Influence of Phosphorus Nutrition on Growth and Carbon Partitioning in Glycine max

Abstract

Soybean plants (Glycine max [L.] Merr var Amsoy 71) were grown in growth chambers with high-phosphorus (high-P) and low-phosphorus (low-P) culture solutions. Low-P treatment reduced shoot growth significantly 7 days after treatment began. Root growth was much less affected by low-P, there being no significant reduction in root growth rate until 17 days had elapsed. The results suggest that low-P treatment decreased soybean growth primarily through an effect on the expansion of the leaf surface which was diminished by 85%, the main effect of low-P being on the rate of expansion of individual leaves. Low-P had a lesser effect on photosynthesis; light saturated photosynthetic rates at ambient and saturating CO2 levels were lowered by 55 and 45%, respectively, after 19 days of low-P treatment. Low-P treatment increased starch concentrations in mature leaves, expanding leaves and fibrous roots; sucrose concentrations, however, were reduced by low-P in leaves and increased in roots. Foliar F-2,6-BP levels were not affected by P treatment in the light but in darkness they increased with high-P and decreased with low-P. The increase in the starch/sucrose ratio in low-P leaves was correlated primarily with changes in the total activities of enzymes of starch and sucrose metabolism.Suboptimal phosphorus supply diminishes photosynthetic C02-fixation rates (2,24) and the expansion of the photosynthetic leaf surface (5,22). It may also lead to decreased cytosolic orthophosphate levels (26). Orthophosphate (Pi) is thought to regulate the activities of several enzymes involved in starch and sucrose metabolism in vivo and the export of C out of the chloroplast via the Pi-translocator (3). However, much of the evidence in support of Pi as a key regulator of carbon partitioning has been obtained with in vitro systems. In this paper, we study the nutritional effects of Pi on growth, photosynthesis, and starch/sucrose metabolism in an intact plant system; soybean plants were treated with sufficient P for optimal plant growth (high-P) and with suboptimal P levels (low-P). The high-P and low-P solutions were similar except that the high-P solution contained 200 Mm KH2PO4 and the low-P solution contained 10 Mm KH2PO4. Both high-P and low-P culture solutions contained 9.125 mM N03-N, 0.625 mM NH4+-N, 4 mm K+, 1.0 mm S, 2.5 mm Ca2+, 1.0 MM Mg2+, 250 Mm Na+, 250 gM Si, 500 Mm C1-, 50 Mm FeHEDTA, 25 Mm B, 1.0 gM Mn, 1.0 gM Zn, 0.4 gM Mo, and 0.4 uM Cu. METHODS AND MATERIALSThe presence of Si in combination with a 10-fold reduction in the Mn concentration in solution ameliorated a putative Mn toxicity symptom in both high-P and low-P plants. Gas ExchangeGas exchange analyses were performed on randomly selected high-P and low-P plants at 18 to 20 d after transplant. Fully expanded trifoliates at the second or third node numbered basipetally from the first primary trifoliate were used for both irradiance and CO2 saturation curve determinations using steady state gas exchange equipment described previously (25). Co2 concentrations of...

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Biomass Yield and Nutrient Responses of Switchgrass to Phosphorus Application

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