Water Stress Promotes Secondary Sexual Dimorphism in Ecophysiological Traits of Papaya Seedlings

Abstract

Plant genders could express different functional strategies to compensate for different reproductive costs, as females have an additional role in fruit and seed production. Secondary sexual dimorphism (SSD) expression is frequently greater under stress than under optimal growth conditions. The early gender identification in papaya may help to reduce orchard costs because the most desirable fruit shape is formed by hermaphrodite plants. We hypothesized that (a) gender ecophysiological phenotyping can be an alternative to make gender segregations in papaya seedlings, and (b) such gender segregation will be more efficient after a short drought exposure than under adequate water conditions. To test such hypotheses, seedlings of two papaya varieties ('Candy' and 'THB') were exposed to two kind of treatments: (1) water shortage (WS) for 45 h, after which they were well watered, and (2) continuously well-watered (WW). Study assessed the ecophysiological responses, such as stomatal conductance (g_s), SPAD index, optical reflectance indices, morphological traits, and biomass accumulation in females (F) and hermaphrodites (H). In WS treatment, the SSD was expressed in 14 of 18 traits investigated, while in WW treatment, the SSD was expressed only in 7 of 18 traits. As tools for SSD expression, g_s and simple ratio pigment index (SRPI) must be measured on the first or second day after the imposed WS was interrupted, respectively, while the other parameters must be measured after a period of four days. In some traits, the SSD was expressed in only one variety, or the response of H and F plants were of opposite values for two varieties. The choice of the clearest responses of gender segregation in WS treatment will be greenness index, combination of normalized difference vegetation index (CNDVI), photochemical reflectance index (PRI), water band index (WBI), SRPI, leaf number, leaf dry mass, and leaf mass ratio. If the WW conditions are maintained for papaya seedling production, the recommendation in gender segregation will be the analysis of CNDVI, carotenoid reflectance index 2 (CRI2), WBI, and SRPI. The non-destructive optical leaf indices segregated papaya hermaphrodites from females under both water conditions and eventually could be adjusted for wide-scale platform evaluations, with planned space arrangements of seedlings, and sensor's set.

Keywords: Carica papaya L.; SPAD; SSD; biomass; gender segregation; morphology; optical leaf properties; stomatal conductance.

Figure 1

Stomatal conductance (g_s) observed in the period from the first to fourth day after the water-shortage interruption, corresponding to 42 to 45 days after sowing (DAS). Seedlings of two varieties (‘Candy’ and ‘THB’) of Carica papaya, including two genders (female: F and hermaphrodite: H), were exposed to well-watered (WW) and water-shortage (WS) conditions from 39 to 41 DAS, and, afterward, all seedlings were well-irrigated. Estimated means ± SE and p-values (bold when significant) are shown (n = 20). Uppercase letters compare water treatments (Wat) for each variety and gender, for each DAS; lowercase letters compare variety (Var) responses for each water treatment and gender, for each DAS; superscripted numbers compare gender (Gen) responses in each water treatment and variety, for each DAS.

Figure 2

SPAD and spectral optical indices associated with Chl content observed in the period of the first to fourth day after the water-shortage interruption, corresponding to 42 to 45 days after sowing (DAS): (A) SPAD, (B) greenness index, and (C) combination of normalized difference vegetation index (CNDVI). Seedlings of two varieties (‘Candy’ and ‘THB’) of Carica papaya, including two genders (female: F and hermaphrodite: H), were exposed to well-watered (WW) and water-shortage (WS) conditions from 39 to 41 DAS, and, afterward, all seedlings were well-irrigated. Estimated means ± SE and p-values (bold when significant) are shown (n = 20). Uppercase letters compare water treatments (Wat) for each variety and gender, for each DAS; lowercase letters compare variety (Var) responses for each water treatment and gender, for each DAS; superscripted numbers compare gender (Gen) responses in each water treatment and variety, for each DAS.

Figure 3

Leaf reflectance indices associated with the pigment contents observed in the period of the first to fourth day after the water-shortage interruption, corresponding to 42 to 45 day after sowing (DAS): (A) carotenoid reflectance index 1 (CRI1), (B) carotenoid reflectance index 2 (CRI2), and (C) flavonoid reflectance index (FRI). Seedlings of two varieties (‘Candy’ and ‘THB’) of Carica papaya, including two genders (female: F and hermaphrodite: H), were exposed to well-watered (WW) and water-shortage (WS) conditions from 39 to 41 DAS, and, afterward, all seedlings were well-irrigated. Estimated means ± SE and p-values (bold when significant) are shown (n = 20). Uppercase letters compare water treatments (Wat) for each variety and gender, for each DAS; lowercase letters compare variety (Var) responses for each water treatment and gender, for each DAS; superscripted numbers compare gender (Gen) responses in each water treatment and variety, for each DAS.

Figure 4

Leaf reflectance indices associated with the water status observed in the period from the first to fourth day after the water-shortage interruption, corresponding to 42 to 45 days after sowing (DAS): (A) photochemical reflectance index (PRI) and (B) water band index (WBI). Seedlings of two varieties (‘Candy’ and ‘THB’) of Carica papaya, including two genders (female: F and hermaphrodite: H), were exposed to well-watered (WW) and water-shortage (WS) conditions from 39 to 41 DAS, and, afterward, all seedlings were well-irrigated. Estimated means ± SE and p-values (bold when significant) are shown (n = 20). Uppercase letters compare water treatments (Wat) for each variety and gender, for each DAS; lowercase letters compare variety (Var) responses for each water treatment and gender, for each DAS; superscripted numbers compare gender (Gen) responses in each water treatment and variety, for each DAS.

Figure 5

Leaf reflectance indices associated with the structure observed in the period from the first to fourth day after the water-shortage interruption, corresponding to 42 to 45 days after sowing (DAS): (A) structure-insensitive pigment index (SIRI) and (B) simple ratio pigment index (SRPI). Seedlings of two varieties (‘Candy’ and ‘THB’) of Carica papaya, including two genders (female: F and hermaphrodite: H), were exposed to well-watered (WW) and water-shortage (WS) conditions from 39 to 41 DAS, and, afterward, all seedlings were well-irrigated. Estimated means ± SE and p-values (bold when significant) are shown (n = 20). Uppercase letters compare water treatments (Wat) for each variety and gender, for each DAS; lowercase letters compare variety (Var) responses for each water treatment and gender, for each DAS; superscripted numbers compare gender (Gen) responses in each water treatment and variety, for each DAS.

Figure 6

Morphological traits and biomass accumulation in seedlings observed at the end of experiment: (A) leaf number, (B) basal stem diameter, (C) leaf dry mass, (D) stem dry mass, (E) root dry mass, (F) total dry mass, (G) leaf mass ratio (fraction of dry mass retained in the leaves in relation to the total dry mass), and (H) root-to-shoot ratio. Seedlings of two varieties (‘Candy’ and ‘THB’) of Carica papaya, including two genders (female: F and hermaphrodite: H), were exposed to well-watered (WW) and water-shortage (WS) conditions from 39 to 41 days after sowing, and, afterward, all seedlings were well-irrigated. Estimated means ± SE and p-values (bold when significant) are shown (n = 20). Uppercase letters compare water treatments (Wat) for each variety and gender; lowercase letters compare variety (Var) responses for each water treatment and gender; superscripted numbers compare gender (Gen) responses in each water treatment and variety.

Figure 7

Mean daily photosynthetic photon flux density (PPFD, µmol m⁻² s⁻¹), air temperature (° C), relative humidity (RH, %), and air vapor pressure deficit (VPD_air, kPa) inside the greenhouse during the pre-experimental period (A, B, C, and D, respectively), followed by a short experimental period, in greenhouse with additional net shading (E, F, G, and H, respectively). The red and blue flashes indicate the beginning and the end of the period of water suspension, which induced water stress in papaya seedlings.

Figure 8

The experimental procedure scheme: (1) Papaya seeds of two varieties; ‘Candy’ and ‘THB’ were seeded (2) in tubes on day after sowing (DAS) = 0. (3) The determination of plant genders was conducted on DAS 29, segregating female from hermaphrodite plants. Steps 1–3 were executed in the greenhouse. (4) The seedlings were transferred under one net (additionally reducing the PPFD by 70%) in the greenhouse on 38 DAS, with the last water spraying effectuated at 5:00 p.m. on 39 DAS in water-shortage (water stress, WS) treatment. The water supply was suspended for 45 h in WS treatment, to 2:00 p.m. on 41 DAS. (5–6) Evaluations of leaf reflectance indices, SPAD index, and (7) stomatal conductance were performed from 42 to 45 DAS, followed by (8) evaluations of morphology and biomass.