How does shading mitigates the water deficit in young Hymenaea courbaril L. plants?

Abstract

Information on tolerance to isolated or combined abiotic stresses is still scarce for tree species, although such stresses are normal in nature. The interactive effect of light availability and water stress has been reported for some native tree species in Brazil but has not been widely investigated. To test the hypothesis that shading can mitigate the stressful effect of water deficit on the photosynthetic and antioxidant metabolism and on the growth of young Hymenaea courbaril L. plants, we evaluated the following two water regimes: a) continuous irrigation - control (I) - 75% field capacity. and b) water deficit (S), characterized by irrigation suspension associated the two following periods of evaluation: P0 - when the photosynthetic rate of plants subjected to irrigation suspension reached values ​​close to zero, with the seedlings being re-irrigated at that moment, and REC - when the photosynthetic rate of the re-irrigated plants of each shading levels reached values ​​similar to those of plants in the control treatment, totaling four treatments: IP0, SP0, IREC, and SREC. The plants of these four treatments were cultivated under the four following shading levels: 0, 30, 50, and 70%, constituting 16 treatments. Intermediate shading of 30 and 50% mitigates the water deficit and accelerates the recovery of H. courbaril. Water deficit associated with cultivation without shading (0%) should not be adopted in the cultivation or transplantation of H. courbaril. After the resumption of irrigation in the REC, the other characteristics presented a recovery under all cultivation conditions. Key message: Intermediate shading of 30 and 50% mitigates the water deficit and accelerates the recovery of H. courbaril.

Keywords: antioxidant enzymes; chlorophyll a fluorescence; photosynthesis; proline; stresses abiotic.

Figure 1

Mean values of the photosynthetic rate A (A), stomatal conductance Gs (B), internal CO₂ concentration C _i (C, D), instantaneous water use efficiency A/E (E), and instantaneous efficiency of carboxylation of CO₂ A/C _i (F) of Hymenaea courbaril L. seedlings under different water regimes + evaluation periods: irrigated at zero photosynthesis (IP0), stressed at zero photosynthesis (SP0); irrigated in recovery (IREC), stressed in recovery (SREC) under different shading levels: 0, 30, 50, and 70%. Capital letters compare the same water regime + evaluation period in the different shading levels. Lowercase letters compare the same shading level in the different water regimes + evaluation periods. Means followed by different letters indicate significant differences by the Tukey`s test (p<0.05) (n= 3).

Figure 2

Mean values ​​of Chlorophyll a (A), Chlorophyll b (B), total Chlorophyll (C) and carotenoids (D) (µg cm²) of Hymenaea courbaril L. seedlings under different shading levels: 0, 30, 50, and 70%. Means followed by different letters indicate significant differences by the Tukey`s test (p<0.05) (n= 3).

Figure 3

Mean values ​​of initial fluorescence - F₀ (A, B), quantum efficiency of photosystem II - F_v/F_m (C, D), efficiency of conversion of absorbed energy – Fv/F0 (E, F), and ineffective photochemical processes of photosystem II - F0/Fm (G, H) in seedlings of Hymenaea courbaril L. under different water regimes + evaluation periods: irrigated at zero photosynthesis (IP0), stressed at zero photosynthesis (SP0); irrigated in recovery (IREC), stressed in recovery (SREC) under the following different shading levels: 0, 30, 50 and 70%. Means followed by different letters indicate significant differences by the Tukey`s test (p<0.05) (n= 3).

Figure 4

Mean values ​​of protein in the leaf (A, B), activity of the enzymes in leaf superoxide dismutase (SOD) (C, D), peroxidase (POX) (E) and root (F) of Hymenaea courbaril L. seedlings under different water regimes+ evaluation periods: irrigated at zero photosynthesis (IP0), stressed at zero photosynthesis (SP0); irrigated in recovery (IREC), stressed in recovery (SREC) under the following different shading levels: 0, 30, 50, and 70%. Capital letters compare the same water regime + evaluation periods in the different shading levels. Lowercase letters compare the same shading level in the different water regimes evaluation periods. Means followed by different letters indicate significant differences by the Tukey`s test (p<0.05) (n= 3).

Figure 5

Mean Values ​​of Proline content (A, B) and Relative water content - RWC (C) in seedlings of Hymenaea courbaril L. under different water regimes+ evaluation periods: irrigated at zero photosynthesis (IP0), stressed at zero photosynthesis (SP0); irrigated in recovery (IREC), stressed in recovery (SREC) under the following different shading levels: 0 30, 50, and 70%. Means followed by different letters indicate significant differences by the Tukey`s test (p<0.05) (n= 3).

Figure 6

Mean values ​​of root lengths (CR) (A, B), leaf area (AF) (C, D) and DQI (E, F) of Hymenaea courbaril L. seedlings under different water regimes+ periods of evaluation: irrigated at zero photosynthesis (IP0), stressed at zero photosynthesis (SP0); irrigated in recovery (IREC), stressed in recovery (SREC) under the following different shading levels: 0, 30, 50, and 70%. Means followed by different letters indicate significant differences by the Tukey`s test (p<0.05) (n= 3).