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Partial wetting - a potential water management saver for peach and apple

Mark O'Connell and Ian Goodwin

Department of Natural Resources and Environment, Tatura, Vic. 3616 www.nre.vic.gov.au Email mark.o’connell@nre.vic.gov.au , ian.goodwin@nre.vic.gov.au

Abstract

Partial wetting (PW) of soil in peach and apple orchards in the Goulburn Valley was investigated during the 2000/01 season. Both, peach and apple irrigation supply exceeded derived crop water requirements by 2-fold, due to low levels of canopy light interception (<35%). For peach and apple, PW did not impact vegetative vigour (shoot or fruit growth), yield, fruit quality, canopy interception, canopy conductance, or leaf water potential, compared to the control. Since yield was not influenced by PW, the 50% water saving afforded by 0.5PW equated to a doubling in crop water use efficiency. This study highlights the need to incorporate effective canopy cover when determining irrigation inputs, in order to better predict crop water requirements.

Introduction

The Goulburn Valley is a major producer of pome and stone fruit in Australia. Improved irrigation practices would reduce water usage and associated off-site environmental impacts, such as waterlogging, salinity and declining river health. Irrigation, using partial wetting (PW) of the root system could increase water use efficiency (WUE), reduce vegetative growth and increase yield and quality traits. Further, this practice has potential for use in high-density orchards, where current irrigation strategies aim to minimise water stress, leading to vigorous growth and internal shading. PW may overcome the reductions in flower initiation, fruit set, fruit size, fruit colour, soluble solids and firmness associated with excessive vigour. Most orchardists use experience to schedule irrigation rather than a measure of crop water use (1). Some irrigate according to estimates of crop water requirement using pan evaporation and locally derived crop coefficients, based on crop development. More recently, it has become apparent that estimates of orchard evapotranspiration should be based on effective canopy cover (2). This study examined the effects of PW on crop water relations, vegetative vigour, yield, fruit quality and effective canopy cover for peach and apple.

Materials and methods

Two complete randomised block design field experiments using peach (5 replicates) and apple (4 replicates) were conducted during 2000/2001 at Tatura (3626'S, 14615E). Peach trees were 7-year-old ‘Okubo Late’ planted at 1 m x 4 m spacing in a mini-tatura trellis arrangement on a red Sodosol. Irrigation treatments were control and two partial wetting treatments (0.5PW and 1.0 PW) which received 50 and 100% of control. Irrigation was based on estimation of crop water requirement (ETc) calculated from FAO procedures (3) and using a crop coefficient (Kc) of 1.15. Apple trees were 7-year-old ‘Pink Lady’ planted at 1.6 m x 4.4 m spacing in a central leader arrangement on a red Chromosol. Irrigation treatments were control and 0.5PW, where a commercial orchardist managed the irrigation. Fruit diameter and shoot length was measured weekly. Fractional photosynthetically active radiation interception (f) was determined using a ceptometer at solar noon. Stomatal aperture was measured with a porometer and leaf water potential was measured with a Scholander pressure bomb. At harvest, skin colour traits and flesh firmness were recorded. Reference crop evapotranspiration (ETo) was computed from pan evaporation data using a pan coefficient (Kp) of 0.8. Thus, derived ETc adjusted for effective canopy cover and soil evaporation equalled (ETo* Kc* f) + (ETo*0.1), where, for peach (2) and apple (3), Kc was 1.5 and 1.25, respectively.

Results and Discussion

Peach fruit growth reached maxima in late December 2000, and apple growth had ceased by March 2001. For peach and apple, shoot and fruit growth was not influenced by irrigation treatment (P<0.05) and no significant differences were found for leaf stomatal conductance and water potential between treatments (P<0.05). Also, no signs of canopy ‘water-stress’ were measured. Irrigation treatments for peach and apple did not influence f (P<0.05). Peak f averaged 33% and 30% for peach and apple, respectively. PW did not alter yield, fruit size, total soluble solids (TSS) or flesh firmness for apple (Table 1), and skin colour traits were similar between treatments. For peach, fruit size was reduced under 0.5PW compared to the control, however, 0.5PW provided higher TSS compared to 1.0PW.

Table 1. Fruit yield and quality characteristics at harvest for peach (8-Jan-01) and apple (20-Apr-01) experiments under the control, full (1.0PW) and partial (0.5PW) wetting treatments.

Irrigated area

Peach (Okubo Late)

Apple (Pink Lady)

Yield (t/ha)

TSS (Brix)

Fruit size
(g)

Flesh firmness (kg)

Yield (t/ha)

TSS (Brix)

Fruit size
(g)

Flesh firmness (kg)

Control

32 a

12.4 ab

154 a

6.4 a

46 a

13.9 a

160 a

7.3 a

0.5PW

32 a

13.5 a

137 b

6.3 a

40 a

14.7 a

149 a

7.6 a

1.0PW

31 a

11.6 b

142 ab

6.1 a

-

-

-

-

LSD (5%)

5

1.4

14.3

0.9

15

0.9

30.8

0.4

Cumulative crop water (rainfall + irrigation) inputs and derived ETc based on effective canopy cover plus a component for soil evaporation indicate 0.5PW for peach experienced deficit conditions prior to harvest (Fig 1). Derived ETc includes the measured maxima for f of 33 and 30 % for peach and apple, respectively. Peach irrigation inputs were scheduled on a full cover (f =100%) and exceeded ETpeach demand by ≈2-fold. Similarly, for apple (commercial orchard) the control treatment exceeded derived ETapple by ≈2-fold. We believe excess inputs beyond ETc are typical and are due to orchardists attempting to reduce perceived production risks prior to harvest. WUE ranged from 14 to 29 t/ML for peach and 8 to 15 t/ML for apple. Since yield was not effected by irrigation treatments, WUE was approximately double for the 0.5PW treatment and equivalent in the 1.0PW treatment.

Figure 1. Reference crop evapotranspiration (ETo), derived crop water requirement (ETpeach, ETapple), rainfall and the control, full (1.0PW) and partial (0.5PW) wetting treatments irrigation inputs for peach and apple experiments.

No impact of PW on crop performance for peach and apple was detected during 2000/01. Future work should aim to match water supply (rainfall + irrigation) to ETc. This would elucidate any osmotic adjustment and/or stomatal control mechanisms under deficit irrigation or PW practices. Further soil and crop field data is needed to test PW on vigour, yield and fruit quality across more diverse environmental conditions. This study highlights that crop water requirements (irrigation inputs) under micro-irrigation require an estimate of effective canopy cover. Otherwise, overestimation occurs as ETc calculations assume complete canopy cover (i.e. f ≈100%).

References

(1) Boland, A-M., Corrie, J., Bewsell, D., and Jerie, P. 2001. Final Report MDBC Project I7044.

(2) Johnson, R.S., Ayars, J., Trout, T., Mead, R., and Phene, C. 2000. Acta Hort. 537:455-460.

(3) Doorenbos, J and Pruit, W.O. 1977 Irrigation and Drainage Paper 24, FAO, Rome.

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