Osmotic adjustment (OA) is a major physiological mechanism associated with maintenance of cell turgor in response to dehydration stress. The objectives of this study were to examine changes in capacity for OA in relation to plant tolerance to drought stress for two cool-season turfgrass species, creeping bentgrass (Agrostis stolonifera L.) and velvet bentgrass (A. canina L.), and to determine major solutes contributing to OA in these grass species. Plants of `L-93' creeping bentgrass and `Greenwich' velvet bentgrass were grown in a growth chamber in polyvinyl chloride (PVC) tubes (5 cm diameter, 40 cm high) filled with a 1:3 (v/v) sterilized mixture of sand and sandy loam soil. The experiment consisted of two soil moisture treatments: 1) well-watered control, irrigated three times per week to maintain soil moisture near pot capacity; and 2) drought stress, irrigation completely withheld. Velvet bentgrass exhibited higher drought tolerance compared to creeping bentgrass, as manifested by higher visual turfgrass quality (TQ) and leaf relative water content (RWC) under drought stress. Both creeping bentgrass and velvet bentgrass exhibited OA in response to drought stress; however, velvet bentgrass exhibited 50% to 60% higher magnitude of OA, which could be related to the maintenance of higher leaf RWC and TQ for greater drought duration compared to creeping bentgrass. OA for both creeping bentgrass and velvet bentgrass was associated with accumulation of water soluble carbohydrates during the early period of drought and increases in proline content following prolonged period of drought; however, inorganic ion content (Ca2+ and K+) did not considerably change under drought stress and did not seem to contribute to OA in these species.
Efficient carbon distribution and utilization may enhance drought survival and recovery ability for perennial grasses. The objectives of this study were to examine changes in carbon partitioning and carbohydrate accumulation patterns in shoots and roots of colonial bentgrass (Agrostis capillaris L.), creeping bentgrass (A. stolonifera L.), and velvet bentgrass (A. canina L.) in response to drought and re-watering following drought, and to determine whether species variation in drought tolerance and recuperative potential is related to differences in the patterns of carbon partitioning and accumulation. The experiment consisted of three treatments: 1) well-watered control; 2) drought, irrigation completely withheld for 18 days; and 3) drought recovery, a group of drought-stressed plants were re-watered at the end of the drought treatment (18 days). Drought tolerance and recuperative ability of three species was evaluated by measuring turf quality and leaf relative water content. These parameters indicated that velvet bentgrass was most drought tolerant while colonial bentgrass had highest recuperative ability among the three species. Plants were labeled with 14CO2 to determine carbon partitioning to shoots and roots. Carbohydrate accumulation was assessed by total nonstructural carbohydrate (TNC) content. The proportion of newly photosynthesized 14C partitioned to roots increased at 12 days of drought compared to the pre-stress level, to a greater extent for velvet bentgrass (45%) than for colonial bentgrass (35%) and creeping bentgrass (30%). In general, the proportion of 14C was highest in roots, intermediate in stems, and lowest in leaves at 12 days of drought treatment for all three bentgrass species. As drought duration and severity increased (18 days), 14C partitioning increased more in leaves and stems relative to that in roots for all three species. Stem TNC content was significantly greater for drought-stressed plants of colonial bentgrass and velvet bentgrass compared to their respective well-watered control plants, whereas no differences in stem TNC content were observed between drought-stressed and well-watered creeping bentgrass. Our results suggest that increased carbon partitioning to roots during initial drought stress represented an adaptive response of bentgrass species to short-term drought stress, and increased carbon partitioning and carbohydrate accumulation in stems during prolonged period of drought stress could be beneficial for rapid recovery of turf growth and water status upon re-watering.
Carbohydrate metabolism is important for plant adaptation to drought stress. The objective of this study was to examine major forms of carbohydrates associated with superior drought tolerance and post-drought recovery in kentucky bluegrass (Poa pratensis) by comparing responses of different forms of carbohydrates with drought stress and re-watering in two cultivars contrasting in drought tolerance. Plants of drought-tolerant ‘Midnight’ and drought-sensitive ‘Brilliant’ were maintained well watered or subjected to drought stress for 10 days by withholding irrigation, and drought-stressed plants were re-watered for 3 days. Physiological analysis (turf quality, relative water content, and electrolyte leakage) confirmed the genetic variability of the two cultivars in drought tolerance. The two cultivars exhibited differential responses to drought stress and re-watering for the content of water-soluble sugars (sucrose, fructose, and glucose) and storage carbohydrates (starch and fructan), and ‘Midnight’ maintained higher sucrose content at 10 days of drought stress and more fructan at 3 days of re-watering. The greater accumulation of sucrose in ‘Midnight’ under drought stress corresponded with higher activities of two sucrose-synthesizing enzymes (sucrose phosphate synthase and sucrose synthase) but was not related to the sucrose-degrading enzyme activity (acid invertase). These results suggested that increased sucrose accumulation resulting from the maintenance of active sucrose synthesis could be associated with superior turf performance during drought stress, whereas increased fructan accumulation could contribute to rapid re-growth and post-drought recovery on re-watering in kentucky bluegrass.
Cold acclimation improves freezing tolerance in various plants, including perennial grass species. The objectives of this study were to determine protein changes in crowns of velvet bentgrass (Agrostis canina) during cold acclimation in association with freezing tolerance. Treatments consisted of: 1) nonacclimated (NA) plants maintained at 18/12 °C (day/night); 2) plants acclimated at a constant 2 °C for 4 weeks with a 10-hour photoperiod [A4 (cold acclimation)]; and 3) plants acclimated at a constant 2 °C for 4 weeks with additional subzero acclimation (SZA) at a constant –2 °C for 2 weeks (A4 + SZA2). Exposing plants to A4 significantly increased freezing tolerance, but additional SZA had no further beneficial effects on freezing tolerance, as demonstrated by the lethal temperature for 50% of the test population (LT50). Thirteen protein spots with increased abundance (up-regulated) or with decreased abundance (down-regulated) during cold acclimation were identified for biological functions. Proteins up-regulated after cold acclimation (A4 or A4 + SZA2) included methionine synthase, serine hydroxymethyltransferase, aconitase, UDP-D-glucuronate decarboxylase, and putative glycine-rich protein. Cold acclimation-responsive proteins involved in amino acid metabolism, energy production, stress defense, and secondary metabolism could contribute to the improved freezing tolerance induced by cold acclimation in velvet bentgrass.