Weed flora and weed growth
The weed vegetation in the
Pokkali ecosystem during the rice growing season was divided into three major groups based on the site of occurrence. They were:
1. Weed flora in the actual rice fields.
2. Weeds seen on the bunds.
3. Weeds in water channels.
The predominant weeds observed in the
Pokkali ecosystem of Kerala were
Diplachne fusca and
Eleocharis dulcis. A total of 40 weeds were identified from
Pokkali ecosystem during low saline phase. Cyperaceae family dominated in the ecosystem with more number of species. The weed flora observed in
Pokkali ecosystem is listed in Table 1.
When compared to the previous reports and surveys in
Pokkali ecosystem, this study could identify more number of weeds in the ecosystem. Tomy
et al. (1984) had reported the presence of fourteen species of weeds in the
Pokkali fields, namely,
Echinochloa crus-galli, Eliocharis fistula, Fimbristylis miliacea, Monochoria vaginalis, Vallisneria spiralis,
Nymphaea sp, Marsilea quadrifoliata, Asteracantha longifolia, Lymnophylla heterophylla, Sphenoclea zeylanica, Cyperus difformis, Ludwigia octovalvis, Salvinia auriculata and
Eichhornia crassipes.
Vidya (2003) reported that the
Pokkali land possesses a unique weed flora having anatomical and morphological modifications for maintaining the salt balance in the plant. A total of 18 weeds species were observed which included three grasses, three sedges, nine broad leaved weeds and three ferns. Accordingly, the most dominant weed species of
Pokkali lands were
Diplachne fusca and
E. crus-galli.
Fimbristylis miliacea and
Eleocharis dulcis were the dominant sedges in these areas. Compared to
C. difformis, C. javanicus was abundantly found in the paddy field bunds.
Eichhomia crassipes, Monochoria vaginalis, Nymphaea nouchali and
Pistia stratiotes were the major broad leaved weeds found in water-logged conditions. Apart from these broad leaved weeds,
Sphenoclea zeylanica and
Sphaeranthus africanus were also worth mention due to the high adaptability to brackish water.
The weed flora observed in the present study is in conformity with the reports of weed survey conducted in coastal rice fields of known high salinity concentrations in Malaysia
(Hakim et al., 2013). They reported that abundant species included Cyperaceae, followed by Poaceae accounting for 47% of the species found.
Grasses
Among grasses,
Diplachne fusca was the most predominant weed.
D. fusca was observed in the fields during early June. It was present all over the fields and on bunds throughout the cultivation season. Peculiarly,
D. fusca was the only grass weed in the
Pokkali tracts which survived both in fields and on bunds. It was found in high density on and in between the mounds.
Echinochloa crus-galli was observed at only one location.
Sedges
Three sedges were reported in the former study by
Vidya (2003), however, the number in the present study has increased to eight, with
Fuirena umbellata, Schoenoplectus lateriflorus, Cyperus exaltatus,
Cyperus javanicus as additional sedges. During the nursery stage, the fields were heavily infested with various sedge species including
Eleocharis dulcis, Cyperus difformis, Fimbristylis miliacea, Kyllinga monocephala etc.
Eleocharis dulcis was present in the fields wherever the establishment of rice was poor. Though sedges dominated in the nursery, they later disappeared with the transplanting of rice seedlings. This might be due to the fact that in nursery stage, mounds provided a lower salinity range and sufficient aeration for their growth.
Eleocharis dulcis was the dominant sedge observed in the nursery and also in the main field after transplanting. Along the bund,
Fimbristylis miliacea was the major weed seen, which failed to survive in the field conditions at high salinity.
Broad leaved weeds
During the nursery stage, on the mounds,
Ludwigia was the major broad leaved weed observed. After transplanting,
Ludwigia was confined to the bunds with higher density. Then on,
Sphenoclea zeylanica was the major broad leaved weed found in the field. As the number of rainy days and crop progressed,
Nymphaea sp.,
Eichhornia crassipes and
Monochoria vaginalis also were observed.
Sphaeranthus africanus was also found in some plots. Floating weed
Pistia stratiotes entered the field during occasional heavy rains and flash floods
. Entry in large numbers suppressed the rice tillers, once the water level dropped.
Alternanthera sessilis and
Ludwigia parviflora remained predominant in the bunds in the cropping season.
Mangroves were found common on the bunds and fallow fields.
Exoecaria agallocha, commonly seen on the bunds has become a menace to the farmers in some areas, though they are advantageous for the following aquaculture.
Ferns
The presence of
Salvinia molesta, Ceratopteris and
Azolla pinnata was reported in
Pokkali fields
(Vidya, 2003). Adding to this, pteridophytes like
Najas graminea and
Hydrilla verticillata also took a lead role in the
Pokkali ecosystem. Heavy infestation of the ferns adversely affected aeration and water movement.
Distribution of weed flora
In this survey, it was observed that wherever the seedling establishment was poor, weed growth was found noticeable. Information on the distribution and importance of weeds in the
Pokkali paddy fields of survey area are given in Table 2. With 75% frequency,
Diplachne fusca was identified as the predominant weed in the area with a Relative importance value (RIV) of 27.25%. Among the sedges,
E. dulcis was the major one (frequency-39%; RIV-16.8%).
S. zeylanica was another well-adapted weed found in these areas; however, the frequency of its occurrence was less. But in abandoned fields, there was over dominance of
S. zeylanica owing to its high adaptability to
Pokkali conditions.
Ceratopteris thalictroides and
Hydrilla verticillata were the major ferns observed in these areas.
Echinochloa crus-
galli was observed in few spots.
Eichhornia was found floating in between the crops and also in the side channels of the fields.
Vidya (2003) reported the presence of three grass weeds in
Pokkali, with maximum intensity for
Diplachne fusca (Frequency- 85%, density - 17.17 m
-2). Apart from this,
Echinochloa crus-
galli was also a major grass weed reported (Frequency- 80%). In the present study, more diverse weed flora was observed and the number of grass weeds increased to seven. The average frequency (75%) and density (9.83 no.m
-2) of
Diplachne fusca has decreased over the years. The reduction in the intensity of
Diplachne fusca favoured the introduction of other grass weeds.
Echinochloa crus-galli, a dominant weed reported in earlier weed survey was now confined to a very few locations and the frequency was seen to have reduced from 80% to 12.5%. The frequency of
Eleocharis dulcis has however reduced from 45% to 39% in the present study. The number of broad leaved weeds was observed almost similar to the earlier situation of 2003.
Alternanthera sessilis was the major broad leaved observed in the ecosystem in the previous records. But in the present survey, the trend has been changed to dominance of aquatic weeds especially
Nymphaea nouchali (Frequency- 36.25%). Occurrence of
Monochoria vaginalis has also reduced in the present scenario.
In
Pokkali ecosystem, distribution and variation in the intensity of precipitation patterns remained much more uncertain over the years. Climate change affected the frequency and intensity of rainfall and resulted in occurrences of extreme events such as floods and delayed rainfall consequently resulting in variations in soil salinity. Reduced salinity in the years of heavy rainfall favoured presence of more number of weeds in the ecosystem.
Adaptive mechanisms of important weeds in Pokkali ecosystem
Diplachne fusca
Diplachne fusca was identified as the most devastating weed in the
Pokkali rice fields. Adaptation of the grass to saline soils indicates the halophytic nature of the plant. The weed competed well with rice gaining heavy biomass accumulation and succulence. Plants grow to a height of about 180 cm with 20-25 number of tillers producing good biomass (Plate 1a,b).
Presence of Kranz anatomy was observed in cross section of leaves of
D. fusca, a typical feature of C4 grasses. As a result of high photosynthetic efficiency, plants grow bigger in size which is attributed to the dilution effect or succulence. This is an important tolerance mechanism in saline environments
(Vidya et al., 2004) to dilute the toxic concentrations of ions taken up.
D. fusca was not affected by the salinity and this species had greater shoot fresh and dry matter yields at all salinity levels
(Ashraf and Yasmin, 1997). High root volume was observed for
D. fusca in
Pokkali ecosystem.
Borzouei et al. (2015) observed that more root volume in salinity was due to higher salt tolerance of plants.
In saline soils, higher Na content and the resultant high ratio between Na and other cations like K, Ca and Mg within the plant become detrimental for crop. Maintaining a low ratio of Na with other cations especially with K, is considered to be an yield determining and salt tolerance criterion in crops. In the present study, analysis of K+/Na+ and Ca+2/Na+ in the plant showed that the ratio of K+/Na+ (3.58) and Ca+2/Na+ (4.47) was high at saline conditions (EC-5 dSm
-1).
Anatomical studies of the weed were done and the observations are illustrated in Plate 1a. Anatomical studies showed the presence of micro-hairs in
D. fusca. Concentration of potentially toxic ions within the tissue can be maintained to lower levels by such excretory mechanisms. Thus, under adverse edaphic conditions, these mechanisms help the plants in growth and survival
(Joshi et al., 1983). In the weed species, the presence of aerenchymatous tissues was observed very conspicuously which help them to withstand the submerged conditions.
Eleocharis dulcis
Eleocharis dulcis (Burm.) Hensch. is an erect, stout and slender, perennial leafless sedge which grows throughout the year in swamps and other moist shallow areas. The plant grows from 3 to 5 feet tall, with heavy biomass and succulence. Since they are devoid of leaves, their photosynthetic requirements are met by the numerous upright tubular septate stems (Plate 2a,b).
The corms of
E. dulcis remain in the field and survive the high saline phase and germinate the following year when low saline phase prevails in the field. The high biomass production of the weed provides better salt tolerance. In Eleocharis, the tolerance to salinity can also be substantiated by the pith formation or aerenchymatous cells developed in the plant under certain ecosystems. A higher ratio of K+/Na+ (14.8) and Ca+2/Na+ (4.78) observed in the present study was yet another salt tolerance mechanism in
E. dulcis at an EC of 5 dSm
-1 in
Pokkali soils.
Sphenoclea zeylanica
S.
zeylanica is an annual weed found invading the paddy fields, growing between the rice plants as colonies. The size of the colony increases each year, making it one of the most serious weeds in rice fields
(Holm et al., 1977). It can cause yield losses as high as 45 per cent
(Ampong-Nyarko and De Datta, 1991), when grown in high densities.
The weeds growing in salinity or submerged conditions usually tend to show higher biomass production compared to other ecosystems. This was exhibited by most of the weeds growing in the saline submerged conditions including
S. zeylanica (Plate 3a) and many other species. The ratio of ionic distribution within the plant was also found significantly high (K+/Na+=4.11 and Ca+2/Na+ 8.26) at an EC of 5 dSm
-1.
The cross section of
Sphenoclea zeylanica revealed the presence of aerenchymatous voids (Plate 3b). The presence of aerenchymatic tissues helps in exchange of gases between the shoot and the root systems. The well-developed aerenchyma in halophytes are generally developed on territories flooded by marine water
(Hameed et al., 2012) and the roots with such a structure are capable of tolerating long periods of salt water flooding.
