4 Growth, yield and size distribution of three main potato crop cultivars
4.2 Materials and Methods (Experiment 1) 55
4.2.5 Calculations 58
4.2.5.1 Tuber yield
Values of FM and DM tuber yield were reported as the mean of each plot on each sampling date. To calculate the accumulated proportion of yield the tubers were sorted into weight grades of 50 g. The sum of weights within each grade divided by the total FM gave the proportion per grade. The sum of grade proportions from the lowest (0 - 50 g) to the highest (1050 - 1100 g) grade established the accumulated proportion which ranged from zero to one.
In addition, accumulated tuber FM and DM yields (measure from 23/11/2011 to final harvest) were fitted against days after planting (DAP) using a Gompertz curve. Later, accumulated tuber DM was also fitted against thermal-time (Section 4.2.5.2) using the same type of curve. The time when 5% and 95% of the final tuber DM was also calculated for each plot based on the curve estimates. Finally, linear growth rates (LGR), tuber t DM ha-1 day-1 or t tuber DM ha-1 °Cd-1 were calculated for each plot of each cultivar. LGR was
calculated by fitting a linear regression between accumulated tuber DM against DAP or Tt using all data-points within the range of 5 and 95% of the maximum yield for each cultivar (Brown, 2004; Teixeira, 2006).
4.2.5.2 Thermal-time calculation
Daily thermal-time (Tt, °Cd) was calculated using daily average air temperature (Tmean)
after crop emergence. Tmean was measured using the average of hourly air temperatures
above a threshold (Tb) of 2 °C (Section 7.3.1). Then the cumulative thermal-time was
Because the Tmean was never over 25 °C during the field experiment period (Section 3.5.2)
Tt did not have to be corrected for supra optimum temperatures (see Section 2.3.1.1).
4.2.5.3 Radiation interceptance
Radiation interceptance (R/Ro) was measured (Section 3.6.2), and the total incident
radiation (Ro) from the Meteorological station (Section 3.5) was used to calculate the
radiation intercepted in MJ m-2 by the canopy.
Daily values of radiation interceptance (R/Ro daily) were estimated with a Piece-wise
regression fitted to each individual plot using a series of 16 measurements taken from 15/11/2011 to 13/04/2012. The regressions were fitted against days after planting (DAP) and Tt (Section 4.2.5.2). The Piece-wise regression consisted of a three linear phase model of canopy formation (Figure 4.5). Immediately after crop emergence, the phase of canopy
growth happened concomitant to the R/Ro rise at a linear rate of increase. Then the crop
had a canopy constant phase when the crop was at maximum radiation interceptance (R/Ro
max). The final phase characterises the canopy senescence in which R/Ro decreases at a
linear rate.
The total amount of accumulated radiation intercepted (Rcum) was calculated by integrating
daily values of radiation interceptance from crop emergence (em) to crop desiccation (des), on 18/04/2012, according to Equation 8:
Equation 8 Rcum =
∫
des
em
R/Ro daily*Ro daily
Where, Ro daily is the daily average total incident radiation.
4.2.5.4 Radiation use efficiency (RUE)
RUEtotal was calculated from the total dry matter (DMtotal) accumulated in the entire plant
(which included above and below-ground parts of the crop but excluded roots). In addition, the radiation use efficiency for tuber dry matter production (RUEtuber) was calculated as a
to the plot averages of DMtotal and DMtuber respectively, against Rcum from 14/10/2011 to
29/02/2012. The split-line regression fitted to DMtuber indicated the moment when RUE
shifted from a low to a higher rate (“break point” of the regression). The “break point” was then considered to be the time of tuber bulking initiation.
RUE was not adjusted for temperature because the average air temperature remained between 12 °C and 30 °C, regarded as the optimum threshold for potato crop growth for the entire crop growth season, from November 2011 to April 2012 (Figure 3.3).
4.2.5.5 Fraction of total DM partitioned in the tubers (harvest index)
The fraction of total dry matter partitioned in the tubers or harvest index (HI) was calculated using Equation 9:
Equation 9 HI = DMtuber / DMtotal
The pattern of crop HI increase was assessed by fitting an exponential curve between the fraction of total DM partitioned to the tubers and DAP for each cultivar from 30/11/2011 to 29/02/2012. This relationship was also performed using total DM partitioned to the tubers and thermal-time accumulated from crop emergence. The measured values of each plot were compared for differences among cultivars.
4.2.5.6 Non-destructive method of estimating individual leaf area
In potato crops individual leaf area estimation using non destructive methods usually
derives from the measures of width (W, cm) and length (L, cm) of the compound leaf.
Later, a factor (f) is used to calibrate these parameters. Leaf area is then expressed as:
Equation 10 A = f * W * L
In the literature f ranges from 0.45 (Vos and van der Putten, 1998), to 0.74 (Fleisher and Timlin, 2006).
An alternative to this approach was tested in this chapter. Leaf expansion and final area
(LA, mm2) of individual leaf position on the main stem was estimated by a split-line
regression fitted against the terminal leaflet length (mm) (Figure 4.10) using the data collected (Section 4.2.4.2).
4.2.5.7 Leaf area index (LAI) and extinction coefficient (k)
Leaf area index (LAI) was estimated from leaf DM using the concept of specific leaf area (SLA; cm2 g-1 DM of leaf). To do this, the relationship between individual leaf area (LA;
cm2) and individual leaf dry matter (g) was established by linear regression
(0.94<R2<0.98). The regression was fitted for individual plots using the individual leaf
area against their DM from six sub-sampled stems (Section 4.2.3). The slopes of the linear regressions gave the SLA of each cultivar. SLA was then multiplied by the average plant leaf DM. Finally, LA was extrapolated for the plant and then to a square meter to obtain LAI.
The extinction coefficient (k) was used to describe the canopy architecture of the crops.
The k value was calculated from the regression of the natural log of the fraction of
radiation interceptance (R/Ro) by the crop against LAI.
4.2.5.8 Leaves, petioles and above-ground ‘stems’ DM
The proportion of DM allocated to leaves, petioles and above-ground main stems was established using the sub-sample measurements (Section 4.2.4.2) and then these were multiplied by the total DM of the sample (6 plants = ~1.7 m2) to give the DM of leaves per
m2, petioles per m2 and above-ground ‘stems’ per m2, respectively. The DM was later
averaged for the plant.
4.2.5.9 Chance of tuber initiation occurrence
The chance of tuber initiation occurrence at a certain node position on the below-ground main stem was calculated based on the six sub-samples averaged on each plot. For each node position it ranged from zero (0% initiated) to one (100% initiated).