Influence of low light intensity and soil flooding on cacao physiology

Abstract

Growth and development of plants frequently are limited by multiple abiotic stresses that occur simultaneously in the environment. ‘Cabruca’ an agroforestry system is a main cropping system invariably adapted for cultivation of cacao in southern Bahia, Brazil. In this system of management, cacao is grown under the shade of native species of the Atlantic Forest. However, growing cacao under the shade of trees is still questionable due to higher incidence of pests and diseases and difficulty of evaporation of excess soil water in high rainfall periods that often causes flooding. The objective of the current study is to evaluate the performance of two clonal genotypes of cacao, contrasting for tolerance to soil flooding (TSA 792 – tolerant and TSH 774 – intolerant), subjected to four levels of light intensity and two water regimes, control and flooded, in order to elucidate the mechanisms of tolerance to these stresses under ‘Cabruca’ environment. All the plants survived the period of flooding, however, with flooding stress visual symptoms like leaf chlorosis, lenticels hypertrophied on the stem base, and decomposition of roots were observed. Obtained results demonstrated morpho-physiological and molecular changes in response to low light intensity and low light intensity×soil flooding interaction. Significant effects (p<0.05) of isolated factors, double interaction and inter- and intra-genotypic differences (p<0.05) were observed for leaf gas exchange, chlorophyll fluorescence, and growth parameters. Differences among light intensities, water regime, plant organ (p<0.001), and for the interactions low light intensity×water regime and water regime×plant organ (p<0.01) were observed with respect to total soluble sugar content. Isolated factors and double and triple interactions (p<0.01) had significant effects on the starch accumulation. Triple interaction was also observed in the expression of genes coding for ADH, LDH and PDC enzymes and for aquaporin PIP1;2. Low light intensity interacts with soil flooding, limiting the damages caused by flooding in cacao plants under lowest light intensity. Flooding was the dominant factor for the most physiological responses observed during the interaction between light intensity and soil flooding whose physiological adjustments were different amoung the clonal cocoa genotypes assessed.