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William J. Carpenter and Susumu Maekawa

Verbena (Verbena × hybrida Voss) seed germination varied with the water content of the substrate. Total germination percentages (G) were highest when substrates were 75% to 100% saturated and progressively declined with increased free distilled water (FDW) on the blotter paper substrate. Natural differences in G among cultivars at favorable substrate moisture levels increased when free water was present. Removing the seed hilums did not increase G of `Romance Scarlet' on a substrate with FDW, but significantly increased the G of `Showtime Blaze' and `Red A'. Au inverse relationship was found between seed moisture contents and G at high substrate moisture levels. `Romance Scarlet', `Showtime Blaze', and `Red A' had 64%, 73%, and 84% seed moisture contents and G of 72%, 18%, and l0%, respectively. The period of sensitivity to excessive water during germination was day 2 for `Red A' and days 2 and 3 for `Showtime Blaze'. The G of `Romance Scarlet' seed was not reduced when placed on a substrate containing FDW for 1 day. Removal of seed hilums from `Red A' significantly increased G during day 2 for seeds on a substrate containing FDW.

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Susumu Maekawa and William J. Carpenter

The germination of verbena (Verbena × hybrids) seed was found to be sensitive to high substrate moisture content. Cultivars varied in sensitivity to excessive substrate moisture content, with `Romance Scarlet' having higher total germination (G) in the presence of free water than `Showtime Blaze' or seedling `Red A'. Hilum cavity measurements of dry seeds showed larger hilum apertures with reduced depths for seeds of `Romance Scarlet' than for the others. Seed imbibition resulted in a rapid and extensive thickening of the hilum wall. The extent of hilum aperture closure varied among cultivars and the quantity of water present. Free water reduced hilum apertures 45% for `Romance Scarlet', 60% for `Showtime Blaze', and 86% for `Red A'. Seeds of `Romance Scarlet' and `Showtime Blaze' failed to germinate with lanolin covering the hilum, while seeds coated with lanolin, except for the hilum, had 67% to 78% G of nontreated seeds. This difference indicates that essential oxygen for the embryo was obtained through the hilum and micropyle of the seeds. Total germination varied with substrate moisture content, with seeds placed horizontally on 2%, 1%, or 0.5% agar having 80%, 75%, and 65% germination, respectively, for `Romance Scarlet' and 59%, 41%, and “24%, respectively, for `Showtime Blaze'.

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Michio Kanechi, Masakatsu Ochi, Michiko Abe, Noboru Inagaki, and Susumu Maekawa

The effects of natural ventilation and CO2 enrichment during the rooting stage on the growth and the rates of photosynthesis and transpiration of in vitro cauliflower (Brassica oleracea L.) plantlets were investigated. In vitro plantlets were established in airtight or ventilated vessels with or without CO2 supplied (≈1200 μg·L-1) through gas permeable films attached to the vessel's cap for 15 days before transplanting ex vitro. Leaves generated in vitro in ventilated vessels had a higher photosynthetic rate than those produced in airtight vessels, which lead to greater leaf expansion and shoot and root dry matter accumulation during in vitro culture and acclimatization. Enhanced photosynthesis in leaves of ventilated plantlets was positively correlated with chlorophyll content. Increasing photosynthetically active radiation from 70 to 200 μmol·m-2·s-1 enhanced the growth of in vitro plantlets under ventilated conditions but it depressed photosynthesis of the leaves grown photomixotrophically with sugar and CO2 enrichment which might be due to the feedback inhibition caused by marked accumulations of sucrose and starch. Higher CO2 levels during in vitro culture enhanced photosynthesis under photoautotrophic conditions, but inhibited it under photomixotrophic conditions. Fifteen days after transplanting ex vitro, high photosynthetic ability and stomatal resistance to transpiratory water loss of ventilated plantlets in vitro had important contributions to rooting and acclimatization. Our findings show that the ventilated culture is effective for accelerating photoautotrophic growth of plantlets by increasing photosynthesis, suggesting that, especially for plantlets growing in vitro without sugar, CO2 enrichment may be necessary to enhance photosynthetic ability.