the present study reveals a comprehensive account of growth responses of two commonly grown ornamental Lantana varieties and wild (naturalised and invasive) L. camara varieties to varying water and light conditions experienced in Sri Lanka and many tropical environments.
The ornamental varieties would have been probably developed through many steps of hybridisation of various Lantana varieties and thus may exhibit varying degrees of expression of traits depending on the parental combinations. The intention of hybridisation would colour and not the degree of tolerance to environmental exploitation. The two ornamental Lantana varieties were blooming oriented, short-term survivors in contrast to the long term surviving wild Lantana varieties, which were more growth oriented. This could be proven by the in ornamental Lantana varieties (especially OW) even under reduced availability of soil moisture, regardless of the short survival period as indicated by the death of individuals.
The present study provides evidence for the low sustainability and survival of white and purple water and light conditions in contrast to the superior capacity shown by the wild growing (naturalised invasive) varieties; L. camara var. splendens (WR) and L. camara var. camara (WY). The low morphological and physiological plasticity observed in ornamental Lantana varieties exhibits their unpreparedness to suggests that the potential invasiveness of these ornamental Lantana varieties could not be as high as the naturalised invasive Lantana taxa in Sri Lanka. However it is important to note that invasive success is considered as a combined outcome of environmental conditions and plant traits (Richardson & Pysek, 2006), thus, the role of species traits is not the only feature that determines biological invasions. Decisive factors such as repeated introductions of species from one or more original ranges into a new environment as well as secondary releases within the new range including cultivation (Mack, 2000), disturbances and cultivated factors (Kowarik, 2003) have acted to promote invasions. The rapid spread of Lantana in India has been also attributed to major land use changes in the country such as habitat degradation, fragmentation, and land conversion creating favourable habitats in terms of higher light availability, moderate soil moisture and other micro environmental parameters (Ray & Ray, 2014). Hence the potential risk of a taxon being invasive may also lie with the spatial and temporal changes of the invisibility of the environment.

CONCLUSION
This study provides evidences for the differences of the level of tolerance to reduced water (drought) and reduced light (shade) between wild and ornamental Lantana varieties. The distinct growth performances of the locally

March 2018
Journal of the National Science Foundation of Sri Lanka 46 (1) varieties of horticultural value are incapable to overcome water and shade stress conditions, which threaten their survival, thus is unlikely to become invasive under the present environment conditions in Sri Lanka.