Ecological And Physiological Responses Of Cotton To Drought During Flowering And Boll-forming Period

Drought is one of the most important abiotic stress factors that limit crop production worldwide. Cotton is grown in a wide region around the world and is negatively affected by water stress. What's worse, changes in climate might lead to expanding of drought-affected areas and enhancement of drought intensity according to projected increase in global air temperature. Therefore, understanding cotton physiological mechanisms in response to water stress is critical for cotton production improvement via stress-tolerant genotypes identification and management practices. Here, pot and pond trials were conducted to investigate physiological and ecological responses of cotton to various soil water regimes Experiments were conducted at the experimental station (32°02?N and 118°50?E) of Nanjing Agricultural University in Nanjing with cotton (c.v. Siza 3). There were three water regimes: a well-watered, a mild drought and a severe drought treatment, with soil water content maintained at 75 ± 5%, 60 ± 5% and 45 ± 5% of the filed capacity, donated as W75,W60 and W45, respectively. We evaluated (1) effects of drought on microclimate, canopy architecture, biomass accumulation and allocation, and boll distribution; (2) the relationship between yield and fiber quality with plant water status; (3) physiological responses of carbon assimilation, allocation and transport in leaf subtending to cotton boll to drought and its relationship with boll biomass accumulation; (4) changes of carbon metabolism, osmotic adjustment and antioxidant capacity in cotton functional leaf during the drought period. Our primary objective of this study was to provide an insight into the physiological and ecological responses of cotton to sustained drought during boll-forming period. The results would be available for better designing irrigation strategy to mitigate drought disaster and optimize the utilization of water resource.The main results are as follows:1. Drought plants produced fewer bolls on higher fruiting branches and more distal fruiting positions than did the well-watered plants.Decreasing soil water content increased leaf temperature while decreased net assimilation rate, leaf area and radiation use efficiency. Drought inhibited plant biomass accumulation and reduced the proportion of biomass that allocated to reproductive origins.Plant biomass in W60 and W45 was decreased by 25% and 50%,respectively, compared with W75. Biomass of reproductive organs in W60 and W45 was decreased by 30% and 60%, respectively. Drought not only reduced lint yield (decreased by 31-35% and 57-60%under (60±5)% FC and (45±5)% FC,respectively) but also altered yield distribution on different fruiting branches (FB). Drought plants produced fewer bolls on higher FB and more distal fruiting positions than did the well-watered plants. More proportion of yield was allocated at lower FB in drought plants than well-watered plants.2. Boll weight, fiber length and strength were decreased linearly with decreased average leaf water potential during boll maturation period.Bolls on different FB developed at different time probably experienced different environments. In the present study, the bolls at various fruiting branches differed in their response to drought. Bolls on higher FB were more affected by soil drought. Boll weight,seed index, fiber length and strength declined linearly with decreasing mean midday leaf water potential during fruit maturation period in the range of about 0.7 g, 1.0 g, 2.4 mm and 3.4 cN tex-1 per MPa, respectively. Micronaire value was greater on FB branches under the same water regime, while no consistent influences of drought on micronaire were obtained.Micronaire was probably affected by the interactive effect of drought and higher temperature.3. Source capacity of leaf subtending to cotton boll and boll biomass on higher FB were more affected by drought.Our research showed drought increased light reception while declined net photosynthesis rate (Pn) of leaf subtending to cotton boll (LSCB),being more pronounced on higher FB. Pn in LSCB rapidly decreased as FB lowered, primarily due to declining light reception and leaf photosynthetic potential. The drought stressed or severely shaded LSCB contributed little to boll development. Drought stimulated the accumulation of carbohydrate in LSCB on various FB. Carbon profiles were, however, altered uniformly at the plant level, being not related to leaf position or age. Carbon production and its use by bolls was out of phase between LSCB and the subtended bolls. Boll biomass was notably decreased on higher FB while maintained on lower FB. This indicated drought promoted carbon allocated into older bolls.4. The accumulated soluble sugars contribute less to osmotic adjustment in cotton leaves under drought condition.Soil water deficit induced global increased soluble carbon metabolites concentrations in cotton leaves, possibly due to growth being more intensively affected than photosynthesis and carbon metabolism. The major contributor to OA depended on the duration of soil water deficit. K+ and amino acid in cotton leaf contributed most to OA at the early and later growing periods, respectively, during the water deficit period.Accumulated sugars in cotton leaf contributed less to OA compared with other measured solutes but showed a slightly increased trend as the duration of soil water stress increased.O2- production rate in cotton leaves was increased by soil water deficit, possibly because of the decreased SOD activity. The increased MDA concentration in cotton leaves indicated that the antioxidant level was not sufficient to prevent long-term damage due to soil water deficit.In this work, we have evaluated the various responses in bolls and LSCB on different FB to drought and identified the functional relationship between midday leaf water potential and boll characters. Our data also revealed changes in cotton growth, leaf Pn and carbohydrate profiles and the modification of OA and ROS-scavenging mechanism in response to the long-term soil water deficit. These results have strong implications on designing better irrigation strategies and improving the functionality of models for cotton.Notably negative influences in yield and fiber quality of drought only occurred on upper fruiting branches or under severe drought condition. We can supply controlled soil drying during later growth stage of cotton in arid area with short growing period, and the sacrifice of boll number per plant could be, to some extent, compensated by increasing cotton planting density. And for the area with too many moisture during boll-forming period,growth regulator could be applied to motivate plant drought responses, thus, inhibit the exceeding vegetative growth and optimize canopy architecture. Base on these practices,cotton would be more suitable for high density planting and mechanical harvesting.