Sap ow analysis

8 sap ow meters were placed in the cabbage crop, four in each experiment, at the beginning of Jan-uary, around 60 days after transplanting. The sap ow sensors within a treatment were given the name

"a", "b", "c" or "d" to distinguish between them, but they were submitted to the same irrigation sched-ule. For each day, we calculated the daytime mean sap ow value and the corresponding daytime mean for ETcband the soil matrix potential. Daytime was considered from 8 a.m. to 18 p.m. The correlation between daytime means was then calculated. We use daily means to avoid the diurnal uctuations in ET which would lead to a strong correlation with the sap ow so that we can focus on the daily trends. We compared only the daytime signals to avoid night uctuations as well.

Due to the removal of external leaves about every 10 days, the interpretation of the sap ow mea-surements is dicult. Indeed, the removal of the leaves resulted in a sharp drop in the sap ow signal, which was then recovered in the next days. The dierent cabbages possessed a stem diameters at sap

ow height of 18 to 27 mm. The pith had a radius of about 5 to 8 mm and the xylem a width of 2 to 5 mm.

Figure 4.3 shows the evolution of the signal for a specic period for treatment 2, which showed good correlation (0.90) between the mean daily daytime sap ow signal and the soil matrix potential. The signal of the sap ow was transformed in a transpiration rate as described in chapter3.1.5. A decrease in the signal can be observed after an irrigation event, especially when the potential falls below -50 kPa.

This daily behavior could however not be observed clearly for treatment 1, where the soil matrix potential remained higher and more constant.

CHAPTER 4. RESULTS OF FIELD EXPERIMENTS T. Müller

Date

31.12 02.01 04.01 06.01 08.01 10.01

Soil matric potential [kPa]

-150 -100 -50 0

50 Treatment 2 b

Watermark 10 cm depth Daytime mean

Date

31.12 02.01 04.01 06.01 08.01 10.01

Sap Flow - [mm/day]

0 5 10 15 20 25 30 35

Sapflow Daytime mean

Date

31.12 02.01 04.01 06.01 08.01 10.01

ETcb [mm/day]

0 5 10 15 20 25

ET_cb Daytime mean

Figure 4.3: Comparison between the soil matrix potential and the transformed sap ow rate taken on cabbage

"b" in treatment 2, at the beginning of the mid-season. The corresponding calculated transpiration rate (ETcb) is also shown. The blue curves represent the 30 minutes time average for the sap ow signal and 15 minutes average for the both other signals. The red curves represent the mean daytime daily averages.

Some additional graphs are shown in gure4.4and also in appendixA.4, which show similar analysis, but for the late period, just before harvesting when leaves were no longer removed, except for treatment 1a, where leaves were removed on January 31. A relationship can also be observed on a longer time scale in all experiments and the corresponding correlations are shown in table 4.4. The correlations between the daily mean values of the sap ow velocity and the meteorological parameters were also tested but weren't very signicant as they remained below 0.5.

Exp 1a Exp 1c Exp 2b Exp 2c

0.74 0.73 0.83 0.41

Table 4.4: Correlation between the mean daily daytime sap ow and the corresponding mean daytime soil matrix potential.

The reduction of sap ow rate (and therefore transpiration) in gure 4.4 is particularly visible in treatments 1c and 2b where a strong reduction of the signal is observed below -100 kPa. The signal also reacted to the small increase in soil matrix potential that occurred around February 10. For treatments 1a and 2c shown in gureA.5in appendixA.4, the signal reacted more slowly with a real decrease in sap

ow signal when the potential was below -200 kPa.

CHAPTER 4. RESULTS OF FIELD EXPERIMENTS T. Müller

These results seem to indicate that water stress occurs below about -50 kPa and is more severe be-low -100 kPa during the mid-season stage. This limit of water stress could explain the decrease in yield observed in experiment 2, where the soil matrix potential dropped below -100 kPa every third day, before an irrigation event is triggered. This value can however not be evaluated with more precision. From this analysis it is also dicult to conclude if the root distribution for treatment 2 adapted to the lower irrigation frequency by increasing deeper root biomass in order to improve its resistance to soil moisture depletion.

21.01 26.01 31.01 05.02 10.02 15.02

ETcb [mm/day]

0 10 20 30

ETcb

ETcb daytime mean

21.01 26.01 31.01 05.02 10.02 15.02

Soil matric potential [kPa]-200 -100 0

Treatment 1 c

Watermark 5 cm depth

21.01 26.01 31.01 05.02 10.02 15.02

Sap Flow - [mm/day]

0 10 20 30

Sapflow Daytime mean

21.01 26.01 31.01 05.02 10.02 15.02

Soil matric potential [kPa]-200 -100 0

Treatment 2 b

Watermark 10 cm depth

Date

21.01 26.01 31.01 05.02 10.02 15.02

Sap Flow - [mm/day]

0 10 20 30

Sapflow Daytime mean

Figure 4.4: Comparison between the soil matrix potential, the transformed sap ow rate and the corresponding transpiration rate (ETcb), for both treatments during the late season of the cabbage experiment. The blue sap

ow curves represent the 30 minutes average, the blue Watermark curve the 15 minutes average and the red curves are the daytime means. For transpiration, the red curve represents the 15 minutes average and the blue curve the daytime average.

CHAPTER 4. RESULTS OF FIELD EXPERIMENTS T. Müller

4.1.4 Root growth

At transplantation the seedling roots had a length of about 6 to 8 cm and about 1 to 2 cm horizontal radius. The rst sampling of the root distribution could only occur during the mid-season due to political instabilities and government transition in the country which prevented eld work so that the analysis had to be postponed. Around 60 days after transplanting, on December 27 the rst root extraction could take place and the image analysis and density proles are shown in gure4.5. At that stage, the roots were already well developed. The roots reached a depth of about 20 to 25 cm. For treatment 1, a wide and dense network of secondary roots was observed in the rst 5 to 10 cm, with only fewer roots penetrating deeper in the soil and a horizontal radial distance from the stem of about 20 to 30 cm. Concerning treatment 2, some main roots reached deeper in the soil to a depth of about 30 cm and more secondary roots could be observed below 10 cm. Nevertheless, most of the roots were contained in the upper 15 cm. The radial distance of the roots was similar to experiment 1. The main dierence between both analysis is that roots were more concentrated near the surface for treatment 1 which can be explained by the lower irrigation depth applied more regularly that replenished the soil more supercially. Indeed, already at 10 cm, the measurement of the soil matrix potential remained steady around -100 kPa for a long part of the mid-season, indicating that the irrigation front did not reach much deeper (gure4.1).

Surface root area: 82.6 cm² Edited black and white image

Relative frequency

(a) Experiment 1 (optimal-low depth).

Surface root area: 108.1 cm² Edited black and white image

Relative frequency

(b) Experiment 2 (stress-high depth).

Figure 4.5: Evolution of the root distribution of cabbage - day 60 after transplanting.

A second extraction of the roots took place 90 days after transplanting, on January 26. There were relatively little dierences in comparison with the previous analysis. As shown in gure 4.6, the roots developed a bit deeper for the experiment 1, with some roots reaching 25 cm. The root system was still mainly contained in the rst 10 to 15 cm, though a bit less dense than before. Experiment 2 did not show much dierence, except a few more deeper roots.

Surface root area: 183.1 cm² Edited black and white image

Relative frequency

(a) Experiment 1 (optimal-low depth).

Surface root area: 146.9 cm² Edited black and white image

Relative frequency

(b) Experiment 2 (stress-high depth).

Figure 4.6: Evolution of the root distribution of cabbage - day 90 after transplanting.