YOUNG WATERMELON PLANT-GROWTH RESPONSES TO END-OF-DAY RED AND FAR-REDLIGHT ARE AFFECTED BY DIRECTION OF EXPOSURE AND PLANT-PART EXPOSED

Differences in the growth regulating ability of end-of-day (EOD) red ( R) and far-red light (FR) as affected by direction of exposure (predom inantly downwardly versus upwardly directed) and plant part exposed (a daxial and abaxial surface of leaf 1, and junction and opposite the ju nction of the attachment of the petiole to the stem) were determined o n young watermelon (Citrullus lanatus (Thumb.) Matsum and Naki cv. Sug ar Baby) seedlings in a controlled environment. Whole-plant exposure t o downwardly or upwardly directed EOD FR stimulated elongation of inte rnode 1 and the petiole of leaf 1 (as measured from the cotyledons) as compared to control seedlings (no EOD light). Upwardly directed EOD F R also induced elongation of internode 2, stem length, and leaf petiol es 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) impl icating phytochrome regulation. Upwardly directed EOD FR increased the leaf angle of leaf I 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 (tis sue area exposed approximately equal to 20cm(2)) enhanced elongation o f internode 1 by 88% as compared to controls. Irradiation of the abaxi al surface of leaf 1 and the junction and side opposite the junction o f 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 concl usion, the growth regulating ability of photomorphogenic light (as pro vided by EOD R and FR) was affected by predominant direction of exposu re.