Water relations of bedding plants
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Water scarcity has brought about the need for more efficient water use in agriculture. To better understand the water use and drought responses of bedding plants, I conducted studies on modeling daily water use of petunia based on plant and environmental factors, the drought physiology of vinca in response to different rates of drought stress development, and gene expression and physiological changes of petunia at specific substrate water contents (θ). Daily water use of two petunia cultivars (Petunia × hybrida ‘Single Dreams Pink’ and ‘Prostrate Easy Wave Pink’) was explained with a regression model based on plant age and environmental factors, such as daily light integral, vapor pressure deficit, and temperature (R2=0.93 and 0.91 for ‘Single Dreams Pink’ and ‘Prostrate Easy Wave Pink’, respectively). Plant age was the most important factor affecting daily water use due to increasing plant size, while the daily light integral was the important environmental factor. A better understanding of plant responses to drought is needed to predict plant responses to deficit irrigation or drought. Reductions in photosynthesis, respiration, and transpiration of vinca (Catharanthus roseus) in response to low θ were less severe in plants subjected to slow drying than in plants subjected to fast drying, suggesting that the rate at which drought stress develops has important implications for the level of acclimation that occurs. Physiology and gene expression of petunia (Petunia × hybrida ‘Apple Blossom’) displayed θ specific responses, showing acclimation of stomatal conductance and photosynthesis under mild drought (θ of 0.20 and 0.30 m3•m-3), but less acclimation and high abscisic acid concentrations in the leaves under severe drought (θ=0.10 m3•m-3). Regardless of θ, stomatal conductance of petunia was highly correlated with leaf ABA concentration. Although putative ABA biosynthesis genes (NCED and AAO3) did not significantly respond to drought, the ABA catabolic gene CYP707A and ABA response gene PLDα responded to the drought, showing significant correlation with ABA level, suggesting that these two genes may be involved in regulating ABA levels and drought signaling. To better understand water relations of bedding plants, precise descriptions of the drought imposition and severity and integrated studies combining gene expression with physiological measurement will be needed for a more comprehensive view of plant responses.