Reference publication:
Ge, Yufeng; Bai, Geng; Stoerger, Vincent; Schnable, James C. (2016). Temporal dynamics of maize plant growth, water use, and leaf water content using automated high throughput RGB and hyperspectral imaging. In: Computers and Electronics in Agriculture, S. 625–632. DOI: 10.1016/j.compag.2016.07.028. http://www.sciencedirect.com/science/article/pii/S0168169916305464

The automated watering and weighing station of the LemnaTec system, together with estimated shoot FW (fresh weight) and DW (dry weight) – see Notes at the end of this document – allows the estimation of daily water consumption (or equivalently evapotranspiration, ET) and water use efficiency (WUE) of each plant. Consider two consecutive days i and i + 1:

where W(i, after) is the total pot weight after water application for Day i; W(i+1, before) is the total pot weight before water application for Day i + 1; FWi and FWi+1 is shoot fresh weight estimated at the point of imaging for Day i and i + 1, respectively; and DWi and DWi+1 is shoot dry weight estimated at the point of imaging for Day i and i + 1, respectively. T is the time interval and is equal to 1 day (daily pot weighing following the same schedule every day).

A numerical example from our data on how daily ET and WUE were calculated is given below. The weight of a certain pot on DAS 17 was 5398 g after watering. The estimated shoot FW and DW were 17.53 and 1.53 g, respectively. On DAS 18, the pot weight before watering was 5304 g. The estimated shoot FW and DW were 24.13 and 2.21 g, respectively. Water consumption between DAS 17 and 18 was calculated as: [(5398 – 17.53) – (5304 – 24.13)] = 100.6 g / day

There was also an accumulation of net shoot dry biomass of: 2.21 – 1.53 = 0.68 g/day

WUE was then calculated as: 0.68/100.6 = 0.0068 g/g (6.8 g of shoot dry matter per 1000 g of water consumption)

1) WUE is commonly defined as a ratio of biomass accumulation to water consumed. Depending on specific research goals, biomass accumulation can be defined as carbon assimilation, crop biomass, or grain yield; and water consumption can be transpiration, evapotranspiration, or total water input to the system. Here we define WUE as the amount of shoot dry matter produced per unit of water loss through evapotranspiration.

2) Estimated shoot FW (fresh weight) and DW (dry weight).
For the calculation of ET and WUE mentioned above, calibration curves for Projected Shoot Area (PSA) or simply Projected Area (PA) based on pixel count of segmented plant images versus actual fresh and dry weights (FW and DW, respectively) need to be generated. For further information refer to the original publication mentioned above.

Correlations between destructively measured leaf area, shoot fresh weight, shoot dry weight and plant projected area (pixel count) of maize plant RGB images for the first stage of the experiment (DAS 6 to 26). Circles represent individual destructively harvested B73 plants. Crosses represent individual FFMM-A plants. The total number of plants is 20.