Differences in the growth regulating ability of end-of-day (EOD) red (R) and far-red light (FR) as affected by direction of exposure (predominantly downwardly versus upwardly directed) and plant part exposed (adaxial and abaxial surface of leaf 1, and junction and opposite the junction of the attachment of the petiole to the stem) were determined on young watermelon (Citrullus lanatus (Thumb.) Matsum and Naki cv. Sugar Baby) seedlings in a controlled environment. Whole-plato exposure to downwardly or upwardly directed EOD FR stimulated elongation of internode 1 and the petiole of leaf 1 (as measured from the cotyledons) as compared to control seedlings (no EOD light). Upwardly directed EOD FR also induced elongation of internode 2, stem length, and leaf petioles 2 and 3 as compared to controls. This EOD FR induced elongation was reversed by following the FR with a red light treatment (FR + R) implicating phytochrome regulation. Upwardly directed EOD FR increased the leaf angle of leaf 1 as compared to controls. Upwardly directed EOD R and EOD FR + R increased the petiole angle of leaf 1. Irradiation of the adaxial surface of leaf 1 with fibre-optic transmitted EOD FR (tissue area exposed approximately equal to 20 cm2) enhanced elongation of internode 1 by 88% as compared to controls. Irradiation of the abaxial surface of leaf 1 and the junction and side opposite the junction of the point of attachment of petiole 1 to the stem with EOD light had no effect on elongation of internodes 1 and 2 or stem length. In conclusion, the growth regulating ability of photomorphogenic light (as provided by EOD R and FR) was affected by predominant direction of exposure and plant part exposed. Of the experimental treatments, whole-plant exposure to upwardly directed EOD light was the most effective treatment for elongating internodes and petioles and in stimulating changes in leaf angle.
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