S.M. Alam

Plant Physiology Division, Nuclear Institute of Agriculture, Tandojam, Pakistan.

Weed seed of sweet clover was evaluated alone or in combination with NaCI for its effect on germination and seedling growth of wheat. There was no effect of weed seed or NaCI salinity on the germination. It was observed that weed seed alone and with 0.2 % NaCI had significant effect on shoot length. Similarly, 0.4 % NaCI alone or in combination with weed seed reduced the shoot length. The root length was reduced by weed seed alone and in combination with NaCI. It was also observed that root length was affected more than the shoot.

Plants influence each others growth by means of exudates (Rice 1984), leachates from residues incorporated in the growing medium (Cotton and Einhellig 1980) or residues in natural undisturbed condition. It is commonly assumed that economical reduction in main crops from weed seeds are the direct result of competition or of the both acting together. Literature reveals that allelopathic interference by weed extracts has been established as one of the several factors that regulate the growth of plants (Alam 1993; Alam et al. 1997; Rice 1984). A variety of organic compounds have been implicated as the possible agents responsible for growth reduction and it has been assumed that in most cases, it is the collective action of several compounds, which cause the growth depression (Rice 1984).

In the process of competition, weed seeds frequently absorb nutrients much more than the crop plant. Evenari (1949) reviewed the various studies conducted and concluded that most weed seeds contain essential oils, alkaloids, or glycosides, which inhibit germination and seedling growth of other crops present in their immediate vicinity. The findings of McCalla and Duley (1948) revealed that soaking corn seeds in aqueous extract of sweet clover for 24 hours reduced germination and growth of tops and roots in petri dishes. Saraswat (1987) reported that sweet clover exhibited allelopathic effect on the growth of rice seedlings. It was also observed that leachates from fresh material of sweet clover produced high inhibition of radicle growth of corn, bean and squash and a reduction of 68 % in the growth of corn has been reported (Anaya et al. 1987). They further reported that reduction is due to allelopathic potential of this weed. In another experiment, they found similar effect of sweet clover.

Coumarin compound and phenolic derivatives of benzoic and cinnamic acids are some of the frequently reported water-soluble allelochemicals. These chemicals may alter several aspects of plant metabolism. These include mitochondrial respiration, photosynthesis rate and chlorophyll content. Some authors have reported coumarin, o-coumaric acid and melilotic acid as the major active compounds in sweet clover (Langer and Hill 1982; Nicollier et al. 1985). Putnam and Weston (1986) reported that corn plant performed poorly, when planted into stubble mulch of white sweet clover.

Gressel and Holm (1964) reported that seed of barnyard grass reduced the germination of alfalfa and turnip by 41 and 43 %, respectively, when compared with the water control. Contrary to this Bhatia et al. (1982) found that stem and roots of Melilotus indica promoted the growth of wheat seedlings. This study was therefore, carried out to see the effect of weed seed of sweet clover on the seedling growth of wheat (cv. Pavon). Some of growth characteristics of cv. Pavon are as: plant height (86.6); growth habit (seedling) (erect); flag leaf attitude (semi-erect); auricle anthocyanin (absent); head shape and color (tapering, light red); head awnness (awned); stem (stiff); seed color and shape (amber, ovate); 1000 grain weight (42.1 g); No. of grains/spike (80); protein content (11.3 %); fat (2.1 %); ash (2.0%); days to heading (77); days to maturity (135); yield potential (5075 kg/ha); average yield (4765 kg/ha); spike length (9.0 cm); Spikelets per spike (17.8); harvest index (36.1%); resistance to stem and leaf rust.