Greenhouse Daily Light Integral for Tomato Production: 22-Agric-A1 Applied Plant, Animal or Human Physiology, May 2015, Q5

Question 5: Greenhouse Daily Light Integral for Tomato Production (30 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given.

Quantity	Value
Minimum outdoor DLI, January, Manitoba	10 mol·m⁻²·d⁻¹
Transmission, double-layered polyethylene cover	76%
Recommended minimum DLI, tomato production	15 mol·m⁻²·d⁻¹

Find. (a) The minimum DLI actually reaching the tomato plants inside the greenhouse; (b) the additional (supplemental) DLI needed to reach the 15 mol·m⁻²·d⁻¹ target.

Approach. Scale the outdoor DLI by the cover's transmission fraction to get the DLI actually reaching the plants, then take the shortfall against the target as the supplemental requirement.

a) DLI reaching the plants. Multiply the outdoor DLI by the cover's transmission fraction — use the January minimum outdoor value, since the greenhouse must be designed to meet the crop's minimum requirement even on the darkest days: $$\text{DLI}_{\text{greenhouse}} = \text{DLI}_{\text{outdoor,min}} \times T_{\text{cover}} = 10 \times 0.76 = \boxed{7.6\ \text{mol}\cdot\text{m}^{-2}\cdot\text{d}^{-1}}$$
b) Supplemental lighting required. Subtract what the cover already delivers from the tomato target: $$\Delta\text{DLI} = \text{DLI}_{\text{target}} - \text{DLI}_{\text{greenhouse}} = 15 - 7.6 = \boxed{7.4\ \text{mol}\cdot\text{m}^{-2}\cdot\text{d}^{-1}}$$ This is the DLI shortfall that supplemental (LED) lighting must make up — carried forward as the design target in Question 6.

Fig. 3 — Light budget for the greenhouse bench: outdoor DLI is cut to 7.6 by the cover, and LED supplement (sized in Question 6) makes up the remaining 7.4 mol·m⁻²·d⁻¹ to reach the 15 target.

Quantity	Result
a) DLI reaching plants in greenhouse	7.6 mol·m⁻²·d⁻¹
b) Supplemental DLI required	7.4 mol·m⁻²·d⁻¹

Concept & Theory

Topic: Daily Light Integral (DLI) as a design metric for greenhouse supplemental lighting.

Key relations: $\text{DLI (mol}\cdot\text{m}^{-2}\cdot\text{d}^{-1}) = \text{PPFD (}\mu\text{mol}\cdot\text{m}^{-2}\cdot\text{s}^{-1}) \times t\text{ (s/day)} \times 10^{-6}$; $\text{DLI}_{\text{transmitted}} = \text{DLI}_{\text{incident}} \times T_{\text{cover}}$.

Why this works: DLI is simply total photon delivery integrated over a day, so cover transmission (a fixed fraction that attenuates instantaneous PPFD uniformly) scales the daily total by the same fraction. Sizing supplemental lighting to the minimum outdoor DLI (worst-case winter day) rather than a seasonal average ensures the crop's minimum requirement is met on every day, not just on average.

Common pitfall: Using the "seldom exceed 15" winter ceiling instead of the stated January minimum of 10 — a supplemental-lighting system sized to an average or typical DLI under-delivers light on the darkest days it's actually meant to cover.

Source: Runkle, Daily Light Integral: A Useful Tool for Greenhouse Growers, Michigan State University Extension.

Question 5 of 6: Greenhouse Daily Light Integral for Tomato Production

Question 5: Greenhouse Daily Light Integral for Tomato Production (30 marks)