High Soil Available Phosphorus Favors Carbon Metabolism in Cotton Leaves in Pot Trials

Phosphorus (P) is an essential macronutrient for plant growth and its metabolism. Because reproductive growth could be influenced by carbon metabolism of the functional leaf (MSL), we hypothesized that soil available phosphorus (AP) deficiency would limit boll dry mass distributions of the whole cotton plant by decreasing photosynthesis of MSL and inhibiting carbon metabolism in the MSL. This study aimed to determine the correlations between carbon metabolism and cotton boll dry mass in relation to soil AP deficiency, and key carbon-metabolism enzymes responsive to P for different low-P-sensitive cultivars. A three-year (2015–2017) pot experiment was conducted with three available phosphorus (AP) levels [3 (P0, AP deficiency), 8 (P1), and 12 (P2, as a control) mg P2O5 kg−1] with two cotton cultivars (CCRI 79 and LMY 28). Low soil AP had significant decreases on leaf P content, net photosynthetic rate (Pn) in the functional leaf (MSL), as well as boll dry mass. The P0 and P1 caused a significant increase in nonstructural carbohydrates (soluble sugar, sucrose, hexose, starch), in especial sucrose (↑8–17% for CCRI 79 and ↑20–32% for LMY 28). Similarly, the lower P levels (P0 and P1) prominently down-regulated the activities of Rubisco, sucrose phosphate synthase (SPS), and amylase, but up-regulated cytosolic fructose-1,6-bisphosphatases and soluble acid invertase (SAI). Moreover, only the absolute value of slope in SAI for LMY 28 was higher than that for CCRI 79 in 2015 and 2016. Our results indicated that the response of carbon-metabolizing enzymes to low AP content was mediated by SAI for CCRI 79 and LMY 28. In our results, Pn, Rubisco, and SAI were the dominant factors affecting the boll dry mass under low soil AP.