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In ecological setting, sodium (Na+) can be beneficial or toxic, depending on plant species and the Na+ level in the soil. While its effects are more frequently studied at high saline levels, Na+ has also been shown to be of potential benefit to some species at lower levels of supply, especially in C4 species. Here, clonal plants of the major tropical C3 crop Theobroma cacao (cacao) were grown in soil where potassium (K+) was partially replaced (at six levels, up to 50% replacement) by Na+, at two concentrations (2.5 and 4.0 mmolc dm−3). At both concentrations, net photosynthesis per unit leaf area (A) increased more than twofold with increasing substitution of K+ by Na+. Concomitantly, instantaneous (A/E) and intrinsic (A/gs) water‐use efficiency (WUE) more than doubled. Stomatal conductance (gs) and transpiration rate (E) exhibited a decline at 2.5 mmol dm−3, but remained unchanged at 4 mmol dm−3. Leaf nitrogen content was not impacted by Na+ supplementation, whereas sulfur (S), calcium (Ca2+), magnesium (Mg2+) and zinc (Zn2+) contents were maximized at 2.5 mmol dm−3 and intermediate (30–40%) replacement levels. Leaf K+ did not decline significantly. In contrast, leaf Na+ content increased steadily. The resultant elevated Na+/K+ ratios in tissue correlated with increased, not decreased, plant performance. The results show that Na+ can partially replace K+ in the nutrition of clonal cacao, with significant beneficial effects on photosynthesis, WUE and mineral nutrition in this major perennial C3 crop.

Flooding is common in lowlands and areas with high rainfall or excessive irrigation. A major effect of flooding is the deprivation of O 2 in the root zone, which affects several biochemical and morphophysiological plant processes. The objective of this study was to elucidate biochemical and physiological characteristics associated with tolerance to O 2 deficiency in two clonal cacao genotypes. The experiment was conducted in a greenhouse with two contrasting clones differing in flood tolerance: TSA-792 (tolerant) and TSH-774 (susceptible). Leaf gas exchange, chlorophyll (Chl) fluorescence, chemical composition and oxidative stress were assessed during 40 d for control and flooded plants. Flooding induced a decrease in net photosynthesis, stomatal conductance and transpiration of both genotypes. In flood conditions, the flood-susceptible clone showed changes in chlorophyll fluorescence, reductions in chlorophyll content and increased activity of peroxidase and polyphenol oxidase. Flooding also caused changes in macro-and micronutrients, total soluble sugars and starch concentrations in different plant organs of both genotypes. Response curves for the relationship between photosynthetically active radiation (PAR) and net photosynthetic rate (P N ) for flooded plants were similar for both genotypes. In flood conditions, the flood-susceptible clone exhibited (1) nonstomatal limitations to photosynthesis since decreased in maximum potential quantum yield of PSII (F v /F m ) values indicated possible damage to the PSII light-harvesting complex; (2) oxidative stress; (3) increased leaf chlorosis; and (4) a reduction in root carbohydrate levels. These stresses resulted in death of several plants after 30 d of flooding.

Coconut palm is of great social and economic importance for millions of people in the tropics and subtropics. Drought is one of the main environmental factors that limit coconut productivity. In this review, physiological and morphological data are presented in an integrated perspective to provide a holistic view of the behavior of coconut trees facing water deficit under both field and controlled conditions. Great capacity to produce homorhizic roots capable of searching continuously for water and nutrients and high protoplasmic tolerance in leaves allows the coconut tree to endure water deficit. High membrane stability, osmoprotection, osmoregulation and enhanced activity of antioxidant enzymes are characteristics often found in leaves of drought-stressed coconut trees. Information on leaf gas exchange, leaf water potential, water-use efficiency and stomatal behavior was compiled highlighting the differences among cultivars and hybrids under water stress. Genotypic differences in physiological and morphological traits can largely explain the agronomic performance of field-grown coconut trees under drought conditions.

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