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Aimin Liu, Joyce G. Latimer and Robert E. Wilkinson

The influence of two fungicides—captan and thiram—on growth and 45Ca absorption by roots of `Starbrite' watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] seedlings was investigated. Unilateral application of Ca+2 and Al in agar induced curvature in roots from untreated and pretreated seeds. In untreated seeds, PCMBS inhibited Ca+2- and Al-induced root curvature by 82% and 92%, respectively. In commercially pretreated seeds (captan + thiram), PCMBS inhibited Ca+2- but not Al-induced root curvature. Captan or thiram also inhibited Ca+2- or Al-induced root curvature, and the effects of captan and thiram on root curvature were additive. Serial concentration (0, 0.01, 0.1, 1, 10, or 100 mg·liter-1) tests indicated that captan inhibited 45Ca absorption the most at 100 mg·liter-1, whereas thiram inhibited 45Ca absorption the most at 0.01 mg·liter-1. The effects of captan and thiram on 45Ca absorption were statistically additive. Thiram seemed to influence Ca+2 uptake by affecting exofacial sulfhydryl groups (a mode of action similar to that of PCMBS). DTT reversed the inhibitory effect of thiram on 45Ca absorption by 34% but did not reverse the effect of captan. A field test showed that acidic soil (pH 4.55) reduced leaf number; leaf, stem, shoot, and whole-plant dry weights; and stem length of 15-day-old seedlings. Although there was no difference in root dry weights or root: shoot ratios of plants from pretreated and untreated seeds planted in soil at pH 6.26, planting commercially pretreated seeds in acidic soil produced plants with greater root dry weights and root: shoot dry weight ratios than those from untreated seeds. Seedlings showed a greater response to seed treatment in early growth stages. Captan and thiram may have influenced growth characteristics by inhibiting Al uptake of seedlings planted in acidic soil. To our knowledge, this is the first report on the influence of the fungicides captan and thiram on mineral ion uptake in roots. Chemical names used: p-Chloromercuribenzenesulfonic acid (PCMBS), dithiothreitol (DTT), N-trichloromethylthio-4-cyclohexene-1,2-dicarboximide (captan), tetramethylthiuram disulfide (thiram).

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Yuki Sago

Tipburn is a severe problem in producing butterhead lettuce under artificial lighting and develops as a consequence of decreased calcium concentrations in leaves. Here, we investigated the effects of light intensity on tipburn development and calcium concentration in leaves by comparing their growth rates. Butterhead lettuce was grown in a plant factory under artificial light at photosynthetic photon flux (PPF) densities of 150, 200, 250, and 300 μmol·m−2·s−1. Fresh and dry weights of shoots, relative growth rate, the number of leaves, and the number of tipburned leaves significantly increased with light intensity. Associations existed between growth and tipburn occurrence. Calcium absorption rate per plant also increased with light intensity in association with increased water absorption rate. Consequently, calcium concentrations in the entire plant and outer leaves increased with light intensity. In contrast, calcium concentration in the inner enclosed leaves did not increase with light intensity. This pattern can be attributed to the higher mass flow of calcium to outer leaves than to inner leaves, driven by transpiration, under high light intensities. Thus, a lack of calcium in the inner leaves resulting from rapid growth may contribute to the frequent tipburn development.