Carbohydrate and photosynthate distribution and nitrogen fixation in beans (Phaseolus vulgaris L.)

Abstract

A series of four studies were conducted in Colombia, South America, in the field and the greenhouse to determine the effect of plant competition and mulching on carbohydrate and photosynthate distribution and nitrogen fixation in beans (Phaseolus vulgaris L.). Mulching beans (determinant growth habit) with rice hulls in the lowland tropics reduced 2:30 p.m. soil temperature at 10 cm 1.3°C prior to flowering and 0.8°C after flowering compared to no mulch. Mulching improved the soil moisture in the top 10 cm 2.18 percentage units prior to flowering and only 0.57 units after flowering. The daily fluctuation of soil temperature was 1.3°C lower under the mulch. Nitrogen fixation (C₂H₂ reduction) showed the typically low rates of the lowland tropics but were three times higher (0.6 vs 0.2 μmoles/plt/hr) in mulched plants and was postively correlated with nodule fresh weight. Nodule specific activity was not affected by mulching. The roots and leaves of mulched plants contained 37% and 42%, respectively, higher quantities of total carbohydrates. A determinant and indeterminant bean cultivar were used in two tropical locations (one cool, high altitude, high rainfall and the other hot, low altitude, low rainfall) to determine the effect of plant population on carbohydrate distribution and nitrogen fixation. Plant development characteristics were similar in both locations. Increasing plant population resulted in lower plant weights and lower shoot/root ratios. The indeterminant cultivar had higher root weights and lower shoot/root ratios than the determinant cultivar. Nitrogen fixation rates and nodule fresh weight were positively correlated (r = 0.88 and r = 0.70) and the fixation rate was over 10-fold higher (0.5 vs 10 μmoles/plt/hr) in the cooler location. The concentrations of both soluble and insoluble carbohydrates were higher in all plant parts in the cooler location but were not markedly affected by plant population density. The distribution of photosynthate to the roots and nodules by the leaves at nodes 4 and 8 of a determinant bean cultivar were determined 35, 48 (flowering), 63 and 70 days after planting. Roots retained 45% 14 of the ¹⁴C-photosynthate translocated from node 4 throughout crop 14 development but accumulated almost none of the ¹⁴C translocated from node 8. From day 35 to 48, activity detected in the lower stem 14 decreased 16% with an equal increase in the nodules. The ¹⁴C-photosynthate from node 8 went initially (day 48) to mid and upper stems and leaves (90%) but subsequently 85% went to mid and upper pods. Nitrogen fixation peaked after flowering and was positively related to the quantity of nodule soluble carbohydrate. In a field study with an indeterminant cultivar, light penetration of the canopy was increased to different depths by defoliating alternate plants and sampling nondefoliated plants. Defoliations to nodes 12, 8, 4 or ground level were made at either 17 days prior to or 3 days after flowering. Nodule dry weight and nitrogen fixation tended to be reduced (not significantly) by all treatments both before and after flowering. Nitrogen content of nodules was reduced by postflowering treatments but no other changes in nitrogen content were observed. The dry weight and carbohydrate content of stems and leaves changed little until one week after flowering when both were increased by preflowering and postflowering treatments; however, only defoliation to ground level resulted in significant increases.