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3 Performance of Seedlings as Reflected in WUE, 13C Composition, Total EVPT and Stem Ψ

3 Performance of Seedlings as Reflected in WUE, 13C Composition, Total EVPT and Stem Ψ

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232



Alveiro Salamanca-Jimenez et al.



[(Figure_4)TD$IG]



0



0.1



0.2



0.4 g N per plant



60

20

Chlorophyll b (mg)



Chlorophyll a (mg)



50

40

30

20

10



10

5

0



0

–10



–50



–100



–10



–500

60

Pigment content (ug/cm2)



12

10

Carotenoids (mg)



15



8

6

4

2

0



–50



–100



–500



Chl a = 1.05*SPAD-24.0 r 2 = 0.97

Chl b = 0.28*SPAD-3.54 r 2 = 0.97

Carot = 0.13*SPAD-1.43 r 2 = 0.91



50

40

30

20

10

0



–10



–50



–100



Soil matric potential (kPa)



–500



35



40



45



50



55



60



65



70



SPAD Reading



Figure 4 Total amount of leaf pigments in coffee seedlings grown under different soil

moisture and N levels, and relationship between chlorophyll meter (SPAD) reading and

total pigments.



À500 kPa and the maximum N rate (0.4 g/plant). The value of δ13C was

taken to indicate relative water stress and largely mirrored WUE, a measure

of the amount of ET required to produce the shoot biomass. The same figure

shows that there was a significant effect of the interaction of water with N on

EVPT (p = 0.0008) and stem Ψ (p = 0.0086): the effect of N dose on ETwas

diminished in drier soils. As might be expected, stem Ψ also responded to our

treatments, decreasing with soil moisture and showing lower values as N

addition increased. This response, reflecting the effects of both soil moisture

and N fertilization, resulted in a strong correlation between stem Ψ and 13C

content (Fig. 6).



3.4 Physiological Traits as Indicators of Plant Performance

Most of the responses presented previously were associated with changes in

photosynthesis as a result of acclimation to the imposed conditions (Fig. 7). A

significant effect of the interaction of water and N was registered only on the

intercellular CO2 concentration (p = 0.0122), while main effects of soil



233



Performance of Coffee Seedlings as Affected by Soil Moisture and Nitrogen Application



[(Figure_5)TD$IG]



0



0.1



0.2



0.4 g N per plant

2.5

WUE (g shoot/L water)



8



EVPT (L water)



7

6

5

4

3

2



1.5



1.0



0.5

–10



–50



–100



–500



–10



–24



0



–25



–20



–26



–40



Stem ψ (kPa)



δ′ 3 C (c/cc)



2.0



–27

–28

–29



–50



–100



–500



–50



–100



–500



–60

–80



–100



–30



–120



–31



–140

–10



–50



–100



–500



–10



Soil matric potential (kPa)



Soil matric potential (kPa)



Figure 5 WUE (water use efficiency), EVPT (evapotranspiration), carbon isotope

composition (δ13C), and stem water potential (Stem Ψ) in coffee seedlings grown

under different soil moisture and N levels.



[(Figure_6)TD$IG]



0.1



0.2



0.4 g N



–10

–24



–25



–25



–26



–26

δ′ 3 C (c/cc)



δ′ 3 C (c/cc)



0

–24



–27

–28



–29

–30

–90



–70



–50



–30



–31

–130



–110



Stem ψ (kPa)

13



C = –0.05-31.09*ψ r 2 = 0.97



13



C = –0.04-30.23*ψ r 2 = 0.99



13



C = –0.03-28.50*ψ r 2 = 0.95



13



13

13

13



–90



–70



–50



–30



Stem ψ (kPa)



C = –0.06-32.12*ψ r 2 = 0.98



13



–500 KPa



–28



–30

–110



–100



–27



–29



–31

–130



–50



C = –0.10-33.45*ψ r 2 = 0.92

C = –0.08-33.18*ψ r 2 = 0.99

C = –0.06-31.68*ψ r 2 = 0.98

C = –0.06-31.90*ψ r 2 = 0.94



Figure 6 Linear regression between stem water potential (Ψ) and 13C content in coffee

seedlings by N and soil moisture levels.



234



Alveiro Salamanca-Jimenez et al.



0.1



0.2



Photosynthesis (μmol/cm2 h)



5.0

4.5

4.0



c



3.5

3.0

2.5



Stomatal Cond (mol H2O/cm2 s)



–10



–50



–100



0.09

0.07

0.05

0.03

–50



280



0.9



260

240

220

200

180

–10



–500



0.11



–10



0.4 g N per plant

Internal CO2 conc (μmol/cm2 h)



0

5.5



Transpiration (mmol H2O/cm2 s)



[(Figure_7)TD$IG]



–100



–50



–100



–500



–50



–100



–500



0.8

0.7

0.6

0.5

0.4

0.3

–10



–500



Soil matric potential (kPa)



Soil matric potential (kPa)



Vapor pressure deficit (kPa)



1.4

1.3

1.2

1.1

1.0

–10



–50



–100



–500



Soil matric potential (kPa)



Figure 7 Physiological traits in coffee seedlings grown under different soil moisture and

N levels.



moisture and N were registered on rate of photosynthesis (p = 0.0014,

p = 0.0344), stomatal conductance (p < 0.0001, p < 0.0001), transpiration

(p < 0.0001, p < 0.0001), and leaf-to-air VPD (p < 0.0001, p = 0.0005). In

general, photosynthesis, internal CO2 concentration, stomatal conductance

and transpiration were higher in the unfertilized plants and decreased with

increasing N dose. The lowest values for these parameters corresponded with

water stress induced by drier soil conditions, which at the same time

increased the VPD in the leaves concomitant with the N fertilization.



Performance of Coffee Seedlings as Affected by Soil Moisture and Nitrogen Application



235



4. DISCUSSION

4.1 N Fertilization and Growth of Young Coffee Plants

Almost all research on coffee N fertilization has focused on the economically

tangible reproductive stage, but few studies have considered the early vegetative stage, even though this stage is critical to a plant’s future performance.

Sadeghian (2008) concludes that insufficient N fertilization during the vegetative phase decreases future bean yields up to 50%. The most basic results of

our study of seedlings, from an agronomic point of view, confirm the

importance of N from the beginning of coffee production. Shoot biomass

and foliar area responded positively to N at all levels of soil moisture, but the

root response did not follow a marked trend line as had been found in a

previous experiment in which N application decreased root biomass

(Salamanca-Jimenez, 2015). Our results highlight the interaction between

N fertilization and soil moisture, in particular, the importance of soil moisture in increasing N uptake. Under higher moisture, NH4+ and NO3À move

with greater ease to roots and are taken up mostly by transpiration-driven

mass flow (Marschner and Rengel, 2012).

We observed that wetter soils allowed the roots of fertilized plants to

explore a greater volume of soil. This may partly explain why the growth of

plants fertilized with 0.4 g N/plant was enhanced at higher soil water contents. Conversely, the root to shoot ratio (R:S) was reduced by increasing N

dose and increased by withholding soil moisture. Similar results were

reported by Bravo and Fernandez (1964) who observed that coffee plant

growth following N application exhibited a linear increase as available water

increased; they also reported that R:S was reduced by N fertilizers but was

not, however, affected by soil moisture.

In general, the optimal combination of water and N for coffee growth

was the wettest soil (À10 kPa) and 0.4 g N per plant. Sadeghian and

Gonzalez (2014), working in one Colombian soil with 13% organic matter,

estimated that a dose of 0.54 g N per plant would produce the highest coffee

seedling biomass, which is similar to the rate of 0.48-0.6 g recommended by

Arizaleta et al. (2002). In contrast to our results which showed an improvement in seedling performance with increasing N dose, Giraldo and Rubiano

(1974) and Salazar (1977) registered a negative effect of N on coffee growth

during the seedling stage, although it is possible that the doses used in these

studies exceeded beneficial levels - from a previous experiment, we also



236



Alveiro Salamanca-Jimenez et al.



observed that excessive application of N (0.8 and 1.2 g N/plant) became

harmful for coffee growth and reduced NUE (Salamanca-Jimenez, 2015).

Although Arizaleta and Pire (2008) state that plant response to N is

associated with soil fertility, we believe that it is more related to plant N

requirements. For this reason, if the goal of fertilization management is to

maximize coffee growth by increasing NUE, N doses must not exceed 0.6 g

N per plant and fertilization during the vegetative phase should be adjusted

proportionally to this optimum value.



4.2 Leaf N Content, Ndff and NUE, and Chlorophyll Content

We observed a positive response of increasing N in leaf N content, Ndff, leaf

pigments and SPAD readings. Similarly, Bravo and Fernandez (1964) showed

that foliar N and chlorophyll content in coffee increased as soil moisture

decreased under greenhouse conditions. The same effect was noted by

Quaye et al. (2009) in corn, who found that the highest N rate (80 kg/ha)

caused a significant increase in biomass yield and N uptake with moisture

levels close to field capacity.

Unlike leaf N, Ndff, or leaf pigments, the percent of N recovered from

fertilizer (NUE) did not exhibit a clear response to treatment conditions.

Only plants grown in the wettest soil and with the highest N dose showed an

increase in NUE with increasing N, but no response was observed for the

other soil moisture levels. This behavior is opposite to that found in a similar

experiment (Salamanca-Jimenez, 2015) where NUE was decreased by

increasing N doses, although in this is case it was likely due to excessive N.

The absolute values of Ndff and NUE in the present study were very

similar to those presented by Salamanca-Jimenez (2015), confirming that the

magnitude of the N uptake response within certain limits is an intrinsic

characteristic of coffee seedlings. Nevertheless, NUE values obtained in pot

studies may be higher than those representative of the field, as roots are

confined in pots and N is typically applied directly to the root system with

no leaching permitted. Much lower NUE values in coffee have been reported

under field conditions by other studies using 15N: Cannavo et al. (2013) registered an N recovery of 13.5% in the whole plant and Salamanca-Jimenez

(2015) reported an average of 3% in the leaves. In both studies, these low NUE

values were associated with considerable loss of N either by volatilization or by

leaching in places with high precipitation. Similarly, Suarez (1996) studying

the effect of four N levels (6, 12, 18, and 24 g N/plant/year) in a high density

(10,000 plants/ha) reproducing coffee plantation established on an Andisol,

found that only 8% of fertilizer N was taken up, exhibiting the following



Performance of Coffee Seedlings as Affected by Soil Moisture and Nitrogen Application



237



distribution: 59.7, 21.7, 9.9, and 8.8% in leaves, branches, stem, and roots,

respectively.

Rian˜o et al. (2004), evaluating N uptake and dry matter accumulation in

coffee plants during the first 15 months after transplanting in three

Colombian fields, found that N uptake reached 8.6 and 19.4 g per plant.

Despite being similar to the 27 g N per plant in 24 months reported by

Suarez (1996), this reflects a very low NUE, considering that one coffee plant

receives between 47 and 58 g of N fertilizer, not including native soil N or

previous applications.

Our study shows that the amount of N addition was more important than

soil water content for chlorophyll and carotenoid concentrations and total

content in coffee leaves. According to Lichtenthaler (1987) chlorophyll a and

b occur in a ratio a/b of approximately 3 but growth conditions and environmental factors can modify this ratio. For example, plants growing under

high levels of sunlight (high-light chloroplasts) exhibit a/b ratios of 3.2-4,

whereas shaded plants (low-light chloroplasts) possess lower values (2.5-2.9).

The a/b values in our plants averaged between 2.4 and 3.1, confirming that

our plants reflected a physiology typical of low light conditions.

It has also been stated that a ratio of chlorophyll a+b/carotenoids lower

than 3.5 can be used as an indicator of senescence, stress, and damage to the

plant and the photosynthetic apparatus, since chlorophyll pigments break

down faster than carotenoids (Lichtenthaler and Buschmann, 2001). In our

study this ratio averaged 8.5, corroborating favorable conditions for high

photosynthetic activity in the top fully expanded leaves of coffee plants, and

was affected more by increasing the N level than by increasing soil moisture.

Lichtenthaler (1987) also states that the individual levels of pigment proteins

also change depending on the developmental stage and the environmental

conditions, which again explains the higher values observed for the ratios

chlorophyll a/carotenoids and chlorophyll a+b/carotenoids in low-light

plants. Carotenoids play an important role in the use of solar energy in

photosynthesis, due to their ability to safely dissipate excess energy

(Demmig-Adams and Adams, 1996).



4.3 Changes in WUE, EVPT and Stem Water Potential (Stem Ψ)

Our results confirm the positive effect of N on WUE and the negative effect

of soil water level on EVPTand stem Ψ. When plants grow under water stress

they use available water more efficiently, but as soil moisture decreases

the water potential differences between the soil and the environment

increase and limit water movement both to and through the plant. In



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