Evidence is accumulating in favor of a linkage at the cellular level between various abiotic stresses. We conducted a study to evaluate the effect of water stress on the heat tolerance of zonal geraniums, Pelargonium×hortorum cv. Evening Glow. Water stress was imposed by withholding irrigation until pots reached 30% (by weight) of well‐watered controls, and by maintaining the pot weight by additions of water for another 7 days. Leaf xylem water potential (XWP, MPa), relative water content (RWC. %), and heat‐stress tolerance (HST; LT50, defined as the temperature causing half‐maximal % injury based on electrolyte leakage) were measured in control, stressed, and recovered plants. Proteins were extracted from the leaves following the above treatments, and SDS‐PAGE and immunoblotting were performed by using standard procedures. Immunoblots were probed with antibodies to dehydrin and 70‐kDa heat shock cognate (HSC70) proteins. Data indicate that XWP and RWC, respectively, were −0.378 MPa and 92.3% for control plants and −0.804 MPa and 78.6% for stressed plants. Water‐stressed plants exhibited a significant increase in HST compared to control (LT50 of 55°C vs 51°C). Water‐stress‐induced HST was not due to heat acclimation (leaf warming in stressed plants). Data also indicate that water‐stress treatment did not increase freezing tolerance of geranium leaves. Increased HST was associated with the accumulation of several heat‐stable, dehydrin proteins (25–60 kDa), and both cytosolic and ER luminal (BiP) HSC70 proteins. Leaf XWP, RWC, and HST reversed to control levels concomitant with the disappearance/reduction of dehydrins and HSC70 proteins in water‐stress‐relieved plants. The possibility of a cellular linkage between water stress and heat‐stress tolerance is discussed.

An appropriate blend of growing media components increases water holding capacity and reduces irrigation frequency. Synthetic commercial materials, referred to as hydrogels, have remarkable hydrating properties, but can add significantly (about 15%) to the cost of growing media. The literature generally states that the physical characteristics of hydrogels, such as polyacrylamide (PAM), are altered by the presence of divalent cations (Ca2+ and Mg2+). Few studies, however, have simultaneously investigated plant growth and development and media characteristics on a daily basis throughout plant production. Thus, the mechanisms explaining the reported beneficial and/or detrimental effects from PAM incorporation remain hidden. In this study, canopy ground cover of two species [pansy (Viola ×wittrockiana Gams) and new guinea impatiens (Impatiens hawkeri Bull)] was measured daily, from transplanting to marketable size, using digital imaging to determine growth differences of plants grown in media containing different amounts of PAM. Media water content was determined with time-domain reflectance probes every 10 minutes in media treatments. Total number of irrigation events, time between irrigation events, root development after 4 and 8 weeks of growth, flower number, flower longevity, and dry masses of the shoot were also measured. Scanning electron microscopy revealed significant structural differences in hydrated PAM depending on water quality. The pansy canopy coverage was significantly greater with hydrogels, and root growth early in production was enhanced with PAM. No such effect was observed for new guinea impatiens. Total flower numbers and flower longevity of new guinea impatiens decreased with increasing amount of PAM (16.7% or higher) in the media. PAM incorporation reduced the need for irrigation early in production for both species, but by the end of production, those new guinea impatiens plants were smaller (less shoot dry mass) and required irrigation as often as plants grown without PAM. This effect coincided with reduced media volume, air capacity, and total porosity in PAM-containing media. Theoretical analysis of the potential benefits from hydrogels confirms the potential benefit early in production with little to no benefit later in production and in post-production. These data will assist growers in determining if the benefits derived from the use of PAM justify the added cost of medium.

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Plant performance and appearance in deficient and toxic levels of nutrients are well characterized. However, less is known about the potential subtleties of plant growth, form, development, nutrient uptake, and biotic stress tolerance in the broad tolerable range. Begonia [Beg (Begonia × tuberhybrida Voss)] and new guinea impatiens [NGI (Impatiens hawkeri Bull.)] were grown over a wide range of N (from 1.78 to 57.1 mm NH4:NO3 ratio at a 1:1 ratio supplied as nutrient solution) in a peat:perlite soilless substrate in greenhouse conditions. Plant growth, development, chlorophyll content, leaf angle, nutrient uptake, tissue caloric value, and susceptibility to Botrytis cinerea Pers.:Fr. disease were evaluated in two experiments. Elevated N supply resulted in decreased plant height (16% in Beg and 7% to 16% in NGI), flower count (3% to 48% in Beg and 7% to 49% in NGI), bud numbers (23% to 80% in Beg), canopy area (11% to 33% in NGI), and mass (21% to 33% in Beg and 18% to 58% in NGI). Chlorophyll content saturated at an N supply of 28.6 mm. N uptake efficiency, shoot N use efficiency, and shoot N utilization efficiency decreased with increasing N supply. Elevated levels of N supply from 7.15 to 57.1 mm also increased the susceptibility of Beg to B. cinerea disease by 10% to 80% in stems and 3% to 14% in leaves. The increase in susceptibility also corresponded with increased tissue energy content (kJ·g−1) and altered leaf orientation. This study indicates many plant changes occur between nutrient extremes that can have a significant impact on growth, development, and the ability to withstand disease.

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