Improving the copper nutrition of peanut on sands with particular reference to South-Central Coastal Vietnam

Thai Thinh  Nguyen

In Vietnam, about 0.5 million ha of sandy soils occur with about 68% in the south-central coastal region (SCCV). These sands in the SCCV are infertile and characterized by high content of sand (range from 80-98%), lesser clay (0.8-10%) and silt content (1-3%), pH H2O from 5.2 to 6.8; low organic carbon (0.2-0.45%); low Olsen-P (<10.0 mg/kg); low cation exchange capacity (<2.0 cmolc/kg); low water holding capacity <10.0 % w/w). In addition, the sands of SCCV are deficient in multiple nutrients, including copper (Cu) which range from 0.2 - 0.6 mg/kg that limit agricultural production. Preliminary studies showed that six out of eight sands from SCCV were severely limited in Cu supply for peanut using the double pot technique. For places like SCCV Vietnam, that have limited laboratory facilities for soil and plant testing of micronutrients, the double-pot approach is an effective diagnostic test for Cu deficiency. Peanut is a valuable cash crop in SCCV but its yield in this region is less than half the national average yield (1.8 vs 3.8 tonnes/ha). Low soil Cu levels may be one of the constraints for peanut production in this region. Mango is also a major cash crop in sandy soils of the SCCV with an increasing current production area of 14,800 ha (MARD, 2023). However, similar to peanut, Cu deficiency in sandy soils may be a constraint for mango production in the SCCV. Foliar spraying Cu before flowering helped to reduce the rate of skin lesions on mango fruits, at only 1.1% as compared to 2.4 - 2.8% in no Cu sprayed control. Application of Cu produced more number of high-quality fruits of grade I (37% of harvested fruit) as compared to 33% without Cu spraying. Fruit yield of 6.3 tonnes fruits/ha was obtained in mango trees receiving foliar Cu whereas in the control fruit yield was less than 5.0 tonnes /ha. Application of Cu into soil at 1.3 mg/kg of soil before sowing was the most effective approach for increasing peanut yield by 67% as compared to the control without applying Cu, followed by foliar spraying of 20 μM Cu at flowering and pegging, at 43% and 32% respectively. Later Cu application (at podding) strongly increased leaf and seed Cu concentrations but was not very effective in increasing the yield. This suggests that flowering and pegging are critical stages for Cu supply. The critical level of Cu in young leaves of peanut at that time is from 1.3 to 1.7 mg/kg for diagnosis of Cu deficiency. Supplementary foliar application at flowering stage (30-35 DAS) increased reproductive growth of the crop such as numbers of pegs, pods, per cent conversion of pegs to pods, pod dry weight and seed dry weight. On a low Cu sand, Cu deficiency had no effect on vegetative dry matter of peanut. This evidence indicates that reproductive growth of peanut is more sensitive to Cu deficiency than vegetative growth. However, there was no evidence that low Cu impaired starch accumulation in pollen grains or the density of pollen grains in flowers, hence pollen viability was not negatively affected by Cu deficiency in peanut. The decreased numbers of pegs and pods in Cu deficient peanut could be due to decreased photosynthesis rate that deprived the growing peg and pod tissue of photo assimilates. There was clear evidence that leaf Cu was positively related to photosynthesis rates, particularly during the podding period. Hence, Cu application enhances the rate of photosynthesis and photosynthate flux to the reproductive growth that is a strong sink for carbon which would directly increase both the number of matured pods and weight of seeds. This might be the mechanism for increased seed yield with Cu supply to soil and by foliar application up to pegging stage to peanut plants on low Cu soil. The mobility of Cu in peanut plants was limited but varied with Cu content within plant. There was evidence that lesser amounts of Cu moved out from the oldest leaves of Cu deficient plants than from plants given sufficient Cu. In addition, the exogenous application of Cu was not remobilized to young leaves, but appeared to be partitioned to pods and seeds that act as a stronger sink for Cu taken up by roots or retranslocated from leaves from the foliar application. There are two sets of conclusions for this thesis. Firstly, Cu deficiency on sands in SCCV is probably widespread and limiting crop production. This could be confirmed for a wider range of crops. Further testing for Cu deficiency on sands in other provinces of coastal Vietnam is proposed. In addition, a set of recommendations on Cu fertilization of crops in central Vietnam is recommended to ensure adequate supply to crops with minimal risk of eventually causing the accumulation of excessive Cu in sands. Secondly, the uptake to Cu into vegetative parts of peanut appeared to be relatively inefficient, while there was evidence of preferential partitioning of carbon and Cu into pods and seeds, but the mechanism(s) was not clear. Further research should focus on the physiological and molecular regulation of carbon and Cu into reproductive growth particularly from flowering to podding stages.

Improving the copper nutrition of peanut on sands with particular reference to South-Central Coastal Vietnam

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