2014 ASHS Annual Conference

Scheduling greenhouse irrigation is a difficult task, because crop water use depends on both crop size and environmental conditions. Most growers schedule irrigation using timers or based on experience, but excessive irrigation is common. The development of models that describe crop water use requirements based on environmental variables, such as temperature, vapor pressure deficit and photosynthetic photon flux can help provide growers with quantitative irrigation guidelines. Light intercepted by plants may be one of the most important environmental variables that affect plant water use and could be a valuable tool for modeling plant water use. The objectives of this study were to measure light interception and quantify its effects on water use of four bedding plant species (impatiens ‘Accent Coral Star’, Dianthus chinensis ‘Ideal Rasberry’, Petunia×hybrida ‘Dreams Burgundy’ and ageratum ‘Blue Hawaii’). Seedlings were transplanted into 15 cm pots filled with a peat:perlite substrate with controlled release fertilizer (16N-2.6P-9.0K) incorporated at a rate of 9 g/pot. Substrate water content was maintained at 0.40 m^3.m^-3 by an automated, soil moisture sensor-controlled, irrigation system. To achieve different levels of light and light interception, half of plants were shaded for 3 consecutive days each week. Canopy light interception was measured three times a week using a ceptometer (AccuPAR LP-80). The total amount of light intercepted by each crop over the course of the study was calculated from canopy light interception and the daily light integral. Water use was calculated from the number of irrigation events recorded by a data logger. Plant water use was correlated with light interception among all the species (R² = 0.66). However, there was interspecies variability in light interception and in the correlation between light interception and daily water use. When analyzed by species, water use of petunia and impatiens was correlated better with light interception (R² = 0.78 and 0.66, respectively) than that of dianthus and ageratum (R² = 0.45 and 0.58, respectively). Despite similar leaf areas among ageratum, petunia and impatiens (7977, 7543 and 10869 cm²/plant, respectively), the maximum measured light interception for ageratum (75%) was lower than that of petunia (85%) and impatiens (93%) at the same age. Accurate light interception data may be harder to collect in ageratum and dianthus (with a more creeping growth habit) than in impatiens and petunia (with a more upright habit), thus affecting the correlation between measured light interception and water use.