Root Development and Source-Sink Relations in Carrot, Daucus carota LII. EFFECTS OF ROOT PRUNING ON CARBON ASSIMILATION AND THE PARTITIONING OF ASSIMILATES

Physiological responses to root pruning were investigated by comparing 14C02 fixation rates, the partitioning of ""?-labelled assimilate, and soluble and insoluble carbohydrate levels in the leaves of carrot plants following the removal of some of the fibrous roots, or fibrous roots and part of the tap root. Root pruning reduced 14C02 fixation by 28-45% but leaf specific activity (14C assimilation g~' leaf fresh weight) was unchanged. The proportion of total assimilate exported to the root system increased following root pruning and this was at the expense of the developing leaves. In younger plants (where the tap root received 10% of the assimilate) the supply of 14C to the tap root was maintained in spite of root pruning. However, shortening the tap root to 3 cm in older plants (in which 30% of the fixed 14C was normally exported to the developing storage organ), reduced its sink capacity and resulted in slightly greater retention of 14C in the mature leaves. Greater concentrations of insoluble carbohydrate were found in the mature leaves following root pruning but soluble sugar content was unaffected. Only small differences were observed in the distribution of I4C between soluble and insoluble carbohydrate fractions when plants were fed 14C02 several days after the root pruning operations. These physiological responses were mainly associated with the removal of fibrous roots and support the view that the fibrous root system is more important than the developing storage organ in regulating growth in young carrot plants. INTRODUCTION The role of the developing storage organ as a metabolic sink was demonstrated in an earlier paper which showed that during the vegetative development of the carrot plant the thickening tap root accepted increasing amounts of assimilates (Benjamin and Wren, 1978). Pruning the root system, by removing some of the lateral roots, reduced the absolute growth rate of both the shoot and the remaining root system, but shortening the tap root had little additional effect. Thus it appeared that, although the root system influenced dry matter production in the carrot plant, the sink activity of the developing storage organ was less important than other root functions. The physiological changes following root removal have now been investigated by comparing assimilation rates and the partitioning of assimilates in plants subjected to different degrees of root pruning. 1 Present address: National Vegetable Research Station, Wellesbourne, Warwick CV35 9EF. This content downloaded from 207.46.13.76 on Fri, 09 Sep 2016 04:27:47 UTC All use subject to http://about.jstor.org/terms 1140 Benjamin and Wren—Root Pruning in Carrot MATERIALS AND METHODS Carrot plants, variety Tip Top (Dobies) were grown in aerated nutrient solution in a Prestcold controlled environment cabinet at 20 °C, 0-6 kPa Pa water vapour pressure deficit and 16 h photoperiods of warm white fluorescent light, with a radiant flux density of 80 W m~2, as described previously (Benjamin and Wren, 1978). Roots were pruned 35 d after sowing. Control plants (WT, WF) retained the whole root system, whilst in treatments 6T,3F and 3T,3F the tap root was cut 6 cm or 3 cm below the cotyledons, respectively. In treatments WT.3F and 3T.3F, fibrous roots originating from the tap root more than 3 cm below the cotyledons were carefully removed. Two experiments were carried out to compare 14C02 fixation and the subsequent partitioning of assimilates following root pruning at 35 d. In Expt 1, four plants from a single treatment were exposed to 100 /iCi 14C02 (produced by adding 2-0 cm3 50% (v/v) lactic acid to 1-0 cm3 Na214C03, specific activity 1 mCi mmol ') during the period following pruning. Plants from the two pruned treatments were fed on consecutive days, 6T.3F plants being fed 5, 9, 13, and 16 d after pruning and 3T,3F plants on days 6, 8, 12, and 15. Non-pruned controls (WT,WF) were fed only at the end of the experiment, on days 19 and 20. Thirty minute exposures to 14C02 were started at 1000 (2 h after the beginning of the photoperiod) and plants were harvested 1 h and 26 h after the start of 14C02 feeding to compare assimilation rates and translocation patterns. In Expt 2, 14C02 was generated by adding 0-5 cm3 1M HC1 to 50 mm3 Na2'4C03 (specific activity 1 mCi mmoL1), in an attempt to improve the speed and efficiency ot 14C02 release (Lovell, Oo, and Sagar, 1972). Six plants were fed on each occasion (duplicates from three different treatments), and feedings were begun at 1000, 1300, and 1600 (8, 11, and 14 h after the start of the photoperiod). In the assimilation experiment (2A), batches of six plants were exposed to 100 juCi 14C02 for 10 min on days 5 and 7. Previous work had shown that virtually all the ,4C was still present in the leaves 1 h after feeding, and these were harvested by plunging into boiling 80% (v/v) ethanol. The mature (leaves 1-4) and expanding leaves (5+) were extracted separately and both ethanol-solu'ble and insoluble activities were determined. After grinding and extracting in cold 80% ethanol, the residue was resuspended in water and 100 mm3 aliquots were placed on to planchettes for counting in a Nuclear Chicago gas flow counter. Total sugar content of the mature leaf blades was also determined using the plants which had been fed 5 d after pruning. Chlorophyll was removed by partitioning with 60-80% petroleum ether before reducing the ethanol-soluble fraction to dryness by rotary evaporation under reduced pressure at 40 °C. The residue was resuspended in water and centrifuged, and the clear solution was diluted and used in the phenol-sulphuric acid reaction to determine soluble sugars (Dubois, Gilles, Hamilton, Rebers, and Smith, 1965). Insoluble, non-structural carbohydrates were determined by hydrolysing the 80% ethanol-insoluble residue with 50 cm3 boiling 0-2 M H2S04 for 1 h (Smith, Paulsen, and Raguse, 1964), prior to determination by the phenol-sulphuric acid reaction. Total sugars are expressed as sucrose equivalents. In the partitioning experiment (2P) three batches of duplicate plants from each treatment were exposed to 100 ,uCi l4C02 for 30 min on days 4 and 6 and then were returned to the growth cabinet for a further 25-j h before harvest. Plants were divided into their constituent organs (leaf blades, petioles including shoot apex, tap root, and fibrous roots), which were killed in boiling 80% ethanol. After storage, the supernatant was decanted and its activity determined by counting with a Nuclear Chicago gas flow counter. The ethanol-insoluble residue was oxidized in a Packard Model 306 sample oxidizer, 14C02 being absorbed into Carbosorb and Permafluor V and its activity determined using a scintillation counter. The measured activities have been corrected to allow for differences in counting efficiency, so that the activities quoted are comparable.