Monitoring supplemental light use
by plants using remote sensing
Krishna Nemali
Purdue University
01/10/17
Supplemental lighting is needed to grow quality plants in the
winter months
DLI (mol/m2)
60
50
Outside greenhouse
40
30
20
10
Inside greenhouse
0
0
25
50
75
100 125 150 175 200 225 250 275 300 325 350
Chronological day
Price per KWH is continuously increasing in the US
Plants are not efficient at using light
48% of absorbed light is
not used
100%
32% used in
photosynthesis
20% of incident light
reflected back and
not absorbed
• We are ‘supplementing’ light to grow quality plants
• Further reducing supplemental light intensity is not
a right approach
• Use energy efficient approaches and ensure
supplemental light is not wasted during production
How to know if plants are wasting supplemental light?
• A typical light sensor
measures the amount of light
incident on plants
• One can measure the fraction
of light reflected by plants
using a light sensor (inverting
over plants) but with less
precision
Photosynthesis is directly affected by light intensity
Light intensity (µmol/m2/s)
Photosynthesis (µmol/m2/s)
7
300
6
250
5
200
4
150
3
100
2
0
SL
SL + DL
SL
1
9
17
25
33
41
49
57
65
73
81
89
97
105
113
121
129
137
145
153
161
169
177
185
193
201
209
217
225
233
241
50
1
0
SL
SL + DL
SL
1
9
17
25
33
41
49
57
65
73
81
89
97
105
113
121
129
137
145
153
161
169
177
185
193
201
209
217
225
233
241
350
Why not measure photosynthesis to monitor changes in
light absorption and use?
 Photosynthesis measurement systems may
be robust but are
•
•
•
expensive ($45 K)
complicated to use
measure small leaf sections
Photosynthesis measurement system
Spectral reflectance sensors (SRS) may provide solution
• Low cost (~ $750)
• Can be connected to any
data monitoring devices
• Measure groups of plants
Before we used spectral reflectance sensors, we tested
their efficacy
• Two sensors namely PRI (photochemical reflective index) and NDVI (normalized
difference vegetation index) were tested
• PRI sensor may measure the extent of supplemental light not used in photosynthesis
• NDVI sensor may measure the extent of supplemental light reflected back by plants
• If the sensors are working as expected, they should affect photosynthesis in plants
• Sensors were tested by comparing them with high cost photosynthesis measurements
Setup to test SRS sensors for efficacy
NDVI sensor
PRI sensor
Photosynthesis
system
Photosynthesis (µmol/m2/s)
Higher NDVI indicates higher light absorption
Higher PRI indicates lower light use
1.8
1.9
1.7
1.8
1.7
1.6
1.6
1.5
1.5
1.4
1.4
1.3
1.3
1.2
1.2
1.1
1
0.865
1.1
1
0.87
0.875
0.88
0.885
0.89
0.895
0.9
0.25
NDVI
0.3
0.35
0.4
PRI
Data collected from petunia plants
0.45
0.5
Questions for the experiment using SRS
• Can plants use supplemental light with similar efficiency
during both daytime and nighttime?
• Can environmental stress like low temperature or drought
further lower supplemental light use by plants?
Light absorption was slightly higher & light use was slightly
lower when supplemental light is provided at night than day
NDVI
PRI
1
1.4
0.9
1.2
0.8
0.7
0.6
0.5
Night
Day
Night
1
Night
Day
Night
0.8
0.6
0.4
0.3
0.4
0.2
0.2
0.1
0
Higher better
1
12
23
34
45
56
67
78
89
100
111
122
133
144
155
166
177
188
199
210
221
232
243
1
12
23
34
45
56
67
78
89
100
111
122
133
144
155
166
177
188
199
210
221
232
243
0
Lower better
Petunia plants grown in 4-inch containers were
subjected to drought stress
Supplemental light intensity
(µmol/m2/s)
Night 2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Night 1
Night 2
1_18
1_20
1_22
1_24
2_2
2_4
2_6
2_18
2_20
2_22
2_24
3_2
3_4
3_6
Night 1
1_18
1_20
1_22
1_24
2_2
2_4
2_6
2_18
2_20
2_22
2_24
3_2
3_4
3_6
100
90
80
70
60
50
40
30
20
10
0
Photosynthesis
(µmol/m2/s)
PRI increased (lower light use) and NDVI decreased
(lower light absorption) as drought stress progressed
NDVI
PRI
0.5
0.905
0.9
0.45
0.895
0.89
0.4
0.885
0.88
0.35
0.875
0.87
0.3
0.865
0.86
0.25
Night 1
0.2
Night 2
0.855
Night 1
Night 2
0.85
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Conclusions
• Supplemental lighting is an expensive but essential input for producing
quality plants
• Plants only use only 1-3% of supplemental light
• Spectral reflectance sensors may offer viable solutions to monitor light
absorption and use by plants
• Plants appear to use light relatively similar at night compared to day
• Transient stress can further reduce supplemental light use efficiency