The allelopathic potential of a plant has been evaluated on the basis of two indicators: specific activity, which is the specific concentration of the allelochemical to exert a half-maximum effect on a receiver plant (EC50), and total activity in a plant, which is the ratio of the concentration of an allelochemical in the producing plant to its EC50. In the present study, a new indicator, total activity in a soil, which takes into account the effects of a soil on the allelopathy activity, is proposed because allelopathic activity is affected by the presence of soils. The total activity in a soil was calculated by multiplying the “total activity in a plant” with a “soil factor.” In this calculation, we assumed simplified cases for comparison, such that the allelopathic plant materials are evenly incorporated in the soils and the allelochemicals are released from the plant materials to the soils at a constant rate. We conducted bioassay experiments in the presence and absence of soils and cited some published data to calculate the specific activities and total activities in a plant and in a soil. The results indicated that the allelopathies of buckwheat caused by (+)-catechin, Leucaena leucocephala by L-mimosine, Xanthium occidentale by trans-cinnamic acid, and Brassica parachinensis by cis-cinnamic acid were not significant in a volcanic ash soil, an alluvial soil, and a calcareous soil, but the allelopathy of sweet vernalgrass caused by coumarin and Spiraea thunbergii by cis-cinnamoyl glucosides was highly effective in those soils. The allelopathies of Juglans species caused by juglone plus juglone precursors and Mucuna pruriens by L-DOPA would depend highly on the soil types. Although some limitations exist for this approach, the total activity approach would allow for a better quantitative estimation of the allelopathic potential of plant materials in soils.

The role of abscisic acid (ABA) in seed germination of two cultivars of lettuce (Lactuca sativa L.; light-sensitive Ritsa and light-insensitive Strada) was investigated. The inhibition of Ritsa seed germination by exogenous ABA was higher than that of Strada seeds, the extent of inhibition of both cultivars being reduced by a short light break. At 25°C the sensitivity of both cultivars to exogenous ABA was higher than at 15°C. The endogenous level of ABA was similar in dry seeds of both cultivars, increasing temporarily in Ritsa seeds during the first 4 h of imbibition in darkness but not in Strada seeds, nor in Ritsa seeds exposed to a short light break. The transitory increase of ABA content in Ritsa seeds imbibed in darkness was accompanied by increased expression of the gene responsive to ABA (Rab) under these conditions. Zorial (Norflurazone), an ABA-biosynthesis inhibitor, decreased ABA content and allowed dark germination of the light-requiring Ritsa seeds. A short light break induced germination of Ritsa seeds when applied at 24 and 48 h after imbibition onset, i.e. after the transitory increase of ABA. GA3, on the other hand was effective when applied at the beginning of imbibition. It seems that light induces germination of the photoblastic Ritsa seeds by both inhibiting ABA synthesis and decreasing seed sensitivity to ABA and inhibitory processes induced by it.

Correlative evidence indicates that sucrose and α-galactosyl-sucrose oligosaccharides (raffinose family oligosaccharides; RFOs) may be involved in seed longevity. Priming treatments (hydration in water or osmotic solutions followed by drying) can improve short-term seed performance but often result in reduced seed longevity. As RFOs are metabolized quickly following seed imbibition, loss of RFOs during priming could lead to more rapid deterioration in dry storage. This hypothesis was tested by measuring sucrose and oligosaccharide contents and potential longevity of primed seeds. Raffinose contents of whole lettuce (Lactuca sativa L.) seeds declined during hydration and priming and were correlated with decreased median potential viability (p50). However, this relationship was less significant when only the embryonic axes were analysed. In tomato (Lycopersicon esculentum Mill.) and impatiens (Impatiens balsamina L.) seeds, planteose was the major galactosyl-sucrose oligosaccharide and only small quantities of RFOs were present. Planteose contents declined during priming in seeds of both species, while sucrose contents increased or remained constant. Post-priming treatments that restored longevity in primed impatiens and tomato seeds were not accompanied by consistent changes in RFO or planteose contents. Our data do not rule out a role for oligosaccharides in seed longevity, but they make it unlikely that changes in oligosaccharide contents alone are responsible for the reduction in longevity due to priming or its restoration by post-priming treatments.