Physiological Processes Contributing to Early Season Crop Vigor in Cotton

Seed characteristics and early season temperature conditions can influence seedling vigor in cotton. What has not been explored fully is the impact of the aforementioned factors on whole crop growth and physiological response. It was hypothesized that large seeded cotton would demonstrate greater early season vigor, thereby promoting early season crop growth through positive impacts on leaf area development or whole-canopy net assimilation rate. To test this hypothesis, a small (DP1614; 72 mg seed^-1) and large (DP1612; 94 mg seed^-1) seeded Upland cultivar and a Pima cultivar (DP348; 138 mg seed^-1) were planted in 2017 and 2018 at three different dates. Measurements consisted of destructively harvesting plants from 2-m sections and measuring leaf area, and dry weights (leaf and stem). From these measurements crop growth indices between 21 and 35 DAP calculated were Crop Growth Rate (CGR), Net Assimilation Rate (NAR), and average Leaf Area Index (LAI). Growth analysis results from both the years showed that CGR and LAI for large seeded cultivars were significantly higher than the small seeded cultivar. Wherever significant differences were found, LAI of Pima was significantly higher than larger seeded upland cultivar. For 2017, NAR for both upland cultivars was higher than the Pima or all planting dates. For 2018, no significant differences in NAR were found except for first planting date (p= 0.0037), where higher NAR was observed for larger seeded cultivars. The study indicated that larger seeded varieties exhibited greater early season crop vigor, as evidenced by higher CGR especially under early season planting conditions when compared to small seeded cultivars. When considered across both years, these differences in early crop growth were very strongly and positively associated with LAI (P < 0.001; r² = 0.97), but only weakly associated with photosynthetic efficiency of the canopy (P = 0.014; r² = 0.32).