A relationship between boron & auxin in C 14 translocation in bean plants

Abstract

The essentiality of boron for higher plants is well established although no definitive role proposed for this element has remained undisputed. So many processes are affected indirectly by boron that the primary effect(s) have remained obscure (9). Gauch and Dugger (9) suggest that a relatively specific role in carbohydrate translocation is indicated. Sucrose translocation is considerably reduced in boron deficient plants before morphological symptoms are evident. Gauch and Dugger (8) maintain that subsequent symptoms are simply the expression of carbohydrate deficiency resulting from the impaired trans-location system. They suggest that a negatively charged sugar-boron complex might more easily traverse cell membranes than non-borated sugar molecules or that boron might be a constituent of the membrane site across which the sugar moves. Skok (20, 21, 22) points out that it is also possible for the relationship of boron to translocation to be indirect. He proposes that the boron effect on translocation is the result of the elements' essentiality to the metabolic activity of meristematic regions which have large substrate requirements. The effect of this particular metabolic activity including that associated with cell enlargement can be called sink effect; its relationship to translocation is now well established (3, 12, 23). If Skok is correct, cessation of growth would be the cause rather than the result of impaired carbohydrate translocation in boron deficient plants. Dugger et al. (4) proposed that boron affects the rate of translocation through its effect on the sugar-starch balance of the leaves. Scott (18) presents a similar hypothesis: "It appears that boron performs a protective function in plants in that it prevents excessive polymerisation of sugars at sites of sugar synthesis". It would appear, however, that boron must have an additional role in the metabolism of meristematic regions. since these last mentioned theories deal primarily with the availability of carbohydrates for translocation; applications of sugars do not prevent the death of terminal buds of boron deficient plants (19, 20). Prior to breakdown of phloem tissue in the more advanced stages of deficiency, the capacity for trans-location should exist in boron-deficient plants although this capacity is not normally expressed. If metabolic activity or growth can be promoted in boron deficient plants by some means such as auxin applications , then translocation should increase if boron is not directly essential to the process. Our experi-meents designed to test this proposition will be described here. MATERIALS & METHODS Bean plants (Phascolus vulgaris L. var. Black Valentine) were used in all experiments in this study. Seeds were soaked for 2 hours in distilled water and then transferred to washed. fine quartz sand in hard rubber trays for germination. After 7 days, plants were transferred to nutrient solutions in black-painted, aluminum foil-wrapped 1-quart Mason jars. The aerated mineral solution consisted of the following salts: 5 x 10-3 AI Ca(NO,)2; 5 X 10-3 M KNO3; 2x 10-3 M MgSO4; 1 X 10-s M KH,P04; 1 X 10-4 M EDTA Na-Fe (5 ppm Fe); 9.2 X 10-6 M MnCl,. 4H,O; 5.7 X 10-7 M Na.,MoO4; 5.1 X 10-7 M ZnC1,; 2.9 x 10-7 M CuCl,. 2H2O. All glassware was either boron-free or low-boron soft glass. Germination and subsequent growth took place in a controlled environment room. The light intensity supplied by cool white fluorescent tubes (96 T8) and 60 wv tungsten bulbs (10: 1 wattage ratio) was approximately 1,000 ft-c at the base of the plant. Temperatures were maintained at 24 + 2 C during the 16 hour light period and 18 ± 2 C during the 8 hour dark period. Under these conditions boron deficiency symptoms were visible in the terminal region approximately 14 days after seed soaking. To half the plants boron, as H3BO,. was added to give a concentration of 4.6 X 10-5 M. Naphthalene-acetic acid (NAA) was used for auxin treated plants because of its greater biological stability when compared to IAA. Application of NAA to the terminal bud consisted of placing a 10 lambda drop of 5 ppm 672