Soil pollution caused by potentially toxic metals has become a worldwide environmental issue. Geogenic processes and anthropogenic activities are two important sources of soil pollution. Soils may inherit toxic metals from parent materials; however, soil pollution mostly results from industrial and agricultural activities. Contamination by metals can be indicated by the changes in chemical, biochemical, and microbial properties of soils and plant responses. The total concentration of toxic metals in soil is still the most widely used indicator for risk assessment although extractable amounts have been reported to be more closely related to plant uptake. Several models have been proposed for assessing soil contamination by toxic metals, but none of them are commonly accepted for application to a wide range of soils. This review paper highlights how toxic metal contamination negatively affects soil and environmental quality, impacts food quality and security, and poses a threat to human health. Further research is needed to not only improve soil contamination diagnosis, modeling, and regulatory standards but also for remediation efficiency.

Cacao is an understory plant cultivated in full-sun monocultures, multistrata agroforestry systems, where cacao trees are planted together with fruit, timber, firewood, and leguminous trees, or within thinned native forests. In agroforestry systems of cultivation, cacao is subjected to excess shade due to high density, excess growth, and the unmanaged pruning of shade trees. Cacao is tolerant to shade, and the maximum photosynthetic rate occurs at an irradiance of around 400 μmol m⁻² s⁻¹. However, excess shade further reduces the irradiance, which is detrimental to photosynthesis and growth functions. Intra-specific variation is known to exist in cacao for the required saturation irradiance. A greenhouse study was implemented with 58 cacao genotypes selected from four geographically diverse groups: (i) wild cacao from river basins of the Peruvian Amazon (PWC); (ii) Peruvian farmers’ collection (PFC); (iii) Brazilian cacao collection (BCC); and (iv) national and international cacao collections (NIC). All of the cacao genotypes were subjected to 50% and 80% shade where photosynthetic photon flux density (PPFD) was 1000 and 400 μmol m⁻² s⁻¹, respectively. Intra-specific variations were observed for growth, physiological and nutritional traits, and tolerance to shade. Cacao genotypes tolerant to shade were: UNG-77 and UGU-130 from PWC; ICT-2173, ICT-2142, ICT-2172, ICT-1506, ICT-1087, and ICT-2171 from PFC; PH-21, CA-14, PH-990, and PH-144 from BCC; and ICS-1, ICS-39, UF-613, and POUND-12 from NIC. Genotypes that tolerate excess shade may be useful plant types for maintaining productivity and sustainability in agroforestry systems of cacao management.

Cacao is an important tropical crop and requires high-fertility soils for better growth and productivity; nevertheless, soils where this crop is grown are, in general, acidic and low in fertility. Therefore, germplasm with tolerance to soil acidity is desirable for cacao genetic improvement. The objective of this study was to evaluate cacao germplasm for early growth, nutrient concentration, and potential tolerance to soil acidity. A greenhouse experiment was conducted to evaluate 60 cacao genotypes with diverse geographic origins. Cacao seedlings were grown for six months in acid soil with and without lime. Growth parameters and the total concentration of N, P, K, Ca, Mg, Fe, Cu, Mn, and Zn were measured in shoots after harvest. Our results indicate that the best early growth predictors of acid soil tolerance are the number of leaves and root area. N, Ca, Mg, and K uptake may have a potential role in tolerance to soil acidity. Finally, the results revealed a large difference among cacao genotypes in terms of their responses to acid soil stress, which led to the selection of ten genotypes: CCN-51, PH-21, CCN-10, PAS-91, ICT-1087, ICS-95, UF-667, TSH-565, PH-144, ICT-1189 that are potentially tolerant to soil acidity and could be used for breeding acid soil-tolerant cacao varieties.

In the early stages of the establishment of plantation crops such as cacao, perennial legume cover crops provide vegetative cover to reduce soil and nutrient loss by erosion. Light intensity at cover crop canopy levels greatly influences their adaptability and optimum growth. As tree crops mature, understory cover crops suffer from inadequate light intensity. A greenhouse experiment was undertaken with nine perennial legume cover crop species (Calopo, Ea-Ea, Jack Bean, Lab-Lab, Mucuna ana, Mucuna preta, Cowpea, Black Pigeon Pea and Mixed Pigeon Pea) to assess the effects of three photosynthetic photon flux densities (PPFDs, µmol m⁻² s⁻¹) 180 (inadequate light), 450 (moderate light) and 900 (adequate light) on growth, physiological and nutrient uptake parameters. PPFD had highly significant effects on leaf, shoot and root growth parameters and increasing the light intensity from 180 to 900 µmol m⁻² s⁻¹ increased all growth parameters with the exception of specific leaf area. In all the legume cover crops, increasing the light intensity significantly increased the net assimilation rates (NAR), SPAD index and net photosynthesis (P_N) and its components, stomatal conductance (gs), transpiration (E) and vapor pressure deficit (VPD). Cover crop species, PPFD and their interactions significantly affected water flux (Vo) and various water use efficiency parameters (WUE_TOTAL, WUE_INST and WUE_INTR). Increasing the PPFD increased the WUE in all of the cover crops. Species and PPFD had highly significant effects on the uptake of macro- and micronutrients. Overall uptakes of all nutrients were increased with increases in the PPFD from 180 to 900 µmol m⁻² s⁻¹. With few exceptions, the nutrient use efficiency (NUE) of the nutrients was significantly influenced by species, PPFD and their interactions. Except for Mn, increasing the PPFD from 180 to 900 µmol m⁻² s ⁻¹ increased the NUE for all the nutrients.

Cacao (Theobroma cacao L.) was grown as an understory tree in agroforestry systems where it received inadequate to adequate levels of photosynthetic photon flux density (PPFD). As atmospheric carbon dioxide steadily increased, it was unclear what impact this would have on cacao growth and development at low PPFD. This research evaluated the effects of ambient and elevated levels carbon dioxide under inadequate to adequate levels of PPFD on growth, physiological and nutrient use efficiency traits of seven genetically contrasting juvenile cacao genotypes. Growth parameters (total and root dry weight, root length, stem height, leaf area, relative growth rate and net assimilation rates increased, and specific leaf area decreased significantly in response to increasing carbon dioxide and PPFD. Increasing carbon dioxide and PPFD levels significantly increased net photosynthesis and water-use efficiency traits but significantly reduced stomatal conductance and transpiration. With few exceptions, increasing carbon dioxide and PPFD reduced macro–micro nutrient concentrations but increased uptake, influx, transport and nutrient use efficiency in all cacao genotypes. Irrespective of levels of carbon dioxide and PPFD, intraspecific differences were observed for growth, physiology and nutrient use efficiency of cacao genotypes.

At early stages of establishment of tropical plantation crops, inclusion of legume cover crops could reduce soil degradation due to erosion and nutrient leaching. As understory plants these cover crops receive limited irradiance and can be subjected to elevated CO₂ at ground level. A glasshouse experiment was undertaken to assess the effects of ambient (450 µmol mol⁻¹) and elevated (700 µmol mol⁻¹) levels of [CO₂] on growth, physiological changes and nutrient uptake of six perennial legume cover crops (Perennial Peanut, Ea-Ea, Mucuna, Pigeon pea, Lab lab, Cowpea) under low levels of photosynthetic photon flux density (PPFD; 100, 200, and 400 µmol m⁻² s⁻¹). Overall, total and root dry biomass, total root length, specific leaf area, and relative growth rates were significantly influenced by levels of [CO₂] and PPFD and cover crop species. With few exceptions, all the cover crops showed significant effects of [CO₂], PPFD, and species on net photosynthesis (P_N) and its components, such as stomatal conductance (gs) internal CO₂ conc. (Ci), and transpiration (E). Increasing [CO₂], from 450 to 700 μmol mol⁻¹ and increasing PPFD from 100 to 400 μmol ּm⁻² ּs⁻¹ increased P_N. Overall, the levels of [CO₂], PPFD and species significantly affected total water use efficiency (WUE_TOTAL), instantaneous water use efficiency (WUE_INST) and intrinsic water use efficiency (WUE_INTR). With some exceptions, increasing levels of [CO₂] and PPFD increased all the WUE parameters. Interspecific differences were observed with respect to macro-micro nutrient uptake and use efficiency. With a few exceptions, increasing levels of [CO₂] from 450 to 700 μmol mol⁻¹ and PPFD from 100 to 400 μmol m⁻² s⁻¹ increased nutrient use efficiency (NUE) of all nutrients by cover crop species.