Further Investigations on the Relation of Photo Period to the Boron Requirement of Plants

Abstract

1. The reduction of intensity of boron-deficiency symptoms, associated with blossoming induced by short photoperiod, has been confirmed and extended. 2. Cocklebur plants grown without boron and exposed to short days fruited and showed no deficiency symptoms. Plants grown in long days without boron remained vegetative and soon displayed both external and internal signs of boron deficiency. This diminution of symptoms in short days was not the result of an increased boron absorption. 3. Continuous exposure to short days is not required for floral initiation in cocklebur, provided an induction period of suitable length is given the plants. Plants grown without boron and given 4 short days, after which they were exposed to long days, showed severe symptoms of boron deficiency; however, plants given an induction period of 10 short days and then returned to long days gave no evidence of boron deficiency. 4. Cocklebur plants exposed to 10 short days displayed reduced cambial activity which was associated with flowering. Boron deprivation in the stem caused tissue disorganization resulting from abnormal activity of the cambium; the cambium in the plants given 10 short days became less active, so that, after return to long days, proliferation and enlargement of cells in the cambial zone, resulting from minus-boron treatment, did not occur. 5. Tomato, sunflower, and buckwheat are day-neutral plants. With the exception of buckwheat, these plants flower after a considerably greater length of time than cocklebur. Plants of tomato and sunflower grown without boron and regardless of length of day showed striking symptoms of boron deprivation. The external symptoms included the typical death of the stem tip, abnormal development of leaves and roots, and fragility of the petioles. The internal structure of the stem was similar to cocklebur in that abnormal cambial activity occurred, resulting in proliferation and enlargement of cells. In plants showing severe symptoms of boron deficiency, necrotic areas were conspicuous. Buckwheat flowered at an early stage. In short days cambial activity ceased sooner than it did in long days. Symptoms of boron deficiency were not apparent in short days, even in those plants grown without boron. In long days, however, maturation of tissues was less rapid, and slight symptoms of boron deficiency were apparent in plants grown without this element. 6. Soybean var. Biloxi is a short-day plant and resembles cocklebur in its response to boron deprivation. This variety fruited in short days and showed no symptoms when grown without boron. In long days, however, severe symptoms in minus-boron cultures were apparent. Pagoda soybean fruited at an early stage in both short and long days; deficiency symptoms were not evident in the minus-boron cultures on either photoperiod. 7. It is suggested that the decrease in cambial activity associated with early floral induction or photoperiodic induction causes a reduction in the severity of boron-deficiency symptoms.