Partial rootzone drying (PRD) and regulated deficit irrigation (RDI) effects on stomatal conductance, growth, photosynthetic capacity, and water-use efficiency of papaya

Scientia Horticulturae 183 (2015) 13–22

Contents lists available at ScienceDirect

Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti

Partial rootzone drying (PRD) and regulated deficit irrigation (RDI) effects on stomatal conductance, growth, photosynthetic capacity, and water-use efficiency of papaya夽 Roberta Samara Nunes de Lima a , Fábio Afonso Mazzei Moura de Assis Figueiredo a , Amanda Oliveira Martins a , Bruna Corrêa da Silva de Deus a , Tiago Massi Ferraz a , Mara de Menezes de Assis Gomes a , Elias Fernandes de Sousa b , David Michael Glenn c , Eliemar Campostrini a,∗ a Setor de Fisiologia Vegetal, LMGV, Centro de Ciências e Tecnologias Agropecuárias, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000, CEP: 28013620 Campos dos Goytacazes, RJ, Brazil b Setor de Irrigac¸ão e Drenagem, LEAG, Centro de Ciências e Tecnologias Agropecuárias, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000, CEP: 28013620 Campos dos Goytacazes, RJ, Brazil c USDA-ARS, Appalachian Fruit Research Station, 2217 Wiltshire Road, Kearneysville, WV 25430, USA

a r t i c l e

i n f o

Article history: Received 14 August 2014 Received in revised form 26 November 2014 Accepted 1 December 2014 Keywords: Carica papaya Soil moisture Root

a b s t r a c t Papaya (Carica papaya, L.) is an important economic crop in tropical and subtropical countries. It is a giant herbaceous species; maintaining adequate tissue turgidity and water availability is necessary to maintain the rigidity of the stem as well as increase productivity. Water-saving irrigation techniques for papaya will be needed in the future due to the possibility of water shortages related to climate change. The morphological and physiological responses of ‘Grand Golden’ papaya to partial root drying (PRD), regulated deficit irrigation (RDI) and no irrigation followed by re-hydration (NI-gh) in a greenhouse study or no irrigation (NI-field) in a field study were compared with full irrigation (FI). In the greenhouse study, plants were grown in pot with roots split equally between two soil columns. In the field study, drip emitters on opposite sides of the plant delivered irrigation water. In FI, the whole root system was irrigated at 100% of water use; in RDI 50% or 70% of FI water use was supplied to the whole root system (greenhouse and field study, respectively); in PRD 100% of FI was supplied to half the root zone while the other was allowed to dry to 50% or 70% of FI (greenhouse, and field study, respectively). In the field study, PRD was achieved by applying different amounts of irrigation water to alternate sides of the root system within the plant row. The application of 50% water use in PRD and RDI in the greenhouse study decreased shoot and root dry weight production, with a more pronounced effect on root dry weight compared to FI. This decrease in biomass was associated with a decrease in the net photosynthetic rate in the day of most intense water stress for the plants in NI-gh. In the field study, a 30% water deficit in both PRD and RDI treatments did not significantly reduce vegetative growth or yield components (number fruit plant−1 , average weight (g) fruit−1 , kg fruit ha−1 , kg fruit plant−1 ), compared to FI. In greenhouse conditions there was evidence of a non-hydraulic signal in the PRD treatments decreasing Gs compared to RDI at comparable soil water tension but it was insufficient to affect shoot growth or yield components in field conditions. There was no difference in the instantaneous leaf water use efficiency (WUE, A/E) of PRD or RDI treatments in the greenhouse or the agronomic water use efficiency (AWUE) (kg fruit L−1 and number fruit L−1 ) in the PRD and RDI treatments in the field but both treatments improved AWUE compared to FI. It appears that papaya can tolerate some water deficits without a significant reduction in yield components indicating that <100% ET0 irrigation replacement can be scheduled but there is little or no benefit to PRD. © 2014 Published by Elsevier B.V.

夽 Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the U.S. Dept. of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable. USDA is an equal opportunity provider and employer. ∗ Corresponding author. Tel.: +55 2227397105. E-mail addresses: [email protected], [email protected] (E. Campostrini). http://dx.doi.org/10.1016/j.scienta.2014.12.005 0304-4238/© 2014 Published by Elsevier B.V.

14

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22

1. Introduction Papaya is widely grown in several countries located in tropical and subtropical regions (Campostrini et al., 2010). Asia is the largest papaya-producing continent, providing 55.5% of the total production, followed by South America (23.0%) and Africa (13.2%) (FAOSTAT, 2012). Gaining in popularity worldwide, papaya is now ranked third with 11.22 Mt, or 15.36% of the total tropical fruit production, behind mango with 38.6 Mt (52.86%) and pineapple with 19.41 Mt (26.58%) (Evans and Ballen, 2012). Papaya commercialization significantly contributes to the economy of developing countries and the revenue generated by papaya domestic trade and export is of great importance for thousands of people in Asia and Latin America. Between 2007 and 2009, three countries, Mexico (40.9%), Brazil (11.2%), and Belize (11.2%), were responsible for 63.3% of the total papaya fruit export (Evans and Ballen, 2012). From 2008 to 2010, the largest papaya-producing countries were India, Brazil and Indonesia, with 38.6, 17.5 and 6.9% of the total production, respectively (FAOSTAT, 2012). Overall, worldwide production of papaya has been increasing approximately 372,000 t year−1 (FAOSTAT, 2012). Adequate irrigation is essential for maintaining or increasing production in the current production areas where there is increasing competition for water resources demanding increased water use efficiency and reduced environmental impact. Studies of papaya related to water deficits are of extreme importance because these deficits affect productivity and quality (Campostrini and Glenn, 2007; Campostrini et al., 2010). A soil water deficit can decrease stomatal conductance (Gs ) in papaya, mediated by hydraulic or non-hydraulic signals (Campostrini et al., 2010; Mahouachi et al., 2007; Marler et al., 1994) resulting in decreased photosynthesis and reduced fruit yield (Campostrini et al., 2010). Water stress for 34 days (of 6-month-old plants in 70 L containers) decreased growth, promoted leaf abscission, and reduced net photosynthesis of papaya plants (Mahouachi et al., 2006). In two papaya cultivars (‘Golden’ and ‘UENF/Caliman 01’) grown for 24 days under water stress (3-month-old plants in 12 L pots), a soil water deficit (20% soil water content) decreased leaf water potential, leaf relative water content, growth, and stomatal conductance (Torres-Netto, 2005). Furthermore, following the 24 days of water stress (considered severe stress), the decrease in soil water availability in the pots caused a significant decrease in the maximal photochemical efficiency of photosystem II (PSII) and photochemical quenching, indicating a negative, non-stomatal effect of water stress on the photochemical metabolism of papaya leaves under the experimental conditions. Marler and Mickelbart (1998) reported a negative effect of soil water deficit on the photosynthesis of papaya plants under field conditions which was associated with reduced Gs . These authors also reported that this negative effect was highly dependent on the time of day, cloudiness, plant size, and the speed of water-stress establishment. Malo and Campbell (1986) and Almeida et al. (2003) showed that soil water content is one of the most important factors controlling papaya fruit productivity in commercial plantations. In addition, papaya does not contain wood, however, this herbaceous plant is able to grow up to 10 m height. Thus, turgor pressure between 0.82 and 1.25 kPa is essential for flexural rigidity of the entire stem (Kempe et al., 2013). The optimization of water resources is needed in the planning of irrigation models enabling higher water-use efficiency (WUE) in papaya. Currently, irrigation is recommended during the whole crop cycle to increase the fruit production (Malo and Campbell, 1986). Given the future scenario of water availability and with water for irrigation expected to become increasingly less available in several regions of the planet due to

global climate changes (Jensen et al., 2010), strategies to reduce irrigation are necessary. It is important to develop irrigation techniques that allow maximization of the WUE of papaya, without compromising photosynthesis and ultimately plant growth and productivity. The application of water to a portion of a plant’s root system is now a common practice in tree fruit production areas throughout the world (Glenn, 2000). Regulated deficit irrigation (RDI) and partial rootzone drying (PRD) have been reported to be promising techniques for increasing WUE in several crops (Kang et al., 2002; Gu et al., 2004). RDI and PRD are water-saving alternatives because they have been observed to result in WUE increases without decreasing productivity. RDI aims at reducing the water supply to 50% of the potential evapotranspiration from the entire root system. PRD consists of irrigating only half of the root system, based on the theoretical assumption that the irrigated root half would maintain the water status of the plant, whereas the nonirrigated half would send chemical signals to the shoot via xylem to reduce Gs (Stoll et al., 2000). In PRD, the two halves of the root are alternately irrigated. The root half that is watered supplies most of the water to the plant (maintaining a high water potential), while the root half experiencing moderate water stress produces chemical signals that reduce Gs and transpiration (Davies et al., 2002; Kang and Zhang, 2004; Schachtman and Goodger, 2008). Studies have showed that the application of PRD resulted in stomatal closure compared to a fully irrigated treatment, with the stomatal conductance decrease related to the plant’s sensitivity to drought and on the intensity of the water stress but without a concomitant decrease in the net photosynthesis rate (Costa et al., 2007; Ahmadi, 2009; Ahmadi et al., 2010; Du et al., 2010). However, other studies showed that PRD might decrease the net photosynthesis rate (Liu et al., 2006b; Kirda et al., 2005; Shao et al., 2010; Yuan et al., 2013). This contrast in results may be due to the use of different species and to different climatic conditions (Zegbe and Behboudian, 2008). Although PRD is used for several crops (Sepaskhah and Ahmadi, 2010), there are no reports regarding the use of PRD or RDI in papaya or the effects of these treatments on photosynthesis (stomatal and non-stomatal effects) and growth of this plant species. Soil water deficits can lead to stomatal and non-stomatal limitations of photosynthesis. Within the non-stomatal effects, little is known of the effects of water deficit on the structure and activity of PSII under PRD and RDI conditions. The activity of PSII can be investigated in vivo through chlorophyll a fluorescence measurements and the analysis of fast fluorescence kinetics (Strasser and Tsimilli-Michael, 2001). The perfect functioning of the photosynthetic machinery depends on an adequate activity of PSII, which can be evaluated through measurements of chlorophyll a fluorescence emission. The optimal PSII activity and structure allows the adequate supply of ATP and NADPH, which are fundamental for the functioning of the Calvin–Benson cycle. It is therefore expected that the PRD and RDI irrigation techniques may allow a reduction of water supply to crops without compromising the PSII photochemical efficiency. A recent study showed that the PSII activity of apple trees, evaluated through the analysis of fast fluorescence kinetics, was slightly reduced in plants subjected to PRD (Yuan et al., 2013). For papaya, in addition to the limited knowledge of stomatal effects on photosynthetic carbon assimilation resulting from the application of PRD and RDI, little is known about the non-stomatal effects, associated with the PSII, of these techniques. Understanding the effects of PRD and RDI on physiological characteristics of papaya associated with photosynthetic efficiency and growth may provide irrigation strategies to increase the WUE of this species. This knowledge will allow growers to decrease detrimental effects of water stress on the production of this important tropical fruit species through the use of different techniques for

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22 Table 1 Total volume of water applied, and volume applied per day of greenhouse-grown papaya (Carica papaya L.) in splitroot pots with four different irrigation treatments: full irrigated (FI), Partial Rootzone Drying (PRD), Regulated Deficit Irrigation (RDI), and non-irrigated followed by 6 days of FI (NI-gh). Total volume of water applied (L)

Volume of water applied per day (L)

FI PRD RDI NI-gh

47.50 23.8 23.8 21.3

2.3 1.1 1.1 1.0

supplying water to the roots while potentially reducing water use. The goal of the present work was to study the effects of water stress associated with PRD and RDI on the growth, gas exchange, and WUE in young plants of papaya ‘Grand Golden’ in a greenhouse and field study. 2. Materials and methods 2.1. Greenhouse study: Material and growth conditions The study was performed in a greenhouse at the Universidade Estadual do Norte Fluminese—UENF, Campos dos Goytacazes, Rio de Janeiro (RJ) (21◦ 44 47 S and 41◦ 18 24 W), Brazil from December 2011 to April 2012, for a total of 111 days from sowing until the end of the experiment. Seeds were obtained from ‘Grand Golden’ papaya fruits (Caliman Agricola S/A), and sown in small pots on 12/14/2011. Forty eight days after sowing, seedlings were transplanted into two 15-L polyvinyl chloride (PVC) pots, connected to each other (the pots were placed side by side and were tied using a wire), with the root split equally between the two pots (the tap root was cut about 20 mm of the collar region), such that water exchange between the two compartments was prevented. To avoid heating of the pot, the pots were wrapped in a reflective aluminized blanket that also covered the soil surface to prevent soil water evaporation. The substrate in the pots was soil, sand, and cattle manure (2:1:2). On the day after seedling transplantation, 53 g pot−1 (∼3 g L−1 ) of Osmocote® fertilizer, containing nitrogen, phosphorus, and potassium (14:14:14), were added. The plants were kept at field capacity (FC) until they were 96 days old. Thereafter, the following treatments were applied: T1—fully irrigated (FI): both sides of the split root were kept at 100% FC, and water was applied daily; T2—severe stress (NI-gh): irrigation was interrupted on both sides of the split root for 14 days, following which the plants were again irrigated for six days; T3—partial rootzone drying (PRD): water was initially applied daily to one side of the root (100% FC), and no irrigation was applied to the other side of the root (100% FC applied to each root half). Beginning seven days after initiating treatments (DAT) (103-day-old plants), irrigation was alternated between the two root sides; T4—regulated deficit irrigation (RDI): both sides of the split root were simultaneously irrigated at 50% FC (i.e., 50% FC applied to each root half). All the treatments (ten replicates for each treatment) were irrigated for 20 days, except NI-gh. NI-gh remained without water for 14 days (110 days old) and was then irrigated for six days, at 100% FC, on both sides of the root. The volume of irrigation to the 15 L pots in all treatments during the experiment was recorded (Table 1).

3-19 3-20 3-21 3-22 3-23 3-24 3-25 3-26 3-27 3-28 3-29 3-30 3-31 4-1 4-2 4-3 4-4 4-5 4-6 4-7 4-8 4-9

Day of the year

0 20 Soil water tension (-kPa)

Treatment

15

40 60 80 100 120 140 160 180 200

Full irrigation (FI) Non-irrigated followed by irrigation (NI-gh) Partial rootzone drying 1 (PRD1) Partial rootzone drying 2 (PRD2) Regulated deficit irrigation (RDI)

Fig. 1. Daily soil water potential (−kPa) planted with ‘Grand Golden’ papaya with four irrigation treatments. PRD-1 and PRD-2 indicate the two sides of the PRD pots.

amount of water applied and the drained volume represented the amount of water stored in the soil (volume required to bring the soil moisture to field capacity). The treatment that received this amount of water represented the control treatment (FI). This procedure was done daily in the late afternoon. Each day, the volume of water required to maintain the soil moisture at field capacity represented the consumption of water by the plant during the previous day. For all the treatments, the water potential on both sides of the root system was monitored using 6450 WD sensors (Watermark, Spectrum Technologies, Aurora, Illinois, USA) placed at a 15-cm depth and at approximately 13 cm from the center of the pot (Fig. 1). The sensors were coupled to a Watchdog 200 data logger (Spectrum Technologies, Inc., Illinois, USA), and the data were stored every 30 min throughout the day. Soil moisture data indicated that soil dehydration began at 3 DAT for both the NI-gh and PRD treatments. The highest soil water tension was observed for treatment NI-gh (−180 kPa) at 14 DAT (110-day-old plants), considered the day of most intense water stress for the plants in this treatment. At 14 DAT, irrigation of the NI-gh treatment was resumed for six days, using the same amount of water as the control treatment. 2.3. Greenhouse study: Growth measurements The youngest leaf of each plant was selected and used for growth measurements until the end of the experiment (20 days after the beginning of the treatment application; leaf ontogeny). The measurements were performed between 700 and 900 HR every 2 days with 10 replicates. The youngest leaf growth was evaluated by measuring the central vein length (CVL) using a tape measure. At the end of the experiment, the plants were divided into shoot (leaves and stem) and root. Shoots and roots were then separately placed in paper bags, labeled, and placed in a forced air circulation oven at 70 ◦ C for 72 h. Total root (TDW) (sum of both root sides of the split root), leaf (LDW), and stem (SDW) dry weights were measured. The total root volume (TRV) was estimated by immersing the roots in a 2-L graduated cylinder containing a known volume of distilled water and measuring the volume of water displaced by the root fresh mass.

2.2. Greenhouse study: Soil moisture To estimate FC, 3 plants in the experiment were used. Initially, the plants were irrigated with a known amount of water sufficient to saturate the soil. After complete drainage (≈30 min), the volume of water drained was measured. The difference between the

2.4. Greenhouse study: Gas-exchange and chlorophyll fluorescence measurements Gas-exchange [net photosynthesis (A), Gs , transpiration (E), and leaf-to-air vapor-pressure deficit (VPDleaf–air )] was measured

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22

Temperature (°C)

Min Temp

average Temp

Max Temp

Relative humidity

35

80

30

70 60

25

50

20

40 15 Planting 1ª Harvest

10

2ª Harvest

30 20

1ª Sampling

Treatments applied

5

2ª Sampling

Relative Humidity (%)

16

10 0

0

Month

Rainfall (mm)

2ª Sampling 275 250 225 200 175 150 125 100 75 50 25 0

Treatments applied Planting

1ª Sampling 2ª Harvest 1ª Harvest

Mon th Fig. 2. Meteorological variables in a field study.

with approximately 392 ± 8.7 ␮mol mol−1 CO2 concentration in the leaf cuvette, 34.5 ± 1.64 ◦ C temperature, and 60 ± 8.7% relative humidity using a Licor 6400 System (Licor, Lincoln, NE, USA). The measurements were performed on ten plants per treatment, every two days after planting, using two fully expanded leaves per plant. The 3rd or 4th leaf from the apex were selected using different leaves than those used for the leaf ontogeny measurements. These leaves would be expected to have maximum A (Wang et al., 2014). A 1500 ␮mol m−2 s−1 light intensity was used on a 6 cm2 leaf surface, using an artificial lighting system composed of a mix of blue and red light-emitting diodes coupled to the equipment. Measurements were performed between 700 and 900 HR Instantaneous leaf (WUE) (A/E) and intrinsic leaf (iWUE) (A/Gs ) were calculated using the A, Gs , and E values measured. At 14 DAT chlorophyll fluorescence emission was measured at 600 HR and 1200 HR in all treatments using a fluorimeter model Pocket PEA (Hansatech, England) with minicuvettes provided by Hansatech Company (Norfolk, England). The leaf tissue was dark adapted for 30 min so that all the reaction centers were in the open state (Qa oxidized) (Bolhàr-Nordenkampf et al., 1989). 2.5. Greenhouse study: Micrometeorological measures During the experiment, micrometeorological variables [maximum photosynthetically active radiation maximum (PARmax )

(1000 ␮mol m−2 s−1 ); temperature maximum (40 ◦ C), average (27 ◦ C), minimum (22 ◦ C); relative humidity maximum (98%), average (80%), and minimum (50%); maximum vapor pressure deficit (VPDmax ) (2.2 kPa), average (0.8 kPa) and minimum (0.2 kPa)] inside of greenhouse were monitored using Watchdog data loggers (Spectrum Technologies, Aurora, Illinois, USA). 2.6. Field study: Plant material and growth conditions The field study was conducted at Caliman Agrícola, Espírito Santo state, Linhares, Brazil (19◦ 23 28 S, 40◦ 04 20 W) from March 2011 to October 2012. Papaya plants, ‘Grand Golden’ were planted in a 1.5 × 3.6 m pattern. The following treatments were established to 5-month-old plants: Control: 100% ET0 was applied on both sides of the root with 1 drip line and 2 emitters per plant (0.75 m emitter spacing). Emitter flow was 2.3 L h−1 (total flow: 4.6 L plant−1 ); PRD100: 100% ET0 was applied to one side only of the root, and every 15 days, water was applied to the alternate side of the root system. Water was applied with 2 drip lines and 2 emitters per plant (0.75 m emitter spacing). Emitter flow was 2.3 L h−1 (total flow: 4.6 L plant−1 ); PRD70: 70% ET0 was applied to one side only of the root, and every 15 days, water was applied to the alternate side of the root system to allow always a part of the root system experience a mild water stress. Water was applied with 2 drip lines and 2 emitters per plant (0.50 m emitter spacing). Emitter flow was

s 1  U2 (es − ea ) ET0 = (Rn − G) +  (s +  ∗ ) (s +  ∗ ) (T + 275∗ )

14 Transpiration (mmol m-2 s-1) or water use efficiency, A/E (µmol CO2 mmol-1H2O)

1.6 L h−1 (total flow: 3.2 L plant−1 ); RDI70: 70% ET0 was applied to both sides of the root system. Water was applied with 1 drip line and 2 emitters per plant (0.50 m emitter spacing). Emitter flow was 1.6 L h−1 (total flow: 3.2 L plant−1 ); and NI-field: non irrigated received only natural rainfall. There was only one soil column in the field study with PRD treatments achieved by applying different amounts of irrigation water to opposite sides of the plant. Air temperature and relative humidity were recorded with data loggers, (Spectrum Technologies, Aurora, Illinois, USA). Calculation of VPD was based on the temperature and relative humidity during the measurement period (Jones, 1992). The meteorological station was installed 300 m the experiment, and the data were used to calculate the reference evapotranspiration (ET0 ) using the Penman equation parameterized by the United Nations Food and Agriculture Organization (FAO) (Pereira et al., 1997) (Eq. (1)). We considered that the daily balance heat flow in soil was zero (G = 0).

Full irrigation (FI) Partial rootzone drying (PRD)

During the experiment, meteorological variables were monitored using Watchdog data loggers (Spectrum Technologies, Aurora, Illinois, USA) (Fig. 2). The maximum photosynthetically active radiation maximum (PARmax ) was about 2400 ␮mol m−2 s−1 in summer (December to March) and 1400 ␮mol m−2 s−1 in winter (June to September).

20

Non-irrigated followed by irrigation (NI-gh)

10 10 8 Transpiration (E) y = 12.698x + 0.8619 R² = 0.84

6

0

-10 4

A/E y = -3.957x + 3.992 R² = 0.55

2

-20

0

-30

(1)

2.7. Field study: Micrometeorological measures

30

Photosynthesis y = -28.308x2 + 34.408x + 4.4224 R² = 0.70

Regulated deficit irrigation (RDI)

12

0

where: s is the slope of the vapor pressure curve (kPa ◦ C−1 );  * is the modified psychrometric constant (kPa ◦ C−1 ); Rn is the net radiation (MJ m−2 d−1 ); G is the heat flow in soil (MJ m−2 d−1 );  is the latent heat of evaporation (MJ kg−1 );  = psychrometric coefficient (kPa ◦ C−1 ); T is the average temperature (◦ C); U2 is the wind speed at 2 m (m s−1 ); es is the saturation vapor pressure (kPa); ea is the partial pressure of vapor (kPa).

17

Photosynthesis (µmol m-2 s-1)

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22

0.2 0.4 0.6 Stomatal conductance, Gs (mol H2O m-2 s-1)

0.8

Fig. 3. Relationship of stomatal conductance (Gs ) to transpiration (E), photosynthesis (A) and water use efficiency (A/E) for papaya treated with four irrigation regimes.

the reaction centers were in the open state (Qa oxidized) (BolhàrNordenkampf et al., 1989). 2.11. Statistical analyses The greenhouse experimental design was completely randomized, with 10 replicates per treatment. The field experimental design had 10 individual plant replicates. An analysis of variance followed by Tukey’s test, at p < 0.05, was used to compare the treatments. Regression analysis related gas exchange, soil water potential, and shoot growth. Analysis of covariance was used to test that PRD stomatal conductance related to soil water potential was significantly different from the other treatments.

2.8. Field study: Growth measurements 3. Results Yield components (number fruit plant−1 , average weight (g) fruit−1 , kg fruit ha−1 , kg fruit plant−1 ) were measured in March (11 months after planting) and June (14 months after planting). Plant height, leaf area and trunk diameter were measured in July (15 months after planting) and October (18 months after planting). 2.9. Field study: Gas exchange Instantaneous gas exchange rates were measured on one leaf from each of 4 plants per treatment using the 4th leaf fully expanded (from the apex). Gas exchange measurements were repeated 15 (july, 2012) and 18 (october, 2012) months after planting (MAP), and used a 6.0 cm2 leaf area. Net photosynthesis rate (A, ␮mol m−2 s−1 ), transpiration (E, mmol m−2 s−1 ), leaf temperature (◦ C) and stomatal conductance (Gs , mol m−2 s−1 ) were measured with a portable gas exchange system (model LI-6400; LI-COR, Lincoln, NE, USA) using 1500 ␮mol m−2 s−1 PAR at 12 h to 14 h. The reference CO2 level was adjusted to maintain a concentration of 390 ␮mol m−2 s−1 inside the leaf cuvette during all measurements. 2.10. Field study: Chlorophyll fluorescence Chlorophyll fluorescence was measured on the same attached leaf used for the gas exchange measurements, using a fluorimeter model Pocket PEA (Hansatech, England) with mini-cuvettes provided by Hansatech Company (Norfolk, England). Chlorophyll fluorescence emission was measured at 1200 and 1400 HR in all treatments. The leaf tissue was dark adapted for 30 min so that all

3.1. Greenhouse study: Soil moisture The NI-gh treatment exhibited the most negative value of soil water potential of the tested treatments, reaching approximately −180 kPa, at 14 DAT (Fig. 1). In PRD, the soil water potential approached −100 kPa on the root side that was not irrigated during both cycles. At 14 DAT, irrigation of the NI-gh treatment was resumed as was irrigation of the root side that was not being irrigated in the PRD treatment. The soil water potential for the RDI treatment was −20 kPa through 14 DAT and approximately −4 kPa for the FI treatment during the whole experiment (Fig. 1). 3.2. Greenhouse study: Gas exchange There were treatment effects on the relationship between Gs and A, E (Fig. 3). Similarly, there were no treatment effects on the relationship between A and Fv /Fm or other chlorophyll fluorescence parameters (data not presented) and Fv /Fm values remained higher than 0.75, indicating efficient PSII activity throughout the study (Bolhàr-Nordenkampf et al., 1989; Mohammed et al., 1995). Fourteen DAT (day of maximum stress) NI-gh had the highest A/Gs and A/E while FI had the lowest values for these two variables; PRD and RDI had intermediate values (Fig. 4). There were no significant treatment effects on chlorophyll fluorescence parameters at 14 DAT (data not presented). Photosynthesis, transpiration and stomatal conductance in FI were positively related to the leaf-air vapor pressure deficit (Figs. 5 and 6). However, in the PRD, RDI and NI-gh

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22 14

7

12 Transpiration, E (mmol H2O m-2 s-1)

6 y = 0.017x + 1.755 R² = 0.59 A/E (µmol CO2 mmol-1 H2O)

5

4

3

FI PRD

10 E for FI y = 4.7684x - 1.5617 R² = 0.50

8

3.5 3 2.5 2

6 1.5

E for PRD, RGI, and NI y = -1.0486x + 6.5503 R² = 0.25

4 Gs for FI y = 0.1303x + 0.2346 R² = 0.15

2

2

4

E, Full irrigation (FI) E, Partial rootzone drying (PRD) E, Regulated deficit irrigation (RDI) E, Non-irrigated (NI-gh) Gs, Full irrigation Gs, Partial rootzone drying Gs, Regulated deficit irrigation Gs, Non-irrigated

1

Gs for PRD, RDI, and NI y = -0.1147x + 0.5334 R² = 0.54

Stomatal conductance, Gs (mol H2O m-2 s-1)

18

0.5

RDI 0

NI-gh

1

0 0

50

100 150 A/Gs (µmol CO2 mol-1 H2O)

0 0

200

250

Fig. 4. Relationship between instantaneous leaf water use efficiency (A/E) and intrinsic leaf water use efficiency (A/Gs ) in ‘Grand Golden’ papaya 14 days after initiating treatments (day of most intense water stress for the plants in NI-gh) in 110-days-old greenhouse-grown plants in splitroot pots under four different water regimes: FI, NI-gh, PRD and RDI. Values represent the means of 0, 3, 6, 9, 12, 14, 17 and 21 days after treatment.

treatments, these gas exchange variables were negatively related to leaf-air vapor pressure deficit due to the imposed water stress. 3.3. Greenhouse study: Leaf ontogeny FI exhibited the highest CVL, reaching the highest value 13 DAT and NI-gh the lowest CVL. PRD and RDI exhibited intermediate responses of CVL over time (Fig. 7). At 2 DAT, the CVL of treatment NI-gh reached 20 cm and remained at this value until irrigation was resumed, when the vein re-initiated growth, reached 25 cm.

1

2 3 4 Leaf -air vapor pressure deficit (kPa)

6

5

Fig. 6. Relationship of leaf–air vapor pressure deficit to transpiration and stomatal conductance in papaya in four irrigation treatments in a greenhouse.

intermediate for the PRD and RDI treatments (P = 0.05) (Fig. 8). The root volumes for treatments PRD and RDI were not significantly different (350 and 360 cm3 , respectively) and were significantly intermediate between the values for the highest FI (700 cm3 ) and significantly the lowest NI-gh (250 cm3 ) similar to the dry weight values (data not presented). Net photosynthesis (A), determined on the day of maximal water stress for NI-gh (14 DAT), was highly correlated with the total dry weight (TDW) at the end of the experiment (Fig. 9). The FI treatment had the highest A and the NI-gh had the lowest A and TDW. The plants from the PRD and RDI treatments exhibited intermediate A and TDW values. ‘Grand Golden’ produced 5.7 g TDW per unit ␮mol m−2 s−1 of assimilated CO2 (Fig. 9). Stomatal conductance was curvilinearly related to soil water potential (Fig. 10) with no treatment differences except for PRD 1 and 2 after water exclusion. Analysis of covariance for stomatal conductance using soil water potential as the covariate indicated that PRD 1 and 2 were significantly different (P ≤ 0.05) from the other treatment responses. The leaf expansion rate (Fig. 11) which was also curvilinearly related to soil water potential.

3.4. Greenhouse study: Root and shoot growth 35

The total dry weight (sum of leaves, stem, and root) was significantly highest in the FI treatment, lowest in the NI-gh and

FI

NI-gh

PRD

RDI

30

Photosynthesis, A (µmol CO2 m-2 s-1)

16 14

4 3.5 3 2.5

A for PRD, RDI and NI-gh y = -2.4043x + 16.33 R² = 0.45

12

2

10 1.5 8

Gs for FI y = 0.1303x + 0.2346 R² = 0.15

6

Gs for PRD, RDI and NI y = -0.1147x + 0.5334 R² = 0.54

1 0.5

4 2

0

0

-0.5 0

1

2 3 4 Leaf -air vapor pressure deficit (kPa)

5

Central vein lenght (cm)

A for FI y = 6.1767x + 3.5648 R² = 0.68

18

A, Full irrigation A, Partial rootzone drying A, Regulated deficit irrigation A, non-irrigated Gs, Full irrigation (FI) Gs, Partial rootzone drying (PRD) Gs, Regulated deficit irrigation (RDI) Gs, non-irrigated (NI-gh)

Stomatal conductance, Gs (mol H2O m-2 s-1)

20

25 20 15 10 5 0 0

2

5

8

11

13

16

20

Days after treatment

6

Fig. 5. Relationship of photosynthesis to stomatal conductance with the leaf-air vapor pressure deficit in papaya treated with four irrigation treatments in the greenhouse.

Fig. 7. Central vein length during the ontogeny of the youngest leaf of each plant (‘Grand Golden’ papaya) in splitroot pots under four different irrigation regimes: FI, NI-gh, PRD, RDI. The narrow and broad arrows indicate the first alternating between the two root sides of the PRD pots and rehydration of NI treatment, respectively (n = 10).

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22

19

Fig. 10. Relationship between stomatal conductance and soil water potential in papaya in four irrigation treatments in the greenhouse.

Fig. 8. Biomass allocation of leaf, trunk and root (g dry weight plant-1 ) tissues in papaya treated with four irrigation regimes in the greenhouse. FI, NI-gh, PRD, RDI. Capital letters refer to mean separation between treatments. Lower case letters refer to mean separation within the three tissue components (P = 0.05).

Incremental leaf growth or A, expressed as a percentage of FI, were both linearly related to Gs for values <0.4 mol H2 O m−2 s−1 (Fig. 12). Stomatal conductance >0.4 mol H2 O m−2 s−1 was associated with values near 100% of FI. 3.5. Field study: Growth and productivity There were no treatments effects on plant diameter (Table 2). Plant height in July was greatest in the PRD70 treatment, in the NI-field and the other treatments were intermediate; however in October, there were no treatment differences. The number of fruit plant−1 , mean fruit weight and yield plant−1 were greatest in the PRD70 treatment and generally least in the NI-field treatment (Table 3). Consequently, AWUE based on both fruit weight

Fig. 11. Relationship between incremental leaf growth (% of full irrigation) and soil water potential in papaya in four irrigation treatments in the greenhouse. 180 Incremental leaaf growth or photosynthesis(A) (% of full irrigation)

FI PRD

NI-gh RDI

200

150 Dry weight (g)

Full irrigation (FI) Partial rootzone drying (PRD)

250

100

50

160

Regulated deficit irrigation (RDI) Non-irrigated (NI-gh)

140

A, Full irrigation A, Partial rootzone drying

120

A, Regulated deficit irrigation A, Non-irrigated

100 80 60 40

y = 226.88x + 6.8987 R² = 0.77

20 0

y = 5.307x + 47.75 R² = 0.666

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Stomatal conductance, Gs (mol H2O m-2 s-1)

0 0

5

10 15 20 Photosynthesis (µmol m-2 s-1)

25

Fig. 9. Relationship of photosynthesis to dry weight 14 days after initiating treatments (the day of most intense water stress for the plants in NI-gh) in 110-days-old ‘Grand Golden’ papaya plants grown in splitroot pots under four different irrigation regimes (FI, NI-gh, PRD, RDI) in a greenhouse. Values represent means of 10 replicates.

Fig. 12. Relationship of stomatal conductance (Gs ) to incremental leaf growth and photosynthesis (A) in papaya in four irrigation treatments in a greenhouse.

and number were greatest for the NI-field treatment and least for the FI treatment; the PRD70 and DI were also significantly different from the FI, while the PRD100 treatment was not significantly different from FI. In the June harvest, all treatments had greater yield components (number fruit plant−1 , average weight (g) fruit−1 ,

20

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22

Table 2 Effect of five irrigation treatments on stem diameter and height of papaya in a field study. July (2012) and October (2012). Treatmenta

Sampling Time July

RDI FI NI-field PRD100 PRD70

Table 4 Effect of 5 irrigation treatments on photosynthesis (A), stomatal conductance (Gs ) and transpiration (E) of papaya in a field study. July (2012) and October (2012). Treatmenta

Sampling

A (␮mol m−2 s−1 )

Gs (mol m−2 s−1 )

E (mmol m−2 s−1 )

RDI FI NI-field PRD100 PRD70

July July July July July

RDI FI NI-field PRD100 PRD70

October October October October October

14.8 12.1 13.2 12.5 12.7 nsy 9.5abz 10.8a 6.5b 9.3ab 7.2b

0.19 0.15 0.21 0.15 0.18 ns 0.12ab 0.13a 0.06c 0.11abc 0.08bc

4.3 3.5 4.4 3.3 4.0 Ns 4.4ab 5.5a 3.2b 4.6ab 3.4b

October

Diameter (mm)

Height (m)

Diameter (mm)

Height (m)

115.91 110.52 113.76 112.80 117.85 nsy

3.48abz 3.31ab 3.08c 3.37ab 3.57a

114.32 115.35 113.87 121.35 116.88 ns

3.61 3.41 3.65 3.67 3.79 ns

a FI = full irrigation; NI-field = no irrigation after treatment initiation; PRD100 = partial root zone drying with 100% water replacement; PRD70 = partial root zone drying with 70% water replacement; RDI = regulated deficit irrigation with 70% water replacement. z Mean values within a column followed by the same letter are not significantly different (P = 0.05) based on Tukey’s multiple range test. y Non-significant difference.

kg fruit ha−1 , kg fruit plant−1 ) than NI-field, however, NI-field had the lowest AWUE instead of the highest due to the continued lack of irrigation. 3.6. Field study: Gas exchange In the July sampling there were no treatment differences in gas exchange (Table 4). However, by October, FI had the greatest A, E, and Gs while NI had the lowest values and the other treatments were intermediate. 4. Discussion Carica papaya L. produces flowers in a continuous manner from 4 to 6 months after emergence, until the end of the plant’s life. This continuous flowering makes irrigation extremely important throughout the year to attain high productivity in commercial cultivation systems. Therefore, the choice of an irrigation technique resulting in an increase in the WUE of papaya, without reducing photosynthesis, growth, and flowering, is of great importance. Although papaya may be considered tolerant to drought, irrigation is essential for papaya growth and production (Kruger and Mostert, 1999). According to Marler et al. (1994), the tolerance of papaya is associated with this species’ ability to delay dehydration through the maintenance of high leaf water content. The RDI and PRD irrigation techniques may trigger different mechanisms associated with water stress and consequently cause different responses of

a FI = full irrigation; NI-field = no irrigation after treatment initiation; PRD100 = partial root zone drying with 100% water replacement; PRD70 = partial root zone drying with 70% water replacement; RDI = regulated deficit irrigation with 70% water replacement. z Mean values within a column followed by the same letter are not significantly different (P = 0.05) based on Tukey’s multiple range test. y Non-significant difference.

photosynthesis and growth (Dodd, 2007; Liu et al., 2006a,b). In addition, the PRD and RDI can increase stomatal closure in response to conditions of high evaporative demand (Collins et al., 2010). In the greenhouse study, the application of PRD and RDI for 20 days decreased dry-mass production in all the plant organs, as well as root volume (root volume data not presented), compared to FI (Fig. 8). This pattern was confirmed by the greater sensitivity to water deficit of the central vein length, measured during leaf ontogeny (Fig. 7). The negative effects of the water deficit associated with these treatments were less pronounced on the leaves (dry matter) since they were not significantly different from FI (Fig. 8). In the field study there were no differences between PRD treatments and FI; only NI-field had significantly lower height in July and yield components (number fruit plant−1 , average fresh weight (g) fruit−1 , kg fruit ha−1 , kg fruit plant−1 ). In apple trees under field conditions with similar treatments to those tested in the present study, the accumulation of shoot dry weight significantly decreased with RDI and less extensively with PRD (Lo Bianco et al., 2012). Similar results were reported by Sun et al. (2012) for apple trees and by Guang-Cheng et al. (2011) for hot pepper. PRD had a negative effect on the allocation of photoassimilates to apple tree leaves (Lo Bianco et al., 2012). However, in papaya the root volume of PRD and RDI was reduced, compared to FI (data not presented). In citrus, PRD (50% of the control treatment) and RDI (50% of the control treatment) treatments applied did not result in significant changes

Table 3 Effect of 5 irrigation treatments on yield components and agronomic water use efficiency (AWUE) of papaya in a field study. March (2012) and June (2012). Harvest

Treatmenta

Number fruit plant−1

Average weight (g FW fruit−1 )

Yield (kg FW ha−1 )

kg FW plant−1

Irrigation + precipitation (L)

AWUE (kg FW fruit L−1 )

AWUE (number fruit L−1 )

March March March March March

FI NI-field PRD100 PRD70 RDI

30b z 33ab 38ab 41a 39ab

22,065ab 23,991b 31,290ab 33,827a 31,117ab

11.9b 13.0ab 16.9ab 18.3a 16.8ab

2698 1025 2698 2189 2189

0.0044c 0.0126a 0.0063bc 0.0084b 0.0077b

0.011c 0.032a 0.014bc 0.019b 0.018b

June June June June June

FI NI-field PRD100 PRD70 RDI

21a 6b 26a 19a 20a

409ab 391b 436ab 437a 430ab P = 0.10 244ab 191b 309a 286a 282a

9620ab 2651b 14,881 10,390a 11,145a

5.2ab 1.4b 8.0a 5.6ab 6.0ab

3755 1107 3755 2949 2949

0.0014 0.0013 0.0021 0.0019 0.0020 nsy

0.006 0.006 0.007 0.006 0.007 ns

a FI = full irrigation; NI-field = no irrigation after treatment initiation; PRD100 = partial root zone drying with 100% water replacement; PRD70 = partial root zone drying with 70% water replacement; RDI = regulated deficit irrigation with 70% water replacement. z Mean values within a column followed by the same letter are not significantly different (P = 0.05) based on Tukey’s multiple range test. y Non-significant difference.

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22

in the root fresh-to-dry weight ratios (Melgar et al., 2010). However, the total root length decreased with RDI, and the root length per unit of dry weight was greater in the dry soil portion of the PRD 50% treatment (Melgar et al., 2010). In maize (Kang et al., 1998; Wang et al., 2012) and tomato (Mingo et al., 2004), PRD increased the growth of lateral roots. In general, the literature indicates that PRD and RDI may or may not affect the production of shoot and root dry weight according to the studied species and growth conditions (greenhouse or field conditions). In papaya, the root dry weight (Fig. 8) and root volume of PRD and RDI (data not shown) decreased when compared with FI, showing that PRD and RDI negatively affected the dry-mass production of papaya plants under greenhouse conditions, however, possibly there was no effect in the field study. The total dry weight of papaya was correlated with the net photosynthesis rate 14 DAT, the day of maximum soil water deficit, (Fig. 9) (r = 0.67) indicating that dry weight production was limited primarily by the degree of soil water deficit imposed by the four treatments. The effects of PRD and RDI on net photosynthesis rate are controversial, and the literature shows a variety of plant responses. Compared to an irrigated treatment, PRD did not affect the net photosynthetic rate in apple trees (Zegbe and Behboudian, 2008), cotton (Du et al., 2008a), or grapevine (Du et al., 2008b). However, PRD decreased photosynthesis in apple trees (Yuan et al., 2013), potato (Liu et al., 2006a,b), and hot pepper (Shao et al., 2010). In grape, RDI did not decrease photosynthetic carbon assimilation (Souza et al., 2005). However, when the photosynthetic rate of whole grapevine plants was analyzed under field conditions, RDI was observed to cause a decrease in photosynthetic carbon assimilation (Tarara et al., 2011). In papaya, the net photosynthesis rate was negatively affected (Figs. 5 and 9) because the stomata of papaya leaves are extremely sensitive to soil and air water deficits (Marler and Mickelbart, 1998; Campostrini and Glenn, 2007; Campostrini et al., 2010; Mahouachi et al., 2006, 2012), and ABA concentrations increase in papaya leaves and roots under soil water deficit (Mahouachi et al., 2007, 2012) which promotes stomatal closure (Davies et al., 2002). In the field study, there were no effects on gas exchange in the July sampling but the PRD70 treatment did have lower A, E, and Gs than the FI treatment but the PRD100 treatment was equivalent to the FI (Table 4). There is evidence in this study that PRD induced stomatal closure (Fig. 10—circled data points), a non-hydraulic response, compared to RDI at comparable soil water tensions, due to the influence of the non-irrigated portion of the root zone. Analysis of covariance for stomatal conductance by treatment using soil water potential as the covariate indicated that the PRD root volume receiving water was associated with a lower Gs than the other treatments (P = 0.05) (Fig. 10). However, the reduced stomatal conductance had insufficient intensity or too short duration to reduce leaf expansion (Fig. 11). Whole plant and leaf growth and gas-exchange parameters were more sensitive to the effects of soil water deficits than the parameters related to PSII activity and chlorophyll content in greenhouse and field studies (data not shown). Leaf growth was extremely sensitive to soil water deficits in the root zone (Fig. 7). The soil water deficit (Figs. 10 and 11), in association with the leaf-to-air vapor-pressure deficit (Figs. 5 and 6), decreased stomatal conductance, which in turn decreased the CO2 supply to the Rubisco carboxylation sites and negatively affected the net photosynthesis rate (Fig. 5). There is some tolerance of the photochemical machinery to water stress since no effects were measured on the various PSII and chlorophyll parameters (data not shown). Havaux (1992) reported high resistance of PSII to leaf dehydration, which is in agreement with the present results.

21

Leaf growth and photosynthesis expressed as a percentage of FI indicated that for stomatal conductance less than approximately 0.4 mol H2 O m−2 s−1 there is a linear decrease in leaf expansion and photosynthesis (Fig. 12) with decreasing Gs . This relationship provides a key threshold that can be used to evaluate irrigation strategies. A 50% water deficit was imposed in this greenhouse study and it adversely affected gas exchange and growth, however, it may be possible to restrict water availability while maintaining stomatal conductance above a set-point of approximately 0.4 mol H2 O m−2 s−1 without a reduction in gas exchange and growth. In the field study, a 30% deficit was imposed by the PRD70 treatment resulting in a significant reduction in A, E, and Gs that exceed the 0.4 mol H2 O m−2 s−1 threshold while the same reduction in irrigation provided in the RDI treatment did not significantly differ from the FI treatment indicating that hydraulic root signals in the PRD70 treatment were effective in reducing gas exchange. However, in the field study, Gs was below the 0.4 mol H2 O m−2 s−1 threshold for all treatments (Table 4) and this was likely due to heat stress, independent of water stress. There was no difference in the WUE of PRD or RDI treatments (Fig. 4) or the AWUE in the PRD70 and RDI treatments (Table 3) but both treatments improved water use efficiency compared to FI. We hypothesized that the difference observed in physiological response to PRD and RDI treatments between the papaya grown in the greenhouse and field may be related to the different volumes of soil explored by the root system. The physiological response of papaya to PRD and RDI was more affected in greenhouse-grown than field-grown papaya because in the greenhouse study, the roots are limited to the volume of the pot. In addition, in field conditions rainfall can increase water availability in the soil and the roots have a greater volume of soil to explore. Thus, environmental variables such as VPD and PAR can more severely affect the gas exchange and growth of plants grown in the greenhouse than plants grown under field condition. In addition, PRD and RDI can increase stomatal sensitivity to VPD (Collins et al., 2010). The significant reduction of yield components in the NI-field treatment, in relation to the FI treatment, for the second harvest (June 2012) was due to a reduced rainfall period that occurred before the second harvest when there was no rainfall in May 2012. Unlike the March harvest, the PRD70 and RDI as well as the PRD100 treatments were significantly different from NI-field indicating that reduced irrigation did not reduce productivity. 5. Conclusion While there was evidence of non-hydraulic signals inducing stomatal closure in the PRD treatments compared to RDI in greenhouse studies, these effects were insufficient to alter dry matter partitioning, biomass, or yield components since there were no significant differences between PRD and RDI at either a 30% or 50% water deficit. A 50% water deficit in the greenhouse study for the PRD and RDI treatments was sufficient to significantly reduce biomass and dry matter partitioning compared to the FI treatment. In the field study, a 30% water deficit in both PRD70 and RDI treatments did not significantly reduce vegetative growth or yield components, compared to FI. It appears that papaya can tolerate moderate water deficits without a significant reduction in yield components indicating that <100% ET irrigation replacement may be scheduled but there is little or no difference between PRD and RDI. Further research will be needed to verify that moderate soil water deficits do not reduce quality. Acknowledgements The authors wish to thank Caliman Agrícola SA, Fundac¸ão de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ)

22

R.S.N. de Lima et al. / Scientia Horticulturae 183 (2015) 13–22

(E-26/101534/2010), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), and Coordenac¸ão de Aperfeic¸oamento de Pessoal de Nível Superior (CAPES). The authors thank CNPq for the fellowships awarded to E. Campostrini (303633/2011-5) and E.F. de Sousa. References Ahmadi, S.H., 2009. Agronomic and Physiological Studies of Partial Root-Zone Drying and Deficit Irrigation on Potato in Different Soil Texture. Department of Basic Sciences and Environment, Faculty of Life Sciences University of Copenhagen, Denmark, pp. 77 (Published PhD Thesis). Ahmadi, S.H., Andersen, M.N., Plauborg, F., Poulsen, R.T., Jensen, C.R., Sepaskhah, A.R., Hansen, S., 2010. Effects of irrigation strategies and soils on field-grown potatoes: gas exchange and xylem [ABA]. Agric. Water Manage. 97, 1486–1494. Almeida, F.T., Bernardo, S., de Sousa, E.F., Marin, S.L.D., Grippa, S., 2003. Growth and yield of papaya under irrigation. Sci. Agric. 60, 419–424. Bolhàr-Nordenkampf, H.R., Long, S.P., Baker, N.R., Öquist, G., Schreiber, U., Lechner, E.G., 1989. Chlorophyll fluorescence as a probe of the photosynthetic competence of leaves in the field: a review of current instrumentation. Funct. Ecol. 3, 497–514. Campostrini, E., Glenn, D.M., 2007. Ecophysiology of papaya: a review. Braz. J. Plant Physiol. 19, 413–424. Campostrini, E., Glenn, D.M., Yamanishi, O.K., 2010. Papaya: environment and crop physiology. In: da Matta, Fabio (Ed.), Ecophysiology of Tropical Tree Crops (Agriculture Issues and Policies), vol. 1, first ed. Nova Science Publishers, New York, NY, pp. 287–308. Costa, J.M., Ortuno, M.F., Chaves, M.M., 2007. Deficit irrigation as a strategy to save water: physiology and potential application to horticulture. J. Integr. Plant Biol. 49, 1421–1434. Collins, M.J., Fuentes, S., Barlow, W.R., 2010. Partial rootzone drying and deficit irrigation increase stomatal sensitivity to vapour pressure deficit in anisohydric grapevines. Funct. Plant Biol. 37, 128–138. Davies, W.J., Wilkinson, S., Loveys, B., 2002. Stomatal control by chemical signaling and the exploitation of this mechanism to increase water use efficiency in agriculture. New Phytol. 153, 449–460. Dodd, I.C., 2007. Soil moisture heterogeneity during deficit irrigation alters root-toshoot signalling of abscisic acid. Funct. Plant Biol. 34, 439–448. Du, T., Kang, S., Zhang, J., Li, F., 2008a. Water use and yield responses of cotton to alternate partial root-zone drip irrigation in the arid area of north-west China. Irrig. Sci. 26, 147–159. Du, T., Kang, S., Zhang, J., Li, F., Yan, B., 2008b. Water use efficiency and fruit quality of table grape under alternate partial root-zone drip irrigation. Agric. Water Manage. 95, 659–668. Du, T., Kang, S., Sun, J., Zhang, X., Zhang, J., 2010. An improved water use efficiency of cereals under temporal and spatial deficit irrigation in north China. Agric. Water Manage. 97, 66–74. Evans, E.A., Ballen, F.H., 2012. An overview of global papaya production, trade, and comsuption. In: Cooperative Extension Service Fruits Crops Fact Sheet FE913. University of Florida, Gainesville, FL. FAOSTAT, 2012. Crop Production, http://faostat.fao.org/site/567/default.aspx#ancor . Glenn, D.M., 2000. Physiological effects of incomplete root-zone wetting on plant growth and their implications for irrigation management. HortScience 35, 1041–1043. Guang-Cheng, S., Ruiqi, G., Na, L., Shuangen, Y., Weng-Gang, X., 2011. Photosynthetic, chlorophyll fluorescence and growth changes in hot pepper under deficit irrigation and partial root zone drying. Afr. J. Agric. Res. 6, 4671–4679. Gu, S.L., Du, G.Q., Zoldoske, D., Hakim, A., Cochran, R., Fugelsang, K., Jorgensen, G., 2004. Effects of irrigation amount on water relations, vegetative growth, yield and fruit composition of Sauvignon blanc grapevines under partial root zone drying and conventional irrigation in the San Joaquin Valley of California, USA. J. Hortic. Sci. Biotechnol. 79, 26–33. Havaux, M., 1992. Stress tolerance of photosystem II in vivo. Antagonistic effects of water, heat, and photoinhibition stresses. Plant Physiol. 100, 424–432. Jensen, C.R., Battilani, A., Plauborg, F., Psarras, G., Chartzoulakis, K., Janowiak, F., Stikic, R., Jovanovic, Z., Li, G., Qi, X., Liu, F., Jacobsen, S., Andersen, M.N., 2010. Deficit irrigation based on drought tolerance and root signalling in potatoes and tomatoes. Agric. Water Manage. 98, 403–413. Jones, H.G., 1992. Plant and microclimate: a quantitative approach to environmental plant physiology, 2nd ed. Cambridge University Press, Cambridge, UK, pp. 19–46. Kang, S.Z., Liang, Z.S., Hu, W., Zhang, J.H., 1998. Water use efficiency of controlled alternate irrigation on root divided maize plants. Agric. Water Manage. 38, 69–76. Kang, S.Z., Hu, X.T., Goodwin, I., Jerie, P., 2002. Soil water distribution, water use, and yield response to partial root zone drying under a shallow groundwater table condition in a pear orchard. Sci. Hortic. 92, 277–291. Kang, S., Zhang, J., 2004. Controlled alternate partial rootzone irrigation: its physiological consequences and impact on water use efficiency. J. Exp. Bot. 55, 2437–2446.

Kempe, A., Lautenschläger, T., Lange, A., Neinhuis, C., 2013. How to become a tree without wood-biomechanical analysis of the stem of Carica papaya L. Plant Biol. 16, 264–271. Kirda, C., Topcu, S., Kaman, H., Ulger, A.C., Yazici, A., Cetin, M., Derici, M.R., 2005. Grain yield response and N-fertiliser recovery of maize under deficit irrigation. Field Crops Res. 93, 132–141. Kruger, J., Mostert, P.G., 1999. Irrigation. In: de Villiers, E.A. (Ed.), The cultivation of papaya. ARC. LNR, South Africa, pp. 50–59. Liu, F., Shahnazari, A., Andersen, M.N., Jacobsen, S.E., Jensen, C.R., 2006a. Physiological responses of potato (Solanum tuberosum L.) to partial root-zone drying: ABA signalling, leaf gas exchange, and water use efficiency. J. Exp. Bot. 57, 3727–3735. Liu, F., Shahnazari, A., Andersen, M.N., Jacobsen, S.E., Jensen, C.R., 2006b. Effects of deficit irrigation (DI) and partial root drying (PRD) on gas exchange, biomass partitioning, and water use efficiency in potato. Sci. Hortic. 109, 113–117. Lo Bianco, R., Talluto, G., Farina, V., 2012. Effects of partial rootzone drying and rootstock vigour on dry matter partitioning of apple trees (Malus domestica cv ‘Pink Lady’). J. Agric. Sci. 150, 75–86. Mahouachi, J., Arbona, V., Gómez-Cadenas, A.G., 2007. Hormonal changes in papaya seedlings subjected to progressive water stress and re-watering. Plant Growth Regul. 53, 43–51. Mahouachi, J., Socorro, A.R., Talon, M., 2006. Responses of papaya seedlings (Carica papaya L.) to water stress and re-hydration: growth, photosynthesis and mineral nutrient imbalance. Plant Soil. 281, 137–146. Mahouachi, J., Argamasilla, R., Gómez-Cadenas, A., 2012. Influence of exogenous glycine betaine and abscisic acid on papaya in responses to water-deficit stress. J. Plant Growth Regul. 31, 1–10. Malo, S.E., Campbell, C.W., 1986. The papaya. In: Cooperative Extension Service Fruits Crops Fact Sheet FC-11. University of Florida, Gainesville, FL. Marler, T.E., George, A.P., Nissen, R.J., Anderssen, P.C., 1994. Miscellaneous tropical fruits. In: Schaffer, B., Anderssen, P.C. (Eds.), Handbook of Environmental Physiology of Fruit Crops, Vol. II: Sub-Tropical and Tropical Crops. CRC Press, Boca Raton, FL, pp. 199–224. Marler, T.E., Mickelbart, M.V., 1998. Drought, leaf gas exchange, and chlorophyll fluorescence of field grown papaya. J. Am. Soc. Hortic. Sci. 123, 714–718. Melgar, J.C., Dunlop, J.M., Syvertsen, J.P., 2010. Growth and physiological responses of the citrus rootstock Swingle citrumelo seedlings to partial rootzone drying and deficit irrigation. J. Agric. Sci. 148, 593–602. Mingo, D.M., Theobald, J.C., Bacon, M.A., Davies, W.J., Dodd, I.C., 2004. Biomass allocation in tomato (Lycopersicon esculentum) plants grown under partial rootzone drying: enhancement of root growth. Funct. Plant Biol. 31, 971–978. Mohammed, G.D., Binder, W.D., Gillies, L., 1995. Chlorophyll fluorescence: a review of its practical forestry applications and instrumentation. Scand. J. For. Res. 10, 383–410. Pereira, A.R., villa Nova, N.A., Sedyama, G.C., 1997. Evapo(transpi)racao. Fundacao de Estudos Agrarios Luiz de Queiroz (FEALQ), Piracicaba, pp. 183. Schachtman, D.P., Goodger, J.Q.D., 2008. Chemical root to shoot signaling under drought. Trends Plant Sci. 13, 281–287. Sepaskhah, A.R., Ahmadi, S.H., 2010. A review on partial root-zone drying irrigation. Int. J. Plant Prod. 4, 241–257. Shao, G., Liu, N., Zhang, Z., Yu, S., Chen, C., 2010. Growth, yield and water use efficiency response of greenhouse-grown hot pepper under time-space deficit irrigation. Sci. Hortic. 126, 172–179. Souza, C.R., Maroco, J.P., Santos, T.P., Rodrigues, M.L., Lopes, C., Pereira, J.S., Chaves, M.M., 2005. Control of stomatal aperture and carbon uptake by deficit irrigation in two grapevine cultivars. Agric. Ecosyst. Environ. 106, 261–274. Stoll, M., Loveys, B., Dry, P., 2000. Hormonal changes induced by partial rootzone drying of irrigated grapevine. J. Exp. Bot. 51, 1627–1634. Strasser, R., Tsimilli-Michael, M., 2001. Stress in plants, from daily rhythm to global changes, detected and quantified by the JIP-test. Chim. Nouv. 75, 3321–3326. Sun, X.P., Yan, H.L., Ma, P., Liu, B.H., Zou, Y.J., Liang, D., Ma, F.W., Li, P.M., 2012. Responses of young ‘Pink lady’ apple to alternate deficit irrigation following long-term drought: growth, photosynthetic capacity, water-use efficiency, and sap flow. Photosynthetica 50, 501–507. ˜ J.E.P., Keller, M., Schreiner, R.P., Smithyman, R.P., 2011. Net carbon Tarara, J.M., Pena, exchange in grapevine canopies responds rapidly to timing extent of regulated deficit irrigation. Funct. Plant Biol. 38, 386–400. Torres-Netto, A., 2005. Atributos fisiológicos e relac¸ões hídricas em genótipos de mamoeiro (Carica papaya L.) na fase juvenil. State University of North Fluminense, Campos dos Goytacazes, Brazil (DSc thesis). Wang, R., Chang, J., Li, K., Lin, T., Chang, L., 2014. Leaf age and light intensity affect gas exchange parameters and photosynthesis within the developing canopy of field net-house-grown papaya trees. Sci. Hortic. 165, 365–373. Wang, Z., Liub, F., Kangc, S., Jensen, C.R., 2012. Alternate partial root-zone drying irrigation improves nitrogen nutrition in maize (Zea mays L.) leaves. Environ. Exp. Bot. 75, 36–40. Yuan, J., Xu, M., Duan, W., Fan, P., Li, S., 2013. Effects of whole-root and half-root water stress on gas exchange and chlorophyll fluorescence parameters in apple trees. J. Am. Soc. Hortic. Sci. 138, 395–402. Zegbe, J.A., Behboudian, M.H., 2008. Plant water status, CO2 assimilation, yield, and fruit quality of ‘Pacific RoseTM ’ apple under partial rootzone drying. Adv. Hortic. Sci. 22, 27–32.